U.S. patent application number 13/886131 was filed with the patent office on 2013-12-26 for indirect evaporative cooler using membrane-contained liquid desiccant for dehumidification and flocked surfaces to provide coolant flow.
This patent application is currently assigned to ALLIANCE FOR SUSTAINABLE ENERGY, LLC. The applicant listed for this patent is Alliance For Sustainable Energy, LLC. Invention is credited to Eric J. KOZUBAL, Jason D. WOODS.
Application Number | 20130340449 13/886131 |
Document ID | / |
Family ID | 49769377 |
Filed Date | 2013-12-26 |
United States Patent
Application |
20130340449 |
Kind Code |
A1 |
KOZUBAL; Eric J. ; et
al. |
December 26, 2013 |
INDIRECT EVAPORATIVE COOLER USING MEMBRANE-CONTAINED LIQUID
DESICCANT FOR DEHUMIDIFICATION AND FLOCKED SURFACES TO PROVIDE
COOLANT FLOW
Abstract
An apparatus for conditioning an inlet air stream. A first stage
is provided with a dehumidifier cooling an air stream input by
absorption of water vapor from the input air stream. A second stage
is provided with an indirect evaporative cooler to receive a cooled
portion of the input air stream and sensibly cool the received
portion of the input air stream to a temperature range near the dew
point temperature. A first portion of the sensibly cooled air
stream is exhausted to a cooled space while a second portion is
directed to a wet side of the indirect evaporative cooler and
receives heat to sensibly cool the input air stream. A flow channel
for the second portion of the sensibly cooled air stream in the
indirect evaporative cooler is defined by a surface of a separation
wall covered with wicking material acting to wick a stream of
liquid coolant.
Inventors: |
KOZUBAL; Eric J.; (Superior,
CO) ; WOODS; Jason D.; (Boulder, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Energy, LLC; Alliance For Sustainable |
|
|
US |
|
|
Assignee: |
ALLIANCE FOR SUSTAINABLE ENERGY,
LLC
Golden
CO
|
Family ID: |
49769377 |
Appl. No.: |
13/886131 |
Filed: |
May 2, 2013 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61662146 |
Jun 20, 2012 |
|
|
|
Current U.S.
Class: |
62/92 ; 62/271;
62/94 |
Current CPC
Class: |
F28D 5/00 20130101; F24F
3/147 20130101; Y02B 30/54 20130101; F25B 15/00 20130101; F24F
5/0035 20130101 |
Class at
Publication: |
62/92 ; 62/271;
62/94 |
International
Class: |
F25B 15/00 20060101
F25B015/00 |
Goverment Interests
CONTRACTUAL ORIGIN
[0002] The United States Government has rights in this invention
under Contract No. DE-AC36-08G028308 between the United States
Department of Energy and the Alliance for Sustainable Energy, LLC,
the Manager and Operator of the National Renewable Energy
Laboratory.
Claims
1. An apparatus for dehumidifying air supplied to a space,
comprising: a first flow channel for receiving a stream of inlet
supply air, wherein the first flow channel is defined in part by a
vapor permeable membrane; a second flow channel adjacent to the
first flow channel receiving a stream of liquid desiccant; and a
third flow channel adjacent to the second flow channel receiving a
stream of exhaust air, whereby heat is transferred from the stream
of inlet supply air to the stream of exhaust air in the third flow
channel via the stream of liquid desiccant, wherein the third flow
channel is defined in part by a first surface of a separation wall,
wherein the separation wall is spaced apart a distance from the
vapor permeable membrane, and wherein a layer of wicking material
is provided on the first surface acting to wick a stream of liquid
coolant.
2. The apparatus of claim 1, wherein the second flow channel is
defined by the vapor permeable membrane and a second surface of a
separation wall, opposite the first surface, that is impermeable to
fluid.
3. The apparatus of claim 1, wherein the stream of liquid coolant
flow through the wicking material layer is gravity driven.
4. The apparatus of claim 1, wherein the wicking material is
knitted nylon fabric, polypropylene woven fabric, polypropylene
non-woven fabric, adhesive-backed flocking fibers, or one or more
fabrics coated with a hydrophilic coating and wherein the liquid
desiccant comprises a salt solution and the liquid coolant
comprises water.
5. The apparatus of claim 1, wherein the layer of wicking material
comprises a hydrophilic coating on the first surface of the
separation wall.
6. The apparatus of claim 1, wherein the exhaust air comprises a
portion of the stream of inlet supply air exiting the first flow
channel at about the second, lower enthalpy.
7. The apparatus of claim 1, wherein the stream of inlet supply air
flows in a first direction in the first flow channel and the stream
of exhaust air flows in a second direction in the third flow
channel, the second direction being in at least one of cross or
counter to the first direction, and wherein the third flow channel
is arranged such that the exhaust air stream flows in an at least a
partially cross or counter flow direction relative to the stream of
inlet supply air in the first flow channel.
8. A method of conditioning a supply air stream, comprising:
dehumidifying the supply air stream to provide a dehumidified air
stream, wherein the dehumidifying includes directing the supply air
stream through a first channel defined by a surface of a vapor
permeable membrane containing liquid desiccant; and transferring
heat from the liquid desiccant to a layer of coolant flowing in a
second channel adjacent to the liquid desiccant.
9. The method of claim 8, wherein the layer of coolant comprises
coolant flowing in a layer of wicking material positioned for heat
transfer from the liquid desiccant and wherein the method further
includes evaporating a portion of the coolant flowing in the layer
of wicking material into an air stream flowing adjacent to the
layer of wicking material.
10. The method of claim 9, wherein the coolant in the layer is
flowing at a flow rate of less than about 50 inches per minute.
11. The method of claim 8, wherein a temperature of the supply air
stream is increased during the dehumidifying less than about
5.degree. F.
12. The method of claim 8, wherein the dewpoint temperature of the
supply air stream is decreased during the dehumidifying to less
than about 55.degree. F.
13. An apparatus for conditioning an inlet air stream, comprising:
a first stage comprising a dehumidifier operable for cooling an air
stream input to an inlet of the dehumidifier by absorption of water
vapor from the input air stream; and a second stage comprising an
indirect evaporative cooler in fluid communication with an outlet
of the first stage to receive a cooled portion of the input air
stream, wherein the indirect evaporative cooler is operable to
sensibly cool the received and cooled portion of the input air
stream to a temperature range less than the wet bulb temperature
and greater than the dew point temperature of the input air stream;
wherein a first portion of the sensibly cooled air stream is
supplied to a cooled space, and wherein a second portion of the
sensibly cooled air stream is directed to a wet side of the
indirect evaporative cooler and receives heat from the received and
cooled portion of the input air stream.
14. The apparatus of claim 13, wherein the dehumidifier performs
the cooling using a membrane-contained liquid desiccant cooled by
an indirect evaporative channel that includes a wicking layer
containing a flow of coolant.
15. The apparatus of claim 13, wherein the dehumidifier performs
the cooling using a membrane-contained liquid desiccant that is
liquid cooled.
16. The apparatus of claim 13, wherein the second portion comprises
10 to 40 percent of the received and cooled portion of the input
air.
17. The apparatus of claim 13, wherein a flow channel for the
second portion of the sensibly cooled air stream in the indirect
evaporative cooler is defined in part by a surface of a separation
wall and wherein a layer of wicking material is provided on the
surface acting to wick a stream of liquid coolant adjacent the flow
channel.
18. The apparatus of claim 13, further comprising a direct
evaporative stage between the second stage and the cooled space,
wherein the direct evaporative stage receives the first portion of
the sensibly cooled air stream and provides additional cooling via
direct evaporative cooling prior to supplying the first portion of
the sensibly cooled air stream to the cooled space.
19. The apparatus of claim 18, wherein the direct evaporative stage
is adapted to contain flow of water with a vapor permeable membrane
or within a layer of wicking material.
20. The apparatus of claim 19, wherein the layer of wicking
material is provided as a surface with a hydrophilic coating.
21. A method for conditioning a supply air stream, comprising:
cooling an air stream by absorption of water vapor from the air
stream to a first wet bulb temperature and first dewpoint
temperature; after the cooling by absorption, using indirect
evaporative cooling to sensibly cool the air stream to a second
temperature; and supplying a first portion of the sensibly cooled
air stream to a space, wherein a second portion of the sensibly
cooled air stream is used to perform the indirect evaporative
cooling of the air stream.
22. The method of claim 21, wherein the second temperature is less
than the first wet bulb temperature and greater than the first
dewpoint temperature and the cooling is performed using
membrane-contained liquid desiccant or with a liquid coolant
flowing on a separation wall covered with a layer of wicking
material or a hydrophilic coating.
23. The method of claim 21, further comprising, prior to the
supplying the first portion to the space, humidifying the sensibly
cooled air stream by providing a flow of water adjacent to the
sensibly cooled air stream, wherein the flow of water with a vapor
permeable membrane or within a layer of wicking material.
24. The method of claim 21, further comprising, prior to the
supplying the first portion to the space, providing additional
cooling to the sensibly cooled air stream via direct evaporative
cooling.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 61/662,146, filed Jun. 20, 2012, which is
incorporated herein in its entirety.
BACKGROUND
[0003] Air conditioning is used worldwide to provide comfortable
and healthy indoor environments that are properly ventilated and
cooled and that have adequate humidity control. While being useful
for conditioning supply air, conventional air conditioning systems
are costly to operate as they use large amounts of energy (e.g.,
electricity). With the growing demand for energy, the cost of air
conditioning is expected to increase, and there is a growing demand
for more efficient air conditioning methods and technologies.
Additionally, there are increasing demands for cooling technologies
that do not use chemicals and materials, such as many conventional
refrigerants, that may damage the environment if released or
leaked. Maintenance is also a concern with many air conditioning
technologies, and, as a result, any new technology that is
perceived as having increased maintenance requirements, especially
for residential use, will be resisted by the marketplace.
[0004] Evaporative coolers are used in some cases to address air
conditioning demands or needs, but, due to a number of limitations,
conventional evaporative coolers have not been widely adopted for
use in commercial or residential buildings. Evaporative coolers,
which are often called swamp coolers, are devices that use simple
evaporation of water in air to provide cooling in contrast to
conventional air conditioners that use refrigeration or absorption
devices using the vapor-compression or absorption refrigeration
cycles. The use of evaporative cooling has typically been limited
to climates where the air is hot and humidity is low such as in the
western United States. In such dry climates, the installation and
operating costs of a conventional evaporative cooler can be lower
than refrigerative air conditioning. Residential and industrial
evaporative coolers typically use direct evaporative cooling with
warm dry air being mixed with water to change the water to vapor
and using the latent heat of evaporation to create cool moist air
(e.g., cool air with a relative humidity of 50 to 70 percent). For
example, the evaporative cooler may be provided in an enclosed
metal or plastic box with vented sides containing a fan or blower,
an electric motor to operate the fan, and a water pump to wet
evaporative cooling pads. To provide cooling, the fan draws ambient
air through vents on the unit's sides and through the dampened
pads. Heat in the air evaporates water from the pads, which are
continually moistened to continue the cooling process. The cooled,
moist air is then delivered to the building via a vent in the roof
or a wall.
[0005] While having an operation cost of about one fourth of
refrigerated air conditioning, evaporative coolers have not been
widely used to address needs for higher efficiency and lower cost
conditioning technologies. One problem with many sump coolers is
that in certain conditions these evaporative coolers cannot operate
to provide adequately cooled air. For example, air may only be
cooled to about 75.degree. F. when the input air is 90.degree. F.
and 50 percent relative humidity, and such cooling may not be
adequate to cool a particular space. The problem may get worse as
temperatures increase well over 100.degree. F. as found in many
locations in the southwest portion of the United States and
elsewhere. As a result, the air conditioning system may need to
include refrigerated air conditioning to cool the outlet air from
the evaporative cooler, which results in a system that is more
expensive to purchase, operate, and maintain.
[0006] Additionally, conventional evaporative coolers provide no
dehumidification of the air and, in fact, often output air at 80 to
90 percent relative humidity, which may only be acceptable in very
dry environments as very humid air reduces the rate of evaporation
for occupants of the building (e.g., reduces comfort levels) and
can cause condensation resulting in corrosion or other problems.
Dehumidification is provided as a second or later stage in some
evaporative coolers such as by wicking a liquid desiccant along a
wall of the air flow channel or chamber, but such systems have not
been widely adopted due to increased operating and maintenance
costs and concerns of having the desiccant expelled with the
conditioned air. In general, maintenance is a concern with
evaporative coolers as the evaporation process can result in
mineral deposits on the cooling pads and other surfaces of the
cooler that need to be cleaned or replaced to maintain the
efficiency of the system, and the water supply line needs to be
protected against freezing during the off season such as by
draining the system. Due to these and other concerns, evaporative
cooling is unlikely to be widely used to provide an energy
efficient, air conditioning alternative for commercial and
residential applications until significant improvements are made
that address maintenance concerns while improving achievable
cooling (e.g., providing adequately cooled output air for direct
use in a building).
[0007] The foregoing examples of the related art and limitations
related therewith are intended to be illustrative and not
exclusive. Other limitations of the related art will become
apparent to those of skill in the art upon a reading of the
specification and a study of the drawings.
SUMMARY
[0008] The following embodiments and aspects thereof are described
and illustrated in conjunction with systems, tools and methods that
are meant to be exemplary and illustrative, not limiting in scope.
In various embodiments, one or more of the above-described problems
have been reduced or eliminated, while other embodiments are
directed to other improvements.
[0009] This is achieved, in part in some applications, by providing
a mass/heat transfer assembly for use in indirect evaporative
coolers or heat exchangers. The assembly is formed of alternating
stacks each including a first (or upper) layer or sheet of membrane
material, a separation wall, and a second (or lower) layer or sheet
of membrane material. The membrane or membrane material for each
layer is permeable to water molecules in the vapor state while the
separation wall is impermeable to water but allows heat transfer
(e.g., is a thin layer and/or is made of materials that conduct
heat). In a first one of adjacent pairs of stacks, coolant such as
water flows between the first membrane layer and the separation
wall and liquid desiccant flows between the separation wall and the
second membrane layer while in the second or next one of the
adjacent pairs of stacks the flow order is reversed. This ordering
is repeated throughout the mass/heat transfer assembly to form
alternating supply and exhaust air flow channels or chambers.
Supply air (or air to be conditioned) is directed through a channel
between a first pair of stacks while a portion of the pre-cooled
exhaust air (e.g., a fraction of the supply air that is cooled by
flowing through the stacks) is directed through a chamber between a
second or next pair of stacks (e.g., typically in a counterflow
arrangement relative to the flow of the incoming supply air).
Liquid desiccant is provided proximate to the supply inlet airflow
while coolant such as water is provided proximate to the exhaust
airflow (i.e., a fraction of supply outlet airflow directed to be
exhausted) with the air only being separated from these flowing
liquids by the water permeable membrane. The supply air inlet
airflow, supply outlet airflow, exhaust airflow, liquid desiccant
flow, and coolant flow are plumbed such as via one or more manifold
assemblies to the mass/heat transfer assembly, which can be
provided in a housing as a single unit (e.g., an indirect
evaporative cooler).
[0010] In a typical embodiment, dehumidification and evaporative
cooling are accomplished by separation of the air to be processed
and the liquid and/or gas substances (e.g., liquid desiccant,
water, desiccated air, and the like) by a membrane. The membrane is
formed of one or more substances or materials to be permeable to
water molecules in the vapor state. The permeation of the water
molecules through the membrane is a driving force behind (or
enables) dehumidification (or dehumidification in some
implementations) and evaporative cooling of one or more process air
streams. As described above, multiple air streams can be arranged
to flow through chambers in the mass/heat transfer assembly such
that a secondary (purge) air stream, such as the exhaust airflow of
pre-cooled supply air, is humidified and absorbs enthalpy from a
primary (process) air stream, such as the supply inlet airflow that
can then be directed to a building as supply outlet airflow (e.g.,
make up air for a residential or commercial buildings or the like).
The process air stream is sensibly cooled and is, in some
embodiments, simultaneously dehumidified by providing a liquid
desiccant flow contained by membranes defining the sidewalls of the
supply inlet airflow channel or chamber.
[0011] The membrane is also used in some embodiments to define
sidewalls of the exhaust (e.g., counter) airflow channel or chamber
such that the membrane controls or separates coolant liquid from
the exhaust air stream. Wicking materials/surfaces or other devices
may be used to contain or control water flow (e.g., direct-contact
wicking surfaces could be used in combination with the use of the
liquid desiccant containment by a membrane), but membrane liquid
control facilitates fabrication of the stacks or manifold structure
useful for heat and mass exchanger/assembly configurations
described herein that provide cooling, dehumidification, and/or
humidification. In such configurations, the air streams can be
arranged in counter-flow, counter-flow with pre-cooled exhaust air,
cross-flow, parallel flow, and impinging flow to perform desired
simultaneous heat and mass transfer in the evaporative cooling
units.
[0012] By way of example, but not limitation, an embodiment
includes an indirect evaporative cooler for cooling a stream of
inlet supply air from a first temperature to a second, lower
temperature using a stream of liquid coolant and a stream of
exhaust or purge air. The cooler includes a first flow channel
through which the stream of inlet supply air flows and a second
flow channel adjacent the first flow channel through which the
stream of exhaust air, at a lower temperature than the inlet or
first temperature of the supply air, flows. The second flow channel
is formed or defined in part by a sheet of a membrane or membrane
material that is permeable to water vapor but that otherwise
contains the liquid coolant. In this manner, the coolant flows on a
side of the membrane (and not in direct contact with) the air in
the second flow channel but mass is transferred as a vapor through
the membrane to the exhaust air when or in response to heat being
transferred from the inlet supply air to the liquid coolant. In
some cases or configurations, as will become clear, the supply air
stream (or inlet supply air) is cooled and dehumidified in this
first stage. A second stage may be provided to sensibly cool the
air stream to a very cool temperature, which could be below the
dewpoint of the original supply inlet air as it was dehumidified
initially or in the first state to allow this.
[0013] A separation wall that is spaced apart from the sheet of
membrane is used to define a flow channel for the liquid coolant,
with the wall being formed from a material (such as plastic) that
is impermeable to the liquid coolant but that conducts or allows
the heat to be transferred from the inlet air supply to the
coolant. A second sheet of membrane may be spaced apart from the
opposite side of this separation wall to define a flow channel for
a liquid desiccant, and during operation, water vapor is
transferred from the stream of inlet supply air through the
membrane to the liquid desiccant, which results in the inlet supply
air being concurrently cooled and dehumidified. The membrane is
effective for resisting or even fully blocking flow of the liquid
coolant and the liquid desiccant while allowing flow of water
vapor, and, in some embodiments, the coolant is water and the
desiccant is a halide salt solution (e.g., a weak desiccant such as
CaCl or the like). The exhaust air in some cases is a redirected
portion of the stream of inlet supply air after it has been cooled
to the second, lower temperature (e.g., as it is exiting the first
flow channel), and the exhaust air may flow in a direction through
the second flow channel that is cross, counter, or a combination of
these relative to the supply air flowing in the first flow
channel.
[0014] In another exemplary embodiment, a method is provided for
conditioning a process or return air for a residential or
commercial building. The method includes first directing the
process air through a first flow channel and second directing a
stream or volume of liquid desiccant adjacent one or more walls
defining the first flow channel, the liquid desiccant is separated
from the process air by a membrane (e.g., the membrane provides the
walls) that contains the liquid desiccant and also allows water
vapor from the process air to flow into and be absorbed by the
liquid desiccant, which dehumidifies the process air. The method
further includes concurrent with the first and second directing,
third directing a stream of purge air through a second flow channel
proximate to the first flow channel (e.g., parallel and adjacent).
The purge air is at a temperature lower than all or at least a
substantial portion of the process air in the first flow channel,
and in some cases, the purge air is a fraction of the dehumidified
process air exiting the first flow channel that is directed in a
counter flow direction relative to the process air through the
second flow channel. The method also includes fourth directing a
stream of liquid coolant adjacent a wall of the second flow
channel. The liquid coolant is also separated from the air by a
membrane that is permeable to vapor from the coolant such that mass
is transferred from the coolant to the purge air. The method
provides for concurrent (or single stage) dehumidification and
cooling of the process air.
[0015] According to another aspect, a mass and heat transfer
assembly is provided for use in an indirect evaporative cooler or
exchanger device. The assembly includes a first stack including an
upper membrane, a lower membrane, and a separation wall between the
upper and lower membranes. The upper and lower membranes are
permeable to water in vapor form and the separation wall is
substantially impermeable to liquid and vapor. Second and third
stacks are provided that also each includes an upper membrane, a
lower membrane, and a separation wall positioned therebetween. In
the assembly, the first stack and second stacks are spaced apart
(such as less than about 0.25 to 0.5 inches apart) to define a flow
channel for receiving a first stream of air (e.g., air to be
conditioned) and the second and third stacks are spaced apart to
define a flow channel for a second stream of air (e.g., purge or
exhaust air directed in cross or counter flow relative to the first
stream of air). In some configurations and/or operating modes, the
device does only evaporative cooling and no dehumidification. Such
that the membranes are only used on the purge side and the other
side of the wall is left bare for the supply air to exchange
heat.
[0016] The first, second, and third stacks may be considered a set
of stacks, and the assembly includes a plurality of such sets of
stacks to define a plurality of air flow channels spaced apart by
the stacks or layers of membranes and separation walls. A divider
or separator may be provided in the flow channels to maintain
spacing of the membranes while allowing flow of the air streams in
the channels. The assembly may further include in the first stack a
liquid coolant flowing between the upper membrane and the
separation wall and a liquid desiccant flowing between the
separation wall and the lower membrane. In the second stack, a
liquid desiccant flows between the upper membrane and the
separation wall while a liquid coolant flows between the separation
wall and the lower membrane. In the third stack, liquid desiccant
flows between the upper membrane and the separation wall while
liquid coolant flows between the separation wall and the lower
membrane. The liquid coolant may be water and during operation
water vapor may be transferred from the coolant through the
membrane to the second stream of air. The liquid desiccant may be a
salt solution (such as weak desiccant such as CaCl, or the like)
and during operation or use of the assembly water vapor may be
transferred from the first stream of air through the membrane to
the liquid desiccant, whereby the first stream of air is
simultaneously dehumidified and cooled to a lower temperature.
[0017] In some cases, the mass and heat transfer assembly may be
configured with no membrane on the coolant (e.g., water) side of
the device. In such a mass transfer assembly, liquid desiccant is
contained by a vapor permeable membrane in the combined stacks, as
discussed above. However, the coolant, which in many cases is
water, is allowed to flow without membrane containment. To this
end, the coolant is maintained or attached on a surface or side of
the separator or separation wall through the use of surface tension
forces on a wicked or flocked surface (e.g., a wicking layer is
attached to the separation wall surface). The flocked surface or
layer of wicking material is attached to the separation wall, and,
thus, there is direct thermal contact between the separation wall
and the liquid coolant (e.g., water flowing through the wicking
material). Water evaporation occurs freely between this
coolant-soaked/containing surface on the separation wall and the
purge or exhaust air stream.
[0018] Further, the mass and heat transfer assembly may include a
humidification stage. The heat and mass transfer assembly may
include an assembly or section where water adjacent to a supply air
stream is membrane contained with a vapor permeable membrane or in
a layer of wicking material. The supply air would be in contact
with the membrane and allow for humidification of the supply air
stream prior to discharge from the mass and heat transfer assembly
(e.g., a humidifier stage provided downstream from the sensible or
indirect evaporative cooler stage).
[0019] In addition to the exemplary aspects and embodiments
described above, further aspects and embodiments will become
apparent by reference to the drawings and by study of the following
descriptions.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Exemplary embodiments are illustrated in referenced figures
of the drawings. It is intended that the embodiments and figures
disclosed herein are to be considered illustrative rather than
limiting.
[0021] FIG. 1 illustrates in schematic form an evaporative cooler
or heat exchanger including an exemplary representative of a
permeable membrane stack or assembly for use in providing indirect
evaporative cooling concurrently with dehumidification in an
integral unit or single stage;
[0022] FIG. 2 illustrates another exemplary representation of an
evaporative cooler showing an assembly of membrane/wall/membrane
stacks used in combination to direct the supply and exhaust
airflows relative to membrane-contained liquid desiccant and
coolant (e.g., cooling water) to achieve cooling and
dehumidification;
[0023] FIG. 3 illustrates an evaporative cooler similar to that
shown in FIG. 2 but being configured with integral counterflow
passages for exhaust/cooled air;
[0024] FIG. 4 is a top view of an exemplary heat exchanger
illustrating air flows through a plurality of channels or chambers
provided by membrane-based assemblies such as those shown in FIGS.
1-3 or other embodiments shown or described herein;
[0025] FIG. 5 (specifically, 5A and 5B) illustrates an exemplary
modeling of an evaporative cooler or counterflow heat/mass
exchanger such as one with the stack assembly shown in FIG. 2 and
flow arrangement shown in FIG. 4;
[0026] FIG. 6 is a graph of air flow and surface temperatures along
the length of the exchanger modeled as shown in FIG. 5;
[0027] FIG. 7 is a graph of humidity ratios of the air along the
length of the exchanger modeled as shown in FIG. 5;
[0028] FIG. 8 is a graph showing concentration of liquid desiccant
flowing through the modeled heat exchanger of FIG. 5;
[0029] FIG. 9 is a psychrometric chart showing the cooling and
dehumidifying process modeled as shown in FIG. 5;
[0030] FIG. 10 is a top view of another exemplary heat exchanger
illustrating air flows through a plurality of channels or chambers
provided by membrane-based assemblies such as those shown in FIGS.
1-3, or other embodiments shown or described herein;
[0031] FIG. 11 is a top view of another exemplary heat exchanger
similar to those shown in FIGS. 4 and 10 showing a differing unit
arrangement with differing exhaust airflows;
[0032] FIG. 12 is a psychrometric chart showing the cooling and
dehumidifying process modeled similar to the modeling shown in FIG.
5 for the configuration of a heat exchanger shown in FIG. 10;
[0033] FIG. 13 illustrates a HVAC system using an indirect
evaporative cooler to provide conditioned air to a building;
[0034] FIG. 14 is a psychrometric chart providing results of one
test of a prototype fabricated similar to the embodiment of FIG. 4
with the stack assembly of FIG. 2;
[0035] FIG. 15 illustrates in schematic form an evaporative cooler
or heat exchanger similar to that shown in FIG. 1 including another
representative permeable membrane stack or assembly;
[0036] FIGS. 16 and 17 illustrate in schematic form two
humidification sections (or portions of a stack that may be
provided in such a section), each of which makes use of a wicking
layer wetted with water or other humidification fluids/sources;
[0037] FIGS. 18, 19, and 20 provide, respectively, a schematic side
view of a two-stage evaporative cooler, a top view of a pair of
first and second stage stacks used to form the cooler, and a
psychrometric chart of the cooling process provided during
operation of the two-stage evaporative cooler; and
[0038] FIGS. 21 and 22 provide, respectively, a top view of a
cooler similar to that of FIG. 19 but with an added direct
evaporative stage and a psychrometric chart of the cooling process
during operation of the cooler.
DETAILED DESCRIPTION
[0039] The following provides a description of exemplary indirect
evaporative coolers with dehumidification and mass/heat transfer
assemblies for such coolers that provide inlet air stream chambers
with sidewalls defined by permeable membrane sheets containing
liquid desiccant. The assemblies also include outlet or exhaust air
stream chambers (such as in counterflow to the inlet air streams)
with sidewalls defined by permeable membrane sheets containing
coolant such as water. In embodiments described below, the membrane
is "permeable" in the sense that moisture in the form of a vapor
(e.g., water in the vapor state) generally can permeate readily
through the membrane such as from an inlet supply air and from
liquid coolant via evaporation. However, the membrane generally
contains or blocks moisture in the form of a liquid from flowing
through as it is instead directed to flow within the channel or
chamber. In some cases, water in the liquid state is contained by
the membrane at pressures less than about 20 psi and more typically
less than about 5 psi. The coolant and the liquid desiccant in some
embodiments are maintained at pressures below about 2 psi, and the
permeable membrane contains moisture such as water in the liquid
state while water vapor permeates the membrane.
[0040] As will become clear from the following description, use of
the assemblies such as for evaporative coolers or mass/heat
exchangers provides a number of benefits. The inlet or process air
stream can be cooled and dehumidified simultaneously or in a single
chamber/stage, and this combined action reduces system size and
cost as well as the number of required components and equipment
(e.g., do not require a multi-stage unit or device to cool and then
to dehumidify and/or further cool with refrigerant or the like).
The combination of liquid desiccant dehumidification with indirect
evaporative cooling provides very high energy transfer rates due to
evaporation and absorption. The design creates a liquid desiccant
system that does not require separate equipment for liquid
desiccant cooling (e.g., a separate cooling tower or chiller). The
stacked arrangements or multi-layered mass/heat transfer assemblies
(or manifolded flow chambers/channels) enable ultra-low flow liquid
desiccant designs. This is due in part to the enhanced geometry of
the assembly and its ability to decrease the liquid desiccant's
temperature to a lower temperature than achievable with traditional
cooling tower technologies. Hence, in the cooler, there are higher
concentration gradients of liquid desiccant (e.g., more than 20
percentage points of lithium chloride (LiCl) and similar gradients
for other desiccants), which provides the following advantages: (a)
a higher thermal coefficient of performance (COP) to regenerate the
desiccant (i.e., to remove water from the desiccant) for reuse in
the cooler; (b) less desiccant storage requirements due to better
utilization; and (c) the ability to use desiccants that are less
expensive than LiCl such as calcium chloride (CaCl), which may not
be used in conventional systems because their absorption properties
are not as favorable as LiCl but lower temperature operation
provided by the cooler embodiments described herein makes the
properties of this and other "weaker" desiccants more acceptable or
favorable.
[0041] The use of membranes as chamber sidewalls facilitates
fabrication of counter-flow and counter-flow with pre-cooled
exhaust air embodiments. Liquid desiccant containment with water
molecule-permeable membranes eliminates liquid desiccant "carry
over" in which small droplets of desiccant are passed into the air
stream as is a concern with direct contact arrangements. The
embodiments described herein also provide considerable reduction or
even elimination of deposited solids during the process of water
evaporation or adsorption (and liquid flow rates can be maintained
at levels that are high enough to further control potential
deposits) whereas fouling leads to increased maintenance and
operating costs with prior evaporative coolers.
[0042] FIG. 1 illustrates in a schematic an evaporative cooler (or
mass/heat exchanger) 100 that is useful for providing concurrent or
simultaneous dehumidifying and cooling of a process or inlet air
stream 120 (e.g., outdoor or process air to be cooled and
conditioned prior to being fed into a building ventilation system).
The cooler 100 is shown in simplified form with a housing shown in
dashed lines, without inlet and outlet ducts, plumbing, and/or
manifolds. Also, the cooler 100 is shown with a single mass/heat
transfer stack 110 whereas in a typical cooler 100 there would be
numerous stacks 110 provided by repeating the configuration shown
(e.g. by alternating the liquid passed through the chamber defined
by the membrane and wall) to provide an assembly with a plurality
of air and liquid flow channels or chambers to provide the desired
mass and heat transfer functions described for the stack 110.
[0043] As shown, an inlet air stream 120 is directed in a chamber
or channel defined in part by a sheet or layer of a membrane 112.
Liquid desiccant 124 flows in an adjacent chamber or channel on the
other side of the membrane 112. The liquid desiccant 124 is
contained by the membrane 112, which is permeable to water
molecules in a liquid or vapor state but generally not to the
components of the liquid desiccant 124. The chamber for the
desiccant flow 124 is also defined by a sheet or layer of material
that is impermeable to fluid flow (i.e., a separation wall) 114 so
as to contain the liquid desiccant 124 in the chamber or flow path.
The chamber for stream 120 is also defined by an opposing membrane
(not shown) that is used to contain another flow of liquid
desiccant. In this manner, heat is passed or removed from the inlet
air stream 120 and transferred to the liquid desiccant flow 124
(and the desiccant behind the opposite sidewall/membrane (not
shown)). Concurrently, the inlet air stream 120 is dehumidified as
water 130 is removed by passing through the permeable membrane 112
into liquid desiccant 124.
[0044] The liquid (or gas) desiccant 124 may take many forms to act
to dehumidify and cool the air stream 120 as it passes over the
membrane 112. Desiccant 124 is generally any hygroscopic liquid
used to remove or absorb water and water vapor from an air stream
such as stream 120. Preferably, the desiccant 124 chosen would be a
regenerable desiccant (e.g., a desiccant that can have the absorbed
water separated and/or removed) such as a glycol (diethylene,
triethylene, tetraethylene, or the like), a salt concentrate or
ionic salt solution such as LiCl, CaCl, or the like, or other
desiccants. The membrane 112 may be formed of any material that
functions to contain liquid desiccant 124 and, typically, coolant
126 (e.g., water or the like) while also being permeable to
molecules of water in liquid or vapor state. For example, polymer
membranes may be used that have pores that are about the size or
just bigger than a water molecule and, in some cases, that are also
adapted to provide water molecules with high mobility through the
membrane 112. In one particular embodiment, the membrane 112 is
formed from a membrane material as described in detail U.S. Pat.
No. 6,413,298 to Wnek, which is incorporated in its entirety herein
by reference. The membrane material may also be obtained from a
number of distributors or manufacturers such as, but not limited
to, Dias-Analytic Corporation, Odessa, Fla., U.S.A. The membranes
112, 118 and separation wall 114 preferably also are formed from
materials that are resistive to the corrosive effects of the
desiccant, and in this regard, may be fabricated from a polymer or
plastic with the wall 114 in some cases being formed of a corrosion
resistant metal or alloy, which provides a higher thermal
conductivity compared with a plastic.
[0045] The embodiment 100 shown is configured for counter-flow of
the pre-cooled exhaust air stream 128 (relative to the inlet air
stream 120). Other embodiments may use cross (at about a 90 degree
flow path) or quasi-counter flow (e.g., not directly counter or
opposite in direction but transverse such as a greater than 90
degree angle flow path relative to air stream 120). The exhaust air
stream 128 flows in a channel or chamber defined by a sheet or
layer of membrane (e.g., second or lower membrane) 118 and an upper
membrane of another stack (not shown). The separation wall 114 and
membrane 118 define a flow chamber or channel for coolant flow 126,
which is typically a flow of water or the like. Heat is transferred
from the liquid desiccant 124 to the coolant 126 through the
separation wall, and the coolant 126 is cooled as heat and mass
(e.g., water or other moisture 132) is transferred to the exhaust
stream 128 via membrane 118. Heat transfer is not shown but
generally is flowing through the membrane 112 to the liquid
desiccant 124, through the separation wall 114 from the liquid
desiccant 124 to the coolant 126, and through the membrane 118 from
the coolant 126 to the exhaust air stream 128. The membranes 112,
118 are relatively thin with a thickness, t.sub.mem, that typically
is less than 0.25 inches and more typically less than about 0.1
inches such as 100 to 130 microns or the like. The membrane 112,
118 may have a tendency to expand outward if unrestrained, and, in
some embodiments, such as that shown in FIG. 3, a divider or "flow
field" support is provided in the inlet air stream 120 and exhaust
air stream 128 (i.e., in the airflow chambers) to maintain the
separation of the adjacent membranes (e.g., a plastic or metallic
mesh with holes or openings for air flow and a zig-zag, S or
W-shaped, or other cross section (or side view) that provides many
relatively small contact points with the membranes 112, 118). The
separation wall 114 also typically is relatively thin to facilitate
heat transfer between the desiccant 124 and coolant 126 such as
with a thickness, t.sub.wall, of less than 0.125 inches or the
like. The flow chambers for the air, desiccant, and coolant are
also generally relatively thin with some applications using
chambers less than 1 inch thick (or in depth) while others use
chambers less than about 0.5 inches, such as about 0.25 inches or
less.
[0046] FIG. 2 illustrates an indirect evaporative cooler 210
utilizing the membrane/separation wall/membrane stack or assembly
configuration to provide a mass/heat transfer exchanger device in
which dehumidification and cooling occur within a single stage and,
therefore, an integral or unitary device. In some embodiments (not
shown), there is no desiccant side membrane or desiccant flow.
Thus, these embodiments are useful for providing an indirect
evaporative cooler in which the membrane contains liquid coolant
but not liquid desiccant and the membrane typically would not be
provided on the supply air side (or in these channels) to provide
better heat transfer surfaces with the separation wall. As shown in
FIG. 2, the cooler 210 includes a mass/heat transfer assembly
formed from stacks or devices 212, 230, 240 and such an assembly of
stack would typically be repeated to provide a plurality of inlet
and exhaust air, coolant, and desiccant flow channels or chambers
in the cooler 210. As shown, each set of stacks (or layered
assemblies or devices) 212, 230, 240 is formed similarly to include
a membrane, a separation wall, and a membrane, with the membrane
being permeable to water on the molecular level to allow mass and
heat transfer and the wall being impermeable (or nearly so) to only
allow heat transfer and not mass transfer.
[0047] Specifically, the stack 212 includes an upper membrane layer
214, a separation wall 216, and a lower membrane layer 218.
Dividers or spacers (not shown) would typically be provided to
space these layers apart to define flow channels for coolant 215
and for liquid desiccant 217. For example, the separators may be
configured to also provide a connection to a supply line for
coolant and for regenerated desiccant, provide a manifold(s) to
direct flow through the various stacks 212, 230, 240, and provide a
connection to a return line for the coolant and diluted desiccant.
The stacks 230 and 240 likewise include an upper membrane layer
232, 242, a separation wall 234, 244, and a lower membrane layer
238, 248. The stack 240 has coolant (such as water) 243 directed in
the chamber between the upper membrane 242 and wall 244 and
desiccant 246 flowing between the wall 244 and lower membrane layer
248 similar to stack 212. In contrast, the stack 230 has liquid
desiccant 233 directed to flow in the chamber defined by the upper
membrane layer 232 and wall 234 and has coolant 236 directed to
flow in the chamber or channel defined by the wall 234 and lower
membrane layer 238.
[0048] The cooler 210 includes ducting and the like (not shown) to
direct supply inlet air 250 through the channel or flow path
between the stack 212 and the stack 230. The arrangement of the
stacks 212, 230, 240 and contained fluids results in the supply
inlet air 250 being passed over the surfaces of the membranes 218,
232 that are containing liquid desiccant 217, 233. As a result,
supply outlet air 254 is output that is dehumidified as moisture in
the air 250 is absorbed by the desiccant 217, 233 via permeable
membrane 218, 232, and the air 254 is also cooled by the
interaction with desiccant 217, 233. The cooling effect in the
cooler 210 is in part effected by a fraction of supply outlet air
254 being redirected in the cooler 210 by ducting/manifolds (not
shown) to flow as pre-cooled exhaust air 255 through the channel or
flow path between stacks 230, 240 to be output as warmer and
moister air 258. Heat passes from desiccant 233 through wall 234 to
coolant 236 (with similar heat transfer occurring in stacks 212,
240), and the coolant 236 is able to transfer heat and mass (e.g.,
water molecules) via membrane 238 to the incoming exhaust air 255.
As discussed above, the stack pattern or set provided by 212, 230,
240 would typically be repeated within the cooler 210 to create a
mass/heat transfer assembly with numerous, parallel flow channels
for air, coolant, and desiccant.
[0049] The cooler 210 is shown as a counter flow exchanger, but
other flow patterns may be used to practice the desiccant-based
dehumidification and cooling described herein. For example, cross
flow patterns may readily be established as well as quasi (or not
fully opposite) counter flow patterns. These patterns may be
achieved by altering the manifolding and/or ducting/plumbing of the
cooler as well as the dividers provided between the stacks.
Additionally, the counter flow passages may be provided integral to
the stack assembly rather than externally as is the case in the
cooler 210. For example, the cooler 310 has a similar stack
arrangement as shown in the cooler 210 of FIG. 2 except that it
includes a counterflow baffle or dividing wall 360 (FIG. 3) on the
end of the flow channels for inlet air 250 and exhaust air 258. The
counterflow divider 360 allows a majority of the cooled air to exit
the stacks as supply outlet air 354 (e.g., more than about 50
percent and more typically 60 to 90 percent or more of the air flow
250). A smaller portion (e.g., a volume equal to the make up
outdoor air or the like) is directed by divider 360 to flow between
stacks 230, 240 as pre-cooled exhaust air 355. FIG. 3 also
illustrates the use of a divider or flow field baffle 370 that
functions to maintain a separation of membranes in the stacks 212,
230, 240 (or at about their original thickness rather than puffed
out or expanded as may occur with some permeable membranes). The
dividers 370 may take many forms such as a mesh with a wavy pattern
(e.g., an S or W-shaped side or cross sectional view), with the
mesh selected to provide as little resistance to air flow as
practical while still providing adequate strength. Also, it is
desirable to limit the number of contact points or areas with the
membranes as these can block moisture transfer from the air 250 and
to the air 355.
[0050] FIG. 4 illustrates an indirect evaporative cooler 400 of one
embodiment. A housing 410 is provided for supporting a mass/heat
transfer assembly such as one formed with the stack sets shown in
FIGS. 1-3 and 15-19. As shown, the housing 410 includes a first end
412 with an inlet 414 for supply inlet airflow 415 and an outlet
416 for exhaust airflow 417. The cooler 410 further includes a
second end 418 opposite the first end 412 that provides an outlet
or vent for directing supply outlet airflow 420 to an end-use
device or system (e.g., an inlet or supply for return air to a
building). The second end 418 is also configured to redirect a
portion 426 of the cooled (and, in some operating modes,
dehumidified) air 426 for use in counter flow cooling of the supply
inlet airflow 415. A prototype of the cooler 400 was fabricated
with a stack assembly as shown in FIG. 2 with 32 desiccant
channels. The prototype was tested with 10 liters per minute (LPM)
flow (or about 0.3 LPM per desiccant channel). Coolant was provided
as water at a water flow rate of about 1.25 to 2.00 times the
evaporation rate. The evaporation rate for this prototype was about
1.33 gallons/ton-hr or about 5 liters/ton-hr, which provides a
water or coolant flow rate of about 6-10 liters/ton-hr of cooling.
Of course, these are exemplary and not limiting flow rates, and it
is expected that the flow rates of liquid desiccant and coolant
will depend on numerous factors and will be matched to a particular
channel design and cooling need as well as other
considerations.
[0051] An indirect evaporative cooler such as the cooler 400 using
stack sets as shown in FIG. 2 (or FIGS. 15-19) may be modeled to
determine the effectiveness of the use of a permeable membrane to
contain coolant and liquid desiccant. FIGS. 5A and 5B provide a
diagram 500 of such modeling showing use of stacks 212, 230, and
240 as discussed with reference to FIG. 2 to cool inlet or process
air and to also dehumidify this air in the same stage or process.
The inputs to the model 500 are shown, and results for a typical
inlet air condition are provided, with results and modeling being
performed in this case with Engineering Equation Solver (EES). The
numeric values shown in boxes or with squares around them are input
values (or assumed typical operating conditions), and the values
outside or without boxes are outputs or results of the modeling.
The modeling results shown in the diagram 500 are believed to be
self-explanatory to those skilled in the heating, ventilation, and
air conditioning (HVAC) arts and do not require detailed
explanation to understand the achieved effectiveness of the
embodiments using membrane containment in indirect evaporative
coolers; however, the following provides a graphical description of
some of the results in the diagram 500.
[0052] FIG. 6 illustrates a graph or diagram 610 showing the
temperatures of the air flows in the channels between the stacks
(e.g., in an evaporative cooler using such mass/heat transfer
assembly described herein). The graph 610 also shows surface
temperatures along the length of the counterflow mass/heat
exchanger (e.g., exchanger 400 with stack arrangements as shown in
FIG. 2). Specifically, the graph 610 shows the temperature of
supply air with line 612, the temperature of exhaust/purge air with
line 614, the temperature of the desiccant side membrane surface
(e.g., at the interface of the membrane and the supply air) with
line 616, the dewpoint temperature of the desiccant side membrane
surface (e.g., at the interface of the membrane and the supply air)
with line 620, and the temperature of the water side membrane
surface (e.g., at the interface of the membrane and the
exhaust/purge air) with line 618.
[0053] FIG. 7 is a graph or diagram 710 showing the humidity ratios
of the air along the length of the counterflow heat/mass exchanger.
Specifically, the graph 710 shows the bulk humidity ratio of the
supply air with line 712, the bulk humidity ratio of exhaust/purge
air with line 714, the humidity ratio of the air in close proximity
to the desiccant side membrane surface (e.g., at the interface of
the membrane and the supply air) with line 716, and the humidity
ratio of the air in close proximity to the water side membrane
surface (e.g., at the interface of the membrane and the
exhaust/purge air) with line 718.
[0054] FIG. 8 illustrates a graph 810 showing with line 815 the
concentration of desiccant (in this particular modeling the
desiccant is LiCl) as it flows concurrent with the supply air flow
down the length of the counterflow mass/heat exchanger. As shown
with line 815, the desiccant is getting weaker as it flows through
the channel between the membrane and the separation wall as it
absorbs water molecules from the air, e.g., the concentration of
the desiccant is dropping from about 44 percent down to about 24
percent in this particular modeling example (which results from the
membrane being characterized as permeable (at a particular input
rate or setting) to water molecules in the flowing air at these
operating conditions).
[0055] FIG. 9 shows the process of model 500 of FIG. 5 in a
psychrometric chart or diagram 910. The supply air shown with line
912 can be seen to be gradually losing humidity (in kilograms water
vapor/kilograms dry air or kg.sub.v/kg.sub.da). The supply air 912
has its temperature initially rise slightly due to the large heat
flow of vapor sorption into the desiccant. As the supply air 912
continues down the length of the exchanger (or flow channel or
chamber between membrane layers or walls of adjacent stacks
containing liquid desiccant), the temperature then drops to a
cooler/drier condition than at the inlet. At the exit of the
exchanger, the supply air 912 is split into two streams. The
majority of the air is supplied to the cooled space, and the
minority of the air (such as less than about 50 percent and more
typically less than about 30 percent of the volume) gets funneled
into the exhaust/purge side (or exhaust/counterflow channels
between the membrane walls containing coolant) of the heat/mass
exchanger or cooler, which is shown with the line 916. The exhaust
air 916 has a low dewpoint, and, thus, it can pick up a large
amount of heat evaporatively. The pre-cooled exhaust or purge air
916 picks up water vapor (and associated heat of vaporization) from
the wet side channel. The air 916 exits out of the unit with a much
higher enthalpy than either the supply inlet or exit shown with
line 912. The diagram 910 also shows the humidity ratio and
temperature of the supply air in close proximity to the desiccant
side membrane surface (ds) with line 918.
[0056] The following table shows results in tabulated form for
modeling of FIG. 5 for inlet and outlet air flows. As shown, a wide
range of temperatures and humidity levels can be chosen and input
into the model 500. In the configuration whose results are shown in
the table, the equivalent wet bulb effectiveness with the desiccant
flow turned off (e.g., in some operating modes it may not be
required or useful to utilize the desiccant to dehumidify the air)
would be 113 percent, which means the cooler is able to cool the
supply air below the inlet wet bulb temperature.
TABLE-US-00001 TABLE Inlet and outlet conditions from model runs
(.degree. C. and kg/kg) Run # T.sub.supply,in T.sub.supply,out
T.sub.exhaust,out .omega..sub.supply,in .omega..sub.supply,out
.omega..sub.exhaust,out 1 27.7 21.11 31.55 0.0133 0.00892 0.0289 2
50.0 33.7 50.7 0.0319 0.0179 0.0834 3 50.0 20.7 41.0 0.0077 0.00406
0.0494 4 30.0 13.1 27.2 0.00262 0.00158 0.0226 5 30.0 18.9 42.55
0.0269 0.0137 0.0547 6 15.0 16.9 25.4 0.0105 0.00418 0.0207 7 15.0
11.9 20.0 0.00528 0.00203 0.0147
[0057] where LiCl Inlet Concentration=44%; flow ratio (flow
exhaust/(flow exhaust+flow supply)=0.3; supply outlet face
velocity=175 SCFM; and ambient pressure=101.3 kPa.
[0058] The cooler 210 of FIG. 2 (or FIGS. 15-19) may be thought of
as a desiccant-enhanced, indirect evaporative cooler that utilizes
a membranes or layers of membrane material that is permeable to
water molecules to provide desired liquid containment. A standard
psychrometric chart (such as one at 14.7 psi ambient pressure and
other typical parameters) may be used to view lines of equal
sensible heat ratios (SHRs) originating at a typical room setpoint.
For vapor compression dehumidification, a SHR of less than about
0.7 is difficult to attain without reheat (e.g., given reasonable
evaporator temperatures). Also, it is psychrometrically impossible
to attain a SHR of less than about 0.6 without reheat, and
attempting such a SHR often leads to frozen evaporator coils that
require defrost cycles. The desiccant-enhanced, indirect
evaporative cooler, such as shown in FIG. 2 at 210, addresses this
problem with a unique, new process (as has been described above and
is presented in more detail below).
[0059] It may be useful at this point to review the process with
reference to FIGS. 2 and 3. FIGS. 2 and 3 show diagrams describing
the inner flow channels of the unit or assembly for use in an
evaporative cooler 210, 310. The mixed return/outdoor air is shown
by the arrow 250 (e.g., return air from a conditioned space along
with outdoor make up air such as 400 cfm/ton supply and 175 cfm/ton
outdoor air or the like). The air 250 is dehumidified by the
desiccant 217, 233 through the membrane 218, 232. This lowers both
the dew point and temperature of this air stream until it is output
at 254 or 354. At the exit of the supply air passage (between the
liquid desiccant-containing membranes), a portion of the air is
fractioned off as shown with arrows 255 and 355 and sent through an
adjacent passage (between the coolant-containing membranes 238,
242) which picks up moisture from the water layer 236, 243 through
the membrane 238, 242. The heat of evaporation is a source of
cooling that acts to remove the sensible heat and heat of
absorption from the supply air stream 250. This air is then
exhausted (purged) out at 254, 354.
[0060] The heat exchanger configuration shown at 400 in FIG. 4 has
been built in the laboratory and was modeled as shown in FIG. 5.
Other options for flow/housing designs are shown in configuration
with the cooler 1000 of FIG. 10 and the cooler 1100 of FIG. 11. The
cooler 1000 is shown to have a housing 1010 with a first portion or
end 1012 and a second portion or end 1020. The first portion 1012
is configured with inlets or vents for receiving supply inlet
airflow 1013 as well as input exhaust airflow 1014, and the first
portion 1012 also includes vents or outlets for outputting exhaust
airflow 1015 from the unit 1000. The second portion 1020 is
configured (e.g., with manifolds and other components to direct air
flow) with outlets for supply outlet airflow 1022 with a portion
1025 being redirected back into the housing 1010 as shown at arrows
1027 to provide counterflow for a fraction of the channel provided
for supply inlet airflow 1013 (with exhaust airflow 1014 provided
as a cross flow in the other or initial portion of the channel) and
then this air is exhausted from the housing portion 1020 at 1028.
The input exhaust airflow 1014 may be return air to be exhausted or
outdoor air (e.g., from the building space). This approach 1000
improves the efficiency by utilizing a smaller purge airflow 1025,
1027, and it is typically preferred to limiting purge air flow to
increase or maintain desirable efficiency.
[0061] Referring again to FIG. 4, operation of the cooler 400 is
expected to have the cooling process shown in the psychrometric
chart 910 of FIG. 9. As shown, line 912 represents the supply air
flow while line 916 represents the purge air flow stream. The
desiccant side air boundary layer is represented with line 918. The
chart shows graphically how the dehumidification driver for the
cooler 400 is advantageously utilized to provide a more effective
cooler. The cooler 400 may use even a weak desiccant such as CaCl
solution to provide significant dehumidification, and this is due
in part to the cold temperatures that are achieved with the
configuration of the cooler 400 that allow weak desiccants to
attain high dehumidification potential.
[0062] The configuration shown with cooler 1000 of FIG. 10 was
modeled to determine the desirability of its performance, and the
results are provided in psychrometric chart 1200 of FIG. 12. In the
chart 1200, line 1210 represents supply air, line 1212 represents
ambient exhaust air, line 1214 represents desiccant side surface
temperatures, line 1220 represents the supply air post cooling,
line 1224 represents the purge air post cooling, and line 1230 is
the sensible heat ratio (SHR) line in which the load on the
building follows. So, for example, a building will have 0.67 units
of sensible heat and 0.33 units of latent heat added to the space
to arrive at the return air condition, which is the middle diamond
at 80.degree. F. and about 70 grains/lb, and that point may be
considered the return air condition. The first point of line 1210
is the "mixed air" condition, which is a 30/70 mixture of outdoor
air and return air. The two-stage approach to cooling provided by
cooler 1000 allows the process to be split into two distinct
sections of dehumidification plus a post cooling stage (e.g.,
sensible cooling only stage in which, for example, there is no
desiccant layer and dehumidification and only evaporative cooling
is provided). The cooler 1000 is, of course, only one example of
numerous configurations that may be implemented to provide two or
more stage cooling using the membrane containment features
described herein, and it shows the possibility of attaining nearly
any SHR desired (e.g., in this case, a SHR of about 0.67). In the
modeling to provide the chart 1200 (FIG. 12), a 1 cubic foot core
(or mass/heat transfer assembly) was used with 176 SCFM, and a flow
ratio of about 0.3 (e.g., 30 percent purge and 70 percent supply
air). Also, the return air was at 80.degree. F. and 40 percent
relative humidity, ambient air was at 86.degree. F. and 60 percent
relative humidity, and the liquid desiccant fed into the assembly
was 44 percent LiCl (but other desiccants such as solutions of salt
(such as, but not limited to, halide salts) and water that are
about 20 to 40 percent salt by weight may be used). The assembly
was able to provide 0.5 tons of building cooling with just this 1
cubic foot at about 7 Btu/lb. As can be appreciated from this
example and modeling, the use of membranes to contain desiccant and
coolant (e.g., to contain liquids) enable indirect evaporative
coolers to be produced that are much more compact than prior
designs, that are easier to maintain (e.g., have less or no fouling
issues), and that are more efficient in producing cooling (e.g.,
with simultaneous dehumidification and cooling to provide an
evaporative cooler that can condition as well as cool process
air).
[0063] FIG. 11 illustrates an evaporative cooler 1100 providing
another counterflow arrangement in which the counterflow cooling
air (or pre-cooled supply air) is directly opposite in direction
but only for a selected length (such as half to 80 or 90 percent or
more of the length) of the stacks or flow chambers (e.g., when full
counterflow is not required or desired). As shown, the cooler 1100
includes a housing 1110 containing a plurality of stacks or sets of
stacks configured as a mass/heat transfer assembly (as discussed
above) with alternating flow channels for supply inlet airflow 1112
and for counterflow air (e.g., redirected supply outlet airflow
1114). The housing 1110 includes venting and/or manifolding for
directing the supply inlet airflow 1112 (e.g., outdoor make up air
and return air) into channels between desiccant containing
membranes and to output the cooled and, often, dehumidified supply
outlet airflow 1114. The cooler 1100 further includes ducting,
manifolding, and the like for redirecting a fraction of the supply
outlet airflow back into the housing 1110 to provide cooling
counterflow air as shown at 1116 (e.g., into flow channels between
coolant containing membranes). The counterflow air 1116 typically
does not travel along the entire length of the housing 1110 but is,
instead, discharged out a side vent at some point along a channel
length (e.g., at a distance about 60 to 80 percent of the length).
Such a configuration is useful to tune a cooler 1100 for particular
operating environments (e.g., to provide a desired amount of
cooling to the supply outlet airflow based on outside air
temperatures and humidities and other operating parameters).
[0064] The stack and membrane technology described herein are
readily applicable to a number of indirect evaporative cooler
designs (with and without use of liquid desiccant for
dehumidification) and applications. However, it may be useful to
discuss the use of the technology within an air conditioning or
HVAC system with the belief that those skilled in the art will
readily understand that the technology is useful in many other such
systems. FIG. 13 illustrates a simplified air conditioning system
1300 in which the membrane technology may be provided to disclose
desiccant dehumidification and evaporative cooling to condition air
within a building 1310 (e.g., a residential or commercial building
or other structure requiring conditioned and cooled air). As shown,
the system 1300 includes a cooler 1320 with a housing 1322 that is
used to house a membrane stack assembly, such as described above
with reference to FIGS. 1-12 and below with reference to FIGS.
15-20. A fan or blower 1324 is provided to draw in outside or make
up air 1325 and move return air 1326 from the building 1310. The
fan 1324 pushes these two air streams as inlet supply air through
the stacks as described above (e.g., adjacent liquid desiccant
contained in membrane in embodiments providing dehumidification or
adjacent separation walls in embodiments with just evaporative
cooling). The cooled (and, typically, conditioned air is output at
1330 as supply to the building 1310 and a portion is returned 1332
as purge or pre-cooled exhaust air that passes on the coolant or
evaporative cooling side of the stacks in housing 1322 and then out
as exhaust 1328. Coolant is provided in the form of a water supply
and drain 1334 to the housing (and through the stack assembly), and
liquid desiccant is provided at 1338 as supply and drain. The
desiccant 1338 is regenerated with a regenerator system 1340
including, in this example, a desiccant boiler 1342.
[0065] The desiccant enhanced indirect evaporative cooler (DE-IDEC)
1320 is the portion of the system 1300 that takes strong desiccant
and water to provide cooling to building 1310. The system 1300
provides both sensible and latent cooling to building 1310 on
demand and in proportion to the demand, e.g., the system 1300 can
provide cooling in the form of 100 percent sensible, 100 percent
latent, or any combination thereof. The DE-IDEC 1320 uses some
portion of outdoor air 1325 with equal exhaust air 1328 to reject
the heat load outside of the building 1310. The DE-IDEC 1320 itself
can sit inside or outside of the building envelope because it has
no wet surfaces and the liquid streams 1334, 1338 are closed loop.
This makes system 1300 acceptable for indoor use and for placement
of cooler 1320 inside the building 1310. The water source (or
coolant source, not shown) for water or coolant 1334 is not
required to be potable, and the system 1300 is compact enough to be
acceptable by building managers. The electricity usage is much less
than that of typical vapor compression systems or units (e.g., less
than 0.2 kW/ton peak compared with 1.2 kW/ton typical for
conventional compression units).
[0066] The regenerator 1340 is another of the significant
components to the operation of the system 1300. This unit 1340
takes the weakened desiccant from the DE-IDEC 1320 and applies heat
with boiler 1342 (see list of heat sources below) to drive off the
moisture contained in the desiccant 1338. The result is a desiccant
1338 that has higher salt concentration and can be re-used by the
DE-IDEC 1320 (e.g., in the membrane contained/defined flow channels
adjacent to supply inlet air 1325, 1326). A list of heat sources
suitable for desiccant regeneration may include: (a) gas or other
fossil fuel; (b) solar heat; (c) waste heat from any waste heat
stream such as combine heat and power plant; and (d) waste heat
from a condenser unit originating from a vapor compression
cycle.
[0067] The inventors performed a test of a prototype fabricated
similar to the cooler shown in FIG. 4 with a stack assembly such as
shown in FIG. 2. FIG. 14 provides results of the testing for this
proof of concept prototype that was constructed and tested at
104.degree. F. and 93 grains/lb inlet air. The prototype was tested
with and without desiccant flow, but with membranes provided to
define liquid desiccant flow channels. Without the desiccant flow,
the indirect evaporative cooler had a wet-bulb effectiveness of
73%. When desiccant was turned on (with 41% LiCl solution as the
desiccant), the effectiveness was 63% and had 12 grains/lb of
dehumidification. This resulted in a sensible heat ratio of 0.73.
The prototype did not reach model expectations as explained above,
and this was likely due to prototype defects creating non-uniform
air, water, and desiccant flow distribution.
[0068] It was recognized that use of the membrane to contain the
liquid desiccant and separate it from air flow is desirable in most
if not all mass transfer/heat exchanger assemblies. For example,
with reference to the indirect evaporative cooler 100 of FIG. 1,
the membrane 112 is used to block flow of the liquid desiccant 124
into the inlet air stream 120 while concurrently allowing water
molecules 130 to flow from the inlet air stream 120 to the
desiccant 124 to dehumidify and cool the inlet or process air
120.
[0069] However, it was further determined that the second membrane
118 is not needed to practice many aspects of the evaporative
coolers described herein. Particularly, an indirect evaporative
cooler may be provided in which each stack only includes a single
water-permeable membrane (such as membrane 112) while coolant flow
is provided on the opposite side of a separation wall (such as wall
114) through other techniques such as by providing a flocking sheet
or layer (or wicking element) on the separation wall 114 opposite
the side of the wall defining the liquid desiccant flow
chamber/channel. The stack may be arranged vertically in such
embodiments of the evaporative cooler to make use of gravity to
encourage coolant flow from the top to the bottom of the stack in
the wicking layer. In other cases, though, the wicking layer or
flocking may be provided on a top or bottom side of a separation
wall (a horizontal stack arrangement) with capillary action (or
other mechanisms) used to obtain a desired coolant flow through the
stack.
[0070] FIG. 15 schematically illustrates an indirect evaporative
cooler (or mass/heat exchanger) 1500, which may be used in place of
the evaporative cooler 100 shown in FIG. 1. The cooler 1500 may be
thought of as a modification of the cooler 100 with retained
components or elements having like reference numerals in FIGS. 1
and 15. Particularly, the evaporative cooler 1500 is useful for
providing concurrent dehumidifying and cooling of a process or
inlet air stream 120. This is achieved with one or more mass/heat
transfer stacks 1510. As shown, the inlet air stream 120 is
directed to flow in a chamber or channel defined in part by a sheet
or layer of a membrane 112, which may take the form described above
for stack 110. Liquid desiccant 124 flows in an adjacent chamber or
channel on the other side of the membrane 112. The chamber for the
desiccant 124 flow is also defined by a separation wall 114, which,
as described above, is impermeable to fluid flow so as to contain
the liquid desiccant 124. The chamber for air stream 120 is also
defined by an opposing membrane (not shown) that is used to contain
another flow of liquid desiccant (e.g., a membrane of another stack
configured similar to stack 1510).
[0071] As with cooler 100 of FIG. 1, the evaporative cooler 1500 is
configured for counter-flow of the pre-cooled exhaust air stream
128 (relative to the inlet air stream 120). In contrast to the
cooler 100, though, the exhaust air stream 128 flows in a channel
or chamber defined on one side by a wicking layer or flocking
element 1520 and on another side by an upper element of another
stack (not shown, but may be another wicking layer or a
membrane).
[0072] Significantly, the wicking layer or flocking 1520 is
attached to a side of the separation wall 114 and acts to wick or
guide flow of a volume of coolant 126 in the stack 1510. In other
words, the second membrane 118 of cooler 100 is removed as it is
not needed to define a coolant flow channel/chamber. Instead, the
wicking layer 1520 may be thought of as defining a channel or flow
path for the coolant 126, which is shown to be counter to the
exhaust air stream 128. The air in stream 128 is in contact with
the wicking layer 1520 and the coolant 126.
[0073] The coolant 126 may be a flow of water or the like, and heat
is transferred from the liquid desiccant 124 to the coolant 126
through the separation wall 114. The coolant 126 flowing or being
wicked by wicking layer 1520 is cooled as heat and mass (e.g.,
water or other moisture 132) is transferred to the exhaust air
stream 128 directly rather than through a membrane as in cooler 100
of FIG. 1. Heat transfer is not shown in FIG. 15 but generally heat
is flowing through the membrane 112 to the liquid desiccant 124 via
water 130 and then through the separation wall 114 from the liquid
desiccant 124 to the coolant 126, and then from the coolant 126 to
the exhaust air stream 128.
[0074] Capillary action may support flow of coolant 126 in wicking
layer 1520 when the stack 1510 is arranged in a horizontal
configuration, but some embodiments will position the stack 1510
including the separation wall 114 and attached/contacting wicking
layer 1520 to be vertical such that gravity facilitated coolant
flow 126 from the top to the bottom of the evaporative cooler 1500.
As with the stack 110, the stack 1510 may be provided in
multi-stack assemblies/coolers such as the cooler 210 with the
stack 1510 being used to provide, or in place of, stack 230 (and/or
other stacks 212, 240). In such an arrangement, the flow channel
for the exhaust air stream 128 typically would be defined by facing
but spaced apart wicking layers 1520 on separation walls 114 (e.g.,
spaced apart, flocked surfaces of two separation walls).
[0075] A variety of flocking materials may be used to implement the
wicking layer 1520 on separation wall 114. The wicking layer 1520
acts to spread out or disperse the flowing coolant 126, e.g., to
avoid rivulets of flowing coolant, which enhances heat transfer
from the wall 114 and also mass/heat transfer to exhaust air stream
128 in the adjacent flow chamber/channel of stack 1510. The
flocking material of the wicking layer 1520 also acts to impede
gravity to get a slower flow in vertical configurations. The
thickness of the layer 1520 may vary but in some cases may be
approximately 0.015 inches thick while other useful implementations
may use flocking in the range of 0.005 to 0.05 inches in thickness.
Exemplary flocking for the wicking layer 1520 include: (a) knitted
nylon fabric; (b) polypropylene woven or non-woven fabric; and (c)
adhesive-backed flocking fibers (typically polyester or
polypropylene), e.g., the layer 1520 may include fibers standing up
along (or arranged transverse to) planar surface of wall 114 and
may have lengths of 0.01 to 0.05 inches or more. In some
embodiments, the wicking layer 1520 may be provided by one or more
fabrics coated with a hydrophilic coating. While in other cases,
the layer of wicking material 1520 is created with a hydrophilic
coating on a surface of the separation wall 114.
[0076] While a wide variety of materials may be used in layer 1520,
there are a number of wicking or flocking characteristics that may
be desirable for operation of the cooler 1500. The wicking surface
of layer 1520 provides a method or mechanism to evenly spread
either desiccant or water (as shown in FIG. 15) over a surface
(e.g., surface or side of wall 114). The wicking surface impedes
the forces of gravity on the flowing liquid to slow the flow rate
down to a range of about 5 to 50 inches per minute, with some
useful implementations using a flow of about 20 in/min. The
flocking also enables low total flow rate of water to be applied.
The total flow rate of water or other coolant enables flow rates
that are between 1.2 to 4.0 times the evaporation rate of water (or
other coolant). Typically, this flow would be set based on water
quality that is being used and would be 1.2 to 2 times the
evaporation rate. In another embodiment, the flow rate of water may
be set higher than in the above examples by use of re-circulating
the water. In this case, the water flow rate may typically be 4
times the evaporation rate and could be in the range of 3-10 times
the coolant evaporation rate.
[0077] As shown in FIG. 15, indirect evaporative cooler 1500
provides a channel pair where a first airflow 120 is cooled and
dehumidified by water absorption 130 through the vapor permeable
membrane 112 to the liquid desiccant 124. The second airflow (in
the second channel of the channel pair provided by stack 1510) 128
removes heat from the first airflow 120 by the evaporation of water
132. The water/coolant 132 is contained within a flocked or wicked
surface (which provides layer of flocking 1520) on wall 114
opposite the flow channel for liquid desiccant 124. The evaporation
of water/coolant 132 from the flocked or wicked surface of wall 114
removes heat from the first airflow 120 by heat conduction and
convection through the membrane-desiccant-separation wall assembly
or stack 1510.
[0078] Generally, the cooling process or method provided by
operation of an evaporative cooler (such as cooler 1500) involves
receiving an input or process air stream. This process air stream
undergoes dehumidification in a first section or portion of the
evaporative cooler (i.e., the desiccant-contained dehumidification
section), and this is followed by sensible cooling in a second
section (i.e., indirect evaporative cooling section). As shown
herein, though, dehumidification and sensible cooling may occur in
a single or integral section or portion of the cooler to occur
concurrently. The process air is then delivered to a work space or
indoor area for use in cooling a space while the purge/exhaust air
is used to remove heat from the coolant and is output/discharged
from the cooler.
[0079] In some cases, it may be desirable for an indirect
evaporative cooler to be provided with a humidification section.
This would allow the above cooling method/process to be modified to
include a step after sensible cooling in which the process air is
humidified adiabatically to further drop the temperature of the air
prior to output from the indirect evaporative cooler into a work
space or building space. In some embodiments, humidification is
provided by having the sensibly cooled air flowing in
channels/chambers with one or both sidewalls defined by vapor
permeable membranes. Particularly, the indirect section (indirect
evaporative cooler) may be followed by a section that provides
direct evaporative cooling, which also humidifies. This acts to
further reduce the temperature of the outlet stream to provide
higher sensible cooling, but such higher cooling comes at the
expense of providing less latent cooling (dehumidification). Such
additional cooling is shown with line 2025 in the psychrometric
chart 2200 of FIG. 22, where the air is moved from an air state "2"
to an air state "2.5" (with this chart 2000 explained in more
detail in the following description). The particular methods or
mechanisms used to provide direct evaporative cooling may be
performed in many ways to practice such a cooler.
[0080] In other cases, though, a flocked surface may be used in the
humidification section. For example, FIG. 16 illustrates a
humidification section (or portion of such a stack/assembly) 1600
in which a separator or separation wall 1610 is provided to define
sidewalls of two adjacent flow channels for process air 1614 (i.e.,
air that has been sensibly cooled in an upstream section of an
evaporative cooler). Both sides of the wall 1610 have been covered
with flocking or wicking material to provide a top wicking
layer/element 1620 and a bottom wicking layer/element 1622 that are
wetted (such as with water) to provide a moisture source or coolant
1630, 1632 for humidification as the air 1614 flows over the wetted
surfaces of layer 1620 and to provide a heat/mass transfer to
exhaust air stream 1640 (but the bottom flocking surface/layer 1622
may be omitted in some embodiments). Note, the air streams 1614 and
1640 (and 1710, 1740 below) may both be supply air.
[0081] FIG. 17 illustrates another humidification section 1700 that
may be used in an indirect evaporative cooler (downstream from the
sensible cooling section). In the humidification section 1700, a
sensibly cooled air stream 1710 flows over a vapor permeable
membrane 1714 separating the air flow 1710 (or the channel it flows
within) from a flow of water or the like 1716 (or the channel in
which it flows). The water flow channel or humidification source is
defined on the other side by a first side/surface of a separation
wall 1720. The other side of the separation wall 1720 along with a
vapor permeable membrane 1730 defines a channel or chamber for flow
of a coolant 1724. The humidification section 1700 further includes
another or second channel or chamber in which exhaust air 1740
flows along the other side of the vapor permeable membrane 1730 and
to remove heat from the evaporative cooler containing
humidification section 1700.
[0082] At this point, it may be useful to describe a two-stage
indirect evaporative cooler 1800 with reference to FIGS. 18-20.
These figures show graphically how the cooler 1800 works on three
levels: (1) FIG. 18 illustrates a heat exchange schematic showing
general air, water, and desiccant flows; (2) FIG. 19 illustrates a
channel pair graphic or schematic that shows an air channel pair
and location of membranes and wicked water surfaces in the first
stage and in the second stage heat and mass exchangers; and (3)
FIG. 20 provides a psychrometric chart 2000 showing each air state
in the cooler 1800.
[0083] FIG. 18 schematically illustrates a two-stage indirect
evaporative cooler 1800 and its air flow pattern during operation.
Air states are numbered in FIG. 18, and these air state numbers are
repeated in FIGS. 19 and 20 (as are reference numerals to
components shown in both FIGS. 18 and 19). In this discussion, air
streams may be referred to or described as moving from one state to
the next such as air stream "1" to "1.5" is the stream of air
moving from a first air state to a second air state as the air is
dehumidified.
[0084] The cooler 1800 is configured in two distinct stages or
assemblies 1810 and 1850 providing a first-stage dehumidifier and a
second-stage indirect evaporative cooler. As shown, the
dehumidifier 1810 is made up of a number of stacks 1814 (as
discussed above and shown in FIG. 19). Each stack 1814 defines a
flow channel or chamber for inlet or process air 1820 to flow
through the dehumidifier 1810 and be output to the second stage
1850 as dehumidified air 1822. The stacks 1814 also define flow
paths for and act to contain liquid desiccant 1816 in the
dehumidifier 1810 (e.g., LiCl, CaCl or the like at 35 to 40 percent
by weight at a flow rate of about 0.34 gallons/minute per space
cooling ton). Further each stack 1814 defines, with a pair of
spaced apart wicking layers or surfaces on separation walls wicking
or flowing water/coolant 1818, flow channels or pathways for
exhaust air 1826 (input at air state "3") to flow through the
dehumidifier 1810 and remove heat from the liquid desiccant 1816
and be output at 1828 (at air state "4").
[0085] The first-stage dehumidifier 1810 is a cross-flow heat and
mass exchanger between two air streams 1820/1822 and 1826/1828.
Desiccant 1816 and water 1818 flow vertically and are gravity
driven. The liquid desiccant 1816 is contained by a polypropylene
microporous membrane or other vapor permeable membrane (e.g., a
Z-series from Celgard LLC or another distributor/manufacturer). In
some implementations of cooler 1800, nozzles may be used to spray a
high water flow rate (water 1818) that creates a two-phase flow of
water and outdoor air in air stream 1826/1828 (air states "3" to
"4"). The dehumidifier 1810 may be designed to provide a low water
flow rate that is spread by wicked surfaces in contact with the air
stream 1826/1828. In some embodiments, a waterside membrane may be
used for controlling biological growth because it creates a barrier
that blocks organisms from implanting or growing onto wet
surfaces.
[0086] The second-stage or indirect evaporative cooler 1850 is
formed with an assembly or number of stacks 1854 (as shown in FIG.
19). Each stack 1854 defines a flow path or channel for
dehumidified air 1822 to flow through the evaporative cooler 1850
to be output as cooled/dehumidified supply air 1860. Further, the
stacks 1854 and/or manifolds or other portions of evaporative
cooler 1850 define flow paths/channels for a portion of the supply
air 1862 to be returned to flow through the cooler 1850 and be
exhausted at 1866 after removing heat from the air stream
1822/1860. Further, the stacks 1854 provide flow paths or channels
for coolant (e.g., water) 1858 such as via gravity flow in wicking
layers on separation walls. The second stage 1850 is designed as a
counterflow indirect evaporative cooler. In testing of some
embodiments, the stage 1850 has a wet bulb effectiveness measured
at 120 to 128 percent at the design mass flow rate. For both stages
1810, 1850 the water 1818, 1858 was gravity driven and provided at
a low flow rate distributed across the heat transfer surfaces of
stacks 1814, 1854 by a wicking material or thickness of
flocking.
[0087] Top views of exemplary implementations of the stacks 1814
and 1854 of the stages 1810, 1850 are shown in FIG. 19 (with
repeated components and flows labeled with like reference numbers).
As shown, the first stage stack 1814 provides a pair of air flow
channels: a first channel/chamber for ventilation or input air 1820
(that typically includes a volume of return air 1821 from the
cooled space) and a second channel/chamber for exhaust air 1826
flowing into the page (cross flow in this example). The first
channel is defined by a first wall assembly formed of a separation
wall 1960 (e.g., a plastic or metal sheet) and a vapor permeable
membrane 1962, which faces the air stream 1820, 1821. A flow of
liquid desiccant 1816 is contained within the wall assembly
provided by separator 1960 and membrane 1962. The first channel is
further defined by a second wall assembly formed of a separation
wall 1966 and another vapor permeable membrane 1964. Again, the
membrane 1964 faces or is exposed to the air stream 1820, 1821 and
a flow of liquid desiccant 1816 is provided and contained between a
side/surface of separation wall 1966 and the membrane 1964.
[0088] The second or paired air flow channel of first stage stack
1814 is defined by the other/opposite side of the separation wall
1966 upon which a wicking layer 1970 is provided. The wicking layer
1970 wicks coolant/water that is directly in contact with flowing
exhaust air to allow heat to be released from liquid desiccant 1816
and air stream 1820, 1821. The second air flow channel is further
defined by another separation wall 1974 (which may be a top wall of
a next stack), and another wicking layer 1972 of flocking or
wicking material is provided on the surface/side of the separation
wall 1974 facing the wicking layer 1970. Coolant such as water is
wicked or gravity fed through the wicking layer 1972 as the exhaust
air flows through the stack 1814.
[0089] With regard to the second stage stack 1854 of the indirect
evaporative cooler 1850, a flow channel is provided for air stream
1822. This channel is provided by a side/surface of a separation
wall 1980 and a spaced apart second separation wall 1982. A second
flow channel is provided in stack 1854 into which a portion 1862 of
the supply air 1860 is returned into the stack 1854 to remove heat
and be exhausted at 1866. A second air flow channel/chamber is
defined by the opposite side of separation wall 1982, which is
covered with flocking/wicking material to provide a wicking layer
1984. Water or coolant is gravity fed through this layer 1984
during use of the stack 1854 in a cooler assembly. The second flow
channel for air stream 1862 is further defined by a second wicking
layer 1988 provided on a facing side or surface of an additional
separation wall 1990. As discussed throughout, numerous first and
second stage stacks 1814, 1854 would be assembled or stacked upon
each other to form a two-stage cooler 1800.
[0090] FIG. 20 is a psychrometric chart 2000 illustrating the
thermodynamics of the cooling processes provided by operation of
the cooler 1800. The return air state is shown at 2060 while the
state of the liquid desiccant is provided with line 2050 in the
chart 2000. Line 2010 shows the thermodynamics as the incoming or
supply air moves from air state "1" to air state "1.5" (as shown in
FIGS. 18 and 19) and is dehumidified using the liquid desiccant
contained in the vapor permeable membranes in the first stage
dehumidifier 1810. Line 2020 illustrates thermodynamics of the
dehumidified air as it passes through the second stage indirect
evaporative cooler 1850 and moves from air state "1.5" to air state
"2" and is subject to sensible cooling. Line 2030 shows the
thermodynamic properties of the return air 1862 that is passed back
through the second-stage cooler 1850 and is then output as purge or
exhaust air 1866. Line 2040 illustrates thermodynamic properties of
exhaust air stream 1826 to 1828 (e.g., outside air) as it passes
through the first-stage dehumidifier 1810. As shown in the chart
2000, the air to be supplied to a building space was dehumidified
and was also reduced from an original temperature between 80 and
85.degree. F. to about 60.degree. F., which is useful for cooling
many residential and commercial spaces.
[0091] The cooler 1800 may be modified by adding a direct
evaporative section or stage as shown in FIG. 21. In the cooler of
FIG. 21, the cooled supply air 1860 from the second stage 1854 is
output to the direct evaporative stage 1999 where the air 1860
undergoes humidification and further cooling before being
discharged at 1997 in air state "2.5." As shown with the
psychrometric chart 2200 of FIG. 22, direct evaporative cooling may
be provided as shown with line 2035 to further reduce the
temperature of the outlet air stream (but, as shown, the air stream
is also humidified) as the air moves from air state "2" to air
state "2.5." The direct evaporative stage may be integrated into
the second stage device 1854 or provided as a separate device
(e.g., with reference to FIG. 18, a separate heat and mass
exchanger in the cooler 1800 downstream of evaporative cooler 1850
or be integrated into evaporative cooler 1850). As shown in FIGS.
20 and 22, the air stream is cooled at least 15.degree. F. such as
to a temperature below 60.degree. F. (e.g., an outlet air stream is
produced with a temperature between 50 and 60.degree. F. or the
like). Also, as shown, dehumidifying is achieved with a relatively
small increase in air stream temperature, e.g., an increase of less
than about 5.degree. F.
[0092] As shown, the supply air 1860 flows in channels defined by
separation walls 1991, 1993, and 1995 with wicking material or
flocked surfaces 1992, 1994 facing into each channel. In this way,
water may be caused to flow next to the air 1860 to provide
humidification to the output supply air 1997 (cooled and humidified
to air state "2.5" as shown in FIG. 22). Air 1997 is colder than
air 1860 from the second stage 1854, therefore less energy is
required to provide a desired level of cooling.
[0093] The cooler 1800 (FIG. 18) may be assembled and implemented
in a variety of ways to practice the cooling methods and techniques
described herein, but it may be useful to describe one tested
assembly or cooler. In the first-stage, flutes were created by
extrusion to form the coolant airstream 1826 to 1828 (state "3" to
state "4"). Water 1818 was distributed via flow nozzles at the top
of the dehumidifier 1810 (e.g., in the airstream 1826 plenum) and
mixed with airstream 1826 to 1828, which ran vertically downward.
Some water evaporated as it traveled through the dehumidifier 1810,
but most was collected at the bottom of the airstream 1828 plenum.
Louvers in this plenum were used to separate the water droplets
from the airstream. Because this design did not have a mechanism to
hold up the water internal to the flutes (e.g., wicked surfaces),
this configuration used a water flow rate that was significantly
higher than the water evaporation rate. Thus, a water reservoir and
pump were used to return the water from the collection sump to the
top flow nozzles.
[0094] The unbacked vapor permeable membrane was welded to the
flutes/extrusions. A liquid manifold distributed desiccant to the
space between the membrane and the flutes/extrusions. Air gaps on
airstream 1820 to 1822 (air state "1" to air state "1.5") were
maintained by strips of spacers with the extruded flutes oriented
parallel to the airflow. The design also incorporated spacers that
mixed the airstream to enhance heat and mass transfer. Flutes were
used to form the channels for airstream 1822 to 1860 (air state
"1.5" to air state "2"). A nylon wick was applied to the outer
walls of the separation wall/plastic sheets. These subassemblies
were then stacked with spacers between each to form the channels
for air flow 1862 to 1866 (air state "2" to air state "5"). A low
flow of water 1858 was distributed into the second-stage channels
from the top. The nylon wick had sufficient water upkeep to allow
this flow rate to be marginally above the water evaporation rate.
Thus, a solenoid valve controlling domestic cold water may be used
to distribute water. Purge water was collected at the bottom of the
plenum of air stream 1866, at which point it was directed to a
drain.
[0095] Wicked surfaces provide a number of advantages for the
indirect evaporative coolers described herein. The wicking ensures
that the walls are fully wetted and that there is no lost
evaporation area. The water feed rate can be held to a factor of
1.25 to 2 times that of the evaporation rate. This technique allows
for "once-through" water use. The water that drains off the heat
and mass exchanger is concentrated with minerals and can then be
drained away. A sump and pumping system are not required, which
improves energy performance and eliminates sump-borne biological
growth. A simple controller can periodically use fresh (low
concentration) water to rinse the heat and mass exchanger (such as
cooler 1800) and clear any built-up minerals. Air streams 1822 to
1860 and 1862 to 1866 are in counterflow in the second-stage 1850.
A sensitivity analysis showed that the cooling effectiveness could
be reduced by as much as 20 percent if proper counterflow was not
achieved. Air stream 1822 to 1860 flowed straight, through extruded
flutes, but airstream 1862 to 1866 used a 90-degree turn before
exiting the second stage 1850. Computational fluid dynamics
software may be used design an air restrictor to ensure proper
counterflow of air stream 1862 to 1866.
[0096] Likewise, the stacks including the membranes and wicking
material may be formed in a variety of ways to implement a mass and
heat exchanger of the present description (such as cooler 1800).
The construction of one prototype revolved around laminated layers
of polyethylene terephthalate (PET) plastic that were adhered with
layers of acrylic pressure-sensitive adhesive. Although this
assembly method may not easily be scaled to high-volume
manufacturing, the achievable geometries are nearly ideal and,
therefore, appropriate for prototypes. This enabled the creation of
a prototype with parallel plate geometry that included airside
turbulators to enhance heat and mass transfer on airstreams.
Another prototype was built using layers of extruded polypropylene
(PP). It is likely that formed aluminum sheets may be used to
create a parallel plate structure to implement a cooler described
herein. For example, the aluminum sheets may be corrugated to form
a wavy flow channel, which would increase heat transfer by the
waviness of the channel (which promotes mixing of the air stream
and impingement of the air into the separator plate wall) and also
act to reinforce the structure by giving the sheets/plates
increased rigidity. Such an arrangement may work better in the
second stage where there is no desiccant (since the desiccant may
corrode the aluminum).
[0097] For the first-stage 1810, the laminated layers enabled the
use of wicked surfaces in the air stream 1826 to 1828 channels. For
the spacer, an off-the-shelf expanded aluminum grating was used,
and the spacer was used in channels for air stream 1820 to 1822 and
air stream 1826 to 1828. The design of the stacks such as stack
1814 used expanded polypropylene hydrophobic membrane backed with a
nonwoven polypropylene fabric to add strength. The backing reduces
vapor diffusion through the membrane but increases tear resistance.
The backing was oriented to the airside gap, where tears can
originate from abrasion by foreign objects or the aluminum spacer.
A desiccant manifold was developed that used laminated layers of
plastic and adhesive to effectively and evenly distribute liquid
desiccant behind the membrane. The second stage 1850 used laminated
construction but, with minimal spacers to create laminar flow, used
parallel plate air channels. The design used strips as airflow
spacers and wicked surfaces on the wet side of the heat and mass
exchanger 1800.
[0098] The above description concentrated or stressed designs of
heat/mass transfer assemblies for use in providing unique indirect
evaporative coolers. Those skilled in the art will recognize that
the coolers described can readily be included in more complete HVAC
systems for residential and commercial use. Such HVAC systems would
include plumbing and components to circulate liquid desiccant to
and from the cooler at desirable and controllable flow rates. These
systems would also include a regenerator for the desiccant (e.g.,
one that heats the liquid desiccant to remove absorbed water such
as heat provided by solar panels, electrical heaters, or the like).
The regenerator also may include a sump and lines for recovering
potable water from the desiccant, and storage would be provided for
the desiccant prior to it being pumped or fed to the cooler.
Portions of the system that come into contact with the desiccant
typically would be fabricated of corrosion resistant materials such
as certain metals or, more typically, plastics. The HVAC system
would also include ducting and other components such as fans or
blowers for moving the return air from the building through the
cooler and back to the cooled spaces, for moving make up air
through the cooler and into the cooled spaces, and for discharging
any purge or exhaust air. A coolant supply system with piping and
pumps/valving (as necessary) would also provide coolant such as
potable water to the cooler stacks (e.g., channels between
membranes and separation walls).
[0099] The embodiments shown typically discussed ongoing use of the
liquid desiccant to dehumidify the supply or process air. However,
in many operating conditions, the cooler may be operated without
desiccant flow, and these operating conditions may be considered
"free evaporative cooling" conditions (or zones on a psychrometric
chart). "Free cooling" is exemplified by cooling efficiency so high
that the cost of energy to run the system is of no consequence. For
example, cooling without drying/dehumidifying may be performed by
coolers described herein when the humidity ratio is below about 80
(and the dry bulb temperatures are above 60.degree. F.) while
cooling and drying may be required above this humidity ratio at
which point the cooler can be operated with flowing liquid
desiccant. Such "free" cooling is practical for relatively large
numbers of days in less humid areas of the world (such as the
southwest portion of the United States).
[0100] Embodiments of an indirect evaporative cooler according to
the above description and attached figures can be provided as a
single unit that provides an integral heat and a mass transfer
device utilizing a number of separation walls. The transfer device
or assembly uses membrane containment and air flows do not come in
direct contact with desiccant or water (coolant). The coolers use
evaporative cooling to drive heat and mass exchange, with heat
being transferred through the separation walls between the liquid
desiccant and the coolant. The heat exchange is between two counter
and/or cross flowing air streams. The mass exchange, such as during
dehumidification, is generally the transfer of water vapor from the
inlet supply air or process air through a water molecule-permeable
membrane to a liquid state (e.g., to absorption by the liquid
desiccant). The evaporative section of the coolers drives heat
through the separation wall and expels that heat by evaporation
from the coolant/water to an air stream.
[0101] It should also be kept in mind that the first and second
stages (dehumidifier and indirect evaporative cooler) may be
provided in a single system or machine and be packaged to be within
a single housing or two or more housings within a single system. In
some cases, the first, second, and third (when included) stages are
provided in a single system with one, two, or more housings or
machines. In other cases, though, the first and second stages are
not packaged into the same machine but are configured to cool the
same building space. The third stage if present may be provided
with the second stage or in a separate machine/housing.
Particularly, the dehumidifier can be packaged into a machine and
take a mixture of indoor and outdoor air, dehumidify that air, then
deliver it to the space. A second machine may pull air from the
space (and maybe some outdoor air) and send it through the indirect
evaporative cooler as described, then deliver colder supply air to
the space. The second machine could exhaust some air so the
dehumidifier machine would supply some make-up air from the
outside. Essentially, the same process takes place as shown for
single systems and/or machines but in two separate machines, one
for each heat and mass exchanger.
[0102] In some embodiments, a method has been described for
conditioning a supply air stream. The method includes the step of
dehumidifying the supply air stream to provide a dehumidified air
stream. Such dehumidifying includes directing the supply air stream
through a first channel defined by a surface of a vapor permeable
membrane containing liquid desiccant. The method further includes
transferring heat from the liquid desiccant to a layer of coolant
flowing in a second channel adjacent to the liquid desiccant.
[0103] In some implementations of the method, the layer of coolant
comprises coolant flowing in a layer of wicking material positioned
for heat transfer from the liquid desiccant. Then, the method may
further include evaporating a portion of the coolant flowing in the
layer of wicking material into an air stream flowing adjacent to
the layer of wicking material. It may be useful in the method for
the coolant in the layer to be flowing at a flow rate of less than
about 50 inches per minute. In some cases, a temperature of the
supply air stream is increased during the dehumidifying less than
about 5.degree. F. In the same or other cases, the dewpoint
temperature of the supply air stream is decreased during the
dehumidifying to less than about 55.degree. F.
[0104] In some embodiments of the indirect evaporative cooler or
supply air conditioning apparatus, a dehumidifier is included in a
first stage, and the dehumidifier performs cooling of an air stream
input to an inlet of the dehumidifier by absorption of water vapor
from the input air stream. Such cooling may be performed using a
membrane-contained liquid desiccant that is liquid cooled. The
phrase "liquid cooled" may take a number of meanings including, but
not limited to, flowing a liquid coolant such as water adjacent the
channel containing the liquid desiccant, and the liquid coolant
flows such that the temperature of the liquid is raised as it
passes through the dehumidifier to carry away heat from the liquid
desiccant.
[0105] The apparatus may also include a second stage comprising an
indirect evaporative cooler in fluid communication with an outlet
of the first stage to receive a cooled portion of the input air
stream. The indirect evaporative cooler may be operable to sensibly
cool the received and cooled portion of the input air stream to a
temperature (or to be within a temperature range) less than the wet
bulb temperature and greater than the dew point temperature of the
input air stream. During operation of the apparatus, a first
portion of the sensibly cooled air stream is supplied to a cooled
space and a second portion of the sensibly cooled air stream is
directed to a wet side of the indirect evaporative cooler and
receives heat from the received and cooled portion of the input air
stream.
[0106] In some implementations of such an apparatus, the
dehumidifier performs the cooling using a membrane-contained liquid
desiccant cooled by an indirect evaporative channel that includes a
wicking layer containing a flow of coolant. In other
implementations, the dehumidifier performs the cooling using a
membrane-contained liquid desiccant that is liquid cooled. The
apparatus may be operated such that the second portion comprises 10
to 40 percent of the received and cooled portion of the input air.
It may be useful for a flow channel for the second portion of the
sensibly cooled air stream in the indirect evaporative cooler to be
defined in part by a surface of a separation wall. Then, a layer of
wicking material may be provided on the surface acting to wick a
stream of liquid coolant adjacent the flow channel.
[0107] In some settings, the apparatus includes a direct
evaporative stage between the second stage and the cooled space.
Then, during operations, the direct evaporative stage receives the
first portion of the sensibly cooled air stream and provides
additional cooling via direct evaporative cooling prior to
supplying the first portion of the sensibly cooled air stream to
the cooled space. Further, the direct evaporative stage may be
adapted to contain flow of water with a vapor permeable membrane or
within a layer of wicking material (e.g., the layer of wicking
material is provided as a surface with a hydrophilic coating).
[0108] While a number of exemplary aspects and embodiments have
been discussed above, those of skill in the art will recognize
certain modifications, permutations, additions, and
sub-combinations thereof. It is therefore intended that the
following appended claims and claims hereafter introduced are
interpreted to include modifications, permutations, additions, and
sub-combinations to the exemplary aspects and embodiments discussed
above as are within their true spirit and scope.
* * * * *