U.S. patent application number 15/117419 was filed with the patent office on 2016-12-29 for flexible liquid desiccant heat and mass transfer panels with a hydrophilic layer.
This patent application is currently assigned to 3M INNOVATIVE PROPERTIES COMPANY. The applicant listed for this patent is 3M INNOVATIVE PROPERTIES COMPANY. Invention is credited to Laurence W. Bassett, Rajeev Dhiman, Thomas J. Hamlin.
Application Number | 20160377302 15/117419 |
Document ID | / |
Family ID | 54009560 |
Filed Date | 2016-12-29 |
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United States Patent
Application |
20160377302 |
Kind Code |
A1 |
Hamlin; Thomas J. ; et
al. |
December 29, 2016 |
FLEXIBLE LIQUID DESICCANT HEAT AND MASS TRANSFER PANELS WITH A
HYDROPHILIC LAYER
Abstract
Provided are flexible panel devices that use desiccants for heat
and mass transfer processes, including but not limited to air
conditioning systems, for example, liquid desiccant air
conditioning (LDAC) applications wherein the liquid desiccant is
contained in a panel that comprises at least one hydrophilic
separation layer, which allows water vapor transfer between the air
and liquid desiccant and enable dehumidification and humidification
of the air. The flexible panel devices can be installed on an
absorber (conditioner) side or a desorber (regenerator) side or
both of a LDAC system. The devices have two flexible layers, at
least one of which comprises a flexible and water vapor permeable
hydrophilic separation layer, that form a desiccant flow channel
and a desiccant flow distributor located therein. The two flexible
layers may both be permeable hydropholic separation layers, or they
may comprise one permeable hydrophilic separation layer along with
another layer that may be a non-porous structure or a water-vapor
permeable hydrophobic separation layer.
Inventors: |
Hamlin; Thomas J.; (Vernon,
CT) ; Bassett; Laurence W.; (Killingworth, CT)
; Dhiman; Rajeev; (North Haven, CT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
3M INNOVATIVE PROPERTIES COMPANY |
St. Paul |
MN |
US |
|
|
Assignee: |
3M INNOVATIVE PROPERTIES
COMPANY
St. Paul
MN
|
Family ID: |
54009560 |
Appl. No.: |
15/117419 |
Filed: |
February 25, 2015 |
PCT Filed: |
February 25, 2015 |
PCT NO: |
PCT/US15/17442 |
371 Date: |
August 8, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61946352 |
Feb 28, 2014 |
|
|
|
Current U.S.
Class: |
165/56 |
Current CPC
Class: |
F24F 3/147 20130101;
F24F 3/1417 20130101 |
International
Class: |
F24F 3/147 20060101
F24F003/147; F24F 3/14 20060101 F24F003/14 |
Claims
1. A heat and mass transfer panel for water vapor exchange with a
liquid desiccant, the panel comprising: a desiccant flow channel
defined by a first flexible layer and a second flexible layer, at
least one of which comprises a flexible hydrophilic water
vapor-permeable separation layer; a desiccant inlet and a desiccant
outlet to the desiccant flow channel; and a flexible desiccant flow
distributor located in the desiccant flow channel.
2. The heat and mass transfer panel of claim 1, wherein both the
first and the second flexible layers comprise a flexible
hydrophilic water-vapor permeable separation layer.
3. The heat and mass transfer panel of claim 1, wherein the first
flexible layer comprises a flexible hydrophilic water-vapor
permeable separation layer and the second flexible layer is a
non-porous layer or a hydrophobic water-vapor permeable separation
layer.
4. The heat and mass transfer panel of claim 1, wherein the
flexible hydrophilic water-vapor permeable separation layer or
layers independently comprise a membrane, a woven mesh, a nanofiber
media, an electrospun fiber media, a glass fiber media, a nonwoven
melt blown fiber media, a corrosion-resistant metal, a ceramic
media, or combinations thereof.
5. The heat and mass transfer panel of claim 1, wherein the
flexible hydrophilic water-vapor permeable separation layer or
layers independently comprise a micro-filtration or an
ultra-filtration membrane comprising a hydrophilic nylon (PA)
membrane, a hydrophilized polyethersulfone (PES) membrane, a
hydrophilized polysulfone (PS) membrane, a hydrophilized
polyvinylidene fluoride (PVDF) membrane, a hydrophilic
polyacrylonitrile (PAN) membrane, a hydrophilized polypropylene
(PP) membrane, a hydrophilized polyethylene (PE) membrane, a
hydrophilized polytetrafluorethylene (PTFE) membrane, a
hydrophilized polycarbonate (PC) membrane, a hydrophilized ethylene
chlorotrifluoroethylene (ECTFE) membrane, or combinations
thereof.
6. The heat and mass transfer panel of claim 1, wherein the
desiccant flow distributor comprises a hydrophilic material that
comprises a polymeric material, a natural fiber, or combinations
thereof.
7. The heat and mass transfer panel of claim 6, wherein the
desiccant flow distributor comprises a hydrophilic polymer material
that comprises a membrane, an open cell foam, a porous nonwoven
material, a porous woven material, or combinations thereof.
8. The heat and mass transfer panel of claim 1, wherein the
desiccant flow distributor comprises one or more draw-and-drip
features at an outlet end of the distributor; wherein the
draw-and-drip features are effective for facilitating uniform flow
through the panel.
9. The heat and mass transfer panel of claim 1 further comprising
an air channel layer.
10. The heat and mass transfer panel of claim 1 further comprising
a desiccant distribution header.
11. A heat and mass transfer module comprising: one or more panels
of claim 1 assembled among one or more air channel layers or air
gaps; and an air inlet and an air outlet.
12. The heat and mass transfer module of claim 11 further
comprising two end plates between which the one or more panels and
the one or more air channel layers are assembled.
13. A method for water vapor exchange between air and a liquid
desiccant, the method comprising: contacting the panel of claim 1
with air having a water vapor pressure different from the
equilibrium vapor pressure in a desiccant flowing through the
desiccant flow channel; wherein the humidity of the air after
contact with the panel is different from the humidity before
contact with the panel.
14. A desiccant flow distributor comprising: a hydrophilic
structure comprising a polymeric material, a natural fiber, or
combinations thereof; and one or more draw-and-drip features at an
outlet end of the structure; wherein the draw-and-drip features are
effective for facilitating uniform flow therethrough.
15. The desiccant flow distributor of claim 14 comprising a
hydrophilic polymeric material that comprises a membrane, an open
cell foam, a porous nonwoven material, a porous woven material, or
combinations thereof.
16. The desiccant flow distributor of claim 14, wherein the one or
more draw-and-drip features comprise a series of edges such that
the linear edge near or at the outlet end of the structure is
longer than the linear edge at the inlet end of the structure.
17. The desiccant flow distributor of claim 14, wherein the one or
more draw-and-drip features comprise a series of shapes defined by
or at the edges of the outlet end.
Description
TECHNICAL FIELD
[0001] This disclosure relates to flexible panel devices that use
desiccants for heat and mass transfer processes, including but not
limited to air conditioning systems. Specifically, devices
disclosed herein are particularly useful in liquid desiccant air
conditioning (LDAC) applications wherein heat and mass transfer is
achieved with a panel that comprises one or more hydrophilic
separation layers that are wettable by the desiccant. A desiccant
flow distributor located in the panel is hydrophilic and is
fabricated to provide excellent wicking and drawing of the
desiccant through the panel.
BACKGROUND
[0002] The use of liquid desiccants for dehumidification of air has
been known for well over 75 years. The application of liquid
desiccants in dehumidification applied in heating, ventilating, and
air conditioning (HVAC) systems has been worked on for many years.
Open absorption systems for air conditioning are desirable due to
their relatively simple design and driving energy at relatively low
temperatures. Liquid desiccant air conditioning (LDAC) is an
exemplary open absorption system.
[0003] Membrane modules have been researched and attempted for use
in LDAC systems. Some module designs incorporated three fluid
paths: one for desiccant, one for air, and one for coolant; and
other designs incorporate two fluid paths: one for desiccant and
one for air. Certain designs have provided benefits on the
performance of the absorber side of the system but not on the
desorber side, and overall commercial success of liquid desiccant
air conditioning (LDAC) systems has been extremely limited.
SUMMARY
[0004] Provided are heat and mass transfer panels, heat and mass
transfer modules, and methods of making and using the same.
[0005] In a first aspect, a heat and mass transfer panel for water
vapor exchange with a liquid desiccant is provided, the panel
comprising: a desiccant flow channel defined by a first flexible
porous layer and a second flexible layer, at least one of which
comprises a flexible hydrophilic separation layer; a desiccant
inlet and a desiccant outlet to the desiccant flow channel; and a
flexible desiccant flow distributor located in the desiccant flow
channel.
[0006] Another aspect provides heat and mass transfer modules
comprising: one or more panels disclosed herein assembled among one
or more air channel layers or air gaps; and an air inlet and an air
outlet.
[0007] Other features that may be used individually or in
combination with respect to any aspect of the invention are as
follows.
[0008] Both the first and the second flexible layers may comprise a
flexible hydrophilic water-vapor permeable separation layer. Or,
the first flexible layer may comprise a flexible hydrophilic
water-vapor permeable separation layer and the second flexible
layer may be a non-porous layer or a hydrophobic water-vapor
permeable separation layer. The flexible hydrophilic water-vapor
permeable separation layer or layers may independently comprise a
membrane, a woven mesh, a nanofiber media, an electrospun fiber
media, a glass fiber media, a nonwoven melt blown fiber media, a
corrosion-resistant metal, a ceramic media, or combinations
thereof. The flexible hydrophilic water-vapor permeable separation
layer or layers may independently comprise a micro-filtration or an
ultra-filtration membrane comprising a hydrophilic nylon (PA)
membrane, a hydrophilized polyethersulfone (PES) membrane, a
hydrophilized polysulfone (PS) membrane, a hydrophilized
polyvinylidene fluoride (PVDF) membrane, a hydrophilic
polyacrylonitrile (PAN) membrane, a hydrophilized polypropylene
(PP) membrane, a hydrophilized polyethylene (PE) membrane, a
hydrophilized polytetrafluorethylene (PTFE) membrane, a
hydrophilized polycarbonate (PC) membrane, a hydrophilized ethylene
chlorotrifluoroethylene (ECTFE) membrane, or combinations
thereof.
[0009] In some embodiments, the desiccant flow distributor is
effective to uniformly spread desiccant under head pressure
conditions of 12 inches (30.5 cm) of water or less. The desiccant
flow distributor may be effective to uniformly spread desiccant
under head pressure conditions in the range of atmospheric pressure
to less than or equal to 12 inches (30.5 cm) of water.
[0010] The desiccant flow distributor may comprise a hydrophilic
material that comprises a polymeric material, a natural fiber, or
combinations thereof. The desiccant flow distributor may comprise a
hydrophilic polymer material that comprises a membrane, an open
cell foam, a porous nonwoven material, a porous woven material, or
combinations thereof. The desiccant flow distributor may comprise a
hydrophilic polymer material that comprises a rail film, an
extruded web material, an apertured polymeric film, or combinations
thereof. The desiccant flow distributor may comprise a hydrophilic
natural fiber that comprises cellulose. The desiccant flow
distributor may comprise an open cell foam of a hydrophilized
polyether urethane or a hydrophilized polyester urethane. The
desiccant flow distributor may comprise one or more draw-and-drip
features at an outlet end of the distributor; wherein the
draw-and-drip features are effective for facilitating uniform flow
through the panel.
[0011] The non-porous layer may comprise polyethylene, cast,
polypropylene, oriented polypropylene, PET (polyethylene
terephthalate), bi-axially oriented PET, bi-axially oriented PET
with aluminum or gold vapor deposited on the surface, PA
(polyamide), PVC (polyvinylchloride), EVOH (ethylene vinyl alcohol)
and/or co-extruded/multilayer film constructions thereof.
[0012] The heat and mass transfer panels may further comprise an
air channel layer. The heat and mass transfer panels may further
comprise a desiccant distribution header.
[0013] In one or more embodiments, the heat and mass transfer
panels, upon contact with air having a water vapor pressure higher
than the equilibrium vapor pressure of the desiccant, are effective
to transfer water vapor from the air to a desiccant flowing through
the desiccant channel. In one or more embodiments, the heat and
mass transfer panels, upon contact with air having a water vapor
pressure lower than the equilibrium vapor pressure of the
desiccant, are effective to transfer water vapor from the desiccant
to the air.
[0014] The heat and mass transfer modules may further comprise two
end plates between which the one or more panels and the one or more
air channel layers are assembled. The end plates may be
mechanically fastened together. In some instances, there are fewer
desiccant inlets than desiccant outlets. In other instances, there
are fewer desiccant outlets than desiccant inlets.
[0015] Further aspects provide methods for water vapor exchange
between air and a liquid desiccant, the methods comprising:
contacting any panel disclosed herein with air having a water vapor
pressure different from the equilibrium vapor pressure in a
desiccant flowing through the desiccant flow channel; wherein the
humidity of the air after contact with the panel is different from
the humidity before contact with the panel.
[0016] When the water vapor pressure of the air is higher than the
equilibrium vapor pressure of the desiccant, the method may further
comprise transferring the water vapor from the air to the
desiccant, and the humidity of the air after contact with the panel
is less than the humidity before contact with the panel. When the
equilibrium vapor pressure of the desiccant is higher than the
water vapor pressure of the air, the method may further comprise
transferring the water vapor from the desiccant to the air, and the
humidity of the air after contact with the panel is more than the
humidity before contact with the panel. The desiccant flow
distributor may comprises one or more draw-and-drip features at an
outlet end of the distributor; wherein the draw-and-drip features
are effective for facilitating uniform flow through the panel.
[0017] Another method is a method of making a heat and mass
transfer panel, the method comprising: forming a desiccant flow
channel defined by a first flexible layer and a second flexible
layer, at least one of which comprises a flexible hydrophilic water
vapor-permeable separation layer; locating a flexible desiccant
flow distributor in the desiccant flow channel; assembling the
first flexible layer, the second flexible layer, and the flexible
desiccant flow distributor; and providing or forming a desiccant
inlet and a desiccant outlet to the desiccant flow channel.
[0018] A further aspect is a desiccant flow distributor comprising:
a hydrophilic structure comprising a polymeric material, a natural
fiber, or combinations thereof; and one or more draw-and-drip
features at an outlet end of the structure; wherein the
draw-and-drip features are effective for facilitating uniform flow
therethrough. The desiccant flow distributor may comprise a
hydrophilic polymeric material that comprises a membrane, an open
cell foam, a porous nonwoven material, a porous woven material, or
combinations thereof. The one or more draw-and-drip features may
comprise a series of edges such that the linear edge near or at the
outlet end of the structure is longer than the linear edge at the
inlet end of the structure. The one or more draw-and-drip features
may comprise a series of shapes defined by or at the edges of the
outlet end.
[0019] These and other aspects of the invention are described in
the detailed description below. In no event should the above
summary be construed as a limitation on the claimed subject
matter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The accompanying drawings are included to provide a further
understanding of the invention described herein and are
incorporated in and constitute a part of this specification. The
drawings illustrate exemplary embodiments. Certain features may be
better understood by reference to the following detailed
description when considered in connection with the accompanying
drawings, in which like reference numerals designate like parts
throughout the figures thereof, and wherein:
[0021] FIG. 1 is a schematic illustrating condensation followed by
hydraulic breakthrough resulting from prior art structures;
[0022] FIG. 2 is a process flow chart for an exemplary flexible
panel fabrication system;
[0023] FIGS. 3A-3B show an exemplary panel in an expanded schematic
view (FIG. 3A) and in an assembled schematic view (FIG. 3B);
[0024] FIG. 4 is a schematic view of a combination of an exemplary
air channel layer and air channel seals;
[0025] FIGS. 5A-5B show an exemplary module in an expanded
schematic view (FIG. 5A) and in an assembled schematic view (FIG.
5B);
[0026] FIG. 6 is an embodiment of a flexible liquid desiccant heat
and mass transfer panel with two air channel layers in a module
assembly having two end plates; and
[0027] FIGS. 7A-7D provide schematic depictions of exemplary
draw-and-drip features having various shapes.
[0028] The figures are not necessarily to scale. Like numbers used
in the figures refer to like components. It will be understood,
however, that the use of a number to refer to a component in a
given figure is not intended to limit the component in another
figure labeled with the same number.
DETAILED DESCRIPTION
[0029] Provided are improved flexible panel devices that use
desiccants for heat and mass transfer processes, including but not
limited to air conditioning systems, for example, liquid desiccant
air conditioning (LDAC) applications allowing heat and water vapor
transfer between the air and liquid desiccant, which enable
dehumidification and/or humidification of the air. The flexible
panel devices may be installed on an absorber (conditioner) side or
a desorber (regenerator) side or both of a LDAC system.
[0030] The devices have two flexible layers, at least one of which
comprises a flexible and hydrophilic separation layer, that form a
desiccant flow channel and a desiccant flow distributor located
therein. The two flexible layers may both be hydrophilic separation
layers, or they may comprise one hydrophilic separation layer along
with another layer that may be a non-porous structure or a
water-vapor permeable hydrophobic separation layer. An air channel
layer is an optional layer.
[0031] The at least one flexible porous hydrophilic separation
layer controls a thin film of liquid desiccant within the pores and
on the surfaces of the separation layer via surface tension and
capillary action. The separation layer(s) are in contact with a
hydrophilic desiccant flow distributor which provides hydrodynamic
control of a falling column of liquid desiccant between the
separation layers. The use of porous hydrophilic separation
layer(s) in flexible liquid desiccant heat and mass transfer panels
has significant advantages over the use of porous hydrophobic
separation layers.
[0032] The following terms shall have, for the purposes of this
application, the respective meanings set forth below.
[0033] A "panel" is a fundamental structure for achieving mass
and/or heat transfer. Panels may provide multiple functionalities
such as water vapor separation, distribution of a desiccant, and
management of condensation. Panels may comprise two layers, at
least one hydrophilic layer and another layer, to form a channel
through which desiccant flows. The hydrophilic layer facilitates
water vapor transfer. The channel may contain a desiccant flow
distributor to facilitate substantially uniform flow through the
channel.
[0034] A "module" is an assembly of several panels to achieve mass
and/or heat transfer in practical commercial quantities.
[0035] "Flexible layers" and "flexible panels" refer to structures
that are non-rigid and can be rolled onto itself and unrolled
without damage. In one or more embodiments, such layers or panels
may be rolled 180 degrees around a radius that is less than or
equal to five (or two and one-half, or even less than or equal to
one) times the thickness of the layer without damage.
[0036] "Hydrophilic" means that the liquid desiccant is able to
invade and wet the pores of the separation layer. This effect can
be quantified by Eq. (1) which gives the criterion for a liquid to
invade a textured solid depending on its intrinsic wettability with
the solid and the details of the textures:
cos .theta. .gtoreq. 1 - .phi. s r - .phi. s ( 1 ) ##EQU00001##
[0037] where .theta. is the contact angle of the liquid with the
solid without any textures (i.e. smooth), .phi..sub.s is the solid
fraction (between 0 and 1), and r (.gtoreq.1) is the ratio of true
surface area of the solid to its projected area. For a porous
solid, r is infinity, which implies that .theta..ltoreq.90.degree.,
i.e. any liquid with contact angle less than 90.degree. will
eventually invade a porous material. Eq. (1) merely predicts
whether or not a given liquid will invade a porous solid, but it
does not predict how quickly this will happen. This feature of a
porous material distinguishes it from solids having textures only
on their surfaces where the condition for liquid invasion is more
restrictive. For example, for .phi..sub.s=0.1 and r=2,
.theta..ltoreq.62.degree., implying that only liquids with contact
angle less than 62.degree. will be able to invade the solid
textures. Thus "hydrophilic" in this context means
.theta..ltoreq.90.degree., preferably .theta. should be as small as
possible and preferably .theta..ltoreq.75.degree..
[0038] A "hydrophilic separation layer," therefore, refers to a
structure that is wettable by the liquid desiccant solutions.
Exemplary such structures include but are not limited to: a
membrane, a woven mesh, a nanofiber media, an electrospun fiber
media, a glass fiber media, a nonwoven melt blown fiber media, a
corrosion-resistant metal, a ceramic media, or combinations
thereof, which are hydrophilic by virtue of the materials used to
fabricate the layer and/or by treatment. Exemplary membranes are
micro-filtration or ultra-filtration membranes.
[0039] In addition, "hydrophobic water vapor-permeable separation
layer" and "hydrophobic separation layer" refer to a structure that
is porous to water vapor but is not wettable by the liquid
desiccant solutions.
[0040] A "liquid desiccant" is a hygroscopic material which has the
ability to both absorb or desorb water vapor into or from solution
based on partial pressure differences. Examples of suitable
desiccants are halide salts (such as lithium chloride, calcium
chloride, and mixtures thereof, and lithium bromide) and glycols
(such as triethylene and propylene glycol).
[0041] A "porous separation layer" is the layer of material in the
flexible panel that controls the interface between the liquid
desiccant and the air (or any gas) to be dehumidified or
humidified. "Control" means to promote both heat and mass transfer
between the liquid desiccant and the air while ensuring that liquid
desiccant is not released, aerosolized, and entrained in the air
stream. Liquid desiccants are typically corrosive salts (i.e.
lithium chloride, calcium chloride) and can cause many problems in
heating, ventilation, and air conditioning (HVAC) systems if they
are allowed into the air stream. Corrosion of equipment components,
corrosion of ductwork, and health and safety concerns have
prevented the widespread use of liquid desiccants in HVAC systems
and improved solutions to liquid desiccant control are needed.
[0042] It has been found that the use of hydrophobic water vapor
permeable separation layers to control the liquid desiccant and air
interface as described in the prior art have some limitations. When
used in LDAC systems in instances where the desiccant enters the
heat and mass exchanger at a temperature lower than the dew point,
one limitation is that condensation can occur on the air side
surface of the separation layers as well as within the pores of the
layer. Beads of liquid discharging to the air side will bead on the
outer surface of the hydrophobic membrane as the bulk of the outer
surface is still hydrophobic and not readily wetted. The formation
of beads will cause air flow to be blocked on the air side and
beads have a much higher chance of being aerosolized or otherwise
carried down the air stream. This phenomenon will continue to
spread as condensation continues to form bridges across the layer.
The ability of the hydrophobic water vapor permeable separation
layer to adequately control the desiccant can only be restored if
the layer is completely dried to eliminate all liquid bridging.
Even if the hydrophobic separation layer is dried, there may be
residual salt left on the internal surfaces of the layer which
could promote rewetting.
[0043] In addition, capillary condensation can occur at
temperatures above the dew point (below the saturation vapor
pressure) within the pores of the layer. The probability of
occurrence increases if the pore structure of the separation layer
is small as can be found in the pore distribution of
micro-filtration membranes, and in ultra-filtration,
nano-filtration, and osmotic membranes. Any condensation that fills
the pores and bridges the thickness of the layer will create a
hydraulic path where desiccant leakage can occur. Turning to FIG.
1, which is a schematic illustrating condensation 14 followed by
hydraulic breakthrough 18 resulting from prior art structures, if
the pressure on the desiccant side 12 of a hydrophobic separation
layer or membrane 10 (P.sub.desiccant) is greater than the pressure
on the air side 16 (P.sub.air), hydraulic leaks 18 will occur and
desiccant will be discharged to the air stream side. This can be
most problematic on modular and desiccant flow circuit designs
where the desiccant pressure is .gtoreq.0.5 psig (14 inches or 35.5
cm) or on designs where there is a totally open desiccant channel
with no desiccant flow distributor.
[0044] Furthermore, any surface contamination due to materials such
as surfactants, greases, or oils can affect the surface energy of
the hydrophobic water vapor permeable separation layers further
complicating the ability of the layer to effectively retain the
desiccant. This can also be most problematic on modular and
desiccant flow circuit designs where the desiccant pressure is
.gtoreq.0.5 psig (14 inches or 35.5 cm) or on designs where there
is a totally open desiccant channel with no desiccant flow
distributor.
[0045] A limitation when using a hydrophobic water vapor permeable
separation layer is that the air/desiccant interface occurs on the
desiccant side of the layer. This means that there is a heat
transfer resistance in the heat and mass exchanger due to the
stagnant layer of air occurring within the pore structure of the
layer. By moving the air/desiccant interface to the outside of the
hydrophilic layer(s), high heat transfer of water through the
hydrophilic layer(s) is achieved.
[0046] The heat transfer resistance, R.sub.th,sep posed by the
hydrophobic separation layer can be determined as: R.sub.th,sep=t/k
where t and k are the thickness and thermal conductivity
(determined by the porosity-weighted sum of the thermal
conductivities of the layer material and air) of the separation
layer. For instance, for a polypropylene separation layer of 50
.mu.m thickness and 70% porosity,
k.apprxeq.0.7k.sub.air+0.3k.sub.PP=0.0796 W/mK and
R.sub.th,sep=6.281.times.10.sup.-4 m.sup.2K/W. By contrast, for a
hydrophilic separation layer of the same thickness and similar
thermal conductivity, k.apprxeq.0.7k.sub.water+0.3k.sub.PP=0.48
W/mK and R.sub.th,sep=1.042.times.10.sup.-4 m.sup.2K/W, which is
over six times smaller than the hydrophobic case and, as a result,
will yield higher heat transfer rates for a given surface area and
temperature difference.
[0047] The mass transfer resistance R.sub.m,sep posed by the
hydrophilic separation layer may be higher or lower than the
hydrophobic case depending on whether the liquid desiccant within
the layer is stationary or mobile. The mobility of the desiccant is
difficult to determine theoretically and can be manipulated by
suitable choices of separation layer and desiccant distributor
materials. In addition, the magnitude of R.sub.m,sep has to be
compared with the other two mass transfer resistances (air-side and
desiccant-side) in the system in order to establish whether
R.sub.m,sep could be a significant bottleneck for mass transfer.
There is an advantage in using a hydrophilic separation layer in
that it stays in intimate contact with the desiccant distributor
due to surface tension and enhances overall heat and mass transfer
through this contact. These considerations make experimental
testing necessary in order to determine mass transfer performance
of the hydrophilic separation layers.
The Panels
[0048] In this invention, the use of a porous hydrophilic
separation layer has eliminated the stagnant air layer present in
the hydrophobic water vapor permeable separation layer as described
in the prior art. A particularly useful material for use as a
desiccant flow distributor is a porous hydrophilic open cell foam.
The effective use of the porous hydrophilic separation layer is
enabled by the use of a gravity feed desiccant flow system in
combination with a porous hydrophilic open cell foam as the
desiccant flow distributor. The structure and shape of the foam can
be manipulated to optimize the desiccant flow between two porous
hydrophilic separation layers or between one hydrophilic separation
layer and another layer that may be a non-porous structure or a
water-vapor permeable hydrophobic separation layer. When liquid is
fed into the top of a panel with this construction, the open cell
foam spreads the liquid uniformly between the separation layers as
it flows downward due to the effect of gravity. The porous
hydrophilic separation layer, unlike a hydrophobic material, wets
out with the liquid and all the pores become filled. The stagnant
air layer is unable to form within the separation layer.
Furthermore, surface tension forces keep the hydrophilic separation
layer and desiccant distributor in intimate contact and prevent any
air entrapment in between. It is important that the open cell foam
desiccant distributor be hydrophilic upon initial wetting with the
desiccant. Once it has been wetted, it should remain wet as long as
the vapor pressure gradient of the environment in which the panel
resides favors water absorption. After initial wetting, even if the
desiccant loaded panels are dried with high temperature dry air,
the panels should re-wet without problem due to the salt left
behind from the evaporation of the water from the drying operation.
In other words, the porous hydrophilic separation layer should be
hydrophilic enough to initially thoroughly wet and fill out all the
pores under the gravity flow condition.
[0049] The effect of the fluid flowing through the open cell foam
desiccant distributor to a discharge at the base of the panel is to
create a controlled desiccant flow with minimal positive pressure.
By positive pressure, it is meant that the liquid pressure between
the separation layers is only slightly above atmospheric pressure
and does not exceed the height of the liquid column provided in the
desiccant distribution header. The combination of the minimal
positive pressure gradient with a hydrophilic porous separation
layer which has a fairly high flow resistance is to create a panel
where the desiccant flow is sequestered within the pores of the
separation layer in an extremely thin, highly spread liquid layer
on the air side of the separation layer. This can be referred to as
a stable film in that under the airflows per unit of active heat
and mass exchanger surface area anticipated in a LDAC, it is
virtually impossible to aerosolize and entrain desiccant in the
airstream. In addition, the wetted separation layers stay in
intimate contact with the desiccant distributor due to surface
tension of the liquid. This eliminates any air gaps and maximizes
heat and mass transfer efficiency.
[0050] Also, when flow into the top of the panel is stopped, for
example, when used in an LDAC and the unit is turned off, the
desiccant can be stabilized within the pore structure of the
hydrophilic separation layer due to capillary forces alone. In some
embodiments, desiccant may drain out, but will uniformly distribute
again upon introduction of desiccant flow. In any case, the
desiccant is prevented from forming drips or leaks which could
enter the air stream and be entrained into the air in the form of
an aerosol.
[0051] The arrangement of a porous hydrophilic separation layer
such as a membrane in contact with a hydrophilic porous material
such as open-cell foam effectively solves the widespread problem of
carryover encountered in liquid desiccant systems. This solution
originates primarily from the fact that the liquid is held tightly
on the surface via surface tension, and within the pores of the
separation layer by virtue of capillary forces, which are far
greater than the shear imposed by the passing air stream. This can
be shown as follows.
[0052] Ishii and Grolmes (M. Ishii & M. Grolmes (1975),
Inception criteria for droplet entrainment in two phase concurrent
film flow, AICHE Journal, Vol 21 2 308-318) showed that a gas
stream passing over a liquid stream can dislodge droplets from the
liquid if the shear imposed by the gas overcomes the surface
tension forces, resulting into the following criterion for droplet
entrainment:
.mu. f v g .gamma. .rho. g .rho. f .gtoreq. 11.78 N .mu. 0.8 Re f -
1 / 3 for N .mu. .ltoreq. 1 15 ( 2 ) ##EQU00002##
[0053] where .mu..sub.f, .rho..sub.f, and .gamma. are the
viscosity, density, and surface tension of the liquid,
respectively, and .rho..sub.g and .nu..sub.g are the density and
velocity of the gas stream.
N .mu. = .mu. f [ .rho. f .gamma. .gamma. g .DELTA..rho. ] 1 / 2
##EQU00003##
is referred as viscosity number and the liquid Reynolds number
Re f = 4 .rho. f v f .delta. .mu. f , ##EQU00004##
where .DELTA..rho. is the density difference between the liquid and
gas, and .delta. is the liquid film thickness. For air-aqueous LiCl
system, N.sub..mu.=0.0097 which is <1/15, indicating that the
criterion of Eq. (2) is applicable. If the liquid completely wets
(i.e. contact angle, .theta.=0 deg.) the separation layer, de
Gennes et al. (P. G. de Gennes, F. Brochard-Wyart, & D. Quere,
Capillary and Wetting Phenomena, Springer Publishing 2004) have
shown that its thickness is governed by the balance of surface
tension and van der Waals forces and is typically on the order of 1
nm. If .theta.>0 deg., microscopic features of the separation
layer will remain emerged (i.e. dry) depending on where the local
contact angle equals the equilibrium value. We can consider the
case of .theta.=0 deg. with .delta.=1 nm. A conservative estimate
of .nu..sub.f would be setting it equal to the bulk fluid velocity
in the porous material (such as open-cell foam), which would be
given by Darcy's law to be about 0.0075 m/s under typical
conditions described previously. This gives
Re.sub.f=7.2.times.10.sup.-6. .nu..sub.g can be estimated by the
typical air CFMs encountered in liquid desiccant systems, giving
.nu..sub.g=0.8 m/s. Substituting all these values in the criterion
of Eq. (2), the left hand side turns out to be
1.327.times.10.sup.-3, whereas the right hand side is about 14.5,
which is five orders of magnitude greater than the left hand side
value. This suggests that desiccant droplet entrainment (i.e.
carryover) is highly unlikely under the typical air flow conditions
encountered in liquid desiccant systems. In fact, even if .delta.=1
mm (for instance, for a few seconds due to condensation of water
vapor on separation layer surface), the left hand side was
calculated to be two orders of magnitude smaller than the right
hand side, indicating that droplet entrainment is highly
unlikely.
[0054] Flow resistance of the open cell foam desiccant distributor
is also an important consideration in the design of the panel. Flow
through such a media is governed by Darcy's law. See A. E.
Scheidegger, The physics of flows through porous media, Third ed.,
University of Toronto Press, Toronto (1974) and K. Boomsma, D.
Poulikakos, The effects of compression and pore size variations on
the liquid flow characteristics in metal foams, J. Fluids Eng.
(2002) 124, pp. 263-272. Darcy's law states that the pressure drop,
.DELTA.P across the media is proportional to media length L, fluid
viscosity .mu., and fluid velocity .nu., and inversely proportional
to media permeability, K:
.DELTA. P L = .mu. K v ( 3 ) ##EQU00005##
[0055] Darcy's law is applicable for slow moving flows
characterized by Reynolds number Re=.rho. {square root over
(K)}.nu./.mu.<O(1), where .rho. is the fluid density.
Experiments were carried out with hydrophilic open-cell foams
available from UFP Technologies (Type HS) to study water flow
behavior. The key parameter varied was media length, L (95 mm and
160 mm); .DELTA.P was kept constant at 500 Pa (i.e. 2'' of water
column). Foam width w and thickness t were 32 mm and 6.35 mm,
respectively. Tests were conducted with water at room temperature
and the resulting flow rate .OMEGA. was measured from which
.nu.=.OMEGA./(wt) was calculated. Foam permeability K was
calculated to be 3.2.times.10.sup.-9 m.sup.2 from Eq. (3) using one
of the flow rate measurements. The calculated value was validated
by the good agreement found between predicted and measured flow
rates at other media lengths. To confirm the applicability of
Darcy's law, Re was calculated and found to be 0.6.
[0056] Equation (3) and the Reynold's number can be utilized to
determine the bulk flow capacity of a specific panel design and
used for optimization of the heat and mass exchanger.
[0057] Assembly of Panels
[0058] It is important that the separation layers be attached or
affixed to the desiccant distributor in some manner to make an
integral panel. Side seams may be used or created using tapes,
adhesives, ultrasonic welding, thermal welding or any other method
of attachment. The seams do not have to be liquid tight seals as
the low pressure of operation and the capillary action of the
materials will control the desiccant within and on the panel. An
exemplary tape is a closed cell foam acrylic tape, for example, one
of 3M VHB.TM. tapes identified as #4955 and 4959F would be
useful.
[0059] Side seams may be eliminated altogether and the panel will
still function provided that head pressure is very low, in the
range of <1'' (2.5 cm) of water column. An example construction
would be to attach or affix the separation layers to the desiccant
distributor by using a light adhesive coating between the layers
and laminating the assembly together. Additional methods of
laminating the separation layers to the desiccant distributor
include but are not limited to heat lamination, open flame
lamination, ultrasonic point bonding, and adhesive point
bonding.
[0060] Should ultrasonic welding be used, some typical parameters
include:
[0061] Branson Ultrasonic Welder using a welding horn having an
approximate size of 10 inches (25.4 cm) by 0.25 inches (6.3 mm)
with the following settings.
[0062] Weld Pressure: 40-60 psi
[0063] Weld Time--0.5 sec to 1.5 sec
[0064] Weld Hold Time--0.5 sec
[0065] Trigger Force--set at 12
[0066] Down speed--set at 30
[0067] Amplitude--set at 100%.
[0068] For commercial purposes, it is desirable to arrange the
layers in an efficient and orderly manner. One exemplary process
for making a flexible LDAC separation panel is as follows: obtain
the materials for the various layers in rolled or bulk form; unwind
and/or feed the layers in a stacked form, attach or affix the
layers together; cut the layers to length; and affix at least a
desiccant inlet and optionally a desiccant outlet. FIG. 2 provides
process flow chart for an exemplary flexible panel fabrication
system prior to attachment of fluid connections which incorporates
an open cell foam slab feeder. In FIG. 2, it is shown that two
porous hydrophilic separation layers 101, 103 are unwound 105 and a
hydrophilic open cell foam 107 is provided by a slab feeder 109
between the two separation layers. Feed rollers 111 and guide rolls
113 form a structure for receipt by a seamer 115 that provides side
seams by any preferred method such as rotary ultrasonic welding,
thermal welding, tape or adhesive application. A cutter 117 cuts
the seamed structures to size to form heat and mass transfer
panels, and the panels are piled in a stack 119.
[0069] FIG. 3A shows an expanded schematic view and FIG. 3B shows
an assembled schematic view of an exemplary panel 100 comprising
two flexible hydrophilic separation layers 102, a desiccant flow
distributor 104, and adhesive tape 106.
[0070] The materials, in particular the open cell foams, used in
this invention are also resilient and a small amount of pressure on
the ends of stacked panels can be applied to insure conformance of
panels relative to each other. The use of open cell foams, for
example, allows for a highly resilient panel and assembled module
design which can tolerate both the hydraulic and thermal expansion
required in the application. It is expected that assembled modules
incorporating the flexible panels will need to withstand conditions
below freezing as well as temperatures up to 140.degree. F. in
operation. This type of flexible construction allows materials to
move relative to each other as the panel or module experiences
changes in temperature and pressure. This minimizes stress
concentrations throughout the assembly and prevents damage to the
structure. The panels are highly durable and can be folded,
dropped, compressed, and impacted without affecting
functionality.
[0071] It would also be useful in the design of a panel to make any
seam used on the leading (and/or trailing) edge of the panel
conform to a rounded shape to minimize any turbulence and drag in
the air channel. This will minimize parasitic losses.
Assembly of Modules
[0072] In general terms, assembly of a module involves placing one
or a plurality of panels and optionally air flow layers/plates in a
standalone unit. The panel or plurality of panels may be contained
within a frame, such as two plates, that allows for desiccant
inflow and outflow through the desiccant channel as well as air
flow along the outer surface of the panels as facilitated by an air
flow layer or plate, which may be affixed to the panel or which may
be provided by assembling panels such that there are air gaps
between them. FIG. 4 provides a schematic view of a combination of
an exemplary air channel layer 108 comprising a hydrophobic open
cell foam for directing air and air channel seals 110 that may
comprise a hydrophobic closed cell foam. FIG. 5A shows an expanded
schematic view and FIG. 5B shows an assembled schematic view of a
exemplary module 150 comprising flexible panels 100 and air channel
layers 108. In FIGS. 5A-5B, there are four flexible liquid
desiccant heat and mass transfer panels that are separated by air
channel layers. Desiccant flow is shown through the desiccant flow
distributors 104 at one end, and air flow is shown perpendicular to
the desiccant flow. FIG. 6 shows an exemplary heat and mass
transfer module 150 comprising two end/support plates 152, plate
connecting and gap adjustment features 154, one flexible LDAC panel
100 and two air flow channel layers (not numbered). A header 156
may be formed from two layers of film that are hot-melt sealed to
both sides of the panel 110 and hot-melt sealed together to form
side seals. A tube may be placed in the header to deliver
fluid.
[0073] It is noted, in addition, that air and desiccant paths can
also be configured in an in-line manner with the air and liquid
desiccant in concurrent or countercurrent flow. Automated or
semi-automated processes can be used to make the panels and
assembled modules in a very cost effective manner. The number of
components in the panel and module assembly has been minimized by
the embodiments herein.
[0074] It is expected that the flexible panels will be relatively
insensitive to the build-up of any dirt and debris on the surface
of the porous hydrophilic separation layer. Air filters such as 3M
Filtrete 2'' Mini-pleat MERV 14 Commercial Air Filters will
generally be used to protect the panels. Any dirt and debris that
impinges and collects on the surface of the separation layer will
wet out and may slowly increase the effective thickness of the
controlled thin film desiccant layer. However, diffusion of water
molecules will still continue and it is expected that only a small
degradation in performance will occur over time. It was
demonstrated in the laboratory that panels could be rinsed clean of
any dirt and debris if necessary.
[0075] The desiccant is biocidal and having the desiccant film
located within and on the surfaces of the hydrophilic membrane and
at the air interface minimizes any chance of bio-fouling or
bio-film build-up on the panel or in the module assembly.
[0076] The individual panels can be easily disassembled from the
module and compressed to "wring-out" desiccant for recovery and
reuse. Panels can be compacted in waste drums for transport and
disposal. Panels and modules can be incinerated.
Materials
[0077] Porous Hydrophilic Separation Layers
[0078] Hydrophilic separation layers may comprise a membrane, a
woven mesh, a nanofiber media, an electrospun fiber media, a glass
fiber media, a nonwoven melt blown fiber media, a
corrosion-resistant metal, a ceramic media, or combinations
thereof.
[0079] Many types of porous hydrophilic separation materials may be
considered for use. Examples include blown melt fiber (BMF)
materials made from nylon or other polymers that have been pre or
post treated to make the fiber surfaces hydrophilic. Extremely
tightly woven mesh materials, nanospun and electrospun fiber media,
and glass fiber media can also be considered. Sintered metal could
be considered although corrosion resistance will need to be managed
by material selection or by use of coatings due to the nature of
the desiccant. Porous ceramic materials are also candidates for the
separation layer. Hydrophilic micro-filtration and ultra-filtration
membranes are very good candidates. Materials particularly well
suited for the application can be selected from the group of
hydrophilic micro-filtration membranes. This group includes but is
not limited to hydrophilic nylon (PA) membranes, hydrophilized
polyethersulfone (PES) membranes, hydrophilized polysulfone (PS)
membranes, hydrophilized polyvinylidene fluoride (PVDF) membranes,
hydrophilic polyacrylonitrile (PAN) membranes hydrophilized
polypropylene (PP) membranes, hydrophilized polyethylene (PE)
membranes, hydrophilized polytetrafluorethylene (PTFE) membranes,
hydrophilized polycarbonate (PC) membranes, and hydrophilized
ethylene chlorotrifluoroethylene (ECTFE) membranes. Membranes can
be naturally hydrophilic, as is the case with PA membranes, or can
be surface modified to render them hydrophilic. Many techniques for
hydrophilization can be used including use of co-polymers and other
additives in the polymer blend, coating with surfactants or other
hydrophilic materials, or grafting of hydrophilic groups to the
membrane surfaces using free radical polymerization techniques or
radiation grafting.
[0080] It is important for the hydrophilic separation layer to have
a high flow resistance as compared to the flow resistance of the
desiccant distributor. This creates preferential bulk flow down the
desiccant distributor located between the separation layers. It is
preferable to use porous hydrophilic separation layers which have
pore sizes of 10 micron or less and with a flow resistance which
minimizes the exterior film coating which forms on the air channel
side due to the very slight hydraulic pressure generated by the
fluid column at the top of the panel. It is more preferable to use
porous hydrophilic separation layers which have pore sizes of 5
micron or less. Pore sizes can be measured for most materials using
a capillary flow porometer. An example porometer is produced by
Porous Materials, Incorporated of Ithaca, N.Y.
[0081] A particularly useful porous hydrophilic separation layer is
a micro-filtration membrane made from nylon 6,6 as described in
U.S. Pat. No. 6,513,666 entitled "Reinforced, Three Zone
Microporous Membrane," which is herein incorporated by reference.
This type of membrane has multiple zones of different pore size and
incorporates a scrim to provide mechanical stability. In the
particular embodiments prototyped to date, a membrane designated as
BLA080 was used. This membrane has a 1.2 micron zone on one side of
the scrim, a 1.2 micron zone within the scrim, and a 0.8 micron
zone on the other side of the scrim. When fabricated into the
module, the orientation of the membrane was controlled so that the
0.8 micron zone is facing the air stream. The smaller pores have
the highest capillary forces so it is advantageous to have this
zone controlling the thin film of desiccant at the desiccant air
interface. The more open 1.2 micron zone facing the desiccant is
better for insuring high mass diffusion of water into the bulk flow
of the desiccant occurring in the desiccant distributor. Pore sizes
and thicknesses of the zones of the membrane may be tailored for
individual applications to achieve desired mass and heat transfer
across the thin liquid film in the membrane.
[0082] An exemplary membrane design would be to make a two zone
membrane comprising a very open pore structure, say 3.0 micron, in
the scrim fill zone, which is approximately 2.5 mils thick, and a
tighter pore size adjacent zone, say 0.8 micron, which is 1 mil
thick or less. The open pore zone would be positioned facing the
desiccant side in a panel and the tight pore zone would face the
air stream. This design will minimize mass transfer resistance in
the panel. A discussion of how to make an engineered
micro-filtration membrane of this type is discussed in patent
application WO 2013/154755 entitled "Thin Film Composite Membrane
Structures," hereby incorporated by reference.
[0083] In addition, unreinforced (no scrim) hydrophilic membranes
would be useful as well. An example of this type of membrane can be
found in U.S. Pat. No. 6,706,184 entitled "Unsupported Multizone
Microporous Membrane," which is incorporated herein by reference.
In particular, polyethersulfone membranes produced with a thin
tight pore size on the air channel side and a more open zone facing
the desiccant distributor side would work well in the
application.
[0084] Desiccant Flow Distributors
[0085] A desiccant flow distributor may include, but is not limited
to a hydrophilic material that comprises a polymeric material, a
natural fiber, or combinations thereof. In some embodiments, the
desiccant flow distributor may comprise a hydrophilic polymer
material that comprises a membrane, an open cell foam, a porous
nonwoven material, and/or a porous woven material. Some embodiments
may use a hydrophilic polymer material that comprises a rail film,
an extruded web material, an apertured polymeric film, or
combinations thereof as the desiccant flow distributor. The
desiccant flow distributor may also comprise a hydrophilic natural
fiber that comprises cellulose.
[0086] Open cell foams are typically made from polyurethanes. Both
polyether and polyester urethane foams are common. They can be
hydrophilized by adding surfactants to the formulation or post
treated to make them hydrophilic. The properties of the foam that
are typically characterized are density, compression deflection,
compression set, pore size expressed in pores per linear inch of
material, tensile strength, tear strength, air flow, and wet-out.
Examples of useful hydrophilic open cell foams include Type HS or
HydroZorb, from UFP Technologies, Georgetown, Mass.
[0087] Desiccant distributors can be rendered hydrophilic by
various methods known in the art. For example: U.S. Pat. No.
6,548,727 (Foam/Film composite medical articles, 2003); U.S. Pat.
No. 5,254,301 (Process for preparing a sheet of polymer-based foam,
1993); U.S. Pat. No. 4,957,810 (Synthetic sponge-type articles
having excellent water retention, 1990); and U.S. Pat. No.
3,781,231 (Physically reinforced hydrophilic foam and method of
preparing same, 1973).
[0088] The shape of the hydrophilic open cell foam is also
important in controlling the uniform flow and distribution of the
desiccant within the panel. An aspect of this invention is to add a
series of features, for example, draw-and-drip features, at the
bottom of the foam in the panel. These features cause the desiccant
to "draw" or "drip" uniformly at the bottom of the panel and
promote uniform flow throughout the panel. Uniform flow is
important in insuring efficiency of heat and mass transfer in the
panel. It is advantageous for the desiccant to uniformly absorb
heat and mass as it flows behind the porous hydrophilic separation
layer. Any type of channeling of the desiccant would cause uneven
heat and mass absorption and reduced heat and mass exchanger
efficiency. The features can be a series of crowns, points, or any
other shape that promotes drawing or dripping of the desiccant at
several points at the bottom of the panel. If the panel is left
straight across at the bottom, the effects of surface tension of
the fluid interacting with the bottom edge of the desiccant
distributor may cause one discharge stream to form, which may
inhibit uniform desiccant flow. Example shapes are provided in
FIGS. 7A-7D. FIG. 7A shows an exemplary desiccant flow distributor
204 having rectangular shapes 205 at intervals to form
draw-and-drip features extending from an end. FIG. 7B shows an
exemplary desiccant flow distributor 304 having square shapes 305
at intervals to form draw-and-drip features extending from an end.
FIG. 7C shows an exemplary desiccant flow distributor 404 having
triangular shapes 405 at intervals to form draw-and-drip features
extending from an end. FIG. 7D shows an exemplary desiccant flow
distributor 504 having semi-circular shapes 505 at intervals to
form draw-and-drip features extending from an end. Other materials
can be considered for use as the desiccant distributor but these
materials must be able to both wick and flow for them to properly
function. Porous wicking materials that can also carry bulk flow
include but are not limited to cellulosic sponges, natural sponges,
fabrics, and gauzes.
[0089] Air Channel or Turbulation Layers
[0090] It is advantageous to utilize a very open cell foam for the
air channel between adjacent panels. A specifically useful type of
very open cell foam is a reticulated open cell foam, which has a
substantially uniform pore size and a lattice that is wide open.
These types of foams are useful for providing a low pressure drop
channel with controlled spacing. In addition, the shape of the foam
adds some tortuosity and promotes mixing of the air. The mixing of
the air helps break up the boundary layer of the air with the
desiccant thin film controlled in the porous hydrophilic separation
layer. This promotes better heat and mass transfer between the air
and the desiccant. It is also advantageous to use a hydrophobic
material of construction in the air channel foam. This is helpful
as it insures that any desiccant, condensation, or any other source
of liquid (water) that gets into the air channel preferentially
wets the porous hydrophilic separation layer and due to the thin
film controlled desiccant layer in and on the separation layer, is
quickly spread and absorbed into the desiccant flow stream.
Essentially the water has a zero contact angle with the desiccant
and it is impossible for a drop to form on the separation layer
surface. In addition, the thickness, pore size, and tortuosity of
the air channel layer can be optimized to balance air flow pressure
drop with heat and mass transfer performance. Excessive pressure
drops in the air channel can lead to parasitic losses due to the
fan energy consumed in a LDAC system. The panel space can be
determined with analytical and computational modeling to optimize
the panel spacing which is related to the power density of an
assembled module.
[0091] An example of an effective material for use as air channel
spacers is polyester filter foam S-10 from New England Foam
Products, LLC, Hartford, Conn.
[0092] It is not required to have a porous air channel spacer in an
assembled module. The panels can instead be affixed at set
intervals or nonporous spacers can be used at each end of the panel
to control the width of the air gaps between panels. In this
manner, air gaps will be provided in an assembled module.
[0093] An exemplary air channel layer may comprise a polypropylene
rail film with adhesive (structured film) as disclosed in U.S. Pat.
No. 6,986,428 to common assignee 3M Innovative Properties Company
and hereby incorporated by reference. This film may be useful to
make an air side separator and can be designed to provide air
channels with low pressure drop and also face support for the
flexible LDAC membrane panel when assembled into a module. This
film can also be made without the adhesive. In other words, the
full geometry of the film can be made of one material such as
polypropylene or polyethylene. It is also possible to make the air
channel layers out of many other plastics or metals in the form of
plates. The air channel layers or plates can be flexible or rigid.
The plates can be machined, thermoformed, extruded, cast, or
produced in a number of other ways. Rail type films may be modified
with surface features to produce mixing of any fluid (i.e. air,
liquid desiccant) which flows down the channels of the film. An
exemplary layer may comprise a polymer film comprising micromixing
surface features such as those disclosed in commonly-assigned U.S.
Ser. No. 61/736,729 filed Dec. 13, 2012, entitled "Constructions
for Fluid Membrane Separation Devices" and incorporated herein by
reference.
[0094] Other materials such as nettings and apertured films can
also be considered for use as air channel spacers.
[0095] Desiccant Distribution Header
[0096] The header should promote uniform flow at the top of the
panel and it is advantageous to feed the desiccant through a slot
or a series of holes. This will insure even distribution at the top
of the panel. This works in coordination with the drawing and
dripping features at the bottom of the panel to insure uniform
desiccant flow behind the porous hydrophilic separation layer. An
exemplary header may be made from nonporous polymer film. For
example, two layers of film may be hot-melt sealed to both sides of
a panel and hot-melt sealed to form side seals. A tube may be
placed in the header to deliver fluid.
[0097] Before describing several exemplary embodiments of the
invention, it is to be understood that the invention is not limited
to the details of construction or process steps set forth in the
following description. The invention is capable of other
embodiments and of being practiced or being carried out in various
ways.
EXAMPLES
Example 1
[0098] A double-sided porous, flexible heat and mass transfer panel
in combination with an air channel layer is made by assembling:
[0099] an air channel layer of a reticulated polyester urethane
foam at 10 pores per inch (PPI), a density of 1.9 lbs/cu ft, 25%
CFD is 0.45 psi, 16 psi tensile strength, elongation 170%, tear
strength 4.5 lbs/in, compression set at 50% deflection is 15,
volumetric flow rate is 23, 0.25'' thick--product S-10 from New
England Foam;
[0100] two porous separation layers of an engineered membrane
comprising: a hydrophilic nylon 6,6 membrane, a multi-zone
structure with a 1.2 micron zone on the desiccant side and a 0.80
micron zone on the air side, membrane is reinforced with a nonwoven
scrim in a center zone--from 3M Purification, U.S. Pat. No.
6,513,666;
[0101] a desiccant flow distributor of a polyether urethane foam,
double cell, hydrophilized with a surfactant, 0.25'' thick--Type HS
from UFP Technologies, having draw-and-drip features in a series of
shapes of triangular point geometry at the bottom of the panel;
[0102] tape side seams comprising multiple layers of a closed cell
acrylic foam backed adhesive tape; and
[0103] a desiccant distribution header comprising a multi-layer
nonporous polymer film, using a general purpose hot melt to adhere
to the membrane and to form side seals of the header.
Example 2
[0104] A single-sided porous, flexible heat and mass transfer panel
in combination with an air channel layer was made by
assembling:
[0105] an air channel layer of a reticulated polyester urethane
foam at 10 pores per inch (PPI), a density of 1.9 lbs/cu ft, 25%
CFD is 0.45 psi, 16 psi tensile strength, elongation 170%, tear
strength 4.5 lbs/in, compression set at 50% deflection is 15,
volumetric flow rate is 23, 0.25'' thick--product S-10 from New
England Foam;
[0106] one porous separation layer of an engineered membrane
comprising: a hydrophilic nylon 6,6 membrane, a multi-zone
structure with a 1.2 micron zone on the desiccant side and a 0.80
micron zone on the air side, membrane is reinforced with a nonwoven
scrim in a center zone--from 3M Purification, U.S. Pat. No.
6,513,666;
[0107] one non-porous separation layer comprising a polyethylene
film;
[0108] a desiccant flow distributor of a polyether urethane foam,
double cell, hydrophilized with a surfactant, 0.25'' thick--Type HS
from UFP Technologies, having draw-and-drip features in a series of
shapes of triangular point geometry at the bottom of the panel;
[0109] tape side seams comprising multiple layers of a closed cell
acrylic foam backed adhesive tape; and
[0110] a desiccant distribution header comprising a multi-layer
nonporous polymer film, using a general purpose hot melt used to
adhere to the membrane and to form the side seals of the
header.
Example 3
[0111] A double-sided porous, flexible heat and mass transfer panel
was made by assembling:
[0112] two porous separation layers of an engineered membrane
comprising a hydrophilic nylon 6,6 membrane, a multi-zone structure
with a 1.2 micron zone on the desiccant side and a 0.80 micron zone
on the air side, membrane is reinforced with a nonwoven scrim in a
center zone--from 3M Purification, U.S. Pat. No. 6,513,666;
[0113] a desiccant flow distributor of a polyester urethane foam,
hydrophilized with a surfactant, 0.25'' thick--type HydroZorb from
UFP Technologies, having draw-and-drip features in a series of
shapes of triangular point geometry at the bottom of the panel;
[0114] ultrasonically-welded side seams using a Branson Ultrasonic
Welder with the following settings:
[0115] Weld Pressure--50 psi
[0116] Weld Time--0.80 sec
[0117] Weld Hold Time--0.5 sec
[0118] Trigger Force--set at 12
[0119] Down speed--set at 30
[0120] Amplitude--set at 100%; and
[0121] desiccant distribution header where an extra membrane was
allowed for above the top of the ultrasonically welded side seals
enabling the formation of a pocket that served as a desiccant
header. A tube or multiple tubes may be inserted into this pocket
and desiccant may be pumped directly into the panel.
Example 4
Testing
[0122] Flow Test 1
[0123] A panel as described in Example 2 and with an active
membrane area of 6'' wide.times.7'' high.times.one side was tested
with water to determine the flow capability of the panel. Active
membrane area is defined as the wetted membrane surfaces which have
fluid flowing behind it on the desiccant distributor side and does
not include any side seams, header area, or the draw-and-drip
features at the bottom of the panel. A peristaltic pump was used to
deliver the water to the top of the panel and the tube was situated
to allow the liquid to drop into the header at the center of the
panel. The maximum flow rate of the pump was 88.5 ml/min and the
panel could handle this flow rate without issue. Liquid did not
build up in the header but rather was drawn into the panel and
spread by the hydrophilic open cell foam desiccant distributor. The
triangular features started to drip as the liquid began reaching
the bottom of the panel. Within two minutes of starting the flow,
the dripping was uniformly distributed across the triangular points
indicating uniform flow.
[0124] Flow Test 2
[0125] A panel as described in Example 3 and with an active
membrane area of 6'' wide.times.7'' high.times.two sides was tested
with water to determine the flow capability of the panel. A
peristaltic pump was used to deliver the water to the top of the
panel and the tube was situated to allow the liquid to drop into
the header at the center of the panel. The maximum flow rate of the
pump was 88.5 ml/min and the panel could handle this flow rate
without issue. Liquid did not build up in the header but rather was
drawn into the panel and spread by the hydrophilic open cell foam
desiccant distributor. The triangular draw-and-drip features
started to drip as the liquid began reaching the bottom of the
panel. Within two minutes of starting the flow, the dripping was
uniformly distributed across the triangular points indicating
uniform flow.
[0126] Flow Uniformity Test
[0127] The flexible panel tested in Flow Test 1 was tested with a
solution of methylene blue dye in water to observe the flow
distribution within the panel. The panel was mounted without an air
channel spacer on one side and with the nonporous film against the
plexiglass end plate on the module holder so the flow patterns
could be observed. A sequence of photos was taken in increments
over the first two minutes of liquid flow and uniform spreading was
observed. This was done with a single point feed at the top of the
panel. It is thought that a header with multiple feed points or use
of a slot feeder would improve the uniformity at the top corners of
the panel to insure uniform flow through the entire face of the
active membrane area.
[0128] Freeze/Thaw Test
[0129] The panel described in Example 2 and flow tested in Flow
Test 1 and in the Flow Uniformity Test was subject to two cycles of
freeze/thaw testing. In the first cycle, the panel was completely
saturated with water and frozen solid to 10.degree. F. in a freezer
compartment over a period of approximately 5 hours. It was frozen
as a panel and not restrained in a holder. It was then removed from
the freezer and while still frozen, rapidly immersed in a pan of
hot water at approximately 125.degree. F. After stabilizing at the
bath temperature, the panel was removed and visually examined for
damage. No damage was observed. The panel then was remounted into a
holder with a foam air channel spacer on the membrane side and the
non-porous film side facing the plexiglass end plate on the holder.
The panel was tested as described in Flow Test 1 and performed
normally.
[0130] This same panel was then mounted in a holder with air
channel spacers on each side. It was fully saturated with water and
then put back in the freezer. It was frozen solid overnight to
10.degree. F. (approximately 16 hours), removed and then rapidly
immersed into 125.degree. F. water as before. Again, no visual
damage was observed when visually inspected and the panel was
retested for flow performance. It tested normally.
[0131] This experiment indicates that the flexible panel design as
described in this invention can withstand a high level of thermal
and mechanical stress without damage or reduction in functional
performance.
[0132] Condensation Control and Desiccant Retention
[0133] The panel as described in Example 2 was assembled into a
holder. Water was introduced into the header as described in Flow
Test 1. A pipette was then used to introduce water droplets as a
surrogate to condensation formation on the membrane surface. This
simulated the case when cold desiccant is introduced into the top
of the panel while warm humid air is delivered down the air
channel. When the surface temperature of the thin film desiccant
controlled in the porous hydrophilic separation layer is below the
dew point of the passing air, condensation will form at the
interface. The surrogate water droplets demonstrated that when they
touched the active membrane surface, they were immediately spread
across the face of the panel. Droplets were unable to form on the
surface. This demonstrates that any condensation formation at the
liquid/air interface will be immediately spread and absorbed into
the thin film layer controlled in and on the panel. In this design,
there is also no chance for capillary condensation as the
separation layer operates in a totally wetted form.
[0134] In addition, a pipette was used to introduce water droplets
into the middle of the hydrophobic foam used to create the air
channel. Drops were able to "hang up" in the middle of the channel
without touching a membrane surface. It is unlikely that
condensation would form in the middle of the air channel, but if
this happened, it was noted that when air was blown down the air
channel, the drops would follow the lattice of the foam for a short
distance until the water hit the surface of the membrane. Since the
membrane is essentially a liquid film, and the lattice is
hydrophobic, the preferential wetting caused the water droplets to
be immediately pulled off of the lattice and onto the panel. The
panel described in this invention will be tolerant of condensation
formation based on the surface tension, capillary action and
hydrodynamic control of a falling column of liquid desiccant
created by the integrated panel design. This testing also
demonstrates that any desiccant is firmly sequestered in the panel
and will not be aerosolized by the flow of air expected in the
application of the panels in an LDAC system.
Example 5
[0135] A double-sided porous, flexible heat and mass transfer panel
was made by assembling:
[0136] two porous separation layers of an engineered membrane
comprising a hydrophilic nylon 6,6 membrane, a multi-zone structure
with a 1.2 micron zone on the desiccant side and a 0.80 micron zone
on the air side, membrane is reinforced with a nonwoven scrim in a
center zone--from 3M Purification, U.S. Pat. No. 6,513,666;
[0137] a desiccant flow distributor--0.025'' thick nylon Naltex
asymmetric diamond netting;
[0138] side seams comprising double sided pressure sensitive
adhesive (PSA) tape;
[0139] a desiccant distribution header was created by having extra
membrane at the top to perform a pocket. A tube was inserted at the
center of the pocket during flow testing.
Example 6
[0140] A double-sided porous, flexible heat and mass transfer panel
was made by assembling:
[0141] two porous separation layers (2) of an engineered membrane
comprising a hydrophilic nylon 6,6 membrane, a multi-zone structure
with a 1.2 micron zone on the desiccant side and a 0.80 micron zone
on the air side, membrane is reinforced with a nonwoven scrim in a
center zone--from 3M Purification, U.S. Pat. No. 6,513,666;
[0142] a desiccant flow distributor--0.010'' thick polypropylene
Naltex asymmetric diamond netting;
[0143] side seams comprising double sided pressure sensitive
adhesive (PSA) tape;
[0144] a desiccant distribution header was created by having extra
membrane at the top to perform a pocket. A tube was inserted at the
center of the pocket during flow testing.
Example 7
Testing
[0145] Flow testing of the panels of Examples 5-6 with methylene
blue dye solution showed liquid distribution patterns behind the
hydrophilic nylon membrane that were less uniform than those
patterns shown for Examples 2-3. Lower fluid flow carrying
capacities compared to Examples 2-3 utilizing the hydrophilic open
cell foam were noted. It is thought that improved capillary action
and liquid spreading could be achieved by the use of diamond
netting or even apertured films having thinner apertures and
channels and more hydrophilic materials relative to what was tested
in Examples 5-6. It is expected, however, that such thinner and
more hydrophilic materials may result in lower fluid carrying
capacities per unit area of active porous hydrophilic separation
layer.
[0146] In contrast, the hydrophilic open cell foams provide a good
balance of properties which allow for superior flow distribution
while still maintaining high bulk flow of the liquid. When applying
this panel design for LDAC applications, it will be extremely
beneficial to accurately control the volume of bulk fluid while
maintaining a stable thin film in and on the porous hydrophilic
separation layer. The bulk fluid flow will provide ample heat
transfer capacity between the conditioner and the regenerator in a
LDAC system while accurately controlling the thin film desiccant
layer within and on the surfaces of the porous hydrophilic
separation layer.
[0147] Reference throughout this specification to "one embodiment,"
"certain embodiments," "one or more embodiments" or "an embodiment"
means that a particular feature, structure, material, or
characteristic described in connection with the embodiment is
included in at least one embodiment of the invention. Thus, the
appearances of the phrases such as "in one or more embodiments,"
"in certain embodiments," "in one embodiment" or "in an embodiment"
in various places throughout this specification are not necessarily
referring to the same embodiment of the invention. Furthermore, the
particular features, structures, materials, or characteristics may
be combined in any suitable manner in one or more embodiments.
[0148] Although the invention herein has been described with
reference to particular embodiments, it is to be understood that
these embodiments are merely illustrative of the principles and
applications of the present invention. It will be apparent to those
skilled in the art that various modifications and variations can be
made to the method and apparatus of the present invention without
departing from the spirit and scope of the invention. Thus, it is
intended that the present invention include modifications and
variations that are within the scope of the appended claims and
their equivalents.
* * * * *