U.S. patent application number 09/932840 was filed with the patent office on 2002-08-29 for heat exchange element with hydrophilic evaporator surface.
This patent application is currently assigned to Ocean Power Corporation. Invention is credited to Campbell, Robert L..
Application Number | 20020117293 09/932840 |
Document ID | / |
Family ID | 22847411 |
Filed Date | 2002-08-29 |
United States Patent
Application |
20020117293 |
Kind Code |
A1 |
Campbell, Robert L. |
August 29, 2002 |
Heat exchange element with hydrophilic evaporator surface
Abstract
The present invention provides a heat exchange element for
particular use in a film heat exchanger, such as a distillation
system. The heat exchange element includes a substrate having a
first surface and an opposing second surface. The first surface
acts as a condenser surface. The heat exchange element further
includes a composition disposed on at least a portion of the second
surface. The composition has an exposed hydrophilic surface so as
to provide a hydrophilic evaporator surface due to the composition
being hydrophilic in nature. In contrast, the condenser surface is
typically hydrophobic in nature. The use of the hydrophilic
composition eliminates the nucleation of vapor bubbles and prevents
gas film (or gas blanket) formation on the evaporator surface. As a
result, the thermal resistance through the heat exchange element is
decreased and the efficiency of the heat exchange element is
increased.
Inventors: |
Campbell, Robert L.; (Rancho
Murieta, CA) |
Correspondence
Address: |
Mr. Robert L. Campbell
Ocean Power Corporation
5000 Robert J. Matthews Parkway
El Dorado
CA
95762
US
|
Assignee: |
Ocean Power Corporation
|
Family ID: |
22847411 |
Appl. No.: |
09/932840 |
Filed: |
August 17, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60226067 |
Aug 17, 2000 |
|
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Current U.S.
Class: |
165/133 |
Current CPC
Class: |
F28F 2245/02 20130101;
B01D 1/22 20130101; F28F 2245/04 20130101; F28F 13/182
20130101 |
Class at
Publication: |
165/133 |
International
Class: |
F28F 013/18 |
Claims
What is claimed is:
1. A heat exchange element comprising: a substrate having a first
surface and an opposing second surface, the first surface acting as
a condenser surface; and a composition disposed on at least a
portion of the second surface, the composition having an exposed
hydrophilic surface so as to provide a hydrophilic evaporator
surface, wherein the substrate is formed of plastic.
2. The heat exchange element of claim 1, wherein the composition
substantially covers the second surface.
3. The heat exchange element of claim 1, wherein the first surface
comprises a hydrophobic surface.
4. The heat exchange element of claim 1, wherein the substrate is
formed of a water and vapor impermeable material.
5. The heat exchange element of claim 1, wherein the substrate is
formed of a plastic material selected from the group consisting of
polypropylene and polyethylene.
6. The heat exchange element of claim 1, wherein the composition
comprises a metal oxide.
7. The heat exchange element of claim 1, wherein the composition
comprises at least one of titanium oxide and nickel oxide.
8. The heat exchange element of claim 1, wherein the second surface
has hydrophilic properties such that liquid flowing therealong
during an evaporation process is maintained below the critical heat
flux value for the liquid.
9. A distillation apparatus comprising: an evaporator having an
evaporator chamber; a condenser having a condenser chamber; and a
heat exchange element disposed between the evaporator and the
condenser such that the heat exchange element is in fluid
communication with the condenser and evaporator chambers, the heat
exchange element comprising: a substrate having a first surface and
an opposing second surface, the first surface acting as a condenser
surface; and a composition disposed on at least a portion of the
second surface, the composition having an exposed hydrophilic
surface so as to provide a hydrophilic evaporator surface, wherein
the substrate is formed of plastic.
10. The distillation apparatus of claim 9, wherein the composition
substantially covers the second surface.
11. The distillation apparatus of claim 9, wherein the first
surface comprises a hydrophobic surface.
12. The distillation apparatus of claim 9, wherein the substrate is
formed of a water and vapor impermeable material.
13. The distillation apparatus of claim 9, wherein the composition
comprises a metal oxide.
14. The distillation apparatus of claim 9, wherein the composition
comprises at least one of titanium oxide and nickel oxide.
15. The distillation apparatus of claim 9, wherein the apparatus is
used to distill sea water into desalinated water.
Description
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/226,067, filed Aug. 17, 2000, which is hereby
incorporated by reference.
FIELD OF THE INVENTION
[0002] This invention relates to a heat exchange element and more
specifically, relates to a heat exchange element including a
condenser surface having hydrophobic properties and an evaporator
surface having hydrophilic properties.
BACKGROUND OF THE INVENTION
[0003] Heat exchange elements are used in heat exchangers in a
variety of settings to accomplish a number of operations.
Generally, a heat exchanger is a device that transfers heat from
one gas or fluid to another or to the environment. Heat exchange
elements are commonly employed in a distillation apparatus for the
evaporation of a liquid and for its subsequent condensation. In the
distillation apparatus, the heat exchange element may be in the
form of flat bag-like elements of a thin sheet material placed
against each other or may have other suitable forms. See, for
example, U.S. Pat. Nos. 5,340,443, 5,512,141, and 5,770,020, all of
which are hereby incorporated by reference. One suitable use for a
distillation apparatus is the production of fresh water from sea
water.
[0004] Conventional sea water distillation systems, especially
vapor compression systems, include an evaporator-condenser type
assembly in which the latent heat of condensing steam is exchanged
to sea water. This latent heat in turn causes the sea water to boil
into a vapor. For purpose of illustration, a conventional heat
exchange element for use in an evaporator-condenser assembly is
shown in FIG. 1 and is generally indicated at 10. The heat exchange
element 10 includes a thermal conductor 12 which is impermeable to
both water and vapor. This thermal conductor 12 acts as a substrate
for the element 10. The thermal conductor 12 has a first surface 14
on one side thereof and a second surface 16 on an opposite side
thereof. The first surface 14 serves as a condenser surface where
vapor is condensed into liquid and the second surface 16 serves as
an evaporator surface where liquid is evaporated. In conventional
heat exchange elements 10, both the condenser surface 14 and the
evaporator surface 16 are hydrophobic surfaces.
[0005] When the conventional heat exchange element 10 is used in a
distillation system, the heat exchange element 10 is connected to
both an evaporator chamber (not shown) and a condenser chamber (not
shown). Typically, the heat exchange element 10 is disposed between
these two chambers. During operation of the distillation system,
water droplets, generally indicated at 18, form on the condenser
surface 16 as the vapor condenses thereon to form a liquid
condensate. A salt water film, generally indicated at 20, forms on
the evaporator surface 16 as the sea water evaporates into a
vapor.
[0006] One of the disadvantages associated with conventional heat
exchange elements, such as the heat exchange element 10, is that
the heat exchange element 10 is likely to have a number of heat
transfer regions present thereon during the heat transfer
operation. Some of these regions include but are not limited to the
following: convective heat transfer to liquid region, subcooled
boiling, saturated nucleate boiling, forced convective heat
transfer through liquid film, liquid deficient region, and
convective heat transfer to vapor. The flow patterns of the fluid
also vary depending upon the heat transfer characteristics of the
region in which the fluid is flowing. In the conventional heat
exchange element 10, these various types of heat transfer regions
are distributed over the heat surface in a stochastic manners
However, it will be understood that not all of these heat transfer
regions may be present on the heat exchange element 10 at any one
time. The stochastic distribution of the regions results in the
formation of bubble sites and dry spots that behave in a
statistical manner in terms of boiling frequency and bubble size.
The result is that heat transfer at a macroscopic level is neither
constant nor periodic.
[0007] For example, vapor bubbles often nucleate on the evaporator
surface 16 resulting in pool or "pot" boiling. The pot boiling or
bubbly flow, which occurs in the saturated nucleate boiling region,
has a lower total heat transfer than other regions. Because heat
transfer rates are linked to efficiency rates, this lower heat
transfer equates to lower efficiency of the overall system. In
addition and as illustrated in FIG. 1, a layer of soft scales,
generally indicated at 22, such as calcium carbonate, frequently
forms on the evaporator surface 16 because a suitable layer of
liquid is not maintained on the evaporator surface 16. The
formation of this layer 22 further reduces the heat transfer
effectiveness in these regions of the evaporator surface 16 and
thus reduces the effectiveness of the system. In order to prevent
such formations, the sea water in the evaporator (not shown) is
generally recirculated in an attempt to make sure that a layer of
liquid flows along the entire evaporator surface 16 to prevent
drying out thereof; however, this requires a significant amount of
energy. Furthermore, a gas film or gas blanket, generally indicated
at 24, may form on the evaporator surface 16 which increases the
thermal resistance of the system. This further reduces the heat
transfer efficiency of the heat exchange element.
[0008] Therefore, there is a continuing need for heat exchange
elements which have improved heat transfer efficiency and require
little, if any, outside energy to prevent soft scale formation.
SUMMARY OF THE INVENTION
[0009] The present invention provides a heat exchange element
having a water and vapor impermeable substrate. The substrate has a
first surface which is typically hydrophobic and an opposing second
surface. A composition is disposed on the second surface such that
the composition provides an exposed hydrophilic surface. In one
embodiment, the composition substantially covers the second surface
so that the second surface essentially comprises a hydrophilic
surface. According to the present invention, the first surface
serves as a condenser surface of the substrate and the hydrophilic
surface and any portion of the second surface not covered by the
composition serve as an evaporator surface. One exemplary and
preferred use for the heat exchange element of the present
invention is in a distillation system for purifying sea water (or
brine) to form fresh desalinated water.
[0010] Advantageously, the use of the hydrophilic composition to
define the evaporator surface eliminates the nucleation of vapor
bubbles on the evaporator surface and also prevents gas film (or
gas blanket) formation on this same surface. As a result, the
thermal resistance through the heat exchange element is decreased
and the efficiency of the heat exchange element is increased.
Furthermore, since the evaporator surface attracts water, little,
if any, power is required to recirculate the sea water or brine
over the evaporator surface. The need to recirculate the sea water
to prevent soft scale formation on the evaporator surface is
reduced or eliminated, thereby, further increasing efficiency.
[0011] In another embodiment, a distillation system is disclosed in
which the heat exchange element of the present invention is
included therein. The distillation system has an evaporator chamber
and a condenser chamber with the heat exchange element of the
present invention being disposed therebetween. The evaporator
surface of the heat exchange element is in communication with the
evaporator chamber and the condenser surface is in communication
with the condenser chamber.
[0012] These and other features and advantages of the present
invention will be readily apparent from the following detailed
description of the invention taken in conjunction with the
accompanying drawings, wherein like reference characters represent
like elements.
BRIEF DESCRIPTION OF THE FIGURES
[0013] Other objects, features, and advantages of the invention
discussed in the above summary of the invention will be more
clearly understood from the following detailed description of the
preferred embodiments, which are illustrative only, when taken
together with the accompanying drawings in which:
[0014] FIG. 1 is a cross-sectional side elevational view of a
conventional heat exchange element;
[0015] FIG. 2 is a cross-sectional side elevational view of a heat
exchange element according to the present invention;
[0016] FIG. 3 is a perspective view of one exemplary heat exchange
element; and
[0017] FIG. 4 is a cross-sectional side elevational view of the
heat exchange element of FIG. 2 in a distillation system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] Referring now to FIG. 2, a heat exchange element is provided
according to the present invention and is generally indicated at
100. Heat exchange element 100 includes a substrate 102 which may
be formed of any number of conventional materials and may have any
number of shapes depending upon the precise application. Although
the substrate 102 may be flexible in nature, it is preferred that
the substrate 102 be formed so that it is rigid. Suitable
substrates 102 also comprise those substrates which are capable of
being metallized. Exemplary materials for forming the substrate 102
include, but are not limited to, plastics, such as thermoplastics;
metals, such as titanium; stainless steel; and copper-nickel
alloys; and any combination thereof. Preferred thermoplastic
materials include, but are not limited to, polypropylene and
polyethylene. In addition, carbon loaded versions of these
thermoplastics are also suitable in the practice of the present
invention.
[0019] The substrate 102 has a first surface 104 and an opposite
second surface 106. The first surface 104 is intended to act as a
condenser surface for producing a condensate. According to the
present invention, a composition, generally indicated at 108, is
disposed on at least a portion of the second surface 106. The
composition 108 is selected so as to impart hydrophilic properties
to at least the portion of the second surface 106 where the
composition 108 is disposed. An evaporator surface 109 is thus
defined by both the hydrophilic composition 108 and the second
surface 106. In the case where the composition 108 entirely covers
the second surface 106, the evaporator surface 109 is defined by
the composition 108. In the case where the composition 108 covers
only a portion of the second surface 106, the evaporator surface
109 is defined by the composition 108 and that portion of the
second surface 106 which does not have the composition 108 disposed
thereon. In the illustrated embodiment, the composition 108 covers
the entire second surface 106. Thus, the entire evaporator surface
109 is a hydrophilic surface.
[0020] In one embodiment, the composition 108 is directly applied
and bonded to the second surface 106 by using conventional
processing techniques, including depositing and bonding the
composition 108 in a plasma environment or by chemical vapor
deposition. See, for example, U.S. Pat. No. 5,763,063, which is
hereby incorporated by reference. It will be appreciated that these
processes are merely exemplary in nature and any number of other
suitable processes may be used to deposit the composition 108 on
the second surface 106. Alternatively, a primer layer (not shown)
may be disposed between the composition 108 and the second surface
106 in order to promote improved bonding therebetween. The primer
layer is selected so that sufficient and effective bonding results
between the primer layer and the second surface 106 and between the
primer layer and the composition 108.
[0021] The composition 108 of the present invention is formed of a
hydrophilic material which is designed to withstand the operating
conditions of the element 100 and also provide the improved
benefits of the present invention. Suitable hydrophilic materials
include, but are not limited to, metal oxides, such as titanium
oxides (e.g., TiO.sub.x) and nickel oxides (e.g., NiO.sub.x).
Preferred metal oxides include, but are not limited to, higher
order titanium oxides and nickel oxides. Metal oxides, and
preferably higher order titanium and nickel oxides, are preferable
since these materials maintain their hydrophilic properties in the
presence of boiling sea water and in reduced pressure environments.
Higher order metal oxides are oxides of the metal with higher
oxygen content (i.e. more oxygen atoms than metal atoms per
molecule) (e.g., HTiO.sub.3 .sup.- or TiO.sub.3.2H.sub.2O). Metal
oxides are generally unaffected by anti-scaling and anti-foaming
agents, which are commonly employed in sea water distillation
systems. Other suitable hydrophilic materials include, but are not
limited to, zeolites (including those at various silicon-aluminum
ratios), aluminophosphates, polymer hydrogels (e.g. acrylate
derivatives, such as Hypan.RTM. (available from Lipo Chemicals,
Inc. of Patterson, N.J.) and diphenyl ethylene (DPE)). Generally,
the hydrophilicity of the composition is sufficient to result in
80% of the surface area of the evaporator portion of the heat
exchanger to be wetted with a thickness of several molecular levels
of the liquid to be evaporated.
[0022] According to one preferred embodiment, the composition is a
titanium oxide or nickel oxide which is coated on the second
surface 106. Generally, the composition is coated on the second
surface at a thickness of from about 0.01 to about 2 .mu.m and
preferably at a thickness of from about 0.1 to about 0.2 .mu.m.
According to another embodiment, the composition is coated on the
second surface at a thickness of from about 0.05 to about 0.4
.mu.m.
[0023] During operation of the heat exchange element 100 in a
suitable heat exchanger (not shown), water droplets 110 form on the
first (condenser) surface 104 as the vapor condenses thereon to
form the liquid condensate. In the particularly preferred
application of the heat exchange element 100 in sea water
evaporator-condenser systems, a salt water film 112 forms on the
evaporator surface 109 as the sea water is heated and evaporates
therefrom. However, unlike the conventional heat exchange element
10 of FIG. 1, few, if any, scales form on the evaporator surface
109. Because the material required to form the scale deposit
remains in suspension with the sea water, scales are not formed. In
addition, the lack of dry spots inhibits amalgamation of scales.
This results in greater heat transfer at the evaporator surface 109
and thus increases the overall efficiency of the system in which
the heat exchange element 100 is used.
[0024] Furthermore, another of the disadvantages associated with
the conventional heat exchange element 10 (FIG. 1) is overcome by
the design of the heat exchange element 100 of the present
invention. More specifically, the gas film or blanket 24 (FIG. 1)
which forms on the evaporator surface of the conventional heat
exchange element 10 does not form on the evaporator surface 109 of
the present invention. Because the gas film 24 is eliminated,
greater heat transfer results at the evaporator surface 109 because
the liquid to be evaporated may more easily come into contact with
the evaporator surface 109. This also results in an overall
increase in the efficiency of the system.
[0025] While not being bound to any particular theory, it is
believed that the hydrophilic evaporator surface 109 offers
benefits relative to conventional evaporator surfaces because the
hydrophilic surface 109 attracts the liquid to be evaporated and
maintains a suitable layer of liquid on the evaporator surface 109
during the evaporation process. During the evaporation process, a
liquid film spreads over the evaporator surface 109. Because the
liquid (e.g., sea water) is attracted to hydrophilic evaporator
surface 109, the liquid spreads into a fairly uniform film over the
entire heat transfer surface (evaporator surface 109). During the
evaporation process, the liquid to be evaporated is continuously
fed to the evaporator surface 109 at a predetermined pressure and
temperature which is sufficient to maintain a desired evaporation
rate over the area of the evaporator surface 109. By using a
hydrophilic material on the evaporator surface 109, the heat
transfer surface is not marked by a variety of heat transfer
regions but rather the heat transfer can be better controlled. The
heat transfer mechanism maintains "evaporation" as opposed to
"boiling" on the evaporator surface 109. In other words, the system
is forced into the forced convective heat transfer through liquid
film region as opposed to the saturated nucleate boiling region.
Thus, the "boiling" action is prevented and the formation of the
gas film or blanket is eliminated because the liquid uniformly
exists on the evaporator surface 109 due to the hydrophilic
attraction therebetween. The evaporator surface 109 does not dry
out due to a uniform liquid layer being maintained thereon.
[0026] In the present invention, the liquid used in the evaporator
process is generally maintained in the two-phase forced convection
region of heat transfer. The liquid is thus maintained below the
point of critical heat flux (CHF), i.e., the point of complete
evaporation of the liquid film. When a liquid reaches CHF, dry
spots form on the evaporator surface 109 because there is a lack of
liquid in contact with the evaporator surface 109 at these dry spot
locations. This causes a reduction in heat transfer and reduces the
overall efficiency of the system. Avoiding the CHF condition places
a limit on the level of evaporation that may be achieved for a
given area of the evaporator (not shown) and at a given heat flux.
However, this limit places little, if any, burden on many systems
of industrial size and typical flow rates found therein.
[0027] It is desirable to maintain the liquid film in the two-phase
forced convection region heat transfer region because in this
region, the liquid flows along the evaporator surface 109 while
evaporation takes place. Dry out conditions are avoided because the
liquid is maintained below CHF and at the same time the liquid
effectively evaporates at the desired rate so as to produce
efficient heat transfer. By staying in this region, the rate of
heat transfer is optimized and the need to recirculate liquid to
prevent drying is eliminated. Also, by staying in this region, the
formation of entrained bubbles is also eliminated. The hydrophilic
nature of the surface produces the foundation of uniformly
distributed water molecules which in turn forms a uniform film of
water above that foundation. The thinner the layer, the less
effective the heat transfer.
[0028] The heat exchange element 100 according to the present
invention may have any number of configurations and shapes so long
as at least a portion of the evaporator surface 109 includes the
hydrophilic composition 108. For example, the heat exchange element
100 may have a flat sheet design, a bag design, or a plate design.
One suitable design is set forth in International Publication No.
WO 98/31529, which is herein incorporated by reference in its
entirety. This application discloses bag-like elements which
stretch during use to provide for bulging during the pressurization
of the interior of the element and is generally illustrated in FIG.
3.
[0029] FIG. 3 shows an individual heat exchange element 150 for use
in a film heat exchanger (not shown). The heat exchange element 150
is formed of two oppositely positioned plastic heat exchange films
152 which attach to one another at the top and the bottom of the
element 150 and on the side which is on the left in the Figure. The
films 152 are additionally bonded to each other along mutually
parallel oblique bonding lines 154, which divide the interior of
the element 150 into parallel ducts 156 extending from one side of
the element to the other. The hot vapor to be condensed is
introduced into the interior of the element 150 from its at least
partly open side 158, which is on the right of the Figure. The
vapor flows in the direction of the arrows set forth in the Figure.
The condensate formed from the vapor in the ducts 156 leaves via
the outlet opening 160 in the lower left comer of the element 150.
The liquid to be evaporated is directed between the elements 150
and is evenly distributed over exterior surfaces 153 of the heat
exchange films 152. In accordance with the present invention, the
exterior surfaces 153 of the heat exchange films 152 are thus
provided with a layer of hydrophilic material resulting in the
exterior surfaces 153 being hydrophilic. It will be appreciated
that this illustrated embodiment is merely exemplary in nature and
any number of heat exchange structures may be used in the practice
of the present invention so long as a portion of the evaporator
surface 109 is made hydrophilic.
[0030] Now referring to FIG. 4 in which an exemplary distillation
system employing the heat transfer element 100 of the present
invention is illustrated and generally indicated at 200. The
distillation system 200 includes a condenser chamber 202 and an
evaporator chamber 204 with the heat exchange element 100 being
disposed therebetween. The condenser and evaporator chambers 202,
204, respectively, are typically not in direct communication with
each other. The hydrophilic evaporator surface 109 of the heat
exchange element 100 faces the evaporator chamber 204 and is in
communication therewith. Similarly, the condenser surface 104 faces
the condenser chamber 202 and is in communication therewith. The
distillation system 200 operates like other traditional
distillation systems in that there is an evaporation process for
evaporation of a liquid and a condensation process for the
subsequent condensation of vapor to a liquid condensate. This
system 200 finds particular utility in sea water distillation
applications for producing purified water from the sea water.
[0031] According to one preferred embodiment, the heat exchange
element 100 is used in a vacuum vapor compression distillation
process, both of the mechanical and thermal types. In thermal vapor
recompression evaporators, the inertia of the steam is used for
recycling part of the evaporators through ejectors. In mechanical
vapor recompression evaporators, all of the vapor is recycled back
as heating steam using vapor compression with high pressure fans or
compressors.
[0032] While the invention has been particularly shown and
described with reference to the preferred embodiments thereof, it
will be understood by those skilled in the art that various changes
in form and detail may be made therein without departing from the
spirit and scope of the invention.
* * * * *