WO2002014774A2

WO2002014774A2 - Heat exchange element with hydrophilic evaporator surface

Info

Publication number: WO2002014774A2
Application number: PCT/US2001/025702
Authority: WO
Inventors: Robert L. Campbell
Original assignee: Ocean Power Corporation
Priority date: 2000-08-17
Filing date: 2001-08-16
Publication date: 2002-02-21
Also published as: AU2001286515A1; US20020117293A1; WO2002014774A3

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 (100) includes a substrate (102) having a first surface (104) and an opposing second surface (106). The first surface (104) acts as a condenser surface. The heat exchange element (100) 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 (108) due to the composition being hydrophilic in nature. In contrast, the condenser surface is typically hydrophobic in nature. The use of the hydrophilic composition eliminated 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.

Description

HEAT EXCHANGE ELEMENT

WITH HYDROPHILIC EVAPORATOR SURFACE

This application claims the benefit of U.S. Provisional Application No. 60/226,067, filed August 17, 2000, which is hereby incorporated by reference.

Field of the Invention

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 hydrophihc properties.

Background of the Invention

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. Patent 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.

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 Figure 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.

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.

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 manner. 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.

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 Figure 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 tins 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.

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

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 hydrophihc surface. In one embodiment, the composition substantially covers the second surface so that the second surface essentially comprises a hydiOphilic surface. According to the present invention, the first surface serves as a condenser surface of the substrate and the hydrophihc 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.

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. 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.

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

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: Figure 1 is a cross-sectional side elevational view of a conventional heat exchange element;

Figure 2 is a cross-sectional side elevational view of a heat exchange element according to the present invention; Figure 3 is a perspective view of one exemplary heat exchange element; and

Figure 4 is a cross-sectional side elevational view of the heat exchange element of Figure 2 in a distillation system.

Detailed Description of the Preferred Embodiments

Referring now to Figure 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 fonned 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 thennoplastic 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.

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, hi the illustrated embodiment, the composition 108 covers the entire second surface 106. Thus, the entire evaporator surface 109 is a hydrophilic surface.

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. Patent 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 maybe 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.

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_x) and nickel oxides (e.g., NiO_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., HTiO₃ ^" or TiO₃ • 2H₂O). 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^® (available from Lipo Chemicals, Inc. of Patterson, NJ.) 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.

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 μm and preferably at a thickness of from about 0.1 to about 0.2 μm. According to another embodiment, the composition is coated on the second surface at a thickness of from about 0.05 to about 0.4 μm.

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 Figure 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.

Furthermore, another of the disadvantages associated with the conventional heat exchange element 10 (Figure 1) is overcome by the design of the heat exchange element 100 of the present invention. More specifically, the gas film or blanket 24 (Figure 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. 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.

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.

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 fomis a uniform film of water above that foundation. The thinner the layer, the less effective the heat transfer.

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 Figure 3. Figure 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 maybe used in the practice of the present invention so long as a portion of the evaporator surface 109 is made hydrophilic.

Now referring to Figure 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.

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 mertia of the steam is used for recycling part of the evaporators through ejectors, mechanical vapor recompression evaporators, all of the vapor is recycled back as heating steam using vapor compression with high pressure fans or compressors.

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.

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.

5. The distillation apparatus of claim 9, wherein the apparatus is used ter into desalinated water.