WO2004018950A1

WO2004018950A1 - Falling film vapor absorber, cooling system, and method

Info

Publication number: WO2004018950A1
Application number: PCT/SG2003/000196
Authority: WO
Inventors: Nihal Ekanayake Wijeysundera; Md Raisul Islam; Juay Choy Ho
Original assignee: National University Of Singapore
Priority date: 2002-08-21
Filing date: 2003-08-21
Publication date: 2004-03-04
Also published as: AU2003259015A1

Abstract

An absorber for an absorption cooling system includes at least two film guides arranged one beneath another so that the film guides guide a falling film sequentially, one after another. At least two adjacent film guides guide the falling film alternately from opposite surfaces of the falling film. Thus, the opposite surfaces of the falling film are alternatively exposed to a vapor. One or more of the film guides may be cooled by a coolant so that the falling film is in turn cooled.

Description

FALLING FILM VAPOR ABSORBER, COOLING SYSTEM, AND METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefits from U.S. Provisional Application No. 60/404,765 filed August 21 , 2002, the contents of which are hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to vapor absorption in cooling systems.

BACKGROUND OF THE INVENTION

Cooling systems (also known as chillers), such as air-conditioners and refrigerators, are commonly used. Most cooling systems exploit the fact that a refrigerant liquid absorbs heat when it evaporates. To cool continuously, the evaporated refrigerant is typically recycled and condensed to the liquid phase again, so that a steady supply of refrigerant liquid can be maintained. Since the vapor is generally condensed at a temperature above the prevailing ambient temperature for efficient heat rejection, the evaporated refrigerant has to be pressurized before it is condensed to liquid. Two common techniques used for pressurizing a refrigerant vapor are compression and absorption.

In a compression-cooling system, the vapor is compressed directly by a compressor such as a mechanical pump. A compression-cooling system typically has an evaporator, a compressor, a condenser and an expansion valve. A refrigerant is cycled through the four stages in the system. The liquid refrigerant evaporates in the evaporator to produce the cooling effect, resulting in a low pressure refrigerant vapor. The low pressure vapor is compressed in the compressor to a high pressure. The high pressure vapor is cooled in the condenser to the liquid phase. The liquid refrigerant is finally expanded to the evaporator pressure through the expansion valve, thus completing the so-called cooling cycle.

In absorption-cooling systems, the compressor is replaced by an absorber and a generator. Instead of being compressed, the low pressure refrigerant vapor is absorbed by a liquid (often referred to as an absorbent solution) including absorbent in the absorber and subsequently re-generated at a higher pressure in the generator by heating the liquid, including the absorbed refrigerant . Compared to compression-cooling systems, absorption-cooling systems can be more environment-friendly because they may require less power to operate and typically do not need to use refrigerants hazardous to the environment such as fluorocarbon.

The performance of an absorption-cooling system depends on the efficiency of its absorber. One factor that affects the rate of vapor absorption is the size of absorbent solution-vapor interface: the larger the interface, the greater the absorption. An absorbent solution in the form of a film generally provides a large interface area. Thus, falling-film absorbers are commonly used in absorption-cooling systems. A falling film is a film of the absorbent solution falling under the force of gravity. As can be appreciated, a free falling film is unstable. To maintain a stable falling film, film guides are often used. A known technique is to guide a falling film with a plate having a large flat vertical surface. The falling film is guided to flow down along the plate with one surface in contact with the flat surface and the opposite surface exposed to the vapor. Surface structures on a film guide, such as fins and other types of protrusions, have been found useful for maintaining stable, large surface size falling films.

Another factor that affects the rate of vapor absorption is the temperature at the interface: the cooler the interface, the greater the absorption. Because an absorption process generates heat, the temperature at the interface may rise as the film falls. A known technique for reducing the interface temperature is to cool the falling film through the film guide using a coolant such as cooled air or water.

A further factor that affects the rate of vapor absorption is the refrigerant concentration at the film surface: the lower the concentration, the greater the absorption. As can be appreciated, the refrigerant concentration at the film surface increases as vapor is absorbed. To reduce refrigerant concentration at the surface, a known approach is to modify the properties of the absorbent film by adding surface-active chemical agents to the absorbent solution, which induce turbulence at the surface of the falling film thus spreading refrigerant out inside the solution and reducing the local refrigerant concentration at the surface. As can be appreciated, turbulence at the film surface also facilitates heat dissipation and distribution.

Similarly, a film guide having an irregular guiding surface with surface structures such as fins and other types of protrusions is also advantageous as the protrusions create flow turbulence and may thus facilitate heat and refrigerant transportation within the solution.

Despite these measures, the absorption efficiency remains low in known falling-film absorbers, which often limits the overall performance of absorption- cooling systems and, ultimately, their usefulness.

Accordingly, there is a need for an improved technique of absorbing vapor and efficient falling-film absorbers and absorption-cooling systems.

SUMMARY OF THE INVENTION

In accordance with an aspect of the present invention, a falling film in a absorption cooling system is guided by at least two film guides arranged one beneath another such that the film guides guide the falling film sequentially, one after another. At least two adjacent film guides guide the falling film alternately from opposite surfaces of the falling film. Thus, the opposite surfaces of the falling film are alternatively exposed to a vapor. One or more of the film guides may be cooled by a coolant so that the falling film is in turn cooled.

Since both surfaces of the falling film are used to absorb the vapor and each surface can be cooled directly, high absorption efficiency can be achieved.

In accordance with an embodiment of the present invention, a falling-film absorber of a cooling system for absorbing a refrigerant vapor into a liquid including an absorbent, includes: a first film guide for guiding a falling film of the liquid having first surface and a second opposite surfaces. The first film guide has a guiding surface for guiding the falling film from the first surface so as to expose the second surface to absorb the vapor while guiding the falling film. The absorber further includes a second film guide located beneath the first film guide. The second film guide has a guiding surface for guiding the falling film from the second surface so as to expose the first surface to absorb the vapor while guiding the falling film.

In accordance with another embodiment of the present invention, a method of absorbing a refrigerant vapor into a liquid including an absorbent, includes: forming a falling film of the liquid, the falling film having a first and a second opposite surface; and alternatively guiding the falling film from the first and second surfaces as it falls, so as to alternatively expose the first and second surfaces to absorb the vapor.

Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the figures, which illustrate, by way of example only, embodiments of the present invention,

FIG. 1 is a schematic diagram illustrating an absorption-cooling system, exemplary of an embodiment of the present invention;

FIG. 2 is a partial cross-sectional view of a falling-film absorber forming part of the absorption-cooling system of FIG. 1;

FIG. 3 schematically illustrates two film guides, forming a part of the falling film absorber of FIG. 2, guiding a falling film; and

FIG. 4 schematically illustrates tubular film guides guiding a falling film, exemplary of a further embodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary absorption-cooling system 40 including a condenser 42, an evaporator 44, an absorber 46, and a generator 48, in fluid communication with each other. As can be understood and will be further described below, a refrigerant fluid (as indicated by reference numerals 50, 52 and 58) assisted by a pump 60, an expansion valve 64 and a pressure reducing valve 62, circulates among condenser 42, evaporator 44, absorber 46, and generator 48; and liquids including absorbent (as indicated by reference numerals 54 and 56) circulate between absorber 46 and generator 48. An expansion valve 64, allows condensed liquid to expand as it passes from condenser 42 to evaporator 44. A pump 60 pumps a liquid mixture 56 including refrigerant and absorbent from absorber 46 to generator 48. Conventional refrigerant fluids and absorbents may be used, and condenser 42, evaporator 44, generator 48, expansion valve 64, pump 60 and pressure reducing valve 62 may all be conventional, interconnected in known way, as for example detailed in (a) Whitman, W.C., W.M. Johnson and J. Tomczyk. Refrigeration and Air Conditioning Technology: Concepts, Procedures, and Troubleshooting Techniques. 4^th edition, pp.1031 -1076, Albany, NY USA: Delmar Publishers, Thomson Learning Press. 2000. (b) ASHRAE Handbook Refrigeration, Absorption Cooling, Heating, and Refrigeration Equipment, 1791 Tullie Circle, N.E., Atlanta, GA 30329, pp. 41.1-41.12. 1998. (c) Wang, S.K. Handbook of Air Conditioning and Refrigeration, pp. 24.1-24.20, McGraw-Hill, Inc., New York USA. 1993. (d) Jones, W.P. Air Conditioning Engineering. 4^th edition, pp. 380-391 , Edward Arnold, London. 1994. As such, the construction of cooling system 40, except absorber 46, will not be further described herein.

An example structure of absorber 46 is illustrated in partial cross-section FIG.2. As illustrated, absorber 46 includes a solution distributor 26 for generating a falling film of liquid 54 (FIG.1) and a plurality of film guides 10A to 10H (collectively guides 10) for guiding the falling film. Liquid 54 is a solution of absorbent and refrigerant. In an embodiment, the absorbent may be lithium bromide. Film guides 10 define two generally parallel paths 24A and 24B for guiding falling film as detailed below. As illustrated, each film guide 10 can be a generally flat plate having a generally flat guiding surface 14 and a cooling surface 16 (FIG.3). Guiding surface 14 of each guide 10 is generally vertical.

Coolant tubes 18 are in thermal contact with cooling surfaces 16. Coolant tubes 18 can be made of any suitable material and be arranged and attached to guides 10 in any suitable manner as can be understood by a person skilled in the art. For example, coolant tubes 18 may be interconnected, either serially or in parallel.

For clarity of illustration, guides 10B and 10C and a falling film 12A are further illustrated in FIG. 3. As illustrated, guides 10B and 10C along path 24A are arranged one above the other and aligned such that the upper guide 10B will direct falling film 12A on to the guiding surface 14 of the guide 10C, which is located directly beneath guide 10B. Guides 10Ato 10D and 10E to 10H are similarly arranged, as illustrated in FIG. 2. The guiding surfaces 14 of two vertically proximate guides 10 (for example guides 10B and 10C) face opposite directions so that falling film 12A can be guided from its opposite surfaces as it falls. Topmost guides 10A and 10E extend from a rounded top-surface 28, and are integrally formed therewith. Each guiding surface 14 of a lower guide (one of 10B to 10D and 10F to 10H) may be curved towards the cooling surface 16 at the upper end of the guide. Advantageously, curved upper ends facilitate the transition of falling film 12 from a higher guide (for example 10B) to a lower guide (for example 10C). Curved upper ends also provide other advantages which will become apparent below.

Guides 10 can be made in any suitable manner using any suitable materials. For example, guides 10 can be produced using techniques typically used for producing film guides used in conventional absorbers, such as those described in (a) Refrigeration and Air Conditioning Technology: Concepts, Procedures, and Troubleshooting Techniques. 4^th edition, supra, (b) ASHRAE Handbook Refrigeration, Absorption Cooling, Heating, and Refrigeration Equipment, supra (c) Handbook of Air Conditioning and Refrigeration, supra and/or (d) Air Conditioning Engineering. 4^th edition, supra.

In operation, condenser 42 (FIG. 1) cools a refrigerant to the liquid phase. Liquid refrigerant 50 travels to evaporator 44 through expansion valve 64. In evaporator 44, liquid refrigerant 50 evaporates at low pressure and absorbs heat from the surroundings, thus producing a cooling effect. Evaporated refrigerant is a low pressure vapor 52 and travels to absorber 46 where refrigerant vapor 52 is absorbed into a liquid 54 in the manner described below. The absorption of refrigerant by liquid 54 creates a mixture 56 having a lower concentration of absorbent than in liquid 54. In an example embodiment, liquid 54 includes 62% absorbent and 38% refrigerant, while mixture 56 includes 55% absorbent and 45% refrigerant. Mixture 56 is transported through pump 60 to generator 48 where it is heated, re-generating refrigerant vapor 58 (but now at a high pressure) and liquid 54, including a higher concentration of absorbent than in mixture 56. Liquid 54 is transported back to absorber 46 by way of pressure reducing valve 62. High pressure vapor refrigerant 58 is also transported to condenser 42, where the refrigerant is again cooled to the liquid phase thus completing the cooling cycle.

In absorber 46, liquid 54 (including a relatively high concentration of absorbent) is dispersed by distributor 26 (FIG. 2) through a number of small holes or horizontal slits into a stream that runs down top surface 28 forming two thin films that are allowed to fall down paths 24A and 24B respectively (film 12A along path 24A is illustrated in FIG. 3 - collectively and individually films are referred to as films 12), along surfaces 14 of guides 10. Falling films 12 absorb refrigerant vapor 52 as they fall along paths 24A and 24B. Each film 12 has opposed surfaces 20 and 22. Opposite surfaces of each falling film 12 are directed onto the guiding surfaces 14 of adjacent guides 10. For example, guide 10B guides falling film 12A from surface 20 because surface 20 is in contact with guiding surface 14 of guide 10B, whereas guide 10C guides film 12A along opposite surface 22 because now surface 22 is in contact with guiding surface 14 of guide 10C, as best illustrated in FIG. 3. Surface 22 of falling film 12A is exposed to vapor 52 as film 12A passes along guide 10B so vapor 52 is absorbed at surface 22. Surface 20 of falling film 12A is then exposed to vapor 52 and absorbs vapor 52 as falling film 12A falls along guide 10C. Thus, as film 12A falls, alternate surfaces 20 and 22 of falling film 12A are exposed for absorbing vapor 52 on guides 10B and 10C. Alternate surfaces are similarly exposed as film 12A is guided by guides 10A and 10B, and 10C and 10D. Coolant tubes 18 (FIG. 2) are filled with a circulating coolant for cooling guides 10. As a result, falling film 12A is also cooled from alternate surfaces 20 and 22.

Another film (not specifically illustrated) similarly travels down following path 24B, with alternate surfaces in contact with guiding surfaces 14 of guides 10E to 10H.

Since refrigerant vapor 52 is absorbed alternately on the opposite surfaces 20 and 22 of a falling film 12 the refrigerant concentration at each of the film surface is low; since the absorbing surfaces of a falling film 12 are cooled directly before being exposed to the vapor 52, the temperature at the vapor-film interface is low; further, the impact on a falling film 12 as it falls from an upper guide 10 to a lower guide 10 creates turbulence in the film, and the drag at the interface between the lower guide 10 and the film causes cross flow inside the film, both of which facilitate refrigerant and heat dissipation within the film. Moreover, as both surfaces of the film 12 alternatively come in contact with the surfaces of guides 10, film interface velocity repeatedly drops to zero. This may lead to an increase in the residence time of the film, which in turn might increase the vapor absorption rate. Therefore, high absorption efficiency can be achieved. In addition, as exposed surface of the film 12 of an upper guide 10 comes in contact with a lower guide 10, the surface tension of the film 12 may facilitate a more uniform spreading of the film 12 on the surfaces of each of the guides 10.

As can be appreciated, absorber 46 may be formed with any arbitrary number of guides 10, defining any desired number of paths 24. Further, individual film guides 10 may have the same or different shapes, as long as they may be arranged such that the falling film can be guided from alternate surfaces. For example, guides 10 may be tubular in shape instead of flat. Tubular guides may be generally circular or rectangular.

To that end, FIG. 4 illustrates a portion of another exemplary absorber 46' in which the film guides include guiding tubes 30 and curved guiding plates 32. Guiding tubes 30 are circular in cross-section with their longitudinal axis horizontally oriented. They are arranged one beneath another defining a single path 24', so that a falling film 12' can fall over each of guiding tubes 30 in sequence. Each curved guiding plate 32 extends upward from an associated guiding tube 30 is shaped and arranged to guide falling film 12' from the surface that is opposite the previously guided surface of falling film 12' and direct falling film to flow smoothly over the next lower guiding tube 30. Advantageously, tubes 30 are generally hollow allowing a coolant to flow therein for cooling falling film 12'. Tubes 30 can be interconnected to form a continuous tube, for example in typical manners used in conventional falling-film absorbers. Alternatively, tubes 30 can be separate and separately cooled.

Guiding tubes 30 and curved guiding plates 32 can be constructed in typical manners for constructing components of conventional absorbers, which are readily understood by a person skilled in the art. Again, example techniques are described in (a) Refrigeration and Air Conditioning Technology: Concepts, Procedures, and Troubleshooting Techniques. 4^th edition, supra, (b) ASHRAE Handbook Refrigeration, Absorption Cooling, Heating, and Refrigeration Equipment, supra (c) Handbook of Air Conditioning and Refrigeration, supra and/or (d) Air Conditioning Engineering. 4^th edition, supra.

Other features, benefits and advantages of the present invention not expressly mentioned above can be understood from this description and the drawings by those skilled in the art.

As can be appreciated by one of ordinary skill in the in the art, there are many possible modifications to the embodiments described herein.

For example, the guiding surface of each film guide 10 does not need to be vertical. One or more film guides 10 may be arranged to have an inclined guiding surface for guiding the falling film. This arrangement may be advantageous when the vertical length of the absorber is limited.

Guiding surfaces of film guides need not be flat and smooth but can have appropriate surface structures or can be curved or rugged so as to further facilitate cross-flow within the falling film.

[0001] As can be appreciated, film guides can be cooled in various manners other than described above. For example, cooling tubes 18 may be rectangular in shape instead of circular in shape, thus increasing thermal contact between the coolant tube 18 and the cooling surface 16 of a film guide.

[0002] Further, guiding tubes 30 need not be hollow, for example when they can be cooled without using a fluid coolant. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.

Claims

What is claimed is:

1. A falling-film absorber of a cooling system for absorbing a refrigerant vapor into a liquid including an absorbent, said absorber comprising:

a first film guide for guiding a falling film of said liquid, said falling film having a first surface and a second opposite surface, said first film guide having a guiding surface for guiding said falling film from said first surface so as to expose said second surface to absorb said vapor while guiding said falling film; and a second film guide located beneath said first film guide, said second film guide having a guiding surface for guiding said falling film from said second surface so as to expose said first surface to absorb said vapor while guiding said falling film.

2. The absorber of claim 1 wherein at least one of said first and second film guides is maintained at a temperature lower than the temperature of said falling film so as to absorb heat from said falling film when said at least one of said first and second film guides is in contact with said falling film.

3. The absorber of claim 2, further comprising a coolant tube in thermal contact with said at least one of said first and second film guides so that a coolant can be circulated in said coolant tube for reducing the temperature of said at least one of said first and second film guides.

4. The absorber of any one of claims 1 to 3, further comprising a first set of film guides including said first film guide, each having a guiding surface for guiding said falling film from said first surface, each of said first set of film guides located one beneath the other.

5. The absorber of claim 4, further comprising a second set of film guides including said second film guide, each having a guiding surface for guiding said falling film from said second surface, each of said second set of film guides located one beneath the other.

6. The absorber of claim 5, wherein said first and second sets of film guides alternatively guide said falling film.

7. The absorber of any one of claims 1-6, wherein at least one of said film guides has a generally flat guiding surface.

8. The absorber of any one of claims 1-6, wherein at least one of said film guides is generally tubular in shape.

9. The absorber of claim 8, wherein said at least one of said film guides has a generally circular cross-section.

10. The absorber of claim 8, wherein said at least one of said film guides has a generally rectangular cross-section.

11.The absorber of claim 1 , wherein said film guides comprise a plurality of guiding tubes.

12. The absorber of claim 11 , wherein said plurality of guiding tubes form a continuous tube.

13. The absorber of claim 11 , wherein said plurality of guiding tubes are adapted to receive a coolant within said guiding tubes.

14. The absorber of claim 11 , wherein said film guides further comprise at least one guiding plate for guiding said falling films between said guiding tubes.

15. The absorber of claim 14, wherein said at least one guiding plate is a curved guiding plate.

16. An absorption-cooling system comprising a falling-film absorber as claimed in claim 1.

17. A method of absorbing a refrigerant vapor into a liquid including an absorbent, comprising:

generating a falling film of said liquid, said falling film having a first and a second opposite surface; and

alternatively guiding said falling film from said first and second surfaces as said film falls, so as to alternatively expose said first and second surfaces to absorb said vapor.

18. The method of claim 17, further comprising cooling said falling film.

19. The method of claim 18, wherein said cooling comprises cooling said falling film alternatively from said first and second surfaces of said falling film.