Abstract:
A refrigeration cycle combines a concentrated absorbent liquid and a refrigerant in its vapor phase at low pressure in an absorber which provides, as an output, a liquid-rich refrigerant in solution with the absorbent liquid. The pressure of the absorbent/refrigerant solution is increased and the pressurized solution is supplied to a pervaporation membrane separator which provides as one output stream a vapor-rich refrigerant and as another output stream a concentrated liquid absorbent. The liquid-rich refrigerant output stream is condensed and subsequently expanded to a low pressure into an evaporator where it evaporates extracting heat from the spaced to be cooled. The concentrated liquid absorbent output stream from the separator is reduced in pressure and recombined in the absorber with the low pressure refrigerant vapor exiting the evaporator.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a refrigeration cycle for extracting heat from a fluid such as air at a low temperature, e.g., below 32° and preferably -25° F. or lower and more particularly to such a cycle which utilizes a refrigerant, an absorbent liquid and a membrane for transforming a low pressure refrigerant vapor into a higher pressure refrigerant/absorbent liquid for circulation back to the evaporator. 
     BACKGROUND OF THE INVENTION 
     Absorption-type refrigeration systems have been used for many years in the residential and commercial air conditioning and refrigeration markets. Compressor-type refrigeration systems have been used even more extensively in these markets for the same purposes. 
     In prior art absorption-type systems low-pressure refrigerant in its vapor state is absorbed by an absorbent liquid in an absorber, the absorber being cooled by water or air. The resulting refrigerant/absorbent solution is pumped to a higher pressure and supplied to a separator unit which utilizes heat (high temperature boils off the refrigerant) or very high pressure (osmotic membrane passes only the refrigerant) to separate the refrigerant from the absorbent. The separated refrigerant is then condensed, expanded to a low pressure (resulting in a low temperature) and delivered to an evaporator which extracts heat from the space being refrigerated. 
     Conventional ammonia absorption refrigeration systems utilizing ammonia, for example, as a refrigerant, and water as the absorbent liquid, and heat for the separation process, are capable of achieving low temperatures less than 32° F. and even down to -25° F. or lower. However, the ratio of refrigeration provided to the energy required by such a cycle, called the coefficient of performance (&#34;COP&#34;), is typically significantly less than 0.50. 
     Osmotic-membrane-absorption refrigeration cycles are also capable of reaching low temperatures and may have a COP higher than conventional ammonia/water heat-separation systems, but require very high pressures, of the order of 2,000 psia or more to force the refrigerant through the pores of the osmotic membrane. Such high pressure must be handled with considerable care and even then may create a potentially hazardous environment. 
     Conventional compressor-type systems rely on electricity as the power source. Although this type system is more efficient than heat-separation-absorption-type systems, i.e., providing a COP of around 4, the electricity to run such systems is approximately ten times more costly than the gas for heat-separation absorption type system. The cost savings of gas vs. electricity is not present in an osmotic-membrane-separation system when the high pressure pump is driven by an electric motor. In addition, compressor-type pumps, the refrigerants, for compression-type systems, may be very destructive of the ozone layer. 
     There is a need in the refrigeration field for an energy efficient system that does not require high working pressures or utilize a working fluid that is destructive of the ozone layer. 
     SUMMARY OF THE INVENTION 
     A heat pump cycle for providing low refrigeration temperatures in accordance with the present invention expands a liquid-rich refrigerant (permeate), such as ammonia, in an evaporator where it is evaporated at low temperature and pressure with heat added, i.e., removed from the space to be cooled, to provide a substantially vaporized low pressure refrigerant. The refrigerant in it&#39;s vapor phase is combined in an absorber with a concentrated absorbent liquid (retentate) such as water while extracting heat to provide a liquid-rich refrigerant in solution with the absorbent (&#34;refrigerant/absorbent solution&#34;). The solution, after the pressure thereof has been increased, is supplied to a pervaporation membrane separator which provides as one output a vapor-rich refrigerant and as another output the concentrated absorbent liquid. 
     The refrigerant vapor is then condensed, reduced in pressure and returned to the evaporator. At the same time the concentrated absorbent liquid from the separator member is reduced in pressure and returned to the absorber to complete the cycle. 
     In order to increase the efficiency of the cycle heat may be exchanged between the concentrated absorbent liquid output from the membrane separator and the pressurized absorbent/refrigerant solution prior to it&#39;s entry into the separator. In addition, the vapor-rich refrigerant output from the separator may be passed through an additional membrane separator to increase the purity of the vapor refrigerant and additional heat exchange may be provided between the various liquid and vapor phases of the constituents to further increase the temperature of the refrigerant vapor in the separator(s) and/or increase the pressure differential across the separator(s) to enhance the COP of the cycle. 
    
    
     The organization and operation of the invention may be best understood by reference to the following description taken in conjunction with the accompanying drawings in which like components are identified by the same reference numerals. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a simplified refrigeration cycle in accordance with the present invention; 
     FIG. 2 is a perspective view of a pervaporation membrane separator which is suitable for use in the invention; and 
     FIG. 3 is a schematic illustration of a more efficient refrigeration cycle in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, and particularly to FIG. 1, an input line 10a of a pervaporation membrane separator unit 10 is connected to the output line 12a of an absorber 12 via pump 14 and a retentate heat exchanger 16. It should be noted that the heat exchanger 16 is not essential to the operation of a simplified system in accordance with our invention (which increases the COP). 
     The absorber, in combining the refrigerant vapor, supplied via input line 12b, with the liquid absorbent supplied via input line 12c, generates heat which is removed by means of an internal heat exchange coil 12d. Heat exchange lines 12e and 12f are provided to conduct the heat exchange fluid through the coil 12d. 
     The output of the absorber on line 12a is a dilute solution of liquid-rich refrigerant, e.g., ammonia, and the absorbent liquid, e.g., water. The pump 14 increases the pressure of this refrigerant/absorbent solution from, say, about 10-15 psia (at the output of the absorber) to, say, about 200 to 350 psia at the input line 16a to the internal coil 16b of heat exchanger 16. The retentate heat exchanger 16 increases the temperature of the refrigerant/absorbent solution as a result of the removal of heat from the concentrated absorbent liquid being returned from the separator output line 10c to the input 12c of the absorber, via a pressure regulator 17, line 16c, internal heat exchanger coil 16d, line 16e, and a restrictor element 19. The pressure regulator 17 maintains the pressure of the liquid absorbent in the separator while lowering the pressure of the absorbent entering the heat exchanger 16. 
     The pervaporation membrane separator 10 provides two output streams in output lines 10b and 10c. The output stream in line 10b constitutes the permeate, i.e., a vapor-rich refrigerant (substantially in the vapor phase) and the output stream in line 10c constitutes a concentrated liquid absorbent (i.e., the retentate). An internal heat exchanger 10d, formed integrally with the separator 10, adds heat from an external source (not shown), via lines 10e, 10f and coil 10g, to increase the temperature of the fluids within the separator to say about 250° to 400° F. to enhance the efficiency of the separator. The vapor-rich refrigerant stream in output line 10b is condensed in a condenser 18. The condenser 18 includes an internal heat exchange coil 18b which receives cooling fluid such as water from a source such as a cooling tower (not shown) via lines 18c and 18d. The liquified refrigerant output stream in line 18a from the condenser passes through an expansion or restrictor element 20, such as a Joule Thompson or JT valve, to lower the pressure (and temperature) of the fluid. Next, the refrigerant stream is passed through an evaporate 22, via input line 22a, at a low temperature, e.g., less than 32° F. and absorbs heat supplied by a suitable heat exchange fluid conducted through an internal coil 22c via lines 22d and 22e from a heat exchanger 24. The heat exchanger 24 is in fluid communication with the space to be cooled such as a refrigerated room etc. The heat transfer process in the evaporator 22 vaporizes all or at least a substantial portion of the refrigerant. The low pressure vaporized refrigerant flows from the output 22b of the evaporator to the absorber 12. 
     The concentrated liquid absorbent in line 10c is also supplied to the absorber 12 via pressure regulator 17, heat exchanger 16, as discussed earlier, and restrictor element 19. The pressure regulator 17 and the restrictor 19 equalize the pressure of the absorbent in line 12c to the fluid pressure within the absorber. In the absorber the vapor refrigerant is combined with the concentrated liquid absorbent to form the dilute solution of the liquid-rich refrigerant and the absorbent liquid to complete the cycle. 
     It should be noted that the concentrated liquid absorbent output from the separator will generally comprise absorbent as well as some refrigerant. By the same token, the separated portion of the refrigerant will generally include some absorbent. 
     Absorbers and pervaporation membranes suitable for use in the new cycle are well known in the art. With respect to the absorbers, see, for example, chapter 40 of the 1994 Ashrai Handbook on Refrigeration Systems and Applications, IP Edition, published by the American Society of Heating, Refrigeration, and Air Conditioning Engineers, Inc. Desalination Systems, Inc. of Vista, Calif. supplies pervaporation membrane units (model Nos. De Sal 3 and 5) suitable for use in the invention and particularly for separating ammonia and water. One such membrane separator unit, with an integrally formed heat exchanger added thereto, is shown in FIG. 2. The refrigerant/absorbent solution is supplied to the annular inlet 10a. The refrigerant, along with a minor portion of absorbent, dissolves into the membrane and passes into an open center tube which is in fluid communication with the refrigerant outlet line 10b. The absorbent, except for a small portion which follows the refrigerant, remains on the outside of the membrane and flows through the outlet line 10c. An example of a pervaporation membrane unit, without a heat exchanger which would typically surround the unit, is illustrated in FIG. 2. The unit includes an outer impermeable shell 10g, an annular inlet 10a and a central hollow core 10h, into which the refrigerant or permeate flows. The core 10g is closed adjacent the inlet 10a and forms the refrigerant/permeate outlet at the end opposite the inlet. The annular outlet 10c mirrors the annular inlet 10a. Concentric layers of feed channel spacers 10i, membrane material 10j and permeate collection material 10k surround the perforated inner core 10h. 
     It is to be noted that a pervaporation membrane operates in a totally different fashion from the prior art membrane separation processes used in refrigeration and heat pump systems. Such prior art membrane systems rely on osmotic pressure to force the refrigerant through the membrane thereby separating the refrigerant from other constituents. For the ammonia-water pair, this conventionally requires pressures of the order of magnitude of 2,000 to 4,000 PSI and higher. Osmotic membranes are porous which allows the ammonia to pass through the membrane. Pervaporation membranes are not porous, but pass constituents through the membrane by dissolving the selected material into the membrane. This allows a much lower driving force, significantly less than 400 PSI, to act as the driver. In the case of an ammonia-water mixture, the pervaporation membrane, selectively passes ammonia and water vapor and rejects liquid water. 
     A refrigeration cycle for applications requiring very low temperatures of the order of -20° to -45° F. or lower is illustrated in FIG. 3. A heat exchanger 30 and a second separator 32 are connected in series between the first separator 10 and the condenser 18 to increase the purity of the refrigerant entering the evaporator 22. The heat exchanger 30 extracts heat from the refrigerant-rich vapor output on line 10b from the first separator 10 to reduce the temperature of the output stream and provide the pressure differential across the pervaporation membrane necessary to induce the refrigerant to pass therethrough. The refrigerant output of the heat exchanger on line 30a is in its liquid phase. A heat exchanger coil 30b, internal to the heat exchanger 30, is connected in series between the output line 14a of the pump 14 and the inlet of the heat exchanger 16 via lines 30c and 30d so that a portion of the heat extracted from the vapor refrigerant is added to the refrigerant/absorbent solution being fed to the heat exchanger 16 to thereby increase the temperature of the liquid entering the separator 10. 
     Another heat exchange coil 30e, internal to the heat exchanger 30, supplies another portion of the heat extracted from the refrigerant to the second separator 32 via a closed circuit fluid circulating system driven by a pump 34. The input and output lines for coil 30c, designated 30f and 30g, respectively, are in fluid communication with an internal heat exchange coil 32c in the second separator 32. 
     The second separator 32 provides two output streams, i.e., a refrigerant-rich vapor stream of increased purity on line 32a and an absorbent-rich liquid on line 32b. The vapor stream in output line 32a is fed to the condenser 18 and the absorbent-rich liquid in output line 32b is fed back to the input (line 12a) of the pump 14 via a pressure regulator 35, line 35a and restrictor 36 to be combined with the refrigerant/absorbent solution exiting the absorber 12. The pressure regulator 35 and restrictor 36 control the pressure of the absorbent-rich liquid so that it is compatible with the pressure of the output stream from the absorber. 
     To improve the efficiency and thereby increase the COP of the cycle, a low temperature compressor 38 may be connected between the outlet 22b of the evaporator and the input line 12b to the absorber 12. The compressor increases the pressure of the refrigerant vapor entering the absorber. 
     The following chart illustrates typical parameters such as pressures, temperatures, and liquid vs. vapor phases of the fluids etc. in the various lines or components (i.e., separators) in FIG. 3. 
     TWO SERIES SEPARATOR STAGES AMMONIA IN SATURATED WATER LOWEST TEMPERATURE -45° F. 
     COP 0.6332 
     
         ______________________________________TWO SERIES SEPARATOR STAGESAMMONIA IN SATURATED WATERLOWEST TEMPERATURE -45° F.COP 0.6332Fluid lines    Pres.   Temp.   Fluid   Compositionor components    PSIA    °F.                    Condt.  Lb.NH.sub.3 Mix /LB.Mix______________________________________First Stage Separator 1010a      270     255     liquid  0.32010e              385     H.T.fluid                            0.00010f              265     H.T.fluid                            0.00010b      240     254     vapor   0.902Refrigerant Flow Heat Exchanger 3030a      238     115     liquid  0.90230g              210     H.T.fluid                            0.00030f              120     H.T.fluid                            0.00030c      290     105     liquid  0.320030d      285     202     liquid  0.3200Second Stage Separator 3232a      228     110     vapor   0.999532b      236     115     liquid  0.004618c              85      H.T.fluid                            0.00018d              95      H.T.fluid                            0.00018a      212     100     liquid  0.9995Evaporator 2222a      9       -45     liquid  0.999522c              -35     H.T.fluid                            0.00022d              -40     H.T.fluid                            0.00022b      8.9     -45     vapor   0.9995Compressor 36    27      130     vapor   0.999512bRetentate Heat Exchanger 1610a      280     255     liquid  0.320016c      266     375     liquid  0.056216e      256     201     liquid  0.0562Absorber 1212c      27      201     liquid  0.056212e      15      85      H.T.fluid                            0.00012d      5       95      H.T.fluid                            0.00012a      27      105     liquid  0.3200______________________________________ 
    
     There has been described a novel and simple refrigeration cycle, method and apparatus for achieving low temperatures which provides a relative high COP while preferably using working fluids that are much more compatible with the atmosphere than the refrigerants used in the conventional vapor-compression systems. While the invention has been described with respect to a preferred embodiment it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and modifications without departing from the spirit and scope of the invention as defined in the appended claims.