EP0138041B1

EP0138041B1 - Chemically assisted mechanical refrigeration process

Info

Publication number: EP0138041B1
Application number: EP84110693A
Authority: EP
Inventors: Arnold R. Vobach
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-09-29
Filing date: 1984-09-07
Publication date: 1989-03-15
Also published as: DE3477259D1; EP0138041A3; EP0138041A2; CA1233655A; JPS60105869A; JPH0532664B2; ATE41506T1

Abstract

There is provided a chemically assisted mechanical refrigeration process including the steps of: mechanically compressing a refrigerant stream which includes vaporized refrigerant; contacting the refrigerant with a solvent in a mixer (11) at a pressure sufficient to promote substantial dissolving of the refrigerant in the solvent in the mixer (11) to form a refrigerant-solvent solution while concurrently placing the solution in heat exchange relation with a working medium to transfer energy to the working medium, said refrigerant-solvent solution exhibiting a negative deviation from Raoult's Law; reducing the pressure over the refrigerant-solvent solution in an evaporator (10) to allow the refrigerant to vaporize and substantially separate from the solvent while concurrently placing the evolving refrigerant-solvent solution in heat exchange relation with a working medium to remove energy from the working medium to thereby form a refrigerant stream and a solvent stream; and passing the solvent and refrigerant stream from the evaporator.

Description

This invention relates generally to refrigeration and more particularly to a new and improved chemically assisted mechanical refrigeration cycle.
The typical mechanical refrigeration system employs a mechanical compressor to raise the pressure and to condense a gaseous refrigerant, which thereafter absorbs its heat of vaporization. Thus, the typical vapor compression cycle uses an evaporator in which a liquid refrigerant, such as Freon-12, boils at a low pressure to produce cooling, a compressor to raise the pressure of the gaseous refrigerant after it leaves the evaporator; a condenser, in which the refrigerant condenses and discharges its heat to the environment, and an expansion valve through which the liquid refrigerant leaving the condenser expands from the high-pressure level in the condenser to the low pressure level in the evaporator.
Much effort has been expanded over the past few decades in developing refrigeration systems which utilize low grade energy sources, such as solar energy, without the need for compressors or pumps. Much of this effort has been directed to the so-called absorption cycle, which accomplishes compression by using a secondary fluid as a solvent to absorb a refrigerant gas. A typical. absorption system includes a condenser, expansion valve and evaporator, as does the vapor compression cycle. However, the compressor is replaced by an absorber-generator pair. Lithium bromide-water or water-ammonia are typical of the solvent-refrigerant mixtures used.
The resorption cycle has also been studied. Introduced in the earlier half of this century, the resorption cycle is similar in operation to the absorption cycle. However, a resorber replaces the condenser and the vapor is absorbed by a special week solution while condensing. This solution is then circulated to the evaporator where the refrigerant boils and the heats of disassociation and vaporization produce the refrigerating effect.
Although the majority of prior systems avoid the use of compressors when using a solvent-refrigerant combination, a few processes have employed a solvent-refrigerant pair with a compressor in the system. The system and process described in US―A―1 675 455 is illustrative. This discloses a chemically assisted mechanical refrigeration process comprising the steps of mixing a solvent and a refrigerant that in combination have a negative deviation from Raoult's Law, in a mixing zone under conditions to form a solution of the refrigerant in the solvent, effecting heat-exchange between said solution and a working medium to remove heat from the solution; passing a stream of the solution through an economizing zone to a refrigeration zone in which the pressure over the solution is reduced to allow evolution of refrigerant, thereby to form a gaseous refrigerant stream and a depleted solution stream; effecting heat-exchange in the refrigeration zone between a working medium and said solution to remove heat from the working medium; passing the depleted solution stream through the economizing zone in heat-exchange relation with the stream of solution passing therethrough to the refrigeration zone; transferring gaseous refrigerant evolved from the depleted solution stream in the economizing zone to the gaseous refrigerant stream; and pumping the gaseous refrigerant and depleted solution from the economizing zone to the mixing zone.
In this process both the gaseous refrigerant evolved in the refrigeration zone and the depleted solvent-refrigerant solution exiting the refrigeration zone are passed through the economizing zone, so that although there is mass transfer of refrigerant between the two streams in the economizing zone, the cooling effect of refrigerant evolution in the economizing zone is partly offset by the retention in that zone of heat in the refrigerant stream.
Moreover, the streams of refrigerant and depleted solution exiting the economizing zone are separately compressed and passed to the mixing zone.
DE-A-2 850 403 also discloses a process of this general type but in which only the depleted solvent-refrigerant solution is passed through the economizing zone, the gaseous refrigerant evolved in the refrigeration zone being passed directly to a compressor in which it is compressed jointly with the stream of depleted solvent-refrigerant solution that has passed through the economizing zone. However in this case also, the cooling effect of evolution of gaseous refrigerant from the depleted solution in the economizing zone is offset by a retention in that zone of heat in the refrigerant stream.
One object of the present invention is to provide an improved process of this general type represented by US-A-1 675 455 and DE-A-2 850 403 and accordingly, in one aspect of the invention provides such a process, in which (as in DE-A-2 850 403) the gaseous refrigerant stream from the refrigeration zone is passed to the mixing zone externally of the economizing zone and there is joint compression of both the gaseous refrigerant and the depleted solution, the process of the invention being characterised by providing a fluid communication between said depleted solution stream in said economizing zone and said gaseous refrigerant stream such that gaseous refrigerant evolving from said depleted solution stream may leave said economizing zone separately from said depleted solution stream and pass to said refrigerant stream.
The compressed refrigerant and depleted solution may be passed to the mixing zone through a precooling zone in heat-exchange relationship with the solution passing from the mixing zone to the economizing zone. Preferably the temperature of the compressed refrigerant and depleted solution are raised in the precooling zone to a temperature approaching that in the mixing zone.
A further object of the invention is to provide apparatus for carrying out this improved process. Thus the invention further provides a chemically assisted mechanical refrigeration apparatus comprising a mixer configured to mix a solvent and a refrigerant that in combination have a negative deviation from Raoult's Law, under conditions to form a solution of the refrigerant in the solvent, and means for effecting heat-exchange between the solution and a working medium to remove heat from the solution; a refrigeration zone adapted for evolution of refrigerant from said solution while in heat-exchange relationship with a working medium to remove heat from that working medium, thereby to produce gaseous refrigerant and a depleted solvent-refrigerant solution; an economizer adapted to accomplish heat-exchange between said solution passing to the refrigeration zone and said depleted solvent-refrigerant solution exiting the refrigeration zone; compressor means for jointly compressing said gaseous refrigerant and said depleted solution, and flow path means external of said economizer for gaseous refrigerant evolved in the refrigeration zone, characterised by a flow path extending between the economizer and the gaseous refrigerant flow path means at a point between the refrigeration zone and the compressor means for transfer of refrigerant evolving from the depleted solution in the economizer to the said gaseous refrigerant flow path means.
The apparatus may also comprise a precooler for accomplishing heat-exchange between the compressed refrigerant and depleted solution passing from the compressor means to the mixer, and the refrigerant-solvent solution passing from the mixer to the economizer, respectively.
An embodiment of the invention will now be described by way of example with respect to the accompanying drawings, in which:

Figure 1 is a schematic view of a chemically assisted mechanical refrigeration cycle in accordance with the invention; and
Figure 2 is a schematic view of an evaporator for use in apparatus according to the invention.

In a presently preferred embodiment, there is provided a chemically assisted mechanical refrigeration process including several steps. The refrigerant and solvent have a negative deviation from Raoult's Law when in combination. A stream of solution including a solvent and a liquefied refrigerant is passed to a refrigeration zone such as provided by an evaporator. The pressure is then reduced over the solution to allow refrigerant to vaporize and separate from the solvent while concurrently therewith the evolving refrigerant and solvent are put in heat-exchange relation with a working medium to remove energy from the working medium and thereby form a depleted solution stream and a refrigerant stream leaving the refrigeration zone. The refrigerant stream includes gaseous refrigerant. The depleted solution stream leaving the refrigeration zone is then passed in heat-exchange relation with the solution stream passing to the refrigeration zone, in an economizing zone, so as to cause transfer of heat between the two solution streams. The refrigerant stream does not pass through the economizing zone but the depleted solution stream in the economizing zone and the refrigerant stream are put in fluid communication with each other so as to accomplish mass transfer of gaseous refrigerant from the depleted solution stream to the refrigerant stream and so facilitate heat transfer in the economizing zone between the two solution streams. The depleted solution and refrigerant streams are subsequently jointly compressed in a compression zone where the pressure over both streams is raised.
The compressed solvent and refrigerant are then passed to a mixing zone under a pressure sufficient to promote substantial dissolving of the refrigerant in the solvent to form the stream of solution for passage to the refrigeration zone. As the mixing zone is in heat-exchange relation with a working medium, energy to removed from the mixing zone.
The compressed refrigerant and solvent passing to the mixing zone from the compression zone may undergo heat-exchange in a precooling zone with the stream of solution leaving the mixing zone.
Figure 1 illustrates an embodiment of this process.
A solvent-liquefied refrigerant solution stream is passed via line 25 to evaporator 10. The refrigerant and solvent of the solvent-liquefied refrigerant stream have a negative deviation from Raoult's Law and may be chosen from a number of combinations of substances, as more fully discussed below.
The pressure over the solution is reduced in the evaporator in order to allow refrigerant to vaporize and separate from the solvent while concurrently placing the evolving refrigerant and solution in heat-exchange relation with the working medium to remove energy from the working medium. As a result, there is formed a depleted solution stream which passes via line 24 and a refrigerant stream including gaseous refrigerant which passes via line 18.
The stream passing via line 24 may contain a material portion of refrigerant without hindering the efficiency of the process. The depleted solution stream leaving the evaporator and passing via line 24 passes through an economizer 26 in which it is placed in heat-exchange relation with the solvent-refrigerant solution stream passing to the evaporator via lines 15 and 25. Further, the depleted solution stream in line 24 is placed in fluid communication with the refrigerant stream of line 18 such that gaseous refrigerant evolving from the stream 24 in the economizer may pass via conduit 92 to the external refrigerant stream 18. This evolution of gas tends to cool the depleted solution stream in the economizing zone provided by economizer 26, thus facilitating heat transfer in this zone, which in turn increases the temperature drop in the solvent-refrigerant solution stream as it passes through the economizing zone. Put another way, any inefficiencies in the evaporator caused by a failure of the refrigerant to separate from the solution are believed diminished since the refrigerant is allowed to further evolve from the solution and the resulting change in energy is transferred indirectly to the working medium passing through the evaporator by virtue of the lowering of temperature of the solvent-refrigerant solution as it enters the evaporator.
Both the depleted solution (solvent) stream leaving the economizing zone and the refrigerant stream are then brought into thermal and physical contact in a joint compression zone as illustrated by compressor 88 in Figure 1. The compression of the refrigerant in thermal contact wihh the liquid solvent in a joint compression zone such as compressor 88 is believed to provide several advantages. Thus, the liquid solvent would generally have a higher heat capacity than the refrigerant and generally act as a coolant in the compressor, thus reducing the amount of work required to compress the refrigerant. Additionally, with joint compression a liquid solvent may be chosen which acts both as a sealant and a lubricant as well as a coolant. Thus, when a refrigerant gas is compressed and the solvent pumped simultaneously by a single compressor-pump, such as compressor 88 in the joint compression zone, several advantages can accrue. For example, the solvent provides internal cooling of the overall apparatus thus permitting compression which is more polytropic than isentropic and hence generally more economical. Additionally, it is believed that the presence of the solvent in the compressor permits higher pressures in the case of a centrifugal compressor, or severs as a lubricant and sealant in case of a rotary compressor.
The resulting combined solvent-refrigerant stream flows via line 90 through a heat-exchanger such as precooler 86 and into mixer 11. The heat-exchanger or precooler 86 serves to further raise the temperature of the solvent-refrigerant combination passing to mixer 11 while concurrently beginning to cool the refrigerant-solvent stream passing via line 15 toward economizer 26. The heat exchanger, such as precooler 86, should be operated so as to allow the temperature of the solvent-refrigerant combination stream entering mixer 11 to approach as closely as possible the temperature of mixer 11 without exceeding the same. Additionally, the precooler should be operated in a such a fashion that the temperature of the solvent-refrigerant combination passing via line 90 in such that the refrigerant will not start to substantially dissolve and give off heat prior to reaching the mixer 11.
In the mixer 11, the combined solvent-refrigerant stream is maintained at a pressure sufficient for the given temperature to promote substantial dissolving of the refrigerant in the solvent to form the stream of solution for passage to the evaporator 10 via lines 15 and 25. Concurrently therewith, the mixer is in heat exchange relation with a working medium which removes energy of heat given off by the dissolving and condensing refrigerant in the mixer 11.
In preferred embodiments of the invention, the evaporator is so constructed as to allow substantial transfer of both the heat of vaporization and the heat of disassociation from the working medium. As efficient heat transfer is promoted through the use of a wetted heat transfer surface, the heat transfer surface may preferably be wetted by the solvent with or without dissolved refrigerant. Thus, in one embodiment the refrigerant-solvent stream may be passed as a thin film over a heat transfer surface with embedded coils containing the working medium.
In the evaporator embodiment shown in Figure 2, a working medium such as chilled water is passed via a line 22 through the shell side of a shell and tube type heat-exchanger while the refrigerant-solvent solution stream entering from a line 21 passes through the tube side. The refrigerant evolves from the solution in the tubes and both depleted solution and refrigerant pass to a liquid-vapor separator 31 where the depleted solution and refrigerant are separated. The liquid-vapor separator 31 may be equipped with a wire mesh 32 to catch entrained droplets, which collect below wire mesh 32. The depleted solution passes via a line 24 to the economizer while the refrigerant passes via line 18 to the compressor.
In another embodiment, the conduit 22 is substantially immersed in liquid in the evaporator, the refrigerant substantially disassociates and boils off from the solution, thus cooling the working medium. In such an embodiment the evaporator may be similar in construction to a shell and tube heat exchanger wherein the working medium circulates through the tubes, which are substantially immersed in liquid.
Alternatively, the working medium may pass through a coil, which passes through the lower portion of the evaporator and so is substantially immersed in liquid. By way of example, the refrigerant-solvent stream may circulate and undergo separation in a single-tube coil of 13 mm (2 inch) diameter for a one to four ton apparatus and then further separate in a liquid-vapor separator.
As would be known to one skilled in the art having the benefit of this disclosure, the evaporator may comprise any one of several modified heat exchangers or evaporators.
Where it is desirable to facilitate the separation of the vaporized refrigerant from depleted solution an eliminator may be employed at the vapor outlet of the evaporator if the vapor and liquid separate into two streams in the evaporator.
The compressor may be any one of several mecahnical types. Regardless of the type of compressor used, in keeping with the spirit of the present invention, its operating cost should generally be less than that of its counterpart in a typical vapor compression refrigeration system for a given application. This is possible due to the increased efficiency of the present system. This increased efficiency over prior mechanical vapor compression cycles is believed to result in part from the fact that the solubility of the refrigerant in the solvent reduces the level of required mechanical compression. The refrigerant need only be pressurized sufficiently to dissolve in the solvent in the mixer at the given operating conditions and concentrations. There is believed to be little or no wasted compression of the refrigerant to pressurize it sufficiently to condense at the mixer temperature as in the usual vapor compression cycle. Additionally, since the refrigerant is at a lower temperature as it leaves the mixer than.in the case of a pure refrigerant cycle, less heat transfer is required and hence less working medium need be circulated to the mixer.
The compressor chosen may vary with operating conditions, the refrigerant-solvent combination chosen or the application to which the system is applied. For example, for embodiments such as shown in Figure 1, a centrifugal, rotary or screw compressor may be preferred.
The refrigerant-solvent combination comprises at least two constituents-a refrigerant and a solvent. The refrigerant and solvent are chosen such that the refrigerant will separate as a gas from the solvent under the operating conditions in the evaporator while preferably absorbing substantial amounts of the heats of demixing, dilution, or disassociation as well as vaporization. Thus, a governing principle for the selection of a refrigerant-solvent combination is that the refrigerant be highly soluble in the solvent, such that the pair exhibits negative deviations from Raoult's Law.
Examples of refrigerants which are beleived to be suitable for use in the present invention with appropriate solvents include hydrocarbons such as methane, ethane, ethylene and propane; halogenated hydrocarbons, such as refrigerants R20, R21, R22, R23, R30, R32, R40, R41, R161 and R1132a; amines, including methylamine, or gases used in certain refrigeration processes such as methyl chloride, sulfur dioxide, ammonia, carbon monoxide and carbon dioxide or any appropriate combinations of these.
The solvent constituent should be a substantially non-volatile liquid at the operating conditions of the cycle or be at least such when in solution with a portion of the refrigerant. Thus, the solvent, for example, nitrous oxide, can be a gas at room temperature.
It is believed that the solvent may be an ether, an ester, an amide, an amine or polymeric derivatives of these, for example, dimethyl formamide and dimethyl ether of tetraethylene glycol as well as halogenated hydrocarbons, such as carbon tetrachloride and dichlorethylene; or appropriate combinations of these A halogenated salt such as lithium bromide may also be a constituent of the solvent.
Also believed to be suitable as solvents are methanol, ethanol, acetone, chloroform and trichloroethane. Organic physical solvents such as propylene carbonate and sulfolane or other organic liquids containing combined oxygen may be used.
Relatively large deviations from Raoult's Law and hence relatively large heats of mixing are obtained when one, or preferably both, of the refrigerant and solvent molecules is polar. The excess solubility is believed to be a consequence of either dipole-dipole attraction (including hydrogen bonding) or induced dipole-dipole attraction.
However, limited experimental data and calculations indicate that certain combinations of refrigerant and solvent may not have a satisfactory coefficient of performance. Thus, calculations on the embodiment shown in Figure 1 indicate that a combination of carbon dioxide and 1,1,1-trichloroethane may not be very efficient. More particularly, calculations generally paralleling those set out below with respect to 1,1,1-trichloroethane and R22 with respect to Figure 1 resulted in a coefficient of performance of 1.83 for assumed evaporator and mixer temperature of -15°C (5°F) and 30°C (86°F), respectively, and 1.49 for assumed evaporator and mixer temperatures of 4.5°C (40°F) and 43°C (110°F). This may possibly be explained by the high critical temperatures and pressures of carbon dioxide of 31.04°C (87.87°F) and 7377.11 kPa absolute (1069.96 psia), respectively.
It is believed that other chemical constituents may be added to the basic pair for other purposes, including foaming, lubrication, inhibition of corrosion, lowering of the freezing point, raising of the boiling point or indication of leaks. However, such added constituents should preferably be chosen so as not substantially to detract from the heat of disassociation or vaporization produced in the evaporator. Further, the constituents are preferably such as to not detract from any negative deviations from Raoult's Law.
The comparative efficiency of the instant invention is illustrated by reference to available data for a refrigerant-solvent pair comprising CHCIF₂ (refrigerant R22) and dimethyl formamide (DMF). According to an enthalpy-concentration diagram disclosed in Jelinek, M., et al Ethalpy-Concentration Diagram-A.S.H.R.A.E. Trans. 84 (1978), Pt. II, pp. 60-67, herein incorporated by reference, an R22-DMF solution is in equilibrium at 391.62 kPA (56.8 psig) and 30°C (86°F) with a weight distribution of 60% R22 and 40% DMF. If pressure is reduced sufficiently, the R22 will boil out of the DMF, absorbing a combined heat of vaporization and heat of mixing of slightly more than 167.36 J/kg (72 But/lb). Alternatively, the heat of mixing can be calculated from Equation (14) in Tyagi, K.P., Heat of Mixing, Ind. Jnl. of Tech., 14 (1976), pp. 167-169, herein incorporated by reference, to be 44.93 J/kg (19.33 Btu/ib) while the heat of vaporization of the R22 is 129.98 J/kg (55.92 Btu/Ib) of solution. Thus, the total heat absorbed, per kilogram of solution entering the evaporator, is 174.91 J (75.25 Btullb), in close agreement with the enthalpy-concentration diagram mentioned above.
Although it may be preferable that the refrigerant-solvent mixture or combination be chosen such that a substantial amount of refrigerant vaporizes from solution in the evaporator, this need not always be the case. For example, a refrigerant with a comparatively high heat of vaporization may be circulated in small proportions relative to the amount of solvent because the refrigerant-solvent leaving the mixer is placed in heat exchange relation with the depleted solution leaving the evaporator.
The operation of the embodiment shown in Figure 1 is further highlighted by the various temperatures shown in the drawing, all of which are in degrees Celsius. These temperatures were calculated based on the following presumptions. It is presumed that a cycle using R22 as a refrigerant and 1,1,1-trichloroethane (TCE) as a solvent was employed with an evaporator temperature of 4.44°C (40°F) and a mixer temperature of 43.33°C (110°F). Based on the resulting calculations from heat balances, it is believed that if the precooler 86 is not present, the theoretical coefficient of performance of the system would be 6.71, which still compares favorably with 5.75 for a pure R22 vapor compression cycle generally used in prior art systems. However, if a heat exchanger such a precooler 86 is present, the refrigerant-solvent combination may be used to cool the refrigerant-solvent solution stream exiting the mixer. Since this combination passing via line 90 is at mixer pressure as it enters the mixer, but below mixer temperature as it begins its passage through precooler 86, it is assumed that solution of refrigerant into solvent will have begun in the precooler 86. With R22 as a refrigerant and 1,1,1-trichloroethane as the solvent at the temperature shown, a theoretical maximum of only half the heat exchange theoretically available for inert liquids is available, and the resultant theoretical coefficient of performance is 7.13. (The theoretical maximum coefficient of performance for a perfect (Carnot) cycle is 7.14).
The results of these calculations in comparison with a pore R22 vapor compression cycle, are set forth in Table 1. Various data necessary to the calculations, vapor densities, discharge temperatures of isentropic compression to determine polytropic discharge temperatures and so forth were taken from American Society of Heating, Refrigerating and Air Conditioning Engineers, Thermophysical Properties of Refrigerants, 1976 and American Society of Heating, Refrigerating and Air Conditioning Engineers, Thermodynamic Properties of Refrigerants, 1980. Where extrapolations had been made, it is believed that they were generally made in the direction of conservative estimates with respect to cycle performance.
Based on 0.4536 kg (one pound) of circulating mass and R22-TCE cycle with an evaporator temperature of 4.44°C (40°F) and a mixer temperature of 43.33°C (110°F), at 43.33°C (110°F) and 652.93 kPa absolute (94.7 psia), 0.310 kg (0.684 lbs) of TCE is in equilibrium in a liquid solution with 0.143 kg (0.316 lbs) of R22. At 4.44°C (40°F) and 170.30 kPa absolute (24.7 psia) 0.119 kg (0.262 lbs) of R22 vaporizes, leaving 0.024 kg (0.054 lbs) of R22 remaining in solution. Enthalpy measurements indicate the evolving R22 absorbs 23.88 kJ (22.65 Btu) as a gross refrigerating effect in the evaporator.
Assuming perfect heat exchange and equal exit temperatures of 20.89°C (69.6°F), the remaining 0.024 kg (0.054 Ibs) of R22 should vaporize in the economizer as the depleted solution entering in at 4.44°C (40°F) flows countercurrent to the incoming refrigerant laden solution streams in lines 15 and 25. The exit temperatures in both cases is approximately 21.11°C (70°F). A temperature closer to 21.67°C (71°F) is attained if precooler 86 is not employed while an exit temperature in each case of about 20.91°C (69.64°F) is reached where precooler 86 is used.
The 0.310 kg (0.684 Ibs) of TCE, with a specific heat of 0.258, enters the compressor at 21.63°C (70.93°F), absent precooler 86, or 20.91°C (69.64°F) with precooler 86 between the compressor 88 and mixer 11, and the entering temperature of the 0.163 kg (0.36 Ibs) of R22 including warmer than 4.44°C (40°F) gas from the economizing zone is calculated as 5.90°C (42.62°F), absent the precooler 86, or 5.83°C (42.5°F) with the precooler 86. Isentropic compression of the gas alone would give a discharge temperature of 64.44°C (148°F), so that the discharge temperature of the liquid and gas is 38.06°C (100.51°F), or in case precooler 86 is used, 37.36°C (99.25°F).
The value of n, the constant of polytropic compression is determined from

Where

T_i=₂₇₉.₂₃ K (502.62°R)
T₂=311.39°K (560.51°R)
P,=₁₇₀.₃₀ kPa (24.7x144 psf)
P₂=652.93 kPa (94.7x144 psf)
n=1.09

The work of compression is
that is, 21.61 kJ per 0.143 kg (2.05 Btu per 0.316 Ib) R22 vaporized. (V and V₂ are taken from the superheat tables of American Society of Heating, Refrigerating and Air Conditioning Engineers, Thermodynamic Properties of Refrigerants, 1980). The density of the stripped TCE leaving the economizer 26 is 1345.23 kg/m^l (83.98 Ib/ft³), the pressure head across the 482.63 kPa (70 psi) differential is 36.70 m (120.42 ft), so that the work of pumping 0.310 kg (0.684 lb) of TCE is 0.112 kJ (0.106 Btu). Hence the total work of compressing the gas and pumping the liquid is 5.02 J/kg '(2.16 Btu/Ib) of mixture.
Since the refrigerant-solvent solution, with a specific heat of 0.264 must be subcooled to -0.59°C (30.93°F), from 4.44°C (40°F) in the evaporator, the net available refrigerating effect is 15.27 kJ/kg (14.48 Btu/Ib) of gas-liquid circulating mass, absent the precooler 86.
The coefficient of performance of the cycle is thus 6.71. Since the theoretical coefficient of performance of the pure R22 cycle at these conditions is 5.75, the embodiment shown in Figure 1 is believed to represent a 16.7% more efficient process than a comparable vapor compression refrigeration cycle, even when an additional heat exchanger such as precooler 86 is not used.
The foregoing, except for initial references above, neglects the fact that the liquid-compressed gas mixture exiting the compressor is still cooler than the 43.33°C (110°F) mixer and has the capacity of subcool the refrigerant-solvent solution exiting the mixer. If there were no absorption of gas by liquid, hence no generation of heat, the precooler 86 would subcool the mixer outflow to 40.90°C (105.62°F). Assuming the actual temperature reduction is only half 43.33-40.90°C (110^0-105.62°F), the refrigerant-solvent solution flows to the economizer 26 at a temperature of 42.12°C (107.81°F) instead of 43.33°C (110°F).
Iterating the previous calculations back through the economizer and the compressor, it is believed all the R22 in the solvent stream in line 24 still comes out. The work of compression-pumping becomes 4.83 kJ/kg (2.08 Btu per pound) of circulating mass. Since the mixer effluent has been cooled a bit the available net refrigerating effect per kg mass is 34.47 kJ (14.83 Btu/Ib).
The coefficient of performance is now 7.13, compared with 6.71 without the precooler 86 and as compared to 5.75 for pure R22. Since the theoretically perfect Carnot efficiency between 4.44°C (40°F) and 43.33°C (110°F) is 7.14, it appears that the precooler provides an even greater efficiency, since 7.13 is about 25% better than 5.75.
The present invention may also be used in conjunction with other systems. For example, a generator-absorber pair might be hooked up in tandem with the compressor to provide a back-up for the same. The generator could function off a secondary source of heat, such as from an exhaust, or a form of solar energy. For example valves could be placed on both sides of compressor 88 in lines 18 and 90 to hook a generator-absorber pair into the system. A portion of the vaporized refrigerant could then pass from line 18 to the absorber, be absorbed in an appropriate secondary solvent and then be pumped in solution to the generator. Upon evaporation of the refrigerant in the generator the now compressed vapor could be passed via line 90 to the mixer 11, while secondary solvent was returned to the absorber.
The secondary solvent may be the same as used in the primary system.
Of course, in order to obtain all of the advantages of the present invention, the generator-absorber pair should not be completely substituted for the compressor 88. Rather, the generator-absorber pair and the mechanical compressor are complementary means of generating pressurized refrigerant gas.
Further, with respect to the illustrated embodiment, as would be known to one skilled in the art having the benefit of this disclosure, there exist a number of alternatives for concurrent compression pumping of the gas and liquid constituents. For example, large multi-stage centrifugal compressors as manufactured by York, frequently are designed to inject liquid refrigerant into the vapor stream as a substitute for flash intercooling between stages. However, in such a case, the liquid flow rate should be as reasonably uniform as possible. Also, helical or rotary screw compressors, such as manufactured by Dunham-Bush may be adapted for use with the chemically assisted mechanical refrigeration system as disclosed herein. However, in the chemically assisted mechanical refrigeration system, the solvent should preferably serve as a coolant, lubricant and sealant. Further, bulky oil separators and oil coolers should be eliminated since the solvent passes on to the mixer with the compressed gas.
For smaller capacities, the Wankel-type compressor, manufactured by Ogura Clutch of Japan, or the rolling piston compressors of Rotorex (Fedders) and Mitsubishi may prove useful. Possibly useful also is the multistage centrifugal compressor-pump of the type manufactured by Sihi. In this device, a gas-liquid mixture enters a first, closed impeller axially and the denser liquid is thrown to the periphery. The lighter gas is ported off to the second and subsequent stages nearer the center of the chamber and both gas and liquid are then carried together through second and subsequent impeller stages.
Moreover, where capital costs permit, a turbine may be installed in the refrigerant-solvent stream between the economizer and evaporator to function as a pressure reducing device, supplementing throttling devices. Under appropriate operating conditions. It is believed that a subcooled stream exiting the economizer is least likely to flash refrigerant gas at this point and the resultant shaft work may be used to power booster pumps, compressors for the system, auxiliary fans or the like.
Additional items of equipment may be employed within the framework of the present invention. For example, control of the system as well as system versatility may be enhanced through the use of appropriate process controls, though the use of essentially manual control devices may suffice for many operations. Additionally, in the embodiment shown in Figure 1, a low pressure drop mixing of gaseous refrigerant and liquid could be achieved by using an inline motionless mixer such as one offered by the Mixing Equipment Co., Inc. of Avon, New York.

Claims

1. A chemically assisted mechanical refrigeration process comprising the steps of mixing a solvent and a refrigerant that in combination have a negative deviation from Raoult's Law, in a mixing zone under conditions to form a solution of the refrigerant in said solvent; effecting heat exchange between said solution and a working medium to remove heat from the solution; passing a stream of said solution through an economizing zone to a refrigeration zone in which the pressure over the solution is reduced to allow evolution of the refrigerant, thereby to form a gaseous refrigerant stream and a depleted solution stream; effecting heat exchange in said refrigeration zone between working medium and said solution to remove heat from said working medium; passing said depleted solution stream through said economizing zone in heat exchange relation with the stream of solution passing therethrough to the refrigeration zone; and pumping said gaseous refrigerant and said depleted solution from the economizing zone to said mixing zone by passing said gaseous refrigerant stream from the refrigeration zone to the mixing zone externally of the economizing zone and jointly compressing the refrigerant and the depleted solution, characterised by providing a fluid communication between said depleted solution stream in said economizing zone and said gaseous refrigerant stream such that gaseous refrigerant evolving from said depleted solution stream may leave said economizing zone separately from said depleted solution stream and pass to said refrigerant stream.

2. A process according to claim 1, further characterised by passing the compressed refrigerant and depleted solution to the mixing zone through a precooling zone in heat-exchange relationship with the solution passing from the mixing zone to the economizing zone.

3. A process according to claim 2, further characterised in that the temperature of the compressed refrigerant and depleted solution is raised in said precooling zone to a temperature approaching that in the mixing zone.

4. A chemically assisted mechanical refrigeration apparatus comprising a mixer (11) configured to mix a solvent and a refrigerant that in combination have a negative deviation from Raoult's Law, under conditions to form a solution of the refrigerant in the solvent, and means for effecting heat-exchange between the solution and a working medium to remove heat from the solution; a refrigeration zone (10) adapted for evolution of refrigerant from said solution while in heat-exchange relationship with a working medium to remove heat from that working medium, thereby to produce gaseous refrigerant and a depleted solvent-refrigerant solution; an economizer (26) adapted to accomplish heat-exchange between said solution passing to the refrigeration zone (10) and said depleted solvent-refrigerant solution exiting the refrigeration zone; compressor means (88) for jointly compressing said gaseous refrigerant and said depleted solution; and flow path means (18) external of said economizer (26) for gaseous refrigerant evolved in the refrigeration zone (10), characterised by a flow path (92) extending between the economizer (26) and the gaseous refrigerant flow path means (18) at a point between the refrigeration zone (10) and the compressor means (88) for transfer of refrigerant evolving from the depleted solution in the economizer (26) to the gaseous refrigerant flow path means (18).

5. Apparatus according to claim 4, further characterised by a precooler (86) for accomplishing heat-exchange between the compressed refrigerant and depleted solution passing from the compressor means (88) to the mixer (11), and the refrigerant-solvent solution passing from the mixer (11) to the economizer (26), respectively.