US3695052A

US3695052A - Concentration control for multiple stage absorption refrigeration systems

Info

Publication number: US3695052A
Application number: US166914A
Authority: US
Inventors: Charles K Griffin
Original assignee: Carrier Corp
Current assignee: Carrier Corp
Priority date: 1971-07-28
Filing date: 1971-07-28
Publication date: 1972-10-03
Anticipated expiration: 1989-10-03

Abstract

A two-stage air cooled lithium bromide absorption refrigeration system having a generator, a refrigerant condenser, a high pressure absorber coupled with a high temperature evaporator, a low pressure absorber coupled with a low temperature evaporator and an air conditioning fan coil unit for passing cooled refrigerant in heat exchange with ambient air is provided with a concentration control system for automatically diluting the absorbent solution under conditions of either low ambient temperatures or an inadequately purged low pressure absorber. The concentration control includes a refrigerant storage tank connected directly to the refrigerant condenser. Intermediate strength absrobent solution discharged from the low pressure absorber is passed through a heat exchanger in the refrigerant storage tank, in heat exchange relation with refrigerant therein, to the high pressure absorber. A refrigerant overflow passage is provided from a refrigerant sump to a location containing absorbent solution. Ambient air passes serially over the low pressure absorber and then over the refrigerant condenser so that when the low pressure absorber is poorly purged or the ambient air temperature is relatively low, these events will be reflected by a drop in the intermediate solution temperature passing through the refrigerant storage tank and a simultaneous drop in the condensing temperature. Consequently, the difference in pressure between the refirgerant storage tank and the condenser will drop and refrigerant will pass into the system from the storage tank and will cause the refrigerant level in the sump to overflow into absorbent soulution through the refrigerant overflow passage to prevent solidification.

Description

United States Patent [15] 3,695,052 Griffin 1 1 Oct. 3, 1972 [54] CONCENTRATION CONTROL FOR high temperature evaporator, a low pressure absorber MULTIPLE STAGE ABSORPTION REFRIGERATION SYSTEMS Primary Examiner-William F. ODea Assistant Examiner--P. D. Ferguson Att0mey1-larry G. Martin, Jr., et al.

57 ABSTRACT A two-stage air cooled lithium bromide absorption refrigeration system having a generator, a refrigerant condenser, a high pressure absorber coupled with a coupled with a low temperature evaporator and an air conditioning fan coil unit for passing cooled refrigerant in heat exchange with ambient air is provided with a concentration control system for automatically diluting the absorbent solution under conditions of either low ambient temperatures or an inadequately purged low pressure absorber. The concentration control includes a refrigerant storage tank connected directly to the refrigerant condenser. Intermediate strength absrobent solution discharged from the low pressure absorber is passed through a heat exchanger in the refrigerant storage tank, in heat exchange relation with refrigerant therein, to the high pressure absorber. A refrigerant overflow passage is provided from a refrigerant sump to a location containing absorbent solution. Ambient air passes serially over the low pressure absorber and then over the refrigerant condenser so that when the low pressure absorber is poorly purged or the ambient air temperature is relatively low, these events will be reflected by a drop in the intermediate solution temperature passing through the refrigerant storage tank and a simultaneous drop in the condensing temperature. Consequently, the difference in pressure between the refirgerant storage tank and the condenser will drop and refrigerant will pass into the system from the storage tank and will cause the refrigerant level in the sump to overflow into absorbent soulution through the refrigerant overflow passage to prevent solidification.

5 Claims, 1 Drawing Figure PAIENIEDuma m2 INVENTOR.

CHARLES K. GRIFFIN ATTORNEY CONCENTRATION CONTROL FOR MULTIPLE STAGE ABSORPTION REFRIGERATION SYSTEMS BACKGROUND OF THE INVENTION This invention relates to a concentration control for a multiple stage absorption refrigeration system.

Air cooled absorption refrigeration systems are sub ject to sudden and widely varying changes in the temperature of air passing over the absorber. In systems of the type utilizing lithium bromide as an absorbent solution, it is necessary to restrict the concentration of the absorbent solution so that it will not solidify under normally encountered operating temperatures. However, the existence of even relatively small quantities of non condensible gases in the absorber reduces the absorption of refrigeration into absorbent solution which may result in solidification of the relatively undiluted solution in the absorber.

Because of the possibility of solidification of the absorbent solution in an air cooled system, it has been necessary to provide some type of concentration control which is effective to limit the concentration of the solution under those conditions where the danger of solidification is imminent. It has, therefore, been previously proposed to provide a refrigerant storage tank having an internal pressure which is a function of the absorber cooling medium temperature and arranged so that refrigerant is discharged from the tank into absorbent solution to prevent solidification thereof upon a drop in ambient cooling medium temperature. It has been discovered, however, that passing absorber coolin g air directly over the exterior of a refrigerant storage tank may result in a sluggishly responding dilution control because the effective heat transfer coefi'icient between air and the refrigerant is relatively low, and it becomes necessary to utilize a relatively large amount of heat transfer surface to obtain the desired speed of response. It has also been discovered that in a multiple stage absorption refrigeration system using relatively strong solution in the low pressure absorber stage, the danger of solidification is surprisingly greater in the intermediate solution passage between the low pressure absorber and the high pressure absorber than in the strong solution passage or the solution heat exchanger, which are the locations most prone to solidification in conventional refrigeration systems. Consequently, the concentration control should be directly responsive to the condition of the intermediate solution for most effective operation.

It is an object of this invention to provide an improved concentration control, specifically designed for use with multiple stage absorption refrigeration systems, which is more responsive and more effective than prior systems.

SUMMARY OF THE INVENTION A multiple stage absorption refrigeration system having a generator, a refrigerant condenser, a low temperature evaporator, a low pressure absorber, a high temperature evaporator and a high pressure absorber is provided with a concentration control arrangement which includes a refrigerant storage tank, a refrigerant sump, a refrigerant overflow passage and means for passing cooling medium first in heat exchange relation with absorbent solution in the low pressure absorber and then in heat exchange relation with refrigerant vapor in the refrigerant condenser. The refrigerant storage tank is connected by a refrigerant condensate passage to receive sutficient liquid refrigerant from the condenser to balance the difference in pressure between the tank and the condenser. Intermediate strength absorbent solution is passed from the low pressure absorber through the refrigerant storage tank, in heat exchange relation with refrigerant therein, to the high pressure absorber.

When either the absorber ambient temperature drops or when absorption of refrigerant vapor in the low pressure absorber is reduced for any reason, such as the accumulation of noncondensible gases, the tem' perature of the cooling medium passing from the low pressure absorber to the condenser is reduced which reduces the condensing temperature. At the same time, the temperature of the intermediate solution passing through the refrigerant storage tank is reduced due to less heat of dilution and condensation being absorbed by the solution in the absorber or due to increased cooling by a lower cooling medium temperature. However, because the condenser is at a higher temperature, its pressure drops faster than that of the refrigerant tank. Consequently, the difierence in pressure between the refrigerant storage tank and the condenser is reduced and the level of refrigerant in the tank drops. The refrigerant discharged from the tank causes the level of refrigerant in the refrigerant sump to rise, which in turn causes refrigerant to overflow into absorbent solution to dilute it, thereby reducing the likelihood of solidification occurring.

When the absorber becomes completely purged or when the ambient temperature again rises, the difference in pressure between the condenser and the refrigerant storage tank will rise so that refrigerant is supplied to the tank from the condenser to balance the pressure difference until it is again needed for dilution.

A concentration control in accordance with this invention provides much faster response with reduced heat transfer surface due to the higher coefficient of heat transfer obtainable between the intermediate solution and the refrigerant, thereby reducing the cost of the system. Further, the system is extremely sensitive to changes in the temperature of the intermediate solution which, it has been discovered, is a more reliable precursor of solidification in a multiple stage absorption refrigeration system than other system conditions previously suggested.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a schematic illustration of an absorption refrigeration system embodying a concentration control in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT A generator It), a refrigerant condenser 11, a high pressure absorber 12, a high pressure refrigerant evaporator 13, a low pressure absorber 14 and a low pressure evaporator 15 are connected in the illustrated embodiment to provide an absorption refrigeration system. In the system illustrated, two evaporator stages of the adiabatic type are employed, which directly cool the refrigerant liquid by evaporating a portion of the refrigerant and the remaining cooled refrigerant passes through an air conditioning heat exchanger 16 such as a fan coil unit to directly cool air being conditioned. Direct expansion or conventional spray type evaporators may be used with suitable modifications in the system. Also, more than two stages of evaporation and absorption may likewise be employed.

Water is employed as the refrigerant and an aqueous solution of lithium bromide is employed as the absorbent although other absorbent refrigerant combinations may be utilized. As used herein, strong solution refers to a concentrated absorbent solution which is strong in absorbing power and weak solution" refers to a diluted absorbent solution which is weak in absorbing power. Intermediate solution" is used to designate absorbent solution which is intermediate in absorbing power between that of strong and weak solution and, in particular, to designate the absorbent solution passing from the low pressure absorber to the high pressure absorber. In the illustrated system, the refrigerant condenser and the absorbers are illustrated as being of the air cooled type but other cooling media such as water may be employed by suitably modifying the heat exchanger constructions.

A quantity of absorbent solution in the bottom of generator is boiled by passing a suitable heat source such as steam, hot liquid or flue gas from a fuel burner through heat exchanger 20 in heat exchange relation with the solution. The strong solution formed in generator 10 by vaporization of refrigerant therefrom passes through strong solution passage 2], pump 22, the outside passage of heat exchanger 24 and strong solution passage 25 to the upper header of low pressure absorber 14. The strong solution flows downwardly through a plurality of heat exchange tubes joined by

headers

30, 31 forming low pressure absorber 14, in contact with refrigerant vapor passed to absorber 14 through vapor passage 17 from low temperature evaporator 15. The absorbent solution is cooled in absorber 14 by passing a cooling medium such as ambient air by fan 18 over the exterior of the heat exchange tubes of absorber l4. Refrigerant vapor from low temperature evaporator 15 is absorbed into the strong solution as it passes downwardly through the tubes of low pressure absorber 14, thereby diluting the strong solution and forming an intermediate strength solution which collects in lower header 31. The cooling medium absorbs the heat of condensation and heat of dilution introduced by the absorption of refrigerant vapor into the solution and assists the absorption process by cooling the solution to a low vapor pressure.

The intermediate solution passes through intermediate solution passage 32, intermediate solution pump 33, intermediate solution passage 34, heat exchanger 35 in refrigerant storage tank 36 and intermediate solution passage 37 into upper header 40 of high pressure absorber 12. The intermediate solution passes downwardly through the heat exchange tubes of high pressure absorber 12 while being cooled by passage of ambient air thereover, and absorbs additional refrigerant vapor passed from high temperature evaporator 13 through vapor passage 47. The resulting weak solution is collected in weak solution header 41 and passes through weak solution passage 42, weak solution pump 43, weak solution passage 44, the interior passage 45 of solution heat exchanger 25 and weak solution passage 46 to weak solution sump 50.

Weak solution sump 50 is preferably located above the heating portion of generator 10 and may be of annular shape surrounding a tubular vapor passage 51 for refrigerant vapor formed in the generator. The weak solution passes from sump 50 by gravity through weak solution passage 52, having a suitable restriction 53, and is discharged into the heating section of generator 10 for reconcentration therein.

Refrigerant vapor formed in generator 10 passes upwardly through tubular vapor passage 51 and refrigerant vapor passage to the vapor header 6] of condenser 11. Condenser 11 may comprise a plurality of vertical heat exchange tubes connected by upper and lower headers located adjacent low pressure absorber 14. While only one refrigerant condenser is illustrated, the system may embody one or more additional parallel connected refrigerant condensers such as one located adjacent high pressure absorber l2 and may employ an additional far. to pass air over high pressure absorber stage 12 and the other condenser section.

Air or other cooling medium is moved by fan 18 or other means so that it passes first over low pressure absorber l4 and then over refrigerant condenser 11 whereby the condensing temperature in condenser 11 is a function of both the ambient air temperature and the heat rejected from the low pressure absorber. Air passing over condenser 11 cools refrigerant vapor therein causing the refrigerant to condense and collect in liquid header 62. Liquid refrigerant passes through condensate passage to a condensate sump 66 con taining a refrigerant metering float valve 67. The refrigerant condensate then passes through condensate passage 68 and is pumped by refrigerant pump 70 through refrigerant passage 69, air conditioning heat exchanger 16 and refrigerant passage 71 into the high temperature evaporator 13.

Refrigerant is evaporated in high temperature evaporator 1.3, thereby cooling the remaining liquid refrigerant. The partially cooled refrigerant passes refrigerant passage 75 containing restriction 76 into low temperature evaporator 15. Additional refrigerant is evaporated from the refrigerant low temperature evaporator 15, thereby cooling the remainder of the liquid refrigerant. The cold liquid refrigerant passes through refrigerant passage 79 into refrigerant sump 80 having a large cross sectional area, lower section 81 and a small cross sectional area upper section 82. The cold refrigerant passes from sump 80 through refrigerant passage 83 and mixes with condensate in passage 68 where it cools the condensate and the cold mixture is pumped through air conditioning heat exchanger 16 by refrigerant pump 70.

A refrigerant overflow passage extends from the small cross section portion 82 of refrigerant sump 80 at a predetermined level therein above the normal refrigerant level, to weak solution passage 42 or some other location containing absorbent solution, preferably at a lower elevation. A solution overflow passage 91 extends from refrigerant sump 50 at a level above the nonnal solution level to upper header 61 of condenser 11 or other location containing refrigerant. Under low ambient temperature conditions, the level of absorbent solution will rise in sump 50 and will overflow into condenser 11 to reduce the vapor pressure of the refrigerant in the system so as to prevent freezing thereof and maintain a constant evaporator temperature. When ambient temperature rises, after the refrigerant has been diluted with absorbent, the level of diluted refrigerant in sump 80 will rise sufficiently to overflow through passage 90 into the weak solution until most of the absorbent solution has been removed from the refrigerant or until the refrigerant is sufficiently concentrated to provide the desired capacity at the higher ambient temperature condition. Refrigerant overflow passage 90 also serves as a portion of the concentration control system as will be subsequently described.

Refrigerant storage tank 36 is connected by a refrigerant condensate passage 95 comprising a depending loop or trap. Tank 36 may be vented by a passage 96 having restriction 97 therein to one of the absorbers to prevent the accumulation of noncondensible gases at the top of the tank. The passage 96 must be highly restricted so that no substantial amount of refrigerant vapor passes therethrough and may comprise a capillary tube instead of restrictor 97. In accordance with this invention, weak solution is passed directly through the heat exchanger 35 in the refrigerant storage tank 36 in heat exchange relation with liquid refrigerant therein. Consequently, the temperature and pressure of refrigerant in tank 36 is directly related to the temperature of the intermediate solution passing between low pressure absorber l4 and high pressure absorber 12. The condensing temperature and pressure in condenser 11 are also directly related to the heat rejected from low pressure absorber 14 because the air or other cooling medium passes in series from low pressure absorber 14 to refrigerant condenser 12. Accordingly, a quantity of liquid refrigerant will accumulate in tank 36 from liquid header 62 which is just sufficient to balance the condensing pressure with the pressure and liquid head in tank 36.

If noncondensible gases accumulate in absorber 14, less refrigerant vapor will be absorbed into the solution in the absorber. This will result in less heat of condensation and heat of dilution being present in the intermediate solution. Consequently, the heat rejected from the absorber will be reduced and the intermediate solution will be cooled to a lower than normal temperature by the air or other cooling medium used to cool the absorber. Under these conditions there is a possibility of solidification of the intermediate absorbent solution occurring unless the concentration of the solution is reduced. The temperature and pressure of refrigerant in tank 36 will also be reduced because it is in heat exchange relation with the cooler intermediate solution. At the same time, the condensing temperature of condenser 11 will drop because the lessened heat rejection from low pressure absorber 14 will result in cooler air passing over condenser 11. As the temperature in both tank 36 and condenser 11 drops, the difference in pressures therebetween will be reduced and less liquid head in tank 36 will be required to balance this new pressure difference. Consequently, refrigerant will flow out of tank 36 through condensate passage 95 into header 62 of the condenser and will be discharged through metering device 67 into refrigerant passage 68 until the lower pressure difference is balanced by the head of refrigerant in tank 36. The increased volume of refrigerant circulating in the system will eventually be reflected in a rise of the level of refrigerant in refrigerant sump 80. When the refrigerant in sump reaches the level of overflow passage 90, it will overflow into the weak solution passage 42 to dilute the solution in sump 50. Eventually, all of the absorbent solution in the system will be diluted by the refrigerant overflowing through passage 90, so that the concentration of the intermediate solution will be reduced sufficiently to prevent solidification occurring.

After the low pressure absorber is completely purged, the additional refrigerant vapor being absorbed therein will increase the temperature of the intermediate solution passing through tank 36 and the increased absorber heat rejection will increase the temperature of air passing over condenser 11. As the temperature of both the condenser and the refrigerant storage tank rise, the pressure difference between them will become greater and refrigerant will pass from header 62 to tank 36 until the increased pressure difference is balanced by the liquid head of refrigerant in the tank. Refrigerant storage tank 36 will then remain substantially full of liquid refrigerant until some other condition occurs requiring dilution.

The concentration control system described is also effective in the event of a sudden drop in ambient air temperature to prevent solidification of the absorbent solution. Under these conditions, both the temperature in absorber l4 and condenser 11 will drop which will result in the pressure difference between tank 36 and absorber 11 being reduced. The reduced pressure difference between tank 36 and condenser 11 will cause refrigerant to flow out of tank 36 through passage 95 and eventually through overflow passage into absorbent solution to prevent solidification under conditions of lower ambient temperatures than the system is normally designed to accommodate.

lt has been found in accordance with this invention that the direct passage of intermediate solution through refrigerant storage tank 36 results in extremely rapid response to changes in absorber conditions without the necessity of employing a large or costly amount of heat transfer surface in the tank. Intermediate solution pump 33 can create substantial intermediate solution velocities in heat exchanger 35 and, consequently, a relatively high heat transfer coefficient is obtained which allows a relatively small size heat exchanger to provide the desired speed of response.

It has also been discovered that the temperature of the intermediate solution is an especially accurate precursor of imminent solidification in a multiple stage system and is, therefore, more effective in controlling the dilution of the machine than other system temperatures.

While the preferred embodiment of this invention has been described, it will be appreciated that the invention may be otherwise embodied in the scope of the following claims.

lclaim:

1. An absorption refrigeration system comprising:

A. a generator for concentrating absorbent solution by heating the solution and vaporizing refrigerant therefrom;

B. a refrigerant condenser for condensing refrigerant vapor formed in the generator;

C. a low temperature evaporator for evaporating refrigerant at a relatively low temperature to produce a refrigeration effect;

D. a low pressure absorber for absorbing refrigerant vapor evaporated in the low temperature evaporator into absorbent solution;

B. a high temperature evaporator for evaporating refrigerant at a relatively higher temperature to produce a refrigeration efiect;

F. a high pressure absorber for absorbing refrigerant vapor evaporated in the high temperature evaporator into absorbent solution; and

G. means to serially pass a cooling medium in heat exchange relation with absorbent solution in said low pressure absorber and thence into heat exchange relation with refrigerant in said condenser;

wherein the improvement comprises:

H. concentration control means for varying the concentration of absorbent solution in the system, said concentration control means including:

1. a refrigerant storage tank for accumulating and discharging liquid refrigerant;

2. concentration control passage means in said system, including refrigerant condensate passage means directly connecting said refrigerant storage tank with said condenser for passing sufficient liquid refrigerant from the condenser to said refrigerant storage tank to automatically balance the pressure in said condenser with the pressure and liquid head of refrigerant in the storage tank, and overflow passage means for passing liquid refrigerant into absorbent solution for mixture therewith when refrigerant is discharged from said refrigerant storage tank; and

3. intermediate solution passage means for passing intermediate strength absorbent solution from said low pressure absorber in direct heat exchange relation with refrigerant in said refrigerant storage tank to said high pressure absorber, whereby the pressure in said refrigerant storage tank is a function of the temperature of said intermediate solution, and the quantity of refrigerant stored in said storage tank and the dilution of the absorbent solution automatically varies as a function of the difference between the condensing temperature and the intermediate solution temperature of said system.

2. An absorption refrigeration system as defined in claim I wherein said overflow passage means includes a refrigerant overflow passage extending from a refrigerant sump to a location in said system containing absorbent solution, and said refrigerant storage tank is located in said system so that discharge of refrigerant from said refrigerant storage tank causes the level of refrigerant in said refrigerant sump to rise and overflow through said refrigerant overflow passage into said location containing absorbent solution and to mix with the absorbent solution.

3. An absorption refrigeration system as defined in claim 1 including a restricted vent passage for venting relatively noncondensible gases from said refrigerant storage tank to a low pressure region in the system.

4. An absorption refrigeration system as defined in claim 1 wherein said refri erant condensate passage means comprises a depen mg loop connecting t e to concentrate it by vaporizing refrigerant therefrom; B. condensing in the refrigerant condenser refrigerant vapor formed in the generator",

C. evaporating liquid refrigerant in the high temperature evaporator and in the low temperature evaporator to produce refrigeration;

D. absorbing refrigerant vapor formed in the high pressure absorber, and absorbing refrigerant vapor formed in the low temperature evaporator into absorbent solution in the low pressure absorber;

E. passing a cooling medium serially into heat exchange relation with absorbent solution in the low pressure absorber and then into heat exchange relation with refrigerant vapor in said refrigerant condenser;

wherein the improvement comprises:

F. passing liquid refrigerant from the refrigerant condenser to the refrigerant storage tank in an amount sufiicient to balance the pressure difference between the refrigerant condenser and the storage tank;

G. varying the quantity of refrigerant stored in the refrigerant storage tank by passing intermediate strength absorbent solution from the low pressure absorber to the high pressure absorber in heat exchange relation with refrigerant in the storage tank to thereby vary the refrigerant pressure in the tank; and

H. mixing liquid refrigerant with absorbent solution to dilute the absorbent solution by overflowing liquid refrigerant from a location containing refrigerant into a location containing absorbent solution upon discharge of refrigerant liquid from the refrigerant storage tank due to a drop in the pressure difference between said refrigerant condenser and said refrigerant storage tank.

0 I3 I l

Claims

1. An absorption refrigeration system comprising: A. a generator for concentrating absorbent solution by heating the solution and vaporizing refrigerant therefrom; B. a refrigerant condenser for condensing refrigerant vapor formed in the generator; C. a low temperature evaporator for evaporating refrigerant at a relatively low temperature to produce a refrigeration effect; D. a low pressure absorber for absorbing refrigerant vapor evaporated in the low temperature evaporator into absorbent solution; E. a high temperature evaporator for evaporating refrigerant at a relatively higher temperature to produce a refrigeration effect; F. a high pressure absorber for absorbing refrigerant vapor evaporated in the high temperature evaporator into absorbent solution; and G. means to serially pass a cooling medium in heat exchange relation with absorbent solution in said low pressure absorber and thence into heat exchange relation with refrigerant in said condenser; wherein the improvement comprises: H. concentration control means for varying the concentration of absorbent solution in the system, said concentration control means including: 1. a refrigerant storage tank for accumulating and discharging liquid refrigerant; 2. concentration control passage means in said system, including refrigerant condensate passage means directly connecting said refrigerant storage tank with said condenser for passing sufficient liquid refrigerant from the condenser to said refrigerant storage tank to automatically balance the pressure in said condenser with the pressure and liquid head of refrigerant in the storage tank, and overflow passage means for passing liquid refrigerant into absorbent solution for mixture therewith when refrigerant is discharged from said refrigerant storage tank; and 3. intermediate solution passage means for passing intermediate strength absorbent solution from said low pressure absorber in direct heat exchange relation with refrigerant in said refrigerant storage tank to said high pressure absorber, whereby the pressure in said refrigerant storage tank is a function of the temperature of said intermediate solution, and the quantity of refrigerant stored in said storage tank and the dilution of the absorbent solution automatically varies as a function of the difference between the condensing temperature and the intermediate solution temperature of said system.

2. An absorption refrigeration system as defined in claim 1 wherein said overflow passage means includes a refrigerant overflow passage extending from a refrigerant sump to a location in said system containing absorbent solution, and said refrigerant storage tank is located in said system so that discharge of refrigerant from said refrigerant storage tank causes the level of refrigerant in said refrigerant sump to rise and overflow through said refrigerant overflow passage into said location containing absorbent solution and to mix with the absorbent solution.

4. An absorption refrigeration system as defined in claim 1 wherein said refrigerant condensate passage means comprises a depending loop connecting the liquid header of the condenser with the refrigerant storage tank.

5. A method of producing refrigeration and operating an absorption refrigeration system having a generator, a refrigerant condenser, a low temperature evaporator, a low pressure absorber, a high temperature evaporator, a high pressure absorber, and a refrigerant storage tank, which comprises: A. heating weak absorbent solution in the generator to concentrate it by vaporizing refrigerant therefrom; B. condensing in the refrigerant condenser refrigerant vapor formed in the generator; C. evaporating liquid refrigerant in the high temperature evaporator and in the low temperature evaporator to produce refrigeration; D. absorbing refrigerant vapor formed in the high pressure absorber, and absorbing refrigerant vapor formed in the low temperature evaporator into absorbent solution in the low pressure absorber; E. passing a cooling medium serially into heat exchange relation with absorbent solution in the low pressure absorber and then into heat exchange relation with refrigerant vapor in said refrigerant condenser; wherein the improvement comprises: F. passing liquid refrigerant from the refrigerant condenser to the refrigerant storage tank in an amount sufficient to balance the pressure difference between the refrigerant condenser and the storage tank; G. varying the quantity of refrigerant stored in the refrigerant storage tank by passing intermediate strength absorbent solution from the low pressure absorber to the high pressure absorber in heat exchange relation with refrigerant in the storage tank to thereby vary the refrigerant pressure in the tank; and H. mixing liquid refrigerant with absorbent solution to dilute the absorbent solution by overflowing liquid refrigerant from a location containing refrigerant into a location containing absorbent solution upon discharge of refrigerant liquid from the refrigerant storage tank due to a drop in the pressure difference between said refrigerant condenser and said refrigerant storage tank.