US3866430A - Method and apparatus for controlling refrigerant flow in cryogenic systems - Google Patents

Abstract

When the conventional sensor for actuating the pressureresponsive expansion valve in a cryogenic or very low temperature phase-change refrigeration circuit is charged with a limited vapor change of phase-change fluid having a boiling point significantly lower than the boiling point of the refrigerant used in the refrigeration circuit, a startlingly unexpected difference in kind in the sensitivity, efficiency and mode of operation of the circuit as a whole results and some wellrecognized deficiencies of such circuits are eliminated. This same principle is applicable to thermal expansion valves for controlling refrigerant flow in such systems as liquid nitrogen chillers. I presently believe that optimum results are achieved when the sensor system is charged with a limited vapor charge having a boiling point 26* F. to 30* F. lower than that of the refrigerant used in the circuit and when the limited vapor charge is 20 to 40 psi at ambient temperature, the quantity of said limited vapor charge being such as to preclude liquefaction thereof at temperatures to be encountered by said system.

Description

United States Patent Webber Feb. 18,1975

[76] Inventor: Robert C. Webber, 8634 Brookville Rd., Indianapolis, Ind. 46239 Filed. July 26, 1974 Appl. No: 492,158

Related US. Application Data [63] Continuation-in-part of Serv No. 425,181, Dec. [7.

I973, abandoned.

[52] US. Cl 62/115, 62/225, 62/514. 73/3682, 236/92 B, 236/99 [51] Int. Cl. F25b 41/04 [58] Field of Search 62/114, 115, 224, 225, 62/514; 73/3682; 236/92 B, 99 R [56] References Cited UNITED STATES PATENTS 2,231,163 2/l94l Johnson 236/92 B 3,091,]20 5/l963 Kounousky.,. 73/3682 328L075 lO/l966 Smyers, Jr 236/99 R 3,299,7l0 l/l967 Pauli et al 73/3682 3.367,l3() 2/l968 Owens i 236/99 R 3,797,266 3/1974 Newton 62/224 OTHER PU BLICATIONS Handbook of Automatic Refrigerant Controls" Alco Valve 0)., St. Louis, Mo., 1955, pp. 2537.

Refrigeration and Air Conditioning, W. F. Stoecker,

McGrawHill. 1958, pp. 146-147.

Ashrae Guide and Data Book," 1963, American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc., N.Y.. 1963, pp. 28], 282, 603, 604.

Primary E.rami/icrWilliam F. ODea Assistant ExaminerPeter D. Ferguson Attorney, Agent, or Firm-William R. Coffey [57] ABSTRACT When the conventional sensor for actuating the pressure-responsive expansion valve in a cryogenic or very low temperature phase-change refrigeration circuit is charged with a limited vapor change of phase-change fluid having a boiling point significantly lower than the boiling point of the refrigerant used in the refrigeration circuit, a startlingly unexpected difference in kind in the sensitivity, efficiency and mode of operation of the circuit as a whole results and some wellrecognized deficiencies of such circuits are eliminated. This same principle is applicable to thermal expansion valves for controlling refrigerant flow in such systems as liquid nitrogen chillers. I presently believe that optimum results are achieved when the sensor system is charged with a limited vapor charge having a boiling point 26 F. to 30 F. lower than that of the refrigerant used in the circuit and when the limited vapor charge is 20 to 40 psi at ambient temperature, the quantity of said limited vapor charge being such as to preclude liquefaction thereof at temperatures to be encountered by said system.

10 Claims, 1 Drawing Figure METHOD AND APPARATUS FOR CONTROLLING REFRIGERANT FLOW IN CRYOGENIC SYSTEMS This is a continuation-in-part application based upon my pending application Ser. No. 425,181 filed Dec. 17, 1973, and now abandoned.

The present invention relates to thermo-expansion valve controls for cryogenic refrigeration systems or very low temperature systems having capacities as low as 80 F. to 320 F. including mechanical phasechange refrigeration systems capable of temperatures as low as 250 F. and such systems as liquid nitrogen systems capable of temperatures as low as -320 F.

A conventional, single stage, phase-change refrigeration circuit includes a compressor, a condenser, an evaporator, and conduit means for leading the refrigerant from the compressor, through the condenser, to the evaporator and thence back to the compressor, an expansion valve being connected in the conduit means between the condenser and the evaporator to control the flow of liquid refrigerant to the evaporator. It is customary to use a pressure-responsive expansion valve dominated by a thermo-responsive, variable-pressure closed system including a bulb secured in heatexchange relation to that section of the conduit means which leads from the evaporator back to the compressor. It is universally customary to charge that closed system with the same kind of refrigerant which is used in the refrigeration circuit.

Reference is made to Handbook ofAuromatic Refrigerant Controls published by Alco Valve Co., St. Louis, Missouri, 1955, pages 25-37; Refrigeration and Air- Condilioning by W. F. Stoeker, McGraw-Hill, 1958, pages 146, 147; and ASHRAE Guide and Data Book 1963 published by the American Society of Heating, Refrigeration and Air-Conditioning Engineers, Inc., New York, 1963, pages 281, 282, 603, 604. Reference is made also to US. Pat. Nos. 2,231,163 issued Feb. 1941 to Johnson in Class 236, subclass 92 B and 3,091,120 issued May 1963 to Kounousky in Class 236, sub-class (fluid digest).

These above-mentioned prior art references discuss the use of thermo-expansion valves which respond to the temperature of the refrigerant gas leaving the evaporator and to the pressure in the evaporator. Three forces govern the valves operation. Those three forces are the pressure created in the valve by the remote bulb containing the refrigerant and which is attached to the refrigerant line leaving the evaporator, the evaporator pressure, and the equivalent pressure of the superheat spring which is contained in the valve. An increase in heat load on the evaporator increases the superheat of the refrigerant gas leaving the evaporator and this, in turn, heats the refrigerant within the sensing bulb of the expansion valve causing the expansion valve to move in an opening direction. A decrease in the heat load on the evaporator decreases the superheat of the refrigerant gas leaving the evaporator and causes the thermostatic expansion valve to move in a closing direction.

While much is known about such expansion valves and the refrigerant charges in the valves for relatively high temperature refrigeration systems, as evidenced by the above-identified references, very little is known about the use of such valves for extremely low temperatures, sometimes referred to as cryogenic temperatures, in the range of 80F. to 320 F. The aboveidentified references do discuss the use of refrigerants in the control expansion valve which are different from the refrigerants in the system being controlled, and they even discuss liquid cross charges. Such prior art references do not, however, even contemplate the extremely low temperatures which are the subject of the present invention, nor do they contemplate limited vapor charges which are the subject of the present invention. 1n fact, the Alco Valve Company handbook identified above, at pages 34 and 35, specifies the need for a drop of liquid in the remote bulb for proper control. This is in contrast to the present invention in which the refrigerant charge on the expansion valve bulb is a limited vapor charge applied at a pressure of 20 to 40 psi at ambient temperature, the quantity ofthe limited vapor charge being such as to preclude liquefaction thereof at temperatures to be encountered by the refrigeration system. In other words, in the systems of the present invention, the refrigerant vapor in the control valve could never be in liquid form.

I have noted that conventionally-charged expansion valves tend to be sluggish and that sometimes they will fail to close as early as they should so that liquid refrigerant floods back to the compressor with deleterious effects or simply fail altogether in very low temperature refrigeration systems.

The primary object of the present invention, then, is to render the control for expansion valves for cryogenic systems more sensitive and more certain to close in response to slight elevations of temperature in the effluent from the evaporator in such systems.

To the accomplishment of the above and related objects, my invention may be embodied in the form illustrated in the accompanying drawing, attention being called to the fact, however, that the drawing is illustrative only, and that change may be made in the specific construction illustrated and described, and/or in the nature and sequence of the steps set forth herein, so long as the scope of the appended claims is not violated.

The single FIGURE forming a part hereof is a diagrammatic illustration of a single stage, phase-change refrigeration system in which my invention may be used.

Referring more particularly to the drawing, it will be seen that l have illustrated a conventional compressor 10, a water-cooled condenser 11, an evaporator 12, and conduit means including a section 13 leading from the compressor 10 to the condenser 11, a section 14, 15 leading from the condenser 11 to an evaporator 12 and a section 16 leading from the evaporator back to the compressor 10. A pressure-responsive expansion valve 17 is formed to establish a chamber having a movable wall 18, and a capillary tube 19 connects the interior of that chamber with a bulb 20 which is secured in any suitable manner in heat-exchanging relation to the exterior of the conduit section 16 close to its connection to the evaporator 12.

A charge of phase-change fluid of suitable characteristics fills the system 10, ll, 12, 13, 14, 15, 17, 16. The compressor 10 delivers hot, gaseous fluid to the condenser 11 where the fluid is cooled and changed to the liquid phase and flows thence through the conduit sections 14 and 15 to the valve 17. When the valve is open, as it will be whenever the pressure in the chamber and impressed upon the movable wall 18 is above a predetermined value, liquid refrigerant will flow to the evaporator 12 where it will undergo a phase change.

The gas emanating from the evaporator 12 when the system is demanding cooling, will be quite warm and will hold up the temperature of the fluid in the bulb 20 so that pressure within the valve chamber will remain relatively high and the valve 17 will remain open. When the demands upon the evaporator are reduced so that the rate of heat absorption from the evaporator drops, the refrigerant emerging from the evaporator will be reduced in temperature. That temperature drop will be applied to the bulb 20 whereby the pressure of the fluid in the bulb 20, tubing 19 and chamber of the valve 17 will drop, ultimately to close the valve 17 until the demands upon the evaporator 12 again increase. T often, however, the bulb 20 will become so cold that all of the fluid in the sensing system will be liquefied with a resultant sharp drop of the pressure in the closed system whereby all control over the valve 17 is lost.

I have found that, if the fluid charged into the system 20, 19 is a phase-change fluid having a boiling point significantly below that of the refrigerant flowing in the refrigeration system, not only is the possibility of a complete phase change in the control system eliminated, but also the control system for the valve 17 is rendered more sensitive throughout its range of operation.

As is set forth in DuPonts Technical Bulletin RT48 LOW-TEMPERATURE REFRIGERANTS, widely used refrigerants include the following, listed with their chemical constitutions and boiling points:

the refrigeration circuit in combination with different refrigerants in the sensingsystem. For instance, when I add 1 1% of R22 refrigerant to R-l2 refrigerant and charge the thermo-sensing system with that mixture. then when a conventional expansion valve is fitted into the circuit under the domination of the closed system so charged, the expansion valve will perform and oper ate satisfactorily and very efficiently at F. below the boiling point of the said refrigerant R-l2.

Similarly, if I add 8% of R502 refrigerant to R22 refrigerant in the closed system and apply the system so charged to an R22 expansion valve, the circuit will operate satisfactorily in a circuit charged with R22 refrigerant, at F. below the boiling point of the R22 refrigerant.

Of course, still lower temperatures in the refrigeration system may be obtained by adjusting the thermostatic expansion valve spring, thus increasing the tension of that spring.

While the invention has been illustrated in a singlestage refrigeration circuit, it will be understood that it is equally applicable to a cascade system of two or more stages. For optimum results, the charge in the closed sensing system should consist of an azeotropic mixture.

1 have found that, using such an azeotropic charge, a conventional thermostatic expansion valve will oper- Boiling Point, F.

Refrigerant Chemical Constitution Refrigerant l2 CCl. ,F Refrigerant llS C ClF Refrigerant 22 CHClF CHF, Refrigerant 23/116 Refrigerant 170. Ethane 2 6 Refrigerant 503 Refrigerant 23/13 Refrigerant 1150, Ethylene C H Refrigerant l4 CF Refrigerant 50. Methane CH Tentative nomenclature assignment. All two component refrigerant combinations listed in the table are azeotropes,

In addition, oxygen has a boiling point of 297.4 F., nitrogen has a boiling point temperature of 320.6 F., hydrogen has a boiling point temperature of 422.9 F. and helium has a boiling point temperature of 450.4 F.

As stated above, it is universal practice to use an Rl 2 refrigerant in the sensing system and in the refrigeration circuit of an Rl2 refrigeration system. Similarly, R22 refrigerant will be used in both parts of an R22 system, R502 refrigerant will be used in the main circuit and in the closed control system of an R502 circuit, etc.

I have discovered that changing the refrigerant charge within the thermo-sensing element or bulb of a system of the character illustrated will radically improve the automatic operation of the refrigeration system and that flood-back of the refrigerant causing the compressor to malfunction, frost-back and wide fluctuate in refrigeration systems having temperature ranges from 350 F to 250 F. without damage to the thermostatic expansion valve.

Using an azeotropic charge of 87% Rl refrigerant and 13% Rl refrigerant, the thermostatic expansion valve will function satisfactorily in refrigeration systems having temperature ranges from 350 F. to 250 F.

It is noted that the solid components of the disclosed system do not differ from those of a conventional phase-change refrigeration system' But in such a system, the refrigerant charge or charges must be construed to be elements of the system, since the function of such a system (to absorb or to emit heat) is absolutely dependent upon the presence of such refrigerants and the presence of such refrigerants is absolutely essential to the accomplishment of a useful function by the recited elements in combination. Thus, the abovedescribed change of the refrigerant charge in the c'ontrol system is equivalent to a change in a mechanical element of a recited combination.

Although they have no bearing on the invention herein disclosed, I have diagrammatically indicated the conventional pressure-actuated valve 22 for controlling cooling water flow to the condenser 11, receiver 23, valve 24 with fusible plug 25, drier 26 and vibration dampers 27 customarily present in simple, phasechange refrigeration systems.

While the above-given examples are not necessarily for very low temperature refrigeration systems, they are examples of systems I have modified to obtain very satisfactory results by placing a refrigerant in the thermo-sensing system which has a boiling point significantly lower than the refrigerant in the refrigeration circuit. For very low temperatures or cryogenic temperatures, however, I have discovered that the charge in the thermo-sensing system must be first, a limited vapor charge, second, a refrigerant having a boiling point to 30 F. below the boiling point of the refrigerant in the circuit of the system being controlled, and third, the limited vapor charge must be between 20 to 40 psi at ambient temperature. The quantity of the lim ited vapor charge must be such as to preclude liquefaction thereof at temperatures to be encountered by the system.

For instance, Ethane has a boiling point of l26.9 F. while Ethylene has a boiling point of l55.0 F. I may build a satisfactory system in accordance with the present invention using Ethane in the refrigeration circuit and using a limited vapor charge of Ethylene in the control loop or in the bulb and housing of the expansion valve at a pressure of approximately 30 psi at ambient temperature. The 30 psi pressure at ambient temperature would still provide 17 lbs. pressure at l40 F. such that a 15 psi valve would still be open. Such valves have adjustment screws and consequently, the Ethylene may be charged into the control loop at a pressure of 30 psi i 10 psi at ambient. Importantly, the Ethylene is charged into the bulb and housing of the thermo-expansion valve in a limited vapor charge.

In order to provide the refrigerant in the control loop with a preferable boiling point 26 to 30 F. lower than or cooler than the boiling point of the refrigerant in the circuit of the basic system, i.e., flowing through the evaporator, I may provide a cross charge of vapor. That is, I may combine, for instance, the vapors of two refrigerants to reduce the boiling point of the vapor charge in the bulb and housing of the thermoexpansion valve. Since Ethylene has a boiling point of l55.0 F. or 28 F. lower than the boiling point of Ethane, I do not have to mix any other lower boiling point refrigerant with the Ethylene unless, for some reason, I want the boiling point difference to be something greater than 28 F.

I can take standard thermostatic expansion valves, manufactured by companies such as Alco, Sporlan, Singer, Automatic Controls, Detroit (American Standard), Parker-Hannifin and others, and charge the sensing bulbs or elements and bellows of such valves with a limited vapor charge of a special mixture of refrigerants to cause the valve to modulate the refrigeration capacity of the system and not overload its compressor (when starting at ambient) and maintain this condition until the temperature within the system has been lowered to, for instance, a temperature as low as F. This is accomplished by the special refrigerant vapor mixture in the sensing element and bellows of the valve and the amount of pressure with which the sensing element and baffle are charged. When the refrigeration system so modified is first energized or put into operation, the suction pressure will not exceed 25 psi, thus preventing overloading the compressor.

One example of my present invention, therefore, may be a cryogenic refrigeration circuit charged with Ethane having a boiling point ofl 27.5 F. with the baffle and sensor of the thermo-expansion valve charged with a limited vapor charge of Ethylene having a boiling point ofl55.0 F., the vapor charge being applied at a pressure of 20 to 40 psi at ambient temperature and with the quantity of the limited vapor charge being such as to preclude liquetication thereof at temperatures to be encountered by the system. The boiling point difference of such a system,.which would be quite satisfactory, would be 225 F.

I presently believe that the preferred boiling point difference should be somewhere in the range of 26 F. to 30 F., i.e., that the boiling point of the refrigerant vapor in the sensor and baffle of the thermo-expansion valve should be 26 to 30 F. lower than the boiling point of the refrigerant flowing through the evaporator.

While, to this point, I have discussed refrigeration systems including compressors and evaporators, it will be appreciated that thermo-expansion valves are also applied to refrigeration systems of the type in which a refrigerant, such as liquid nitrogen, is released to flow through evaporator coils. In such systems, the expansion valve is disposed between the source of the refrigerant and the evaporator to control the flow of the refrigerant through the evaporator. For such a liquid nitrogen system, I may charge the sensor and bellows of the control valve with a limited vapor charge of nitrogen, which has a boiling point of 320.6 F. and hydrogen, which has a boiling point of422.9 F., so that the vapor charge will have a boiling point 26 to 30 F. lower than the boiling point of nitrogen. Just as I do for mechanical refrigeration systems, I limit the quantity of the limited vapor charge to preclude liquefaction, and I provide the charge at a pressure of 20 to 40 psi at ambient temperature.

I claim:

1. A method of controlling refrigerant flow in a cryogenic refrigeration circuit including a compressor, a condensor, an evaporator, conduit means connected to lead refrigerant from the compressor to the condensor, thence to the evaporator, and thence back to the compressor, and a charge of phase-change refrigerant in said circuit, which includes the steps of providing a pressure-responsive valve to control refrigerant flow through said conduit means, providing a thermoresponsive variable-pressure system operatively con nected to actuate said valve, said system including a bulb arranged in heat-exchanging relation with a remote point in said conduit means, and charging said system with a limited vapor charge of a refrigerant having a boiling point significantly lower than the boiling point of the refrigerant in the said refrigeration circuit, the limited vapor charge being 20 to 40 psi at ambient temperature, and the quantity of said liquid vapor charge being such as to preclude liquefaction thereof at temperatures to be encountered by said system.

2. The method of claim 1 in which the boiling point of the refrigerant vapor used to charge the variablepressure system is approximately 20 to 30 F. lower than the boiling point of the refrigerant used in said circult.

3. The method of claim 2 in which the refrigerant used to charge the variable-pressure system has a boiling point 26 F. to 30 F. lower than the boiling point of the refrigerant used in said circuit.

4. The method of claim 3 in which the limited vapor charge is applied at approximately 30 psi at ambient temperature.

5. A cryogenic refrigeration circuit including a compressor, a condenser, an expansion valve having a movable valve head, an evaporator. conduit means for conducting refrigerant from said compressor to said condenser, thence to said expansion valve, thence to said evaporator and thence back to said compressor, thus establishing a circuit, a charge of phasechange refrigerant in said circuit, and means for controlling refrigerant flow through said valve comprising a closed control system including a chamber in said valve having a movable wall operatively associated with said valve head, a bulb held in heat-exchanging association with a section of said conduit means through which refrigerant flows from said evaporator back to said compressor, a capillary tube providing open communication between said chamber and said bulb, and in which the improvement comprises, in said closed system, a limited vapor charge of phase-change fluid having a boiling point significantly lower than the boiling point of the phase-change refrigerant in said refrigeration circuit, the quantity of said limited vapor charge being such as to preclude liquefaction thereof at temperatures to be encountered by said system, said limited vapor charge having a pressure of 20 to 40 psi at ambient temperature.

6. The invention of claim Sin which the limited vapor charge has a boiling point approximately 20 to 30 F. lower than the boiling point of the refrigerant in said circuit.

7. The invention of claim 6 in which the limited vapor charge has a boiling point approximately 26 to 30 F. lower than the boiling point of the refrigerant in said circuit.

8. A method for controlling flow of refrigerant through an evaporator of a cryogenic cooler including the steps of providing a pressure-responsive valve to control refrigerant flow through said evaporator, providing a thermo-responsive variable-pressure system operatively connected to actuate said valve, said system including a bulb arranged in heat-exchanging relation with the exhaust side of said evaporator, and charging said system with a limited vapor charge of a refrigerant having a boiling point significantly lower than the boiling point of the refrigerant flowing through said evaporator, the limited vapor charge being 20 to 40 psi at ambient temperature, and the quantity of said liquid vapor charge being such as to preclude liquefaction thereof at temperatures to be encountered by said system.

9. The method of claim 8 in which the boiling point of the refrigerant vapor used to charge the variablepressure system is approximately 20 to 30 F. lower than the boiling point of the refrigerant flowing through the evaporator.

10. The method of claim 9 in which the limited vapor charge is applied at approximately 30 psi at ambient temperature.

{ENZYME STATES PATENT OFFICE (IERTiFHJATE OF CORRECTION FAR 40 3,866,430

DATE; February 18 1975 Ei iV'Etfl 'JRifl Robert C. Webber 3? is certified m em. appea s n the above-identified patent and that said Letters Patent a? hen .1;, tazz'ecefl as shown b low In the Abstract, line 4, "vapor change" should be vapor charge Column 6, line 10, "-l27.5 F." should be -l26.9 F. 1 line 16, "liquefaction" is misspelled; line 19, "22.5 F."

should be 28.l F. line 27, after "compressors" insert condensers line 48, (claim 1, line 3) "condenser" is misspelled; line 49 (claim 1, line 4) "condenser" is misspelled.

Signed and sealed this 27th day of May 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C I- ASON Commissioner of Patents Attesting Officer and Trademarks

Claims (10)

1. A method of controlling refrigerant flow in a cryogenic refrigeration circuit including a compressor, a condensor, an evaporator, conduit means connected to lead refrigerant from the compressor to the condensor, thence to the evaporator, and thence back to the compressor, and a charge of phase-change refrigerant in said circuit, which includes the steps of providing a pressure-responsive valve to control refrigerant flow through said conduit means, providing a thermo-responsive variablepressure system operatively connected to actuate said valve, said system including a bulb arranged in heat-exchanging relation with a remote point in said conduit means, and charging said system with a limited vapor charge of a refrigerant having a boiling point significantly lower than the boiling point of the refrigerant in the said refrigeration circuit, the limited vapor charge being 20 to 40 psi at ambient temperature, and the quantity of said liquid vapor charge being such as to preclude liquefaction thereof at temperatures to be encountered by said system.

2. The method of claim 1 in which the boiling point of the refrigerant vapor used to charge the variable-pressure system is approximately 20* to 30* F. lower than the boiling point of the refrigerant used in said circuit.

3. The method of claim 2 in which the refrigerant used to charge the variable-pressure system has a boiling point 26* F. to 30* F. lower than the boiling point of the refrigerant used in said circuit.

4. The method of claim 3 in which the limited vapor charge is applied at approximately 30 psi at ambient temperature.

5. A cryogenic refrigeration circuit including a compressor, a condenser, an expansion valve having a movable valve head, an evaporator, conduit means for conducting refrigerant from said compressor to said condenser, thence to said expansion valve, thence To said evaporator and thence back to said compressor, thus establishing a circuit, a charge of phasechange refrigerant in said circuit, and means for controlling refrigerant flow through said valve comprising a closed control system including a chamber in said valve having a movable wall operatively associated with said valve head, a bulb held in heat-exchanging association with a section of said conduit means through which refrigerant flows from said evaporator back to said compressor, a capillary tube providing open communication between said chamber and said bulb, and in which the improvement comprises, in said closed system, a limited vapor charge of phase-change fluid having a boiling point significantly lower than the boiling point of the phase-change refrigerant in said refrigeration circuit, the quantity of said limited vapor charge being such as to preclude liquefaction thereof at temperatures to be encountered by said system, said limited vapor charge having a pressure of 20 to 40 psi at ambient temperature.

6. The invention of claim 5 in which the limited vapor charge has a boiling point approximately 20* to 30* F. lower than the boiling point of the refrigerant in said circuit.

7. The invention of claim 6 in which the limited vapor charge has a boiling point approximately 26* to 30* F. lower than the boiling point of the refrigerant in said circuit.

8. A method for controlling flow of refrigerant through an evaporator of a cryogenic cooler including the steps of providing a pressure-responsive valve to control refrigerant flow through said evaporator, providing a thermo-responsive variable-pressure system operatively connected to actuate said valve, said system including a bulb arranged in heat-exchanging relation with the exhaust side of said evaporator, and charging said system with a limited vapor charge of a refrigerant having a boiling point significantly lower than the boiling point of the refrigerant flowing through said evaporator, the limited vapor charge being 20 to 40 psi at ambient temperature, and the quantity of said liquid vapor charge being such as to preclude liquefaction thereof at temperatures to be encountered by said system.

9. The method of claim 8 in which the boiling point of the refrigerant vapor used to charge the variable-pressure system is approximately 20* to 30* F. lower than the boiling point of the refrigerant flowing through the evaporator.

10. The method of claim 9 in which the limited vapor charge is applied at approximately 30 psi at ambient temperature.

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