US2106591A

US2106591A - Refrigerating system

Info

Publication number: US2106591A
Application number: US7210A
Authority: US
Inventors: Harold F Briggeman; Roger A Fuller
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1935-02-19
Filing date: 1935-02-19
Publication date: 1938-01-25
Anticipated expiration: 1955-01-25

Description

Jan. 25, 1938. H. F. BRIGGEMAN ET AL 2,105,591

REFRIGERATING SYWSTEM Filed Feb. 19, 1935 WO TEMP. VALVE.

lrwerwtos: Harold T- Bruggeman, Dger AFUHGT] Attorrwe y Patented `an. 25, 1938 REFRIGERATING SYSTEM Harold F. Briggeman and Roger A. Fuller, Fort Wayne, Ind., assignors to General Electric Company, a corporation of New York Application February 19,1935, serial No. 7,210

13 Claims.

Ourinvention relates to refrigerating systems of the type having two or more evaporators supplied with a refrigerant from a single source.

It is frequently desirable to cool a. number of rooms, compartments or the like, of varying sizes and further to maintain each of them at a dierent average temperature, or to maintain rooms of various sizes at the same average temperature. For example, in a meat market, it may be desirable to provide apparatus for cool- 'ing a large meat storage room located in the rear of the store and also apparatus to cool a relatively small display case located in the front part of the store. Separate evaporators of proper capacity maybe provided for the storage room and for the display "case, the temperature of each evaporator being regulated according to the requirements of the compartment which it cools. If all of the evaporators are supplied with a refrigerant from a common compressor .and condenser unit or other sourceof supply, the cost of the apparatus as Well as the cost of its operation will be minimized.

It is an object of our invention to provide an improved refrigerating system of the type having two or more evaporators supplied with a refrigerant.from a single source, the system being adapted to maintain said evaporators at different average temperatures.

It is a further object of our invention to provide a refrigerating system of the type having two or more evaporators of different capacities supplied with a refrigerant from a single source, the system being adapted' to maintain said evaporators at either different or the same average temperatures.

Further objects and advantages of our invention will become apparent as the following description proceeds and the-features of novelty which characterize our invention will be'pointed out with particularity in the claims annexed to and forming Aa part of this specification.

For a better understanding of our invention, reference may be had to the accompanying drawing in which Fig. l is a schematic representation 'of a refrigerating system embodying our invention and Fig. 2 is a schematic representation of a modied control arrangement for one of the evaporators included in the system shown in Fig. 1.

Referring to the drawing, we have shown 'in 'y Fig. 1 a refrigerating system embodying our invention. This refrigerating system is provided with a refrigerant liquefying unit including a compressor I of the reciprocating type driven byan electric motor I I through a belt I2 which is connected to a pulley I3 on the motorII and a fly-wheel I4 on the crank shaft of the compressor IIl. 'I'he refrigerant liquefying unit also includes a water-cooled condenser I5, to which the compressed gaseous refrigerant ,passes through a conduit I6 from the compressor Il). Cooling water enters the casing of the condenser I through the inlet |5a and leaves through the outlet I5b as indicated by the arrows in the drawing. The gaseous refrigerant is liquefied in thecondenser I5 and the liquid refrigerant passes therefrom through a refrigerant supply conduit Il. The refrigerant supply conduit I'I is provided with branches I8 and I9 through which the liquid refrigerant is supplied to the evaporators and 2i. In this form of our invention, the evaporator 20 is a forced draft type evaporator of relatively large capacity, being provided with a motor driven fan 2Ilafor circulating air over the surfaces thereof. The evaporator 20 is arranged to be maintained at a relatively high temperature as compared to the evaporator 2l. The evaporator 20 may be used to cool a storage chamber, for example, and the smaller evaporator 2I may be used to cool a display case. The evaporator 2| is of the fin type, air being circulated over its surfaces by natural convection.

An automatic thermostatic expansion valve 22 is provided in the inlet of the evaporator 2li and controls the admission of liquid refrigerant thereto by throttling the same. The automatic thermostatic expansion valve 22 is of the conventional type, and includes a bulb 23 connected thereto by conduit 24, the bulb 23 containing a uid such as sulphur dioxide, which is cooled by the portion of. the evaporator 20 adjacent the outlet thereof. A bellows or similar pressure responsive member is included in the thermostatic expansion valve 22, and is subjected to the pressure of the refrigerant in the evaporator 20. When the pressure decreases the bellows lor the like opens the valve to admit refrigerant to the evaporator 20, and as the pressure increases the bellows or the like gradually closes valve 22, thus cutting off the supply of liquid refrigerant to the evaporator 20. The thermostatic expansion valve 22 is so arranged that on a rise in temperature at the outlet of the evaporator 20 the expansion of the fluid in the bulb 23 causes the valve 22 to open and conversely on a drop in this temperature the fluid contractsand causes the valve to close. Since for every refrigerant vapor pressure in the evaporator there is a corresponding temperature, it will be seen that the valve 22 is controlled by the difference between the temperature in the evaporator 20 and the ternperature of the bulb 23, and in consequence maintains this diiference at a constant value. Maintaining this difference in temperature really amounts to controlling the superheat of the gas at the outlet of the evaporator, that is, the warming of the refrigerant Vapor above the temperature at which it is vaporized. "It is necessary to maintain this superheat constant in order that no liquid refrigerant will pass to the compressor l0, so as to protect the latter and to maintain eicient operation of the system.

A two-temperature valve 25 of the conventional pressure operated snap-acting type is provided at the outlet of the evaporator 20., The valve 25 is arranged to close the outlet of the evaporator 20 when the pressure therein reaches a predetermined low value and to open again when the pressure in the evaporator 20 reaches a predetermined higher value. The two-temperature valve 25 thus serves to maintain the evaporator 20 within the limits of the above mentioned predetermined temperatures. Vaporized refrigerant passing through the two-temperature valve 25 enters a suction conduit 26, passes therethrough to the main suction conduit 21 through which it -returns to the intake of the compressor l0. It

will thus be seen by the use of the two-temperature valve 25, we have provided an arrangement by which the evaporator 20 may be maintained at a relatively high temperature, while at the same time -all of the cooling surfaces of the evaporator 20 may be utilized. It is thus unnecessary to starve the evaporator in order to maintain it at arelatively high temperature, that is, it is unnecessary under conditions of light load on the evaporator 20 to limit the refrigerant supplied thereto to such an amount that only a part of the surface of this evaporator is utilized for cooling.

In the preferred form of our invention, in which the evaporator 20 is used to cool a storage room or the like, the two-temperature valve 25 is set to open at a temperature above 32 F. and to close at a temperature below 32 F. The pressure settings might be 12 lbs. per square inch and 0 lbs. per square inch gauge pressure, for example, if SO2 is used as the refrigerant. The evaporator .20 is thus operated alternately above and below 32, that is, on what is known as a defrosting cycle, Such operation isadvantageous in that the relative humidity of the air in the room or space cooled by the evaporator 20 is increased and the dehydration of articles preserved therein is decreased.

The low temperature evaporator 2| is provided with an automatic thermostatic expansion valve 28 at the inlet thereof, which is similar to the automatic thermostatic expansion valve 22 described above. The thermostatic expansion valve 28 includes a bulb 29 connected thereto by a. conduit 30, the fluid in the bulb and conduits 29 and 3|) being cooled by a portion of the evaporator 2| adjacent the outlet thereof so as to maintain substantially constant the superheat of the vaporized refrigerant withdrawn from the evaporator. In order to maintain the illustrative conditions high and low temperatures of the evaporators 20 and 2| noted above, it is necessary to provide some control arrangement for maintaining the temper-ature of the evaporator 2| at a relatively lower value than the temperature of the evaporator 20. If a two-temperature valve, similar to the valve 25, were placed in the outlet of the evaporator 2|,

however, diculties under certain conditions of operation would arise. For example, if the evaporator 20 were subjected to a sudden heavy load as by the placing of a large quantity of material to be cooled in proximity thereto, the pressure in the evaporator 20 would remain for a long period of time at a value too high to cause closing of the Valve 25, and the compressor I0 would continue to operate during all this period. At the same time, if such a two-temperature valve were arranged at the outlet of the evaporator 2| the valve would remain open, as it would be set for a lower value of pressure for closing. During a long period of the time mentioned, the evaporator 2| would be subjected to a low suction pressure and the compartment cooled by the evaporator 2| would thus be subjected to sustained -low temperature for too long aperiocl. In order to overcome this diiculty, we have provided a valve 3| located in the supply conduit of the evaporator 2| in series with the thermostatic expansion valve 23. The valve 3| is of the shut-oil` type, that is, it moves from the fully open to the fully closed position as distinguished from the gradual opening of' the valve 28, for example, with its consequent throttling action. The valve 3| is opened by a solenoid l32 which is energized by current supplied through the conductor 33. The electric circuit through the conductor 33 is opened and closed by a thermostatic operating mechanism 34 which includes a fluid containing bulb 35 connected thereto by a conduit 36. The bulb 35 is responsive to the temperature of the evaporator 2| and in the illustrative form of ourinvention is located on the surface thereof adjacent its center. When the evaporator 2| reaches a predetermined high temperature, the fluid in the bulb 35 and the conduit 36 expands, as it is warmed by the evaporator 2|, and actuates a bar 31a to close contacts 31 of the thermostatic switching device 34, thus completing the operating circuit of the solenoid 32. When the s'olenoid 32 is thus energized, it opens the valve 3| and admits the liquid refrigerant to the evaporator 2|. This liquid refrigerant vaporizes inthe evaporator 2| and when the latter, as a consequence, reaches a. predetermined low temperature, the contraction of the iiuid in the bulb 35 and conduit 36 causes the contacts 31 to open, thus de-energizing the solenoid 32. Upon a deenergization of the solenoid 32, the valve 3| is closed by a spring 32a and the supply of liquid refrigerant to the evaporator 2| is stopped. y After the. vaporization of the liquid refrigerant remaining in the evaporator l2|, the temperature of the latter will rise until it again reaches the predetermined temperature at which the contacts 31 are closed and the cycle repeated. In the illustrative form of our invention the valve 3| might be setto open and close at 36 F. and 22 F. respectively. When the evaporators 20 and 2| are subjected to stable load conditions, the solenoid operated valve 3| functions primarily as a protective device rather than as a control device.

The refrigerant vaporized in the evaporator 2| passes therefrom through a suction conduit 38 to the common suction conduit 21 through which it returns to the intake of the compressor I0. A'

check valve 39 is provided in the suction conduit 38 in order to prevent vaporized refrigerant from the evaporator 20 entering the evaporator 2| when the latter has been exhausted to a low pressure. i

A back pressure control device 40 has been provided in the' suction conduit 21 for controlling the motor Il. The back pressure control device 43 includes a diaphragm 4| which is subjected to the pressure existing in the suction conduit 21 through a pressure connection 42 and is biased by a compression spring 43. When the pressure in the suction conduit 21 reaches a predetermined high value, the 4diaphragm 4| is moved upwardly against the bias of the spring 43, thus moving the contact 44 into engagement with the stationary contact 45 and completing the supply circuit of the electric motor through the conductors 46. The electric motor is thus started and it drives the compressor I3 until the pressure in the suction conduit 21 reaches a predetermined low value in consequence of which the diaphragm 4| moves downwardly carrying the movable contact 44 therewith, thus opening the elecpressure control device 43 more nearly in accord-A ance with the requirementsof the colder evaporator 2|, since if the compressor |3 reduces the pressure in the suction line 21 below the point for which valve 25 is set the latter will close while the refrigeration of the evaporator 2| may continue. If the valve 3| is arranged to open and close at 36 F. and 22 F.; respectively, the control device 43 may be set to open and close at l0' lb. pressure and 6 in. vacuum respectively.

It will be understood that the various valve settings may be altered in order to provide for `desired temperatures of the evaporators as required ,in various applications. The

valves

22, 25, 23 and 3| may be set to maintain the evaporators 23 and 2| within the same temperature range despite their different capacities if such operation is desired.

In Fig. 2 of the drawing, we have shown a modified control arrangement for an evaporator 53, which corresponds to the evaporator 2| in the system shown in Fig. 1. The evaporator 53 is of the iin type and is of relatively vsmall capacity as compared'to the evaporator 23. Liquid refrigerant is supplied to the evaporator 53 through a conduit 5|, the admission of liquid refrigerant to the evaporator 53 being controlled by a thermostatic expansion valve 52. A'I'he valve 52 is similar to the valve 23 described above and is provided with a bulb 53 connected thereto by a conduit 54, the bulb 53 being located at the outlet of the evaporator 53, so as to maintain substantially constant the superheat of vapor withdrawn from this evaporator. AA shut on valve 55 is provided in a suction line 53 in order 'to control the ow of refrigerant through the evaporator 53, and thus maintain the same within a predetermined range of relatively low temperatures.` The valve 55 is opened by a solenoid 51, which is energized by current supplied through a conductor 53. The electric circuit through the conductor 53 is opened and closed by a thermostatic operating mechanism 53, which includes a duid containing bulb 33 connected thereto by a conduit 6|. 'I'he bulb 53 is responsive to the temperature of the evaporator 53 and in the form of our invention illus'- trated is located on the surface thereof adjacent its center. When the evaporator 53 reaches a predetermined high temperature, the uid in the bulb 33 and the conduit' 3| expands as it is warmed by the evaporator 53 and closes contacts 32 of the thermostatic switching device 53 by a bar 53, thus completing the operatingcircuit of the solenoid 51. When the solenoid'51 isthus energized, it opens the valve 55 against the action of a spring 55a, thus permitting the flow of refrigerant through the evaporator 53 to the suction line 53. 'I'he remainder of the refrigerating system for use with 'the modified control arrangement shown in Fig. 2 is the same as that shown in Fig. 1.

While we have shown a particular embodiment of our invention in connection with a compression refrigerating machine, we do not desire our invention to be limited to the particular construction shown and described, and we intend in the appended claims to cover all modifications within the spirit and scope of our invention. v

What we claim as lnew and desire to secure by Letters Patent ofthe United States is: i.

l. A refrigerating system having a plurality of refrigerant evaporators, each of said evaporators having a refrigerant outlet, means including a refrigerant liquefying unit yfor supplying liquid refrigerant to saidevaporators, conduits connecting the outlets of said evaporators to said liquefying unit in parallel relationship, control means responsive to the refrigerant pressure prevailing at the outlet of one of said evaporators and including a valve at the outlet of said one of said evaporators for maintaining the temperature thereof within a predetermined range of relatively high tem'peratures, means including a solenoid operated shut oi valve located in the conduit connecting. the outlet of another of .said

Aevaporators to said refrigerant liquefying unit for shutting off the flow of refrigerant therethrough,

and means for controlling said solenoid to control said shut o valve in accordance 'with the temperature produced by said last mentioned evaporator and for maintaining the temperature thereof within a predetermined range of relatively low temperatures, said second named control means being independent of said first named control means.

2. A refrigerating system having a plurality of` refrigerant evaporators, each of said evaporators having a refrigerant inlet and outlet, means including a refrigerant liquefying unit for supplying liquid refrigerant to said evaporators, supply and exhaust conduits connecting the inlets and outlets of said evaporators to said liquefying unit in parallel relationship, means including a thermostatic expansion valve responsive to the temperature at the outlet of one of said evaporators and located at the inlet thereof for throttling the flow Vof refrigerant therethrough, control means renoid operated stop valve located in the exhaustconduit of the .last mentioned evaporator forl shutting oil' the ow of refrigerant therethrough,

and means for controlling said solenoid to control said shut oil valve in accordance with the temperature produced by said la'st mentioned evaporator and for maintaining the temperature thereof within a predetermined range of relatively low temperatures, said second named control means being independent of said i'lrst named control means.

3. A refrigerating system having a plurality of refrigerant evaporators, each of said evaporators having a refrigerant inlet and outlet, means including a compressor and supply conduits for supplying liquid refrigerant to said evaporators, a common suction conduit connecting the outlets of said evaporators to said compressor in parallel relationship, means responsive to the pressure in said common suction conduit for starting said compressor at a. predetermined maximum pressure and stopping said compressor at a predetermined minimum pressure, means including a thermostatic expansion valve responsive to the temperature at the outlet of one of said evaporators and located at the inlet thereof for throttling the flow of refrigerant therethrough, control means responsive to the refrigerant pressure prevailing at the outlet of one of said evaporators and including a two-temperature stop valve at the outlet of said one evaporator for maintaining the temperature thereof within a predetermined range of relatively high temperatures, means including a second thermostatic expansion valve responsive to the temperature at the outlet of another of said evaporators and located at the inlet thereof for throttling the iiow of refrigerant therethrough, means including a solenoid operated shut off valve located in a supply conduit of the last mentioned evaporator for shutting off the flow of refrigerant through said last mentioned evaporator, and means for controlling said solenoid to control said shut oi valve in accordance with the temperature produced by said last mentioned evaporator and for maintaining the temperature thereof within a predetermined range of relatively low temperatures, said second named control means being independent of said first named control means.

' 4. In a refrigerating system having Va plurality of refrigerant evaporators, one of said evaporators having a relatively large capacity and another of said evaporators having a relatively small capacity, each of, said evaporators having a refrigerant inlet and outlet, means including a refrigerant liquefying unit for supplying liquid refrigerant to said evaporators, conduits connecting the outlets of said evaporators to said liquefying until in parallel relationship, means including an automatic expansion valve located at the inlet of said evaporator of relatively large -said shut of! valve in accordance with the temcapacity for throttling the flow of refrigerant therethrough, control means responsive to the refrigerant pressure prevailing at the outlet of said evaporator of relatively large capacity and including a two-temperature stop valve located at the outlet of said evaporator of relatively large capacity for maintaining, the temperature thereof within a predetermined range of temperatures, means including a second automatic expansion valve located at the inlet of said evaporator of relatively small capacity for throttling the flow of refrigerant therethrough, means including a solenoid operated shutoff valve for` shutting of! the flow of refrigerant through said evaporator of relatively small capacity, and means for controlling said solenoid, to control perature produced by the last mentioned evaporator and for maintaining the temperature thereof within said predetermined range of temperatures, said second namedl control means being independent of said rst named control means.

5. A refrigerating system having a plurality of refrigerant evaporators, each of said evaporators having a refrigerant outlet, means including a refrigerant liquefying unitfor supplying liquid refrigerant to said evaporators, conduits connecting the outlets lof said evaporators to said liquefying unit in parallel relationship, control means responsive to the refrigerant pressure prevailing at the outlet of 'one of said evaporators and including a valve at the outlet of said one of said evaporators for maintaining the temperature thereof Within a predetermined range of relatively high temperatures, means including an expansion valve located at the inlet of another of said evaporators for throttling the ow of refrigerant therethrough, and a second control means shutting oi the iiow of refrigerant through said last mentioned evaporator and dependent upon the temperature produced by said last mentioned evaporator for maintaining the temperature thereof Within a predetermined range of relatively low temperatures, said second control means being independent of said rst named control means.

6. A refrigerating system having a plurality of refrigerant evaporators, each of said evaporators having a refrigerant outlet, means including `a refrigerant liquefying unit for supplying refrigerant to said evaporators, conduits connecting the outlets of said evaporators to said liquefying unit in parallel relationship, control means responsive to the refrigerant pressure prevailing at'the outlet of one of said evaporators and including a valve at the outlet of. said one of said evaporators for maintaining the temperature'thereof within a predetermined range of relatively high temperatures, means including an expansion valve located at the inlet of another of said evaporators for throttling the flow of refrigerant therethrough, means including a solenord operated shut off valve for shutting off the flow of refrigerant'through said last mentioned evaporator, and means for controlling said solenoid to control said shut olf valve in accordance vvith the temperature produced by said last mentioned evaporator and for maintaining the temperature thereof within a predetermined range of relatively loW temperatures, saidsecondnamed 4control means being independent of said first named control means.

7. A refrigerating system having a plurality ofrefrigerant evaporators, each of said evaporators having a refrigerant outlet and a refrigerant inlet, means including a refrigerant liquefying unit for supplying liquid refrigerant to said evaporators, exhaust and supply conduits connecting the outlets and inlets of said evaporators to said liquefying unit in parallel relationship, control means responsive tothe refrigerant pressure prevailing at the outlet of one of said evaporators and including a valve at the outlet of said one of said evaporators for maintaining the temperature thereof within a predetermined range of relatively high temperatures, means including an expansion valve located at the inlet of another of said evaporators for throttling the flow of refrigerant therethrough, means including asolenoid ,operated shut off valve located in the supply conduit of said last mentionedevaporator for shutting off the flow of liquid refrigerant therethrough, and means for controlling said solenoid to control said shut oif valve in accordance with the temperature produced by said last mentioned evaporator and for maintaining the temperature thereof within a predetermined range of relatively low temperatures, said second named control means being independent of said ilrst named control means. y

8. A refrigerating system having a plurality o'f refrigerant evaporators, each of said evaporators having a refrigerant outlet, means including a refrigerant liquefying unit for supplying liquid to said evaporators, conduits connecting the outlets of said evaporators to said liquefying unit in parallel relationship, control means responsive to the refrigerant pressure prevailing at the outlet of one of said evaporators and including a two-temperature stop valve located at the outlet of said one of said evaporators for maintaining the temperature thereof within a predetermined range of relatively high temperatures, means including an expansion valve located at the inlet of another of said evaporators for throttling the flow of refrigerant therethrough, and a seicond control means shutting o the flow of re-A frigerant through said last mentioned evaporator and dependent upon the temperature of said last mentioned evaporator for maintaining the temperature thereof within a predetermined range of relatively low temperatures, said second control means being independent of said rst named control means.

9. YA refrigerating system having a plurality of refrigerant evaporators, each of said evaporators having a refrigerant outlet, means including a compressor for supplying liquid refrigerant to said evaporators, a common suction conduit connecting the outlets of said evaporators to said compressor in parallel relationship, means responsive to the pressure in said common suction conduit for starting said compressor at a predetermined maximum pressure and for stopping said compressor at a predetermined minimum pressure, control means responsive to the refrigerant pressure prevailing at the outlet of one of said evaporators and including a valve at the outlet of said one of said evaporators for maintaining the temperature thereof within a predetermined range of relatively high temperatures, means including an expansion valve located at the inlet of another of said evaporators for throttling the flow of refrigerant therethrough, and a second control means shutting 0E the ilow of refrigerant through said last mentioned evaporator and dependent upon the temperature of said last mentioned evaporator for maintaining the temperature thereof within a predetermined range of relatively low temperatures, said second control means being independent of said first named control means.

10. A refrigerating system having a'. plurality of refrigerant evaporators, -each of said evaporators having a-refrigerant outlet, means includving a refrigerant liquefying unit for supplying liquid refrigerant to said evaporators, conduits connecting the outlets of said evaporators to said liquefying unit in parallel relationship, control means responsive to the refrigerant pressure prevailing at the outlet of one of`said evaporators and including a valve at the outlet of said one of said evaporators 'for maintaining the temperature thereof alternately above and below 32 F., means including an expansion valve located at the inlet of another of said evaporators for throttling the fiow of refrigerant therethrough, and a second control means shutting oil? the iiow of refrigerant through said last mentioned evaporator and dependent upon the temperature of said last mentioned evaporator for maintaining the temperature thereof within a predetermined Arange of relatively low temperatures, said second control means being independent of said first named control means.

11. A refrigerating system having a. plurality of refrigerant evaporators, each of .said evaporators having a, refrigerant outlet, means including a refrigerant liquefying unit for supplying liquid refrigerant to said evaporators, conduits connecting the outlets of said evaporators to said liquefying unit in parallel relationship, control meansl responsive to the refrigerant pressure prevailing at the outlet of one of said evaporators and including a vtwo-temperature stop valve at the outlet of said one of said evaporators for maintaining the temperature thereof alternately above and below 32 F., means including an expansion valve 'located at the inlet of another of said evaporators for throttling the flow of refrigerant therethrough, and a second control means" shutting oif thefiow of refrigerant through said orators, conduits yconnecting the outlets of said A evaporators to said liquefying unit in parallel re. lationship, control means responsive to the re-v frigerant pressure prevailing at the outlet of said evaporator of relatively large capacity andv including a valve at the outlet of said evaporator of relatively large capacity for maintaining the temperature thereof within a predetermined range of relatively high temperatures, means including an expansion valve located at the inlet of said. evaporator of relatively small capacity for throttling the flow of refrigerant therethrough, and a second control means shutting off the ow of refrigerant through said evaporator of relatively small capacity and dependent upon the temperature produced by said last mentioned evaporator for maintaining the temperature thereof within a predetermined range of relatively low temperatures, said'second control means being independent of said rst named control means.

13\. A refrigerating system having a plurality of refrigerant evaporators, one ofsaid evaporators having a relatively large capacity and another` of said evaporators having a relatively small capacity,` each of said evaporators having a refrigerant outlet, means including a refrigerant liquefying unit for supplying liquid refrigerant to said evaporators, conduits connecting the outlets of said evaporators to said liquefying unit in parallel relationship, .control means responsive to the refrigerant pressure prevailing at the outlet of said evaporator of relatively large capacity and including a two-temperature stop valve lo'- cated at the outlet of. said evaporator of relatively large Vcapacity for maintaining the temperature thereof Within a predetermined range of relatively high temperatures,y means including an expansion valve located at the inlet of said evaporator of relatively small capacity for throttling the flow of refrigerant therethrough, and a second control means shutting off the ilow of refrigerant through said evaporator of relatively small capacity and dependent upon the temperature of said last mentioned evaporator for maintaining the temperature thereof within a predetermined range of relatively low temperatures, said second control means being independent o said first named control means.

HAROLD F. BRIGGEMAN. ROGER A. FULLER.