US3111814A

US3111814A - Refrigerator defrost control system

Info

Publication number: US3111814A
Application number: US18720A
Authority: US
Inventors: Frank A Schumacher; Ralph E King
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1960-03-30
Filing date: 1960-03-30
Publication date: 1963-11-26
Anticipated expiration: 1980-11-26

Description

Nov. 26, 1963 F. A. scHUMAcHER ETAL REFRIGERATOR DEFRosT CONTROL SYSTEM 2 Sheets-Sheet 1 Filed March 30, 1960 FIGB F. A. scHUMAcHER ETAL 3,111,814

REFRIGERATOR DEFROST CONTROL SYSTEM 2 Sheets-Sheet 2 Nov. 26, 1963 Filed March 50. 1960 INVENTORS FRANK A. SCHUMACHER BYE RALPH E. KING THEIR ATTORNEY United States Patent O 3,111,814 REFRGERATR DEFRGST CNTROL SYSTEM Frank A. Schumacher and Ralph E. King, Louisville, Ky., assignors to General Electric Company, a corporation of New York Filed Mar. 30, 1961i), Ser. No. 18,720 9 Claims. (Cl. 152-156) 4The present invention relates to refrigerators and more particularly to refrigerating systems including a low tempera-ture evaporator and control means for automatically initiating and terminating the periodic defrosting of the evaporator.

In the operation of a refrigerator including a low temperature evaporator, that is an evaporator which normally operates at temperatures below freezing, it is desirable periodically to remove frost collecting on the evaporator since any appreciable layer of frost has an insulative effect that decreases the efficiency of the refrigerating system. In addition it is also desirable that the entire evaporator structure be warmed to defrost temperatures during each defrost cycle as otherwise ice, in increasing amounts, will continue to accumulate on those portions of the evaporator 'which are not completely defrosted. On the other hand since much of the heat supplied to the evaporator 'during each `defrost cycle must be removed by the refrigerating system during the succeeding refrigerating cycle, it is also `important that the defrost cycles be properly timed and the heating of the evaporator during each cycle, be terminated as quickly as possible.

it is a primary object of the present invention to provide a refrigerating system including defrost control means for periodically initiating defrosting of the evaporator as a function of time and terminating the defrost operation as a function of the defrosted evaporator temperature. l

Additional objects and advantages of the invention will become apparent as the following description proceeds and the features of novelty which characterize the invention will he pointed out with particularity in the claims annexed to and forming a part of this specication.

In carrying out the objects of the present invention, there is provided a refrigerating system including an evaporator which normally operates at below-freezing temperatures. In order to remove `frost from the evaporator, means are provided for periodically warming the evaporator by means of a warm fluid, preferably hot refrigerant gas, circulated in heat exchange with the r evaporator. lFor lthe purpose of initiating and terminating these defrost cycles, there is provided a defrost control means including a body of high thermal mass arranged to be cooled by the evaporator during normal operation of the refrigerating system and to be warmed by the circulating fluid after it has been heat exchanged p with the evaporator during the defrost operation.

Thermally operated switch means responsive to the temerature of the 'body are provided for initiating a defrost cycle at a predetermined low temperature and for terminating the defrost cycle when the bo-dy reaches a predetermined high temperature. By this arrangement, time required for cooling the control body to the predetermined low temperature determines lthe period between defrost cycles while the warming of that body during defrost by defrost uid which has passed in heat exchange relationship with the evaporator causes the temperature of this body to follow the temperature of the evaporator thus assuring that the defrost cycle will continue so long as the evaporator is ybelow defrosting temperatures but that it will terminate when the evaporator temperature, reflected -by the defrost fluid flowing l Ziel l 1,814 Patented Nov. 26, 1963 from the evaporator, reaches a temperature above freezing.

For a `better understanding of the invention reference may be had to the accompanying drawings in which:

LFIG. 1 is a side elevational View, in section, of the freezer compartment of a refrigerator cabinet incorporating the present invention; Y

FG. 2 is a side elevational view of a portion of the cabinet of FIG. 1, illustrating one modification of the defrost control employed in the practice of the present 1nvention;

FIG. 3 is a View similar to FIG. 2 illustrating, in section, another modification of the `defrost control o-f the present invention;

PIG. 4 is a front elevation view, partly in section, of the defrost control shown in FIG. 3;

PEG. 5 is a top view of the control of FIG. 3 taken along line 5 5 of FIG. 4; and

FIG. 6 is a diagrammatic illustration of one Itype of hot gas defrost refrigerating system employed in the practice of the present invention.

Referring now to `the drawing there is shown in FIG. 1 a household refrigerator of the two-temperature type comprising a freezer compartment 1 positioned 4below a fresh food storage compartment (not shown). :The cabinet incl-udes an outer shell 2 and an inner shell or liner 3 spaced from the outer shell and defining the freezer compartment. This compartment is maintained at sub-freezing temperatures by an evaporator unit 4 which normally operates at below-freezing temperatures. The refrigerator also includes a machinery compartment positioned below the freezer compartment and containing a compressor 5 and a condenser 6. The evaporator unit t is supported along one wall of that compartment r a u and compr1ses a housing '7 1n which there is arranged an evaporator coil comprising, as illustrated in FIG. 6, a refrigerating tube or conduit 1o and a defrost `tube or conduit 11 extending parallel lto the refrigerating tube and in heat exchange relationship therewith.

The defrost tube or conduit 11 is designed -to conduct a warming fluid such as hot gaseous refrigerant into heat exchange relationship with the evaporator unit including the refrigerating tubing 1li for the purpose of removing frost from the refrigerant tubing and other frost collecting portions of the evaporator structure 4. While it is contemplated that any source of `warm defrosting fluid can be employed in the practice of the present invention and that this fluid may be used to defrost the evaporator lstructure either by passing it through the normal refrigerant tubing 1t) or in heat exchange therewith by means of a separate defrost line, the invention will be specifically described in its application to a refrigerating system employing a separate hot gas defrost tube 11 and will be more particularly described in its application to the system described and claimed in Patent 2,928,256, issued March 15, 1960 in -the name of Clyde l. Nonomaque and assigned to the same assignee as the present invention. With reference to FIG. 6, the refrigerating system shown in the Nonomaque patent comprises a hermetic motor compressor unit 14, a condenser 15, a fixed ilow restrictor or capillary tube 16 and a cooling or evaporator unit or structure 4 including the refrigerant tubing 1@ and the defrost line 11. An accumulator 18 forming part of the evaporator structure is connected to the outlet end of the evaporator tubing 1@ and is connected by a suction line 17 to the hermetic compressor unit 14. The motor compressor unit 1 comprises a motor 19 for driving a compressor y21, the two being sealed in a hermetic casing 22. As the suction line 17 is connected to the case 22, the case is part of the low pressure side of the normal refrigerating system and is therefore lled with low pressure refrigerant in cooling contact with the motor 19. The compressor 2l withdraws low pressure refrigerant from the case and discharges high pressure refrigerant directly through a discharge line 23 into the condenser E5. r1Chus the compressor, condenser, capillary ilow restrictor, evaporator passage l@ and hermetic casing form a series-flow normal refrigerating circuit.

For the purpose of warming and defrosting the evaporator structure there is provided an auxiliary defrost circuit which is connected to the normal refrigerating circuit in parallel relationship with the capillary ow restrictor 16, the evaporator tubing l0, and the accumulator f8 so that the auxiliary defrost circuit, which includes the defrost conduit il, forms a heating or defrost circuit for' circulating hot compressed refrigerant gas from the compressor through the defrost conduit ll in heat exchange with the evaporator structure. In the illustrated embodiment of the invention, the inlet end of the auxiliary circuit comprises a conduit 29 connected to the discharge line 23 leading from the compressor to the condenser and a normally closed valve 3@ for permitting fiow of refrigerant through the auxiliary circuit during the defrost cycle. The conduit 29 is connected to the defrost passage 11 while the outlet end of the defrost passage 1l is connected through a restrictor 31 to the suction line 17.

The system thus far described is constructed in accordance with the teachings of the aforementioned Nonomaque patent. Also in accordance with the teachings of that patent, the auxiliary circuit flow restrictor 31 preferably has a lower flow restriction than the capillary tube l so that when the valve 36 is opened for defrost operation of the system, refrigerant will preferentially flow from the compressor through the auxiliary circuit. On the other hand, the restriction of the flow restrictor 3l is preferably sufficient to maintain the compressed 'refrigerant gas in the defrost passage lll at condensing pressures during defrost operation of the system. By this arrangement, hot compressed refrigerant gas warms the evaporator to defrosting temperatures as it condenses in the defrost tubing 11.

ln accordance with the present invention there is provided a defrost control 34 for initiating and terminating the hot gas defrosting of a low temperature evaporator comprising in one form, as illustrated in FIG. 2 of the drawing, a body 35 of high thermal mass, such as a steel block, and a thermally actuated switch 36 in heat exchange with the body 35. The body 35 is located in position in which it is gradually cooled by the evaporator 4 during normal refrigerating operation of the system. To

obtain the desired slow and controlled cooling of the body 35, it is preferably positioned within the insulated cabinet wall adjacent the evaporator unit 4 and between the outer shell 2 and the liner 3. It is enclosed within and separated from the inner wall 3 and the evaporator unit 4 by means of insulation 37 so that the rate of cooling by the evaporator 4 during normal operation of the refrigerating system will be at such a rate that it will normally take a significant amount of time, as for example 8 hours, to reach a predetermined low temperature of, for example, F. This time can, of course, be controlled by the spacing of the body from the evaporator, the amount of insulation between the body and the liner 3, and the mass of material involved.

The switch 36 responsive to the temperature of the body 35 controls the operation of valve 30 and is designed to close at this predetermined low temperature of body 35 thereby causing the solenoid valve 3i) to open and initiate a defrost cycle.

An electrical control system suitable for this purpose and for controlling the normal operation of the system is illustrated in FIG. 6. For normal refrigerant control, the circuit comprises a pair of supply lines or

conductors

38 and 39 for energizing the compressor motor 19 through a temperature operated switch 4t) in the supply line 38. A temperature sensing element 4l in contact with the evaporator structure 4 operates the switch #tti so that during normal operation of the system the compressor motor' is energized, for example, whenever the evaporator' structure' reaches a predetermined maximum temperature of 0 F.,- and de-energized whenever that structure attains a predetermined loW temperature of, for example, a minus 20` F. The defrost control portion of the electrical circuit comprises the switch 36 and the solenoid component of` the valve 3d which are connected in series to the supply' lines 3S and 39 in such a manner that the energization off the solenoid of valve 30 is also under control of the rtefrigerant control switch 4). Thus the circuit permits'r` energization and opening of the solenoid val-ve 3i) only when the switch 40 is also closed to energize th`ef compres' sor motor 19. Once the defrost cycle is initiated, ftlesensf ing element 4l will of course sense only higher or drosting evaporator temperature so that switch 4@ will renfmfi closed and the compressor will continue to operate during the entire defrost period and a normal refrigerating cycle immediately following the defrost period.

For the purpose of terminating the defrost cycle in accordance with the present invention, a portion 43 of the defrost circuit downstream from the defrost portion lll is employed to warm the body 35 to the terminating temperature during the defrost cycle. For convenience the portion 43 of the defrost circuit heat exchanged with the body 35 may be the inlet end section of the ow restrictor 3l which operates at substantially the same temperature as the defrost tubing 1l, this portion being brazed or soldered in good heat exchange contact with the body 35. Since this portion of the defrost circuit carries condensed fluid which has given up its heat to the evaporator and has been cooled thereby, the temperature of the fluid passing through the defrost circuit portion 43 follows and. 1s indicative of the evaporator temperature or more specifically of the coldest part of the evaporator being defrosted. in other words, during the defrost, the temperature of the thermal member 35 follows and lags the temperature or the defrost fluid cooled by the defrosting evaporator and this temperature is an accurate reflection of the temperature of the coldest or last to defrost part of the evaporator. Once all of the frost is removed from the evaporator structure 4, the temperature of the defrost fluid leaving the evaporator structure will rapidly rise thereby furnlshmg more heat to the thermal mass or metal body 35 and warming the body to temperatures above freezing. For terminating defrost, the thermal switch 36 is therefore set to open at an above-freezing temperature of for example 50 F. which assures elimination of all frost from the evaporator. Opening of switch 36 Causes the solenoid valve 3th to close. The compressor is then returned to the control of the switch d@ and will continue to operate until the sensing element 4l controlling the switch d@ again senses a predetermined low evaporator temperature of for example a minus 20 F.

Since, at the termination of the defrost cycle, the body 35 has been heated to an elevated temperature of for example 50 F., the time that will `elapse before a defrost cycle is again initiated is the time required for the body 35 to cool to the defrost initiating temperature of 15 F. as a result of heat leakage from the block to the evaporator structure 4 during normal operation of the refrigerating system.

The length of the time intervals between defrost can be controlled by the selection of material comprising the body 35, that is its mass and thermal characteristics or by its spacing location with reference to the evaporator 4 and the amount of insulation between it and the evaporator in order to obtain shorter or longer refrigerating periods between defrost cycles. In some cases the thermal mass provided by a metal body may not be suicient to provide a time interval of the desired length between defrost cycles. Accordingly, there may be employed in place of a metal body, a thermal mass in the form of a liquid material which freezes lat some temperature be-- tween the `defrost initiating and the defrost terminating temperatures to which the body is subjected. The heat of fusion olf such a liquid increases the effective thermal mass of the control 35 so that the time between defrost cycles is correspondingly increased.

Such an arrangement is shown in FIGS. 3, 4 and 5 of the drawing in which the thermal mass comprises a heat storage body of liquid 5t) confined within a container 51 positioned within the insulation 37 of the cabinet. The liquid may be water although any other material which has a melting point between for example, a minus F. anda plus 50 F. may be employed. In this modification of the invention the portion 43 of the ldefrost circuit is in the form of a plurality o-f loops extending into the container in contact ywith the body 50 in order to obtm'n a direct heat exchange between the thermal mass and the defrost circuit. Since as is well known the body of liquid under such conditions may become subcooled, that is to be cooled below its normal solidification temperature without changing phase and .since the liquid cannot be seeded to effect the phase change, means may also be provided which will act as a nucleus for the freezing process. For this purpose, one wall of the container 51 may include a cavity 55 as illustrated in FlG. 4 of the drawing which is somewhat remote from the defrost circuit portion 43 so that once the material contained in the cavity S5 has `solidified or frozen for the first time, it will not thaw or be converted to a liquid during the heating or melting of the remaining material during the defrost cycle. The small amount of solid material contained in this cavity will therefore seed the remaining body of material and prevent super-cooling thereof during the normal refrigerating periods.

From the Iabove description it will be seen that with either modification of the invention, there has been provided means for obtaining the desired interval between defrost cycles and for terminating each defrost cycle without the use of mechanical timers or the like normally used for such purposes. The control body of high thermal mass in effect yfunctions as a thermal clock with the time required for `cooling of that body to the predetermined minimum temperature following each defrost cycie `determining the period between defrost cycles. As the body is quickly heated during defrost to the predetermined elevated temperature following the complete defrosting of the evaporator and is heated to such a temperature only upon the complete defrosting of all of the evaporator, complete removal of frost from the evaporator without significantly prolonging the defrost period beyond that required for such removal is also assured. ln other words, there is provided a control for the defrost cycle which is self-compensating as to the Idefrost load, that is the amount of frost to be removed from the evaporator unit.

While there has been sh wn and described various embodiments of the present invention it is to be understood that the invention is not limited to these particular forms and it is intended by the appended claims to cover all modifications within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is.

l. A refrigerating system comprising an evaporator normally operating at temperatures below the freezing point of water, defrost means 'for periodically warming said evaporator to a defrosting temperature, and control means 'for controlling the operation of said defrost means comprising a 'body of high thermal mass separate from but arranged to be cooled by said evaporator during normal oper-ation of said evaporator, and switch means responsive to a predetermined low temperature of said body for initiating operation of said defrost means and responsive to a predetermined high temperature of said body for terminating operation of said defrost means, said defrost means including a defrost circuit -for circulating heated fluid first into heat exchange with said evaporator and 6 then into heat exchange with said body whereby said body is warmed to said predetermined high temperature by the heat remaining in said fluid after heat exchange thereof with said evaporator.

2. A refrigerating system comprising an evaporator, means including a compressor for circulating refrigerant through said evaporator for normal operation of said evaporator at temperatures below the freezing point of water, defrost means for periodically warming said evaporator to a defrosting temperature, and control means for controlling the operation of said -defrost means comprising a body of high thermal mass material separate from but arranged to be cooled by said evaporator during normal operation of said evaporator, and switch means responsive to a predetermined low temperature of said body for initiating operation of said defrost means and responsive to a predetermined high temperature of said body for terminating operation of said defrost means, said defrost means including a defrost circuit for circulating hot refrigerant from said compressor first into heat exchange lwith said evaporator and then into heat exchange with said body whereby said body is warmed to said predetermined high temperature by the :heat remaining in said fiuid after heat exchange thereof with said evaporator.

3. A refrigerating system comprising an evaporator normally operated at temperatures bellow the freezing point of water, defrost means for periodically warming said evaporator to a defrosting temperature comprising a defrost circuit including in series-flow connection a first section in which refrigerant vapor is brought int-o heat exchange with said evaporator and condensed and a second section, and control means for controlling the operation of said defrost means comprising a 4body of high thermal mass material separate from but arrange-d to be cooled by said evaporator during normal operation of said evaporator, switch means responsive to a predetermined below-freezing temperature of said body `for initiating operation of said defrost means and responsive to `a predetermined above-freezing temperature of said body to terminate operation of said defrost means, said second section of said defrost circuit being in heat exchange with said body whereby during operation of said defrost means said body is warmed to said predetermined above-freezing temperature by the heat remaining in the condensed refrigerant flowing from said evaporator first section.

4. A refrigerating system comprising an evaporator normally operating at temperatures below the freezing point of water so that -frost collects thereon, defrost means for periodically defrosting said evaporator comprising a defrost circuit for circulating a warm fluid in heat exchange with said evaporator and including in series-flow connection a first section for warming said evaporator and a second section, and control means for controlling the operation of said defrost means comprising a body of high thermal mass material spaced from but arranged to 'be cooled by said evaporator during normal operation of said evaporator, switch means responsive to a predetermined belowfreezing temperature of said body for initiat- .ing operation of said defrost means and responsive to a predetermined above-freezing temperature of said body to terminate operation of said defrost means, said second section of said defrost circuit being heat exchanged with said body whereby said body can be warmed to said predetermined above-freezing temperature only by the heat remaining in the circulating fluid after heat exchange thereof with said evaporator.

5. In a refrigerating system of the type comprising a refrigerating circuit including a compres-sor, a condenser, a yrestrictor and an evaporator connected in series-flow refrigerating circuit for normal operation of said evaporator at frost collecting temperatures, and a defrost circuit including in series-flow connection a defrost portion in heat exchange relationship with said evaporator and a second portion, said defrost circuit being connected at its inlet end to said refrigerating circuit between said compressor and said restrictor and at its outlet end to said refrigerant circuit between said evaporator and said compressor, normally closed valve means in said defrost circuit, defrost control means for controlling the operation of said valve comprising a body of high thermal mass material arranged to be cooled by said evaporator during normal operation of said evaporator, switch means responsive to a predetermined low temperature of said body for opening said valve to initiate flow of compressed refrigerant through said defrost circuit and responsive to a predetermined high temperature of said body for closing said valve and terminating operation of said defrost means, said second portion of said defrost circuit being in heat exchange relation with said body to warm said body to said predetermined high temperature during iow of refrigerant through said defrost circuit.

6. ln a refrigerating system of the type comprising a normal refrigerating circuit including an evaporator, means including a compresso-r for circulating refrigerant through said evaporator for normal ope-ration thereof at frostcollectinrg temperatures and a defrost circuit for periodically warming said evaporator to defrost temperatures by means of hot compressed refrigerant circulated from said compressor to said evaporator and back to said compressor; control means for initiating and terminating the defrost operation of said system comprising a body of high thermal mass spaced from *but cooled by said evaporator during normal operation of said system, thermally operated switch means responsive to the temperature of said body to initiate defrost operation of sai-d system when said body attains a predetermined below-freezing temperature and to terminate defrost operation of said system when said body attains an above-freezing temperature, said defrost circuit including a section thereof conducting refrigerant from said evaporator to said compressor in heat exchange with said body whereby during defrosting operation of the systems said body will follow the increasing temperature of the evaporator.

7. A refrigerator including a cabinet includinginsulated walls dehning a storage compartment, an evaporator in said compartment adjacent one wall thereof, means for circulating refrigerant through said evaporator for normally maintaining said evaporator at temperatures below the freezing point of water, defrost means for periodically warming said evaporator to a defrosting temperature comprising a hot gas defrost circuit including in series-flow connection a first section for circulating Warm refrigerant in heat exchange with said evaporator and a second section, and control means for controlling the operation of said defrost means comprising a body of high thermal mass material arranged in the insulation contained in said one `cabinet wall whereby it is gradually cooled by said evaporator during normal operation of said evaporator, switch means responsive to a predetermined below-freezing temperature of said body for initiating operation of said defrost means and responsive to a predetermined above-freezing temperature of said body to terminate operation of said defrost means, said second section of said defrost circuit being in heat eX- change with said body whereby said body is warmed to said predetermined above-freezing temperature by the heat remaining in the circulating refrigerant after heat eX- change thereof with said evaporator.

8. A refrigerator including a cabinet including insulated w-alls defining a storage compartment, an evaporator in said compartment adjacent one Wall thereof, means including a compressor, a condenser, a restrictor and said evaporator in series connection for circulating refrigerant through said evaporator to normally maintain said evaporator at temperatures below the freezing point of water, defrost mean-s for periodically defrosting said evaporator comprising a hot gas defrost circuit for circulating compressed refrigerant vapor into heating relation with said evaporator and including, in closed seriesfrow connection, said compressor, a first section in heat exchange with -said evaporator, a second section and flow restricting means for maintaining said first section at condensing pressures, and control means for controlling the dow of refrigerant through said defrost circuit comprising a body of high thermal mass material arranged in the insulation contained in said one cabinet wall adjacent said evaporator whereby said body is gradually cooled by said evaporator during normal operation of said evaporator, switch means responsive to a predetermined belowfreezing temperature `of said body for effecting dow of refrigerant through said defrost circuit and responsive to a predetermined above-freezing temperature of said body to terminate said flow, said second section of said defrost circuit being in heat exchange with said body whereby said body is warmed to said predetermined abovefreezing temperature by the heat remaining in the condensed refrigerant 'after heat exchange thereof with said evaporator.

9. A refrigerator comprising a cabinet including insulated walls defining a storage compartment, an evaporato-r in said compartment adjacent one of said walls, a

efrigerating system comprising a refrigerating circuit including a compressor, a condenser, a restrictor and said evaporator connected in series-flow refrigerating circuit for normal operation of said evaporator at frost collecting temperatures, and a defrost circuit including in seriesilow connection a defrost portion in heat exchange relationship with said evaporator and a second portion, said defrost circuit being `connected at its inlet end to said refrigerating circuit between said compresso-r and said restrictor and at its outlet end to said refrigerant circuit between said evaporator and said compressor, normally ciosed valve means in said defrost circuit, defrost control means for controlling the operation of said valve comprising a body of high thermal mass material arranged in said one of said walls to be cooled by said evaporator during normal operation of said evaporator, switch means responsive to a predetermined low temperature of said body for opening said valve to initiate flow of compressed refrigerant through said defrost circuit and responsive to `a predetermined high temperature of said body for closing said valve and terminating operation of said defrost means, said second portion of said defrost circuit being in heat exchange relation with said body to warm said body to said predetermined high temperature during flow of refrigerant through said defrost circuit.

References Cited in the tile of this patent UNITED STATES PATENTS

Claims

1. A REFRIGERATING SYSTEM COMPRISING AN EVAPORATOR NORMALLY OPERATING AT TEMPERATURES BELOW THE FREEZING POINT OF WATER, DEFROST MEANS FOR PERIODICALLY WARMING SAID EVAPORATOR TO A DEFROSTING TEMPERATURE, AND CONTROL MEANS FOR CONTROLLING THE OPERATION OF SAID DEFROST MEANS COMPRISING A BODY OF HIGH THERMAL MASS SEPARATE FROM BUT ARRANGED TO BE COOLED BY SAID EVAPORATOR DURING NORMAL OPERATION OF SAID EVAPORATOR, AND SWITCH MEANS RESPONSIVE TO A PREDETERMINED LOW TEMPERATURE OF SAID BODY FOR INITIATING OPERATION OF SAID DEFROST MEANS AND RESPONSIVE TO A PREDETERMINED HIGH TEMPERATURE OF SAID BODY FOR TERMINATING OPERATION OF SAID DEFROST MEANS, SAID DEFROST MEANS INCLUDING A DEFROST CIRCUIT FOR CIRCULATING HEATED FLUID FIRST INTO HEAT EXCHANGE WITH SAID EVAPORATOR AND THEN INTO HEAT EXCHANGE WITH SAID BODY WHEREBY SAID BODY IS WARMED TO SAID PREDETERMINED HIGH TEMPERATURE BY THE HEAT REMAINING IN SAID FLUID AFTER HEAT EXCHANGE THEREOF WITH SAID EVAPORATOR.