US3195321A - Refrigeration system including defrosting means - Google Patents

Description

y 1965 A. s. DECKER ETAL 3,

REFRIGERATION SYSTEM INCLUDING DEFROSTING MEANS Original Filed Nov. 14, 1960 2 Sheets-Sheet 1 INKENTORS Alan S. Decker y Cecil Boling (bagMwr/g4,%r

ATTORNEYS u y 2 1965 A. s. DECKER ETAL 3,195,321

REFRIGERATION SYSTEM INCLUDING DEFROSTING MEANS Original Filed Nov. 14, 1960 2 Sheets-Sheet 2 INVENTORS Alan S. Decker BY Cecil Boling C x/ &5 l/wws a g 4rab ATTORNEYS United States Patent 3,195,321 REFRIGERATZON SYSTEM INCLUDING DEFROSTING MEANS Alan 5. Becker, West Hartford, and Qecil Boling, Litchfield, Conn, assigners to Dunham-Eush, inc, Hartford, Conn., a corporation of Qonneeticut Continuation of abandoned application Ser. No. 69,030, Nov. 14, 196i}. This application May 28, 1964, Ser. No.

Claims. (c1. 52-473) This invention relates to refrigeration, and more in particular to defrosting the evaporators of refrigeration systems of the type which are used to maintain refrigerated compartments at low temperatures, for example, from somewhat below freezing to -40 F. This application is a continuation of Serial No. 69,030, now abandoned, filed November 14, 1960.

An object of this invention is to improve the operation of refrigeration systems, particularly in solving the problems relating to the accumulation of frost on the evaporators. A further object is to provide simple and efficient structure for defrosting evaporators. Another object is to provide efiicient and dependable means for defrosting evaporator surfaces. Another object is to provide for defrosting evaporators in refrigerated compartments at subfreezing temperatures with minimum transfer of heat to materials within the compartment other than the surfaces where frost or ice is to be removed. Another object is to provide for the utilization of the heat in the compressed refrigerant to elevate the temperature of the sub-freezing evaporator surfaces without objectionable dissipation of this heat elsewhere. Another object is to insure against the passage of liquid refrigerant back to the compressor. Another object is to provide for the accurate control of the back pressure during the defrosting operation. A further object is to provide systems of the above character with structure which is of minimum size and weight, simple and efiicient, inexpensive to manufacture and maintain, and which is adaptable to use to meet various demands and practices. These and other objects will be in part obvious, and in part pointed out below.

In the drawing:

FIGURE 1 is a somewhat schematic representation of one embodiment of the invention;

FIGURE 2 is an enlarged sectional view of the heat accumulator unit of the embodiment of FIGURE 1; and,

FIGURE 3 is an enlarged view of a portion of the evaporator of the embodiment of FIGURE 1. Referring to FIGURE 1 of the drawings, the refrigeration system 1 comprises a compressor 2, a compressed refrigerant discharge in line 3, a condenser 4, a receiver 6, a liquid line 8, a heat exchanger it a liquid line 11, an expansion valve 12 having a sensing bulb 13, an evaporator 14, an evaporator fan 16 driven by an electric motor 18, a refrigerant return line 24 which extends through heat eX- changer iii, a heat accumulator unit 22 and a refrigerant gas return line 24 extending to the compressor. Evaporator 14 is mounted near the top of a refrigerated cornpartment 26 which is maintained at sub-freezing temperature, illustratively 0 F. Positioned beneath evaporator 14 is a drain pan 28 from which a condensate drain line 30 extends for carrying the condensate to waste.

During operation of the refrigeration system, evaporator 14 is maintained at sub freezing temperature and air at compartment 26 is circulated through the evaporator by fan 16. This causes frost to accumulate upon the evaporator surfaces, and this frost is removed by defrosting the evaporator. The present invention is concerned with the carrying on of the defrosting operations.

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In addition to the components referred to above, the refrigeration system includes: a hot gas bypass line 32 extending from the compressed refrigerant line 3 to the left-hand header 33 of units 22, and having a strainer 34 and a solenoid valve 36 therein; a defrosting circuit formed by a hot refrigerant line 38, extending. from the right-hand header 37 of unit 22 into compartment 26 directly beneath condensate drain line 30, and an internal tube circuit 40 of evaporator 14; a branch or loop 43 of the defrosting circuit extending from line 38 along the bottom of pan 28 to supply heat thereto An adjustable restrictor valve or regulating valve 42 at the discharge end of the internal tube circuit 40; a line 44 extending from valve 42 to the refrigerant return line 20; and, a solenoid valve 46 in the liquid refrigerant line 11. Referring to FIGURE 3, evaporator 14 is formed by a plurality of tube assemblies 48, each of which is formed by an internal tube 50 and an internal passageway 51, an external or outer tube 52, an external fin assembly 54 along which the air passes to be cooled, and an internal fin assembly 5d. The internal fin assembly ispositioned in the annular passageway 58 between tubes 50 and 52 and, during manufacture, tube 50 is expanded so as to place fin assembly 56 under radial compression. This construction is of the type covered by United States Patents Nos. 2,611,585 and 2,611,587. In the present embodiment, the interconnected annular passageways 58 constitute the evaporator, and the interconnected passageways 51 in tubes 50 form the internal tube circuit 40 through which hot refrigerant is passed during the defrosting operation to heat the evaporator surfaces and remove the frost accumulations. Refrigerant flows through passageways 58 only during the refrigeration cycle, and refrigerant flows through passageways 51 only during the defrosting cycle. During the defrosting cycle, the heat is transmitted from tubes 56 radially outwardly by the fin assembiies 56 across the annular passageways 58 to the eX- ternal or outer tubes 52. During the operation of the refrigeration system to cool compartment 26, the liquid refrigerant is evaporated in the annular passageways 58, and heat is removed from the air by the fin assembly 54 and tubes 52 and passes efficiently to the refrigerant, partially through the fin assemblies 54. Frost and ice form upon the surfaces of tubes '52 and the fin assembly 56, and the defrosting operation is for the purpose of removing such formation. The heat accumulator unit 22 (see FIGURE 2) has a cylindrical shell 61 with end bells 62 and 64, and a pair of header plates 66 and 68 which form a central chamber '79 and the headers 33 and 37 to which the lines 32 and 38 connect. The refrigerant return or suction line 20 from the evaporator is connected to the upper right-hand end of chamber 70, and the suction line 24 connects the lower left-hand end of this chamber to the compressor. A plurality of tubes 72, mounted in header plates 66 and 68, provide passageways between the headers. as to provide an open passageway along the top of the chamber for the flow of gas from line 20. Hence, during the refrigeration cycle, the refrigerant gas passes freely through unit 2t During the defrosting operation, the refrigerant flows from the compressor through tubes 72 in heat exchange relationship with the refrigerant in chamber 76 flowing from the evaporator toward the compressor.

Positioned in chamber '70 to the right of the connection with line 24 is a bafiie 74 which is supported by rods not shown, and has its upper edge 76 above the center line of chamber 70. Thus, the return refrigerant The upper portion of chamber '70 is free of tubes so which flows from the evaporator through line 26 enters the right-hand end of chamber 70, and as it flows to. the left it is diverted to the top of the chamber by baffle 76. It then flows downwardly and through line 24- back to the compressor. Hence, any liquid in the return refrigerant gas is dropped out into chamber 70 to the right of bafiie 74, while the gas passes to the left over the bafiie. The diameter of baflie 74 is slightly less thanthe internal diameter of shell 60, and there is anarrow arcuate slot 78 between the bafile and the shell wall. Hence, during the refrigeration cycle, the oil returning from the evaporator to the compressor flows through this slot and passes. with the refrigerant gas through line 24 back to the come presson, However, as will be explained more fully: below, during the defrosting operation, liquid refrigerant passes from the evaporator and accumulates in chamber 70 at the right of baffle 74. 'This liquid refrigerant is evaporated by. heat from the hot compressed gas in tubes,

72. Any smallamount of liquid refrigerant which flows through slot 78 is immediately evaporated, and no liquid refrigerant is returned to the compressor. The evaporated refrigerant gas passes upwardly and to. the left over sensed at bulb 13. Compressor 2 compresses the gas refrigerant from the evaporator and discharges itthrough line 3 to condenser 4 where it is condensed; and, the

liquid refrigerant then passes to the receiver and from there to the evaporator through line 8, heat exchanger 10, line 11 and expansion valve.12. The gasrefrigerant from the evaporator flows from line through chamber 70 above the upper edge of baffle 74 and thence down to line 24, while the returning oil flows through,

the arcuate slot 78 to line 24. During the refrigeration, cycle, no refrigerant flows through the circuit formed by line 32, the headers and tubes of unit 22, line 38 and the internal tube circuit of the evaporator. Therefore, there is no heating of thereturning refrigerant by hot refrigerant, gas as occurs during the defrosting operation. Also, unit 22 is sized to produce negligible pressure drop in the returning refrigerant path.

Asindicated above, frost or ice forms upon the evapo-. rator surfaces, and when the accumulator is sufiicient to make it'desirable, a defrosting cycle is initiated by first closing solenoid valve 46, and no additional liquid refrigerant is supplied tothe evaporator passageways 58.. Compressor 2 remains operative so that it starts and stops under thecontrol of its low-pressure cut-out switch on its suctionside. That is,.the compressor starts and continues to operate whenever the pressure in line 24 rises above a predetermined value, and it is stopped automatically when the pressure in line .24 is reduced below that value.

' When'the defrosting cycle is started by the closing of valve 46, there is liquid refrigerant in the evaporator passageways 58 of evaporator 14, and fan 16 and the compressor continue to operate'until this refrigerant evaporates andthe suction pressure drops in line 24. .Fan 16 is then stopped and solenoid valve 36 is opened so as to permit the high pressure gas from line 3, condenser 4 and receiver 6 to flow through line 32 to header '33 of heat accumulator unit 22, tubes 72, header 37, and thence through line'38 to the. internal tube circuit 40 of evaporator 14 and the pan heating loop 43. At this;

regulating or restrictor valve 42. The pressure in line as is substantially the same as the suction pressure at the compressor, so that valve 42 acts initially somewhat as an expansion valve through which the refrigerant passes to the lower pressure side as the high pressure builds up. The flow of refrigerant through valve .42 increases the suction pressure with the result that compressor 2 restarts under the control of its low-pressure switch, and draws the refrigerant back toward the compressor. This returning refrigerant passes through chamber 70 of unit 22 along the same path as that of the refrigerant gas returning to the compressor during the refrigerating cycle. However, during the defrosting cycle, a substantial amount of liquid refrigerant passes through valve 42 to line 20, and this liquid refrigerant accumulates in chamber 70 at the right of bathe 74, and is in heat exchange relationship with the compressed gas refrigerant flowing away from the compressor in tubes -72; Therefroe, the compressed refrigerent is cooled by the returning refrigerant, while the returning refrigerant is heated. The heating of the returning refrigerant insures against slug back, that .is, no liquid refrigerant will pass back to the cornpressor. As the operation continues, the high'pressure refrigerant is supplied'to the defrosting circuit, and the cooling in unit 22 and in the defrosting circuit condenses the refrigerant so that there may be a column of liquid 7 refrigerant'in parts or all of line 38, loop 43, and the internal tube circuit .46 of evaporator .14. Valve d2 causes the pressure to build up, and the entire defrosting circuit is subjected to heating by the refrigerant, either before or afterthe refrigerant is condensed.

The temperature of the refrigerant flowing to the right through line 38 is below the temperature of the hot compressed refrigerantfiowing. from the compressor, but it is substantially above freezing, and therefore it immediately warms drain line 35 and drain pan 28, sothat water resulting from melting ice or frost from the evaporator will flow to waste. Tubes which form the internal tube circuit 46 are also heated, 'andthe heat is transmitted through the internalfin assembly 56 to the I external tube 52 and to the fin assembly 54. Upon continned operation, the surfaces of the evaporator are raised to temperatures above freezing so as to start the melting of. thefrost and ice heating is generally upwardly from the bottom of the evaporator, so that the frost and ice isfirst cleared from the bottom of thecoil andthen progressively upwardlyn This insures, against refreezing of the melted water, as tends to occur when the coil is heated at the top first, or at the same rate at the top and bottom. During this time, line 30 and pan 28 are maintained at a temperature above freezing, so that the water resulting from the defrosting operation flows downwardly and through the'trap to waste.

Throughout the entire defrosting operation, valve 42 maintains a relatively high pressure or high side condition throughout the defrosting circuit, including the internal tube circuit 40 of evaporator 14. and in line 38. Tube 44 and the suction line 20 are on the suction side of valve 42,.and. liquid refrigerant passes through valve 42 and line 26 into chamber of unit 22, at the right-hand side of baiiie 74. This liquidrefrigerant' is heated and evaporated by the super-heated refrigerant gas from the compressor flowing through tubes 72. The refrigerant gas passes to the left along the top' of chamber 76, above tubes 72 and over bafiie 74, and'thence downwardly past the left-hand ends of tubes 72 into line 24. The left-hand ends of tubes 72 contain the super-heated refrigerant gas flowing from the compressor, andv this insures that the temperature of the refrigerant gas flowing from -unit22 is elevated sufficiently to insure that it containsno liquid refrigerant, and that its temperature is sufficiently high to vaporize any liquid refrigerant which seeps under the bafiie. A

- An important aspect of the present invention is the mode of operation which permits the reduction in the temperature of refrigerant so that part or a major portion of the refrigerant is condensed prior to passing into the defrosting zone. The heat which is available for defrosting is used at a relatively low temperature to elevate the temperature of the surfaces upon which frost tends to accumulate. The temperature of the liquid refrigerant flowing through line 38 is not sufliciently high to cause an excessive amount of heat to be dissipated and lost while the refrigerant is passing to compartment 26, or as it flows through line 38 within the compartment. Also, drain line 30, drain pan 28, and the other components and materials in contact with the hot refrigerant line 38 and the evaporator are not heated excessively. This increases the rate of the defrosting operation; and, it also avoids adding unnecessary heat to components in compartment 26, so as to aid in maintaining a satisfactory uniform temperature in compartment 26 and so as to reduce the refrigeration load when the defrosting cycle has been completed.

The continued operation of the compressor circulates the refrigerant through the circuit formed by line 32, the tubes 72 of unit 22, line 38, the internal tube circuit 49 of the evaporator, valve 42, line 44, line Zti, chamber '70 of unit 22 and line 24 back to the compressor. However, the heat circuit is from the compressor to unit 22 at the relatively high temperature of the compressed gas, and then to the defrosting zone at the reduced temperature caused by the evaporation of the returning refrigerant. The compressed refrigerant is discharged from the compressor at the relatively high superheated tem perature, and the mode of operation insures that all of the return liquid refrigerant is evaporated in unit 22 so that none returns to the compressor. The defrosting operation is then carried on at the reduced range of temperatures, and this insures maximum efficiency in performing the defrosting operation. For any particular installation and set of operating conditions, restrictor or back pressure regulator valve 42 is adjusted to maintain the maximum back pressure which does not overload the compressor motor. When the valve is turned toward its fully open position, the back pressure rises, and when the valve is turned toward its fully closed position, the back pressure drops. Therefore, the valve is adjusted while the system is carrying on a defrosting operation and, if the compressor motor tends to become overloaded, valve 42 is closed slightly and this reduces the load because of the reduction in back pressure. Generally, it is desirable to maintain the maximum acceptable back pressure so as to reduce the defrosting time, but the system operates satisfactory in all respects, even though the back pressure is less than the permissible maximum.

In the illustrative embodiment of the invention, for one set of conditions of operation the temperature of liquid refrigerant flowing through line 38 was 65 F, at the beginning of the defrosting cycle, and rose to a maximum of 80 F. at the end of the defrosting cycle. The evaporator surfaces were raised to temperatures within the range of 40 F. to 50 F. With this operation, the back pressure was the equivalent of 20 to 25 F. during defrosting, and compartment 2d was maintained at a temperature of F.

It has been pointed out above that the defrosting opera tion is started in the illustrative embodiment by a time clock which first closes solenoid valve 46, after which the evaporator is pumped down, and the compressor stops automatically. Shortly thereafter, the time clock stops the evaporator fan 16 and opens the hot gas solenoid valve 36 which permits refrigerant gas from condenser 4 to flow into the defrosting circuit. Refrigerant may also boil off in receiver 6 and flow up through the condenser to the defrosting circuit. This occurs very rapidly, and shortly thereafter, sufficient refrigerant flows through valve 42 to the low-side or suction side of the compressor to cause the compressor to restart. When the evaporator coil is completely free of ice and frost, a temperature sensitive switch located on the evaporator terminates the defrosting operation 'by closing solenoid valve 36, and opening solenoid valve 46. The compressor continues to operate so as to evaporate the refrigerant in unit 22, and the entire defrosting circuit, including the internal circuit of the evaporator and line 38. The drop in suction pressure and the simultaneous rise in the high-side pressure causes the refrigerant to flow through the refrigeration circuit from receiver 6 through expansion valve 12, and to the evaporator, so that the normal refrigeration cycle is started. When the surface temperature of the evaporator has been reduced to its normal temperature, fan 16 is restarted. The delay in the restarting of fan 16 for the period during which the temperature of the evaporator surfaces is being reduced is obtained by utilizing the differential between the switch opening and the switch closing temperatures of the temperature sensitive switch on the evaporator. That is, this temperature-sensitive switch operates to move into one position when the temperature of the evaporator surfaces rises sufficiently to insure that the frost and ice have been removed, and that terminates the defrosting operation by closing valve 36 and opening valve 46; and, this switch moves to its other position only after the evaporator surface is reduced a substantial amount in temperature, and this restarts fan 16.

We claim:

1. In a refrigeration system which includes a compressor for compressing gaseous refrigerant and a condenser for condensing the compressed refrigerant thereby to produce liquid refrigerant and also includes an evaporator having a flow path within which the liquid refrigerant is evaporated thereby to produce gaseous refrigerant and means forming a refrigerant-return flow path through which the gaseous refrigerant is returned from the evaporator to the compressor and means providing a flow path from the condenser to the evaporator which includes restrictor means to maintain a pressure-drop condition between an elevated pressure in the condenser and a reduced pressure in the evaporator, said evaporator having evaporator surfaces upon which frost tends to accumulate, means forming a defrosting flow path for refrigerant which is separate throughout from the flow path of the evaporator and through which refrigerant flows during the defrosting cycie in heat exchange relationship with said evaporator surfaces, defroster restrictor means downstream from said defrosting circuit and providing a defrosting refrigerant outlet from said defrosting circuit and maintaining an elevated pressure condition within said defrosting circuit during the defrosting operation, a

heat interchange unit having two fiuid circuits through which separate streams of refrigerant pass in heat exchange relationship with each other, one of said fluid circuits forming a portion of the refrigerant-return flow path from the evaporator to the compressor, means connecting said defrosting-refrigerant outlet to said refrigerant-return flow path whereby during the defrosting operation the refrigerant flows from the defrosting circuit along said refrigerant-return flow path through said one of said fluid circuits of said heat interchange unit, diverting means to supply compressed refrigerant at an elevated pressure from the compressor through the other of said fluid circuits of said heat interchange unit and thence along said defrosting flow path and through said defrosting restrictor means at which the pressure drops and from which the refrigerant returns along said refrigerant-return fiow path to the compressor, and means to control the supplying of the compressed refrigerant from the compressor through said diverting means.

2. Apparatus as described in claim 1 wherein said heat interchange unit comprises a horizontally positioned shell having a central chamber which forms said one of said flow paths and within which liquid refrigerant may accumulate and a plurality of tubes extending through said chamber and forming the other of said flow paths through which the compressed refrigerant fiows during the defrosting cycle.

3. The combination as described. in claim 1, wherein said evaporator comprises, a plurality of interconnected tube assemblies each of Which-comprises, an inner tube and an outer tube concentrically positioned with an annular space therebetween, an internal fin assembly positioned within said annular space and providing a heat exchange relationship between said tubes, and an external fin assembly on said outer tube, and where said inner tube of each of said tube assemblies is connected as part of said defrosting circuit and said annular space of each of said tube assemblies comprises a portion of the evaporator.

4. In a refrigeration system, the combination of, an

evaporator having an evaporator flow; path and external,

surfaces upon which frost tends to accumulate, a compressor which Withdraws refrigerant from said evaporator and compresses it, a condenser Which receives compressed refrigerant from said compressor and condenses it, a receiver which receives liquid refrigerant from said condenser, an expansion valve through which liquid refrig erant flows at a reduced pressure into said evaporator, a heat interchange unit comprising a tube and shell construction which forms two separate fluid circuits through,

which two separate streams of refrigerant flow in heat exchange relationship, an evaporator defrosting circuit extending from-the outlet of one of said fluid circuits through a defrosting flow path separate from said evap-: orator fioW path and inheat exchange relationship with the evaporator surfaces upon which frost tends to ,ac-

cumulate, a defrosting-restrictor valve providing a defrosting refrigerant outlet from said defrosting circuit, a common return line from said evaporator and said defrosting-refrigerant outlet through the other of said fiuid circuits of said heat interchange unit and thence to said compressor, and means to supply compressed refrigerant from said compressor during the defrosting operation through said one of said fluid circuits of said heat eX- change unit to said defrosting circuit.

5. A refrigeration system as described in claim 4 wherein said heat interchange unit includes an accumula- ,tion chamber which forms said other of said fluid circuits and Within which liquid refrigerant is accumulated and evaporated and passed along said common return line.

References Cited by the Examiner UNITED STATES PATENTS 2,068,677 1/37 Higharn 62-509 2,385,667 9/45 Webber 62-509 2,526,379 10/50 Maseritz 62-275 2,637,983 5/53 Malkofi? 62-278 2,645,101 7/53 LaPorte 62-275 2,693,678 11/54 Toothman 62-278 2,693,683 11/54 'Toothrnan 62-278 2,698,522 1/55 LaPorte 62-5 2,759,339 8/56 Kunderr 62-278 2,770,104 11/56 SWeynor 62-513 2,783,621 3/57 Staebler 62-278 2,799,999 7/57 SWEIIISOIL' 62-503 2,876,630 3/59 Boling 62-278 2,882,696 4/59 Hermann et al 62-278 2,888,252 5/59 Heathman 62-513 2,895,306 7/59 Latter 62-152 2,909,907 10/59 Swanson 62-278 2,961,848 11/60' Nonomaque "62278 3,021,693 2/62 Anne 62-278 OTHER REFERENCES Pages 61-65, 124, 126 of March 1959 issue of American Society ofHeating, Refrigerating and Air-Conditioning Engineers. Five Defrost Methods by I. H. Rain- Water.

ROBERT A. OLEARY, PrimarvExnrrziner.

Claims (1)

1. 4. IN A REFRIGERATION SYSTEM, THE COMBINATION OF, AN EVAPORATOR HAVING AN EVAPORATOR FLOW PATH AND EXTERNAL SURFACES UPON WHICH FROST TENDS TO ACCUMULATE, A COMPRESSOR WHICH WITHDRAWS REFRIGERANT FROM SAID EVAPORATOR AND COMPRESSES IT, A CONDENSER WHICH RECEIVES COMPRESSED REFRIGERANT FROM SAID COMPRESSOR AND CONDENSES IT, A RECEIVER WHICH RECEIVES A LIQUID REFRIGERANT FROM SAID CONDENSER, AN EXPANSION VALVE THROUGH WHICH LIQUID REFRIGERANT FLOWS AT A REDUCED PRESSURE INT SAID EVAPORATOR, A HEAT INTERCHANGE UNIT COMPRISING A TUBE AND SHELL CONSTRUCTION WHICH FORMS TWO SEPARATE FLUID CIRCUITS THROUGH WHICH TWO SEPARATE STREAMS OF REFRIGERANT FLOW IN HEAT EXCHANGE RELATIONSHIP, AN EVAPORATOR DEFROSTING CIRCUIT EXTENDING FROM THE OUTLET OF ONE OF SAID FLUID CIRCUITS THROUGH A DEFROSTING FLOW PATH SEPARATE FROM SAID EVAPORATOR FLOW PATH AND IN HEAT EXCHANGE RELATIONSHIP WITH THE EVAPORATOR SURFACES UPON WHICH FROST TENDS TO ACCUMULATE, A DEFROSTING-RESTRICTOR VALVE PROVIDING A DEFROSTING REFRIGERANT OUTLET FROM SAID DEFROSTING CIRCUIT, A COMMON RETURN LINE FROM SAID EVAPORATOR AND SAID DEFROSTING-REFRIGERANT OUTLET THROUGH THE OTHER OF SAID FLUID CIRCUITS OF SAID HEAT INTERCHANGE UNIT AND THENCE TO SAID COMPRESSOR, AND MEANS TO SUPPLY COMPRESSED REFRIGERANT FROM SAID COMPRESSOR DURING THE DEFROSTING OPERATION THROUGH SAID ONE OF SAID FLUID CIRCUITS OF SAID HEAT EXCHANGE UNIT TO SAID DEFROSTING CIRCUIT.

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