US2313390A

US2313390A - Reverse cycle heating system

Info

Publication number: US2313390A
Application number: US289939A
Authority: US
Inventors: Alwin B Newton
Original assignee: Honeywell Inc
Current assignee: Honeywell Inc
Priority date: 1939-08-14
Filing date: 1939-08-14
Publication date: 1943-03-09
Anticipated expiration: 1960-03-09

Description

REVERSE CYCLE HEATING SYSTEM Filed Aug. 14, 1939 Y fin nenfor Alwin B. Newton.-

Patented Mar. 9, 1943 REVERSE CYCLE HEATING SYSTEM Alwin B. Newton, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application August 14, 1939, Serial No. 289,939

13 Claims. (01. 62-4) This invention relates to an automatic control system for a reverse cycle heating system having heating means in the form of a condenser for dissipating heat to a space for heating the same, an evaporator for absorbing heat from the outside air and a mechanism for circulating refrigerant through the condenser and evaporator.

An object of this invention is to provide first control means for controlling the mechanism to maintain desired conditions in the space along with second control means for automatically defrosting the evaporator to cause most eflicient operation of the system, the arrangement being such that when the second control means is causing defrosting of the evaporator the first control means is ineffective to operate the mechanism.

A further object of this invention is to tervide a control means responsive to the pressure on the high pressure side of the system for preventing the building up of excessive pressures in the system.

Another object of this invention is to provide heating means for heating the evaporator during the defrosting cycle to hasten the defrosting action and preferably the system is so arranged that the heating means is only effective when the'first control means is demanding heat.

Other objects and advantages will become apparent to those skilled in the art upon reference to the accompanying specification, claims and drawing in which:

Figure 1 is a diagrammatic illustration of one form of this invention, and

Figure 2 is a diagrammatic illustration of a modified control system that may be substituted for the control system of Figure 1.

Referring now to Figure 1, the space or enclosure to be heated is designated at H) and is provided with anoutside wall II; is heated by an air conditioning unit l2. Return air is drawn from the space l0 through a return air duct l3 and fresh air is drawn from outside of the space through a fresh air duct M the fresh air and the return air being mixed The space l0 and passed over a heating coil which may take the form of a condenser I8. A fan I5 operated by a motor It draws the mixture of fresh air and return air over the heating coil i8 and discharges the heated air through a discharge duct ll into the space l0. Refrigerant is circulated through the condenser l8 by a refrigerating apparatus generally designated at 20 which may comprise a compressor 2| operated by an electric motor 22. Compressed refrigerant is discharged from the compressor 2I- through a high pressure line 23 to the condenser l8 and condensed refrigerant is collected in a receiver 24. Liquid refrigerant flows from the receiver 24 through a liquid line 25 into an evaporator 26 located outside of the space it. Evaporated refrigerant is withdrawn from the evaporator 26 through a suction line 21 by the compressor 2|. The flow of refrigerant to the evaporator 26 may be controlled by a thermostatic expansion valve 28 having a thermal bulb 29 responding to the temperature of the refrigerant leaving the evaporator 26. Heating means which may be in the form of strip heaters 30 are provided for heating the evaporator 26 under certain conditions to hasten the defrosting action of the evaporator 26. The structure thus far described is conventional in the art and therefore a further description thereof is not considered necessary, it being sufficient to state that when the compressor 2| is placed in operation heat is absorbed by the evaporator 26 and is liberated by the condenser l8 for heating the space 10.

The compressor motor 22 and hence the compressor 2| and the heating means 30 for the evaporator 26 are controlled by a temperature responsive controller 32 responding to the temperature within the space l0 and by a relay generally designated at 33. The relay 33 is controlled by a unitary control arrangement 34 responsive to the pressure in the condenser l8 and hence the temperature of the condenser and to the pressure in the evaporator 26 and hence to the temperature of the evaporator 26. The relay 33 may also be controlled by a timer generally designated at 35.

The temperature responsive controller 32 may comprise a bellows 38 charged with a volatile fluid for operating a lever 39 against the action of an adjustable tension spring 40 for operating a mercury switch 4|. For purposes of illustration, it is assumed that when the temperature within the space H) decreases to the mercury switch M is closed, and when the temperature increases to 72 the mercury switch 4| is opened.

The relay generally designated at 33 may comprise an operating coil 43 for operating a switch arm 44 with respect to

contacts

45 and 46. When the operating coil 43 is energized, the switch arm 44 is moved into engagement with contact 45, and when the operating coil 43 is deenergized the switch arm 44 is moved into engagement with the contact 48 by means of springs, gravity, or other means, not shown.

The unitary control arrangement 34 may be of the type shown and described in application Serial No. 196,447, filed by Albert Li. Judson and Carl G. Kronmiller on March 17, 1938. For purposes of illustration in this application, the unitary control arrangement 34 is shown to include a control device 48 responsive to changes in pressure in the evaporator 25, a control device 49 responsive to changes in pressure in the condenser I8, and a relay 58 controlled by the two control devices.

The control device 48 may comprise a bellows 52 connected by a pipe 53 to the suction line 21 so that the bellows 52 is operated in accordance with changes in evaporator pressure or temperature. The bellows 52 operates a lever 54 pivoted on a fulcrum member 55 against the action of an adjustable tension spring 58. One end of the tension spring 55 is connected to the lever 54 and the other end is connected to an adjusting screw arrangement 51. The lever 54 carries an insulating pad 58 upon which is mounted a bridge member 58. The bridge member 59 carries a contact 58 which is adapted to be moved into engagement with a contact member 6| carried by a terminal 82. The bridge member 59 also carries a contact 63 which is adapted to be moved into engagement with a contact member 54 car ried by a terminal 65. Concentrically arranged cams 86 and 51 are provided for independently positioning the contact members SI and 54 with respect to the contacts 58 and 63. For purposes of illustration, it is assumed that when the suction pressure, that is, the pressure in the evaporator 28, increases to pounds the contact 68 is first moved into engagement with the contact member 8|, and when the pressure increases to 33 pounds the contact 53 is moved into engagement with the contact member 54. Upon a decrease in pressure, the contact 53 first disengages the contact member 84 at 33 pounds and then the contact 58 disengages the contact member 6| at 0 pounds. The pressure value at which the contact 88 en gages and disengages the contact member 8| will 'be determined by the climatic conditions and it is desirable to adjust this value to correspond to a temperature value about 15 or 20 below the coldest weather which is liable to occur in that particular locality. An evaporator pressure of 0 pounds as illustrated herein corresponds to a temperature value of 22 below zero. Therefore the design temperature of theparticular locality will be approximately below zero.

The control device 49 may comprise a bellows 88 connected by a pipe I8 to the high pressure line 23 so that the bellows 69 is expanded and contracted in accordance with. changes in pressure or temperature in the condenser I8. The bellows 89 operates a lever II fulcrumed on a fulcrum member I2 against the action of an adjustable tension spring I3. One end of the tension spring I3 is connected to the lever 'II and the other end is connected to an adjusting screw arrangement 14. Adjustably mounted on the lever II is an abutment member I5 formed of insulating material and provided with abutment surfaces I5 and TI. The abutment surface I5 is adaptedto engage a contact member 18 carried 'by a terminal 19 to move the contact member 18 out of en gagement with a contact 88. The abutment surface I1 is adapted to engage a contact member 8| carried by the terminal for moving the contact member 8| out of engagement with a contact 82. For purposes of illustration, it is assumed that as the pressure in the condenser increases the contact member II is moved out of engagement with the contact 82 at 150 pounds and then the contact member I8 is moved out of engagement with the contact 88 at 280 pounds. Upon a decrease in pressure, the contact member I8 first engages the contact 88 at 200 pounds and then the contact member 8| engages the contact 82 at 158 pounds.

The relay 58 may comprise an operating coil 88 for.

operating bridge members

88 and 81. when the operating coil .85 is energized, the bridge member 85 is moved into engagement with con- "

tacts

88 and 89 and the bridge member 81 is moved into engagement with contacts 98 and 9|. when the operating coil 85 is deenergized the

bridge members

88 and 81 are moved out of engagement with their respective contacts by means of springs, gravity, or other means, not shown. The unitary control arrangement 34 may also be provided with a terminal 92 to facilitate wiring connections.

The timer 35 may comprise a rotor 95 influenced by a field winding 88. The rotor 95 Operates a cam 98 through a reduction gear train 91 and the cam 98 in turn operates a lever 99 which operates a mercury switch I88. The mercury switch I88 is normally closed and is opened momentarily at definite intervals by the high dwell on the cam 98. The timer 35 may open the mercury switch I88 at any desired intervals, and for purposes of illustration in thisapplication it is assumed that the timer 35 opens the mercury switch I88 every two hours.

Power is supplied to the control system, tothe compressor motor 22, and to the strip heaters 38 by means of line wires I82 and I83 leading from some source of power, not shown. It is here noted that the field winding 95 of the timer 35 is connected directly across the line wires I 82 and I83 so that th cam 98 is continuously operated.

Assume now that the relay 33 is energized so that the switch arm 44 is engaging the contact 45 and that the temperature within the space I8 decreases to 70 to close the mercury switch 4|. A circuit is thereupon completed from the line wire I82 through wire I85, mercury switch 4|,

wire I86, switch arm 44, contact 45, wire I81, compressor motor 22, and wire I88 back to the other line wire |83. Completion of this circuit energizes the compressor motor 22 and hence the compressor 2| to circulate refrigerant through the condenser I8 and the evaporator 25 to liberate heat to the space I8. when the temperature within the space I8 rises to '72 to open the mercury switch 4|, the above outlined circuit is broken and the compressor motor 22 and hence the compressor 2| are stopped. In this manner desired temperature conditions are maintained in the space I8 by cyclically operating the compressor 2 I.

Assume now that the relay 33 is deenergized so that the switch arm 44 is engaging the contact 46. If under these conditions the temperature within the space I8 should decrease to 70 to close the mercury switch 4|, a circuit is completed from the line wire I82 through wire I85,

' wire I03.

mercury switch 4|, wire I06, switch arm 44, con

tact 46, wire I09 through the strip heaters 30 arranged in parallel and through wire back to Thus when the temperthe other line wire I03. 7 ature within th space I0 is below a desired value and the relay 33 i deenergized, the strip heaters 30 ar energized to hasten the defrosting action of the evaporator 26.

Assume now that themercury switch I00 of the timer 35 is closed, that the evaporator pressure has risen to 33 pounds following defrosting of the evaporator 26 to move the contacts 60 and 63 into engagement with the contact mem bers 6| and 64, and that the pressure in the concontact 60, bridge member 59, contact 63, con-" tact members 64 and 8I, contact 82, conductor H5, contact 89, conductor 6, operating coil 85,

conductor terminal 92, and wire 8 back to the other line wire I03. Completion of this cir-' cuit causes energization of the operating coil 85 of the relay 50 to move the

bridge members

86 and 87 into engagement with their respective contacts.

Movement ofthe bridge member 81 into engagement with the contacts 90 and 9| completes a circuit from the line wire I02 through wire I20, contact 9|, bridge member 81, contact 90, wire |2|, operating coil 43 of the relay 33, and wire I22 back to the other line wire I03. Com-' pletion of this circuit energizes the relay 33 to move the switch arm 44 into engagement with the contact 45 so that the space temperature responsive controller 32 may control the operation of the compressor 20. In other words, when the evaporator pressure has increased to a defrosting value of 33 pounds and thecondenser pressure has decreased to 150 pounds, the relay 33 is energized to place the compressor 2| under the control of the space temperatureresponsive a controller 32.

Movement of the bridge member 86 into engagement with the

contacts

88 and 89 completes a maintaining circuit for the operating coil 85 of the relay 50 which is independent of the con 3 tact 63 and contact member 64 and the contact 82 and contact member 8|.- This maintaining circuit may be traced from the line wire I02 through wire I I2, mercury switch I00, wire |I3,

contact 80, contact member 18, terminal 19, con-' ductor II4, terminal 62, contact member 6|, contact 60, bridge member 59, conductor I24, contact 88, bridge member 86', contact 89, conductor H6, operating coil 85, conductor 1, terminal 92, and wire 8 back to the other line Completion of this maintaining circuit maintains the operating coil 85 of the relay 50 energized until either the evaporator pressure decreases to 0 pounds or the condenser pressure increases to 200'pounds. When either of these contingencies occur, the above outlined maintaining circuit is interrupted to deenergize'the operating coil 85 of the relay 50 which in turn deenergizes the operating coil 43 of the relay 33. The operating coil 85 of the relay 50 and hence the operating coil 43 of the relay 33 cannot again be energized until the above outlined starting circuit is completed which occurs only when the evaporator pressure increases to 33 pounds and the head pressure decreases to 150 pounds. I

It is here noted that the mercury switch I00 of the timer 35 is connected in'series in both the starting and maintaining circuits of the operating coil o'f the relay 50 so that when the I mercury switch I 00 is momentarily opened the operating coil 85 isdeenergized and cannot again 10 pressure rises to 33 pounds and the condenser pressure decreases to 150 pounds.

The operation of Figure 1 may be summarized briefly as follows: when the evaporator pressure 26 has defrosted and when the condenser pressure decreases to 150 pounds, the relays 50 and 33.are energized to place the controlv of the compressor 2| under the control of the space temperature responsive controller 32. The space temperature responsive controller 32 then cycles the compressor 2| to maintain desired temperature conditions within the space I0. If either the switch I00 of the timer '35 is opened or if the evaporator pressure should decrease to 0 pounds indicating that a large amount of frost is present on the evaporator 26 or the pressure in the condenser I8 should become excessive, illustratively 200 pounds, then the relays 50 and [33 are deenergized to interrupt the control of the compressor 30 2| bythe space temperature responsive 'controller32. The operation of the compressor 2| is therefore stopped and the evaporator 26 is allowed to defrost. Deenergization of the relay 33 places the control of the strip heaters 30 under the control of the space temperature responsive controller 32, and if the space temperature responsive-controller' 32 should be calling for heat, then the strip heaters 30 are energized to hasten the defrosting action. If the space temperature 0 responsive controller 32 is satisfied, there is no need for hastening the defrosting action since under'the majority of conditions the evaporator '26 may be defrosted without the assistance of the strip heaters 30; This affords quite a saving in operating costs of this type of system. After the evaporator 26 has defrosted to .cause the evaporator pressure to rise to 33 pounds and after the condenser pressure has decreased to 150 pounds, the compressor 2| is again placed under the control of the space temperature responsive conditions within the space I0. feature of this invention is that orator'26 has been defrosted sothat if the temperature responsive controller 32 is demanding heat the compressor may immediately be placed in operation. If it is not desirable to defrost theevaporator26 at timed intervals, the timer 35 may be eliminated and under these conditions the evaporator will be defrosted only when a relatively large amount of frost has been built up on the evaporator 26 to cause thepressure thereof to decrease to 0 pounds. Under some conditions, this type of operation would be entirely satisfactory.

Referring now to Figure 2, there is disclosed another control arrangement'which-may be .substituted for the control arrangementofFigure 1 for accomplishing substantiallythe same results as are accomplished with the control arrangement of Figure ;l with the exception that the timer for defrosting at timed intervals is omitted. In Figure 2, the compressor is designated at 22,

be reene'rgized until such time as the evaporator.

rises to'33 pounds indicating that the evaporator controller 32 to maintain desired temperature I An important the defrosting cycle is terminated immediately after the evapthe strip heaters at 30, and the space temperature of Figure 2 also includes a controller I33 responsive to evaporator pressure and a controller "I responsive to condenser pressure.

' The controller I33 responsive to evaporator pressure may include a bellows I33 connected by a pipe I34 to the suction line for operating a lever I35 against the action of an adjustable tension spring I33. The lever I35 operates a mercury switch provided with left electrodes I33 and I33 and with right electrodes I43 and I. For purposes of illustration, it is assumed that when the evaporator pressure increases to 33 pounds, the mercury switch I31 is tilted to the position shown in Figure 2 to cause bridging of the electrodes I33 and I33, and when the evaporator pressure pressure decreases to pounds the mercury switch I31 is tilted to the opposite position to bridge the electrodes I43 and I.

The controller I3I may comprise a bellows I43 connected by a pipe I44 to the high pressure line for operating a lever I45 against the action of an adjustable tension spring I46. The lever I4I operatesa switch I41, and for purposes of illustration it is assumed that when the condenser pressure decreases to 150 pounds the mercury switch I41 is closed as in Figure 2, and when the pressure increases to 200 pounds the mercury switch I41 is tilted to the openposition.

Power is supplied to the compressor motor 22 and to the strip heaters 30 by means of line wires I53 and I5I leading from some source of power not shown. 7

Assume now that the evaporator has defrosted to cause the evaporator ressure to rise to 33 pounds to cause bridging of the electrodes I38 and I39, that the condenser pressure has decreased to 150 pounds to close the mercury switch I41 and that the temperature within the space being heated decreases to 70 to close the mercury switch 4I. Under these conditions, a circuit is completed from the line wire I53 through mercury switch 4I, wire I52, electrodes I39 and I38, wire I53, mercury switch I41, wire I54, compressor motor 22, and wire I55 back to the line wire I5I. The compressor is thereby placed in operation and remains in operation until such time as the space temperature responsive controller 32 becomes satisfied. In this manner, desired temperature conditions are maintained within the space. If the pressure on the high pressure side of the refrigerating apparatus should increase to 200 pounds the mercury switch I41 is opened to stop operation of the compressor motor 22 thereby providing safe operation of the system. If the evaporator pressure should decrease to 0 pounds indicating that a relatively large amount of frost has accumulated on the evaporator then the mercury switch I31 is tilted to the opposite position to cause bridging of the electrodes I40 and HI. This interrupts the circuit to the compressor motor 22 to stop operation of the compressor whereby the evaporator is allowed to defrost. If under these latter conditions the space temperature responsive controller 32 should demand heat then a circuit is completed from the line wire I through mercury switch 4I, wire I52, electrodes I43 and I, wire I56, strip heaters 33, and wire I51 back to the other line wire I5I. Completion of this circuit energizes the strip heaters 30 as long as the space temperature responsive controller 32 is demanding heat to hasten the defrosting action of the evaporator. After the evaporator has been defrosted, the suction pressure rises to 33 pounds to 7 2,818,390 responsive controller at 32. The control system tilt the mercury switch I31 to the position shown in Figure 2 to again place the control of the compressor motor 22 under the space temperature responsive controller 32.

While for purposes of illustration a controller responsive to evaporator pressure and a controller responsive to condenser pressure have been disclosed, it is obvious that these controllers may respond directly to evaporator temperature or condenser temperature inasmuch as the pressure and temperature in the evaporator and condenser are coextensive. Thus this invention in its broader aspects contemplates a control device responsive to a condition which is a measure of evaporator temperature and hence accumulation of frost on the evaporator and also a controller responsive to a condition which is a measure of the temperature of the condenser. The various temperature and pressure values utilized for purposes of illustration herein are not to be construed in a limiting sense since these pressure and temperature values may be varied in accordance with difierent types of installations.

Although for purposes of illustration two forms of this invention have been disclosed, other forms may become apparent to those skilled in the art upon reference to this disclosure and therefore this invention is to be limited only by the scope of the appended claims.

I claim as my invention:

1. In a reverse cycle heating system for a space having an evaporator for absorbing heat, a condenser for dissipating heat for heating the space and a mechanism for controlling the circulation of refrigerant through the condenser and evaporator, the combination of, control means for normally controlling the mechanism to maintain desired temperature conditions in the space, means for periodically interrupting at regular timed intervals the control of the mechanism by the control means for defrosting the evaporator, and means for returning the control of the mechanism to the control means as soon as the evaporator has been defrosted.

2. In a reverse cycle heating system for a space having an evaporator for absorbing heat, a condenser for dissipating heat for heating the space and a mechanism for controlling the circulation of refrigerant through the condenser and evaporator, the combination of, control means for normally controlling the mechanism to maintain desired temperature conditions in the space, means for periodically interrupting at regular timed intervals the control of the mechanism by the control means for defrosting the evaporator, and means responsive to a condition which is a measure of evaporator temperature for returning the. control of the mechanism to the control means as soon as the evaporator temperature increases to a value indicating that the evaporator has been defrosted.

3. In a reverse cycle heating system for a space having an evaporator for absorbing heat, a condenser for dissipating heat for heating the space and a mechanism for controlling the circulation of refrigerant through the condenser and evaporator, the combination of, control means for normally controlling the mechanism to maintain desired temperature conditions in the space, means for interrupting at timed intervals the control of the mechanism by the control means for defrosting the evaporator, and means responsive to the pressure in the evaporator for returning the control of the mechanism to the control means as soon as the evaporator pressure increases to a value indicating tht the evaporator has been defrosted.

4. In a reverse cycle heating system for a space having an evaporator for absorbing heat, a condenser for dissipating heat for heating the space and a mechanism for controlling the circulation of refrigerant through the condenser and evaporator, the combination of, control means for normally controlling the mechanism to maintain desired temperature conditions in the space, defrosting control apparatus for interrupting the control of the mechanism by the control means at timed intervals and when the evaporator becomes appreciably frosted to defrost the evaporator and for returning the control of the mechanism to the control means as soon as the evaporator has been defrosted.

5. In a reverse cycle heating system for a space having an evaporator for absorbing heat, a condenser for dissipating heat for heating the space and a mechanism for controlling the circulation of refrigerant through the condenser and evaporator, the combination of, control means for normally controlling the mechanism to maintain desired temperature conditions in the space, means for interrupting at timed intervals the control of the mechanism by the control means for defrosting the evaporator, means for also interrupting the control of the mechanism by the control means when the evaporator becomes appreciably defrosted, and means for returning the control of the mechanism to the control means as soon as the evaporator has been defrosted regardless of how the control of the mechanism by the control means was interrupted.

6. In a reverse cycle heating system for a space having an evaporator for absorbing heat, a condenser for dissipating heat for heating the space and a mechanism for controlling the circulation of refrigerant through the condenser and evaporator, the combination of, control means for normally controlling the mechanism to maintain desired temperature conditions in the space, means for interrupting the control of the mechanism by the control means for defrosting the evaporator and for returning the control of the mechanism to the control means as soon as the evaporator has been defrosted, and means for interrupting the control of the mechanism by the control means when the temperature of the condenser becomes excessive.

7. In a reverse cycle heating system for a space having an evaporator for absorbing heat, a condenser for dissipating heat for heating the space and a mechanism for controlling the circulation of refrigerant through the condenser and evaporator, the combination of, control means for normally controlling the mechanism to maintain desired temperature conditions in the space, means for interrupting the control of the mcha nism by the control means for defrosting the evaporator, means for interrupting the control of the mechanism by the control means when the temperature of the condenser rises to a predetermined high value, and means for returning the control of the mechanism to the control means only when the evaporator has been defrosted and the temperature of the condenser decreases to a predetermined low value.

8. In a reverse cycle heating system for a space having an evaporator for absorbing heat, a con- :lenser for dissipating heat for heating the space "ind a mechanism for controlling the circulation If refrigerant through the condenser and evap- .irator, the combination of, heating means for thereof, control means operative upondemand for heat for operating themechanism to circulate refrigerant to maintain desired temperature conditions in the space, means for interrupting contro1 of the mechanism by the control means for defrosting the evaporator, and means for operating the heating means while the evaporator is defrosting to hasten defrosting of the evaporator only when the control means is demanding heat.

9. In a reverse cycle heating system for a space having an evaporator for absorbing heat, a condenser for dissipating heat for heating the space and a mechanism for controlling the circulation of refrigerant through the condenser and evaporator, the combination of, heating means for heating the evaporator to hasten defrosting thereof, control means operative upon a demand for heat for operating the mechanism to circulate refrigerant to maintain desired temperature conditions in the space, means for interrupting the control of the mechanism by the control means for defrosting the evaporator and for returning the control of the mechanism to the control means as soon as the evaporator has been defrosted, and means for operating the heating means while the evaporator is defrosting to hasten defrosting of the evaporator only when the control means is demanding heat.

10. In a reverse cycle heating system for a space having an evaporator for absorbing heat, a condenser for dissipating heat for heating the space and a mechanism for controlling the circulation of refrigerant through the condenser andevaporator, the combination of, heating means for heating the evaporator to hasten defrosting thereof, control means operated upon a demand for heat for operating the mechanism to circulate refrigerant to maintain desired temperature conditions in the space, means for interrupting at timed intervals the control of the mechanism by the control mean for defrosting the evaporator, and means for operating the heating means while the evaporator is defrosting to hasten defrosting of the evaporator only when the control means is demanding heat.

11. In a reverse cycle heating system for a" space having an evaporator for absorbing heat, a condenser for dissipating heat for heating the space and a mechanism for controlling the circulation of refrigerant through the condenser and evaporator, the combination of, heating means for heating the evaporator to hasten defrosting thereof, control means operative upon a demand for heat for operating the mechanism to circulate refrigerant to maintain desired temperature conditions in the space, means for interrupting at timed intervals the control of the mechanism by the control means for defrosting the evaporator, means for returning the control of the mechanism to the conrol means as soon as the evaporator has been defrosted, and means for operating the heating means while the evap-. orator is defrosting to hasten defrosting of the evaporator only when the control means is demanding heat.

12. In a reverse cycle heating system for a space having an evaporator for absorbing heat, a condenser for dissipating heat for heating the space and a mechanism for controlling the circulation of refrigerant through the condenser and evaporator, the combination of, heating means forheating the evaporator to hasten defrosting thereof, control mean operative upon a heating the evaporator to hasten defrosting demand for heat for operating the mechanism to circulate refrigerant to maintain desired temperature conditions in the space, .means responsive to the formation of frost on the evaporator for interrupting the control of the mechanism space having an evaporator for absorbing heat, a

condenser for dissipating heat for heating the space and a mechanism for controlling the cit culation of refrigerant through the condenser and evaporator, the combination or, heating means fonheating the evaporator to hasten defrosting thereof, control means operative upon demand for heat for operating the mechanism to circulate refrigerant to maintain desired temperature conditions in the space, means for interrupting control of the mechanism by the control means for defrosting the evaporator, and means for Operating the heating means when the evaporator is defrosted.

ALWIN B. NEWTON.