Control Circuit For Preventing Rapid Recycling In Automatic Systems

Abstract

Undesirable rapid recycling of an automatic system such as a recycling pumpdown refrigeration or air conditioning system is prevented by a control circuit which inhibits initiation of an operating cycle for a predetermined interval of time following completion of the preceding operating cycle. When an operating cycle can begin, signals for initiating the cycle may be further delayed by another, preferably randomly selected time interval so that simultaneous starting of a large number of units in a multiunit installation is prevented.

Description

BACKGROUND OF THE INVENTION

This invention relates to circuitry for the control of automatic systems, and more particularly to circuitry for preventing undesirable rapid recycling in such systems. The invention has particular application, for example, in the control of refrigeration and air conditioning systems of the pumpdown variety, hereinafter referred to as pumpdown cooling or air conditioning systems.

Many automatic systems operate intermittently rather than continuously. Because of the increased strain on the system associated with starting and/or stopping, it is usually desirable to prevent such intermittently operating systems from recycling too rapidly (i.e., starting too soon after stopping or stopping too soon after starting). Most such systems are designed so that under normal operating conditions the system will not recycle too rapidly. Rapid recycling may occur, however, if the system is exposed to unusual or unanticipated operating conditions or if the characteristics of certain system components are subject to change with age or wear. In that event it is desirable to provide positive means for peventing rapid recycling of the apparatus.

One type of system in which the problem of rapid recycling is particularly acute is the pumpdown air conditioning system. An air conditioning system of this type typically includes an evaporator, a condenser, a compressor for pumping vaporized refrigerant from the evaporator to the condenser, and a liquid refrigerant line including an expansion valve for returning liquified refrigerant from the condenser to the evaporator. Cooling occurs as a result of the evaporation of the liquid refrigerant in the evaporator. Cooling is initiated by the opening of a thermostatically controlled valve in the liquid refrigerant line. Operation of the compressor is controlled by the refrigerant pressure on the evaporator side of the system. The compressor operates to keep the pressure on this side of the system relatively low at all times. This means that the compressor operates continuously during cooling and may in addition operate occasionally to "pump down" the refrigerant pressure in the evaporator between cooling cycles. Normally, these pumpdown compressor cycles will not be required too frequently. More frequent pumpdown cycles will be required, however, if there is residual refrigerant in the evaporator or if any of the valves in the system leak refrigerant into the evaporator. If for any reason the time between pumpdown cycles becomes too short, the resulting rapid starting and stopping of the compressor will shorten the life of the compressor motor and may cause the compressor motor to overheat and eventually burn out.

It is therefore an object of this invention to provide apparatus for preventing rapid recycling in automatically controlled systems.

It is a more particular object of this invention to provide control circuitry for preventing rapid recycling in pumpdown air conditioning systems.

SUMMARY OF THE INVENTION

These and other objects of the invention are accomplished by means of recycling system control circuitry which inhibits the initiation of a cycle of system operation for a predetermined interval of time following completion of the preceding operating cycle. In a pumpdown air conditioning system, for example, the control circuit inhibits the flow of current through the pressure switch controlling compressor operation for a predetermined time interval (called the lockout time interval) after the compressor last stopped. While compressor operation is thus inhibited, the control circuit also prevents initiation of a cooling cycle by inhibiting thermostatic control of the liquid line refrigerant valve. When the lockout time interval has elapsed, both the pressure switch and the thermostat are again enabled so that either a pumpdown cycle or a cooling cycle can begin at any time thereafter.

The control circuit of this invention is also designed to prevent operation of the controlled system for a predetermined time interval after the system is first turned on or after power is restored following any power service interruption. The control circuit may also include a time delay network for delaying the start of any operating cycle by a predetermined, preferably randomly selected delay interval so that simultaneous starting of a large number of units in a multiunit installation is prevented.

Further features and objects of the invention, its nature and various advantages will be more apparent upon consideration of the attached drawing and the following detailed description.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a conventional pumpdown air conditioning system;

FIG. 2 is a block diagram of a control system constructed in accordance with the principles of this invention for controlling a pumpdown air conditioning system like the system of FIG. 1; and

FIG. 3 is a schematic diagram of a circuit for realizing the control system of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

When the pumpdown air conditioning system of FIG. 1 is cooling, liquid line valve 14 is open and compressor 10 operates to recirculate refrigerant through the system in the direction indicated by the arrows. Compressor 10 pumps refrigerant in the form of a gas from evaporator 18 to condenser 12. The refrigerant condenses to a liquid in condensor 12 and returns to evaporator 18 through expansion valve 16. In evaporator 18 the refrigerant vaporizes again and the cycle continues. Cooling stops when liquid line valve 14 is closed and refrigerant can no longer recirculate. Valve 14 therefore controls the cooling operation and is conveniently an electrically operated valve controlled by a thermostat, e.g., a normally closed, solenoid-operated valve which opens in response to a thermostat call for cooling.

During cooling, compressor 10 operates continuously to maintain pressure on the condensor side of the system at a higher level than pressure on the evaporator side of the system. Compressor 10 is controlled by pressure switch 20 which is responsive to refrigerant pressure on the suction or low pressure side of the system. Pressure switch 20 closes and compressor 10 begins to operate whenever the suction pressure rises above a predetermined cut-in pressure. Compressor 10 continues to operate until the suction pressure drops below a predetermined cut-out pressure and switch 20 reopens. Accordingly, when valve 14 closes at the end of a cooling cycle, compressor 10 will continue to operate until pressure on the evaporator side of the system has been pumped down below the cut-out pressure. At the start of the next cooling cycle, compressor 10 will not begin to operate again until valve 14 has opened and pressure on the evaporator side of the system has risen above the cut-in pressure. Between cooling cycles, compressor 10 may operate periodically to pump down the evaporator. A pumpdown compressor cycle will occur any time the suction pressure rises above the cut-in pressure.

The pumpdown mode of operating compressor 10 is employed to keep refrigerant from accumulating and liquifying in the evaporator. Since the compressors conventionally used in such systems are intended only for pumping gases, they can be severely damaged if required to pump liquid refrigerant out of the evaporator. Normally it will only be necessary for compressor 10 to pump down evaporator 18 infrequently between cooling cycles. However, if valve 14 or any of the valves in compressor 10 leak refrigerant into the evaporator, more frequent pumpdown cycles will be required. If the time between pumpdown cycles becomes too short, the frequency starting and stopping may cause the motor of compressor 10 to overheat and eventually to burn out. This is prevented in accordance with the principles of this invention by providing a control system which ensures that the time interval between pumpdown cycles will be at least a predetermined length. A block diagram of such a control system is shown in FIG. 2.

When the control system of FIG. 2 is first turned on, current from control circuit power supply 50 is applied to lockout time delay network 52 and bistable network 54. Bistable network 54 is a network or device having stable set and reset states and which is initially reset. In its initial reset state, bistable network 54 inhibits the flow of current from power supply 50 through the coil of relay 56 until triggered to the set state by time delay network 52. Time delay network 52 triggers bistable network 54 a predetermined lockout time interval T.sub.1 after current is first applied to it. Once triggered, bistable network 54 continues to energize relay 56 until reset by reset device 74 or by an interruption in the supply of current from power supply 50. Before time delay network 52 triggers bistable network 54, time delay hold off network 64, responsive to the state of bistable network 54, prevents compressor starting time delay network 66 from timing even if the contacts of pressure switch 20 are closed. As in the pumpdown air conditioning system of FIG. 1, pressure switch 20 is responsive to the pressure of refrigerant on the low pressure side of the system (as indicated by line 19). The contacts of switch 20 close when refrigerant pressure on the low pressure side of the system rises above a predetermined cut-in pressure and remain closed until that pressure drops below a predetermined cut-out pressure. As long as the bistable network 54 is reset, time delay network 66, which may be similar to time delay network 52, is prevented from triggering bistable network 76, similar to bistable network 54. Bistable network 76 therefore inhibits the flow of current from power supply 50 through pressure switch 20 to the coil of relay 68, or (by way of path 77) to the coil of relay 56. Accordingly, neither of relays 56 and 68 can be energized until bistable network 54 is triggered by lockout time delay network 52. The normally open contacts of both of these relays therefore remain open. Power from power supply 70 cannot be applied to compressor motor 72 and compressor 10 in the pumpdown air conditioning system cannot operate regardless of the state of pressure switch 20. Pressure switch 20 is therefore effectively disabled or locked out during lockout time interval T.sub.1. Similarly, power from power supply 58 cannot be applied to valve solenoid 62 and valve 14 in the air conditioning system cannot open regardless of the condition of thermostat 60. Like pressure switch 20, thermostat 60 is therefore effectively disabled during lockout time interval T.sub.1. With both pressure switch 20 and thermostat 60 disabled, the entire system is locked out and neither a cooling cycle nor a pumpdown cycle can begin.

When time delay network 52 triggers bistable network 54 T.sub.1 time units after current is first applied to network 52, bistable network 54 energizes relay 56 and the normally open contacts of that relay close. This places solenoid 62 under the control of the thermostat 60, i.e., enables thermostat 60. If the contacts of thermostat 60 are closed, indicating a need for cooling, current from power supply 58 is applied to solenoid 62, thereby opening valve 14 and initiating a cooling cycle. If the contacts of thermostat 60 are open when relay 56 is energized, a cooling cycle will not begin until called for by thermostat 60; provided that relay 56 is still energized at that time.

When bistable network 54 is first triggered by time delay network 52, time delay hold off network 64 no longer prevents time delay network 66 from timing. This enables pressure switch 20. If the contacts of pressure switch 20 are closed, indicating a need to start compressor 10 either as part of a cooling cycle or a pumpdown cycle, compressor starting time delay network 66 begins timing. If the contacts of pressure switch 20 are open when time delay network 66 is first able to start timing, timing does not begin until pressure switch 20 closes. In any event, bistable network 76 continues to inhibit the flow of current from power supply 50 to relay 68 and (by way of path 77) to relay 56 until triggered by time delay network 66 a predetermined compressor starting time delay interval T.sub.2 after time delay network 66 begins timing. This has no effect on relay 56 which remains energized by bistable network 54. It does mean, however, that relay 68 cannot be energized until T.sub.2 time units following the triggering of bistable network 54 or the closing of switch 20, whichever happens later. When relay 68 is energized, compressor power supply 70 is connected to compressor motor 72 and compressor 10 begins to operate. The effect of time delay network 66 is to delay the starting of compressor motor 72 by T.sub.2 time units following any lockout time interval or following actuation of pressure switch 20 while the system is not locked out. Thus a cooling cycle (controlled by thermostat 60) can begin immediately after a lockout time interval (i.e., as soon as bistable network 54 is triggered); but a compressor operation (controlled by pressure switch 20) cannot begin until T.sub.2 time units after the end of a lockout time interval, whether as part of a cooling cycle or a pumpdown cycle. In large installations including several separately controlled air conditioning units, it is desirable to provide control circuits with randomly distributed compressor starting time delay intervals T.sub.2 to decrease the probability of two or more compressors starting simultaneously, and overloading the power supply. In many applications, T.sub.2 can be made relatively short as compared to T.sub.1.

When bistable network 76 is first triggered, reset device 74 resets bistable network 54, thereby cutting off the flow of current through network 54 to relay 56. Relay 56 is kept energized by current through bistable network 76. At the same time, time delay hold off network 78, responsive to the triggered or set condition of bistable network 76, shuts off time delay network 52, thereby preventing network 52 from timing and retriggering bistable network 54. Hold off network 78 keeps time delay network 52 shut off as long as bistable network 76 remains triggered.

Once compressor motor 72 is started, it continues to run until pressure switch 20 reopens. When pressure switch 20 reopens, bistable network 76 is reset by the lack of current from power supply 50 and relays 56 and 68 are deenergized and their contacts open. Hold off network 78 permits lockout time delay network 52 to begin timing. The control system is restored to its initial condition and the sequence of events described above begins again. The system is locked out when relays 56 and 68 open and it remains locked out until bistable network 54 is triggered again by time delay network 52, i.e., T.sub.1 time units after pressure switch 20 opens and compressor motor 72 stops. Just as in the case of the initial lockout time interval, during this lockout time interval neither a cooling cycle nor a pumpdown cycle can begin. There is therefore an interval of at least T.sub.1 time units at the end of either a cooling or a pumpdown cycle and the restarting of motor 72 at the start of the next cooling or pumpdown cycle. By an appropriate choice of lockout time interval T.sub.1, the too frequent starting of compressor motor 72 can be entirely prevented.

As is suggested by the foregoing, the system is also locked out for T.sub.1 time units after the restoration of power following any power service interruption. This is desirable in that it prevents placing too heavy a load on the power supply while service is being reestablished. When the power service is first interrupted, bistable networks 54 and 76 are shut off or reset due to the lack of current from power supply 50. When power is restored, the control system responds exactly as when it is first turned on, i.e., bistable network 54 is reset until lockout time delay network 52 completes its timing cycle. While bistable network 54 is reset, both thermostat 60 and pressure switch 20 are locked out and neither a cooling cycle nor a pumpdown cycle can begin.

One possible circuit for realizing the control system of FIG. 2 is shown schematically in FIG. 3. Control circuit power supply 100 supplies a positive D.C. (direct current) potential of approximately 175 volts. Elements 102 and 104 are the coils of relays 56 and 68, respectively, in the system of FIG. 2. Pressure switch 20 is connected as shown. The elements surrounded by broken lines 108 and 110 correspond to time delay networks 52 and 66, respectively, in the system of FIG. 2.

When power supply 100 is first turned on, transistor 170 is turned on by a small current flowing through relay coils 102 and 104 and voltage dividing resistors 172 and 174. This current is insufficient to cause the contacts of either relay to close. While thus turned on, transistor 170, in conjunction with resistor 176, holds transistor 122 off. Current from power supply 100 also flows through relay coil 102, diode 112, and serially connected resistors 114, 116, and 118 to timing capacitor 120. Device 124 is a programmable unijunction transistor such as type D13T1 or 2N6027 manufactured by the General Electric Company. It is initially cut off because the voltage applied to its anode by way of resistor 178 and reflective of the potential across capacitor 120 is initially lower than the voltage applied to its gate by voltage dividing resistors 126 and 128. Because transistor 122 and device 124 are both cut off, capacitor 120 begins to charge at a rate determined for the most part by the product R.sub.1 C.sub.1 where R.sub.1 is the sum of the values of resistors 114, 116, and 118 and C.sub.1 is the value of capacitor 120. Silicon controlled rectifier (SCR) 130 is cut off by device 124. Leakage current flowing through devices 124 and 130 is conducted to ground by resistor 132. Since SCR 130 is cut off, the amoung of current flowing through coil 102 remains insufficient to close the contacts of relay 56. Accordingly, valve 14 in the apparatus of FIG. 2 is locked out.

While SCR 130 is cut off as described above, transistor 136 is held on by voltage dividing resistors 138 and 140. As long as transistor 136 is on, timing capacitor 142 cannot charge and time delay network 110 is cut off (i.e., prevented from timing). While time delay network 110 is thus cut off, SCR 134 is cut off in the same way that SCR 130 is initially cut off by time delay network 108. As long as SCR 134 is cut off, no significant current can flow through pressure switch 20. Pressure switch 20 is therefore effectively disabled.

As timing capacitor 120 charges, the potential drop across it increases and the anode voltage of device 124 approaches the gate voltage. When these two voltages are approximately equal, device 124 conducts momentarily and time delay network 108 has completed its timing operation. Device 124, in conjunction with resistors 126, 128, and 178, therefore acts as a threshold detector for timing elements 114, 116, 118, and 120. The triggering of device 124 triggers SCR 130 and a greatly increased current flows from power supply 100 through relay coil 102, diode 112, and SCR 130 to ground. This increased current is sufficient to cause coil 102 to close the contacts of relay 56, thereby placing valve solenoid 62 under the control of thermostat 60. Once SCR 130 begins to conduct, it is unaffected by further changes in its gate voltage. SCR 130 therefore performs the function of bistable network 54 in the apparatus of FIG. 2. Capacitors 144 and 146 filter signals applied to devices 124 and 130, respectively to prevent premature triggering of those devices.

When SCR 130 first conducts, transistor 136 is cut off, thereby enabling time delay network 110. If pressure switch 20 is open, nothing further happens until the pressure switch closes. As soon as pressure switch 20 closes, current from current source 100 flows through relay coil 104, pressure switch 20, and serially connected resistors 150 and 152 to timing capacitor 142. Device 154 is initially cut off in the same way that similar device 125 in time delay network 108 is initially cut off. Since transistor 136 and device 154 are both cut off, capacitor 142 begins to charge at a rate determined for the most part by the product R.sub.2 C.sub.2, where R.sub.2 is the sum of the values of resistors 150 and 152 and C.sub.2 is the value of capacitor 142. As in the case of time delay network 108, SCR 134 is cut off while device 154 is cut off. Leakage current through these devices is conducted to ground by resistor 162. As capacitor 142 charges, the voltage applied to the anode of device 154 by way of resistor 156 approaches the voltage applied to the gate of device 154 by voltage dividing resistors 158 and 160. When the anode voltage is approximately equal to the gate voltage, device 154 conducts momentarily, causing SCR 134 to conduct. Sufficient current is then drawn through coil 104 to cause the contacts of relay 68 to close, thereby starting compressor motor 72 in the apparatus of FIG. 2. Once SCR 134 begins to conduct it is unaffected by further changes in its gate voltage. SCR 134 therefore performs the function of bistable network 76 in the apparatus of FIG. 2. As in the case of time delay network 108, capacitor 164 filters signals applied to device 154 to prevent premature triggering of that device.

The sudden drop in potential at node 106 when SCR 134 first conducts causes capacitor 166 to cut off SCR 130. The current flowing through relay coil 102 is diverted through diode 168, pressure switch 20, and SCR 134 so that the contacts of relay 56 remain closed. Simultaneously, voltage dividing resistors 172 and 174 turn off transistor 170, increasing the potential at the base of transistor 122 and causing transistor 122 to conduct. While transistor 122 is on, time delay network 108 is shut off and SCR 130 cannot be retriggered. The control circuit is now stable with the contacts of both of relays 56 and 68 closed and essentially all of the current in coils 102 and 104 flowing to ground through pressure switch 20 and SCR 134. The control circuit remains in this condition until pressure switch 20 reopens.

When pressure switch 20 reopens, the flow of current through coils 102 and 104 is substantially reduced and the contacts of relays 56 and 68 open. Transistors 136 and 170 turn on and transistor 122 turns off. The circuit is therefore restored to its initial condition with thermostat 60 and pressure switch 20 locked out. Time delay network 108 begins timing again and the sequence of events described above is repeated.

Three components in the circuit of FIG. 3 not yet described are diodes 180, 182, and 184. These diodes are included in the circuit as flyback diodes to prevent excessive voltage from appearing across SCRs 130 and 134 when these devices change state.

Typical values for the elements in the circuit of FIG. 3 are as follows:

Resistor 150 220 k-ohms 152 180 to 680 K-ohms 156 100 ohms 158 27 K-ohms 160 27 K-ohms 162 1 K-ohms 172 390 K-ohms 176 390 K-ohms 114 220 K-ohms 116 3.9 M-ohms 178 100 ohms 126 27 K-ohms 132 1 K-ohms 128 27 K-ohms 138 220 K-ohms 140 10 K-ohms 118 100 K-ohms 174 2 K-ohms Capacitor 142 10 .mu.f 166 .1 .mu.f 120 100 .mu.f 146 .003 .mu.f 144 .1 .mu.f 164 .1 .mu.f

Transistors 122, 136, and 170 are all type 2N5172, available, for example, from the General Electric Company. Devices 124 and 154 are General Electric Company type D13T1. Devices 130 and 134 ar General Electric Company type C103B. All diodes are type 1N4004, available, for example, from the Motorola Company. In the circuit with the foregoing components, time delay network 108 triggers after approximately 5 minutes and time delay network 110 triggers after from 2 to 10 seconds, depending on the value chosen for resistor 152. The value of resistor 152 can be randomly selected to produce control circuits in which the delay of network 110 is randomly distributed over the preceding range.

It is to be understood that the embodiments shown and described herein are illustrative of the principles of the invention only, and that various modifications may be implemented by those skilled in the art without departing from the spirit and scope of the invention. For example, other circuit components can be substituted for those mentioned specifically above. Time intervals T.sub.1 and T.sub.2 can be changed to meet the requirements of particular applications by varying the values of the various resistors and capacitors in the system. In particular, if staggered compressor starting is not desired, compressor 10 can be enabled at essentially the same time that pressure switch 20 is enabled by making compressor starting delay interval T.sub.2 very short.

Claims

What is claimed is:

1. In a recycling pumpdown cooling system including an evaporator, a condenser, a compressor for selectively pumping refrigerant from the evaporator to the condensor to recirculate refrigerant through the system during cooling cycles and to keep refrigerant from accumulating in the evaporator between cooling cycles, a refrigerant line including valve means for returning refrigerant from the condenser to the evaporator when the valve means is open, valve control means for opening the valve means in response to a signal indicating that cooling is required, and compressor control means actuated by refrigerant pressure in said system for controlling the pumping operation of the compressor, a control circuit for preventing operation of said system for a predetermined interval of time following completion of a compressor pumping operation comprising:

first bistable circuit means having stable set and reset states, said first bistable circuit means being initially reset;

second bistable circuit means responsive to the compressor control means and to said first bistable circuit means, and having stable set and reset states for enabling the compressor when set, said second bistable circuit means being reset when said compressor control means is not actuated and being enabled to change from the reset state to the set state in response to the actuation of the compressor control means when said first bistable circuit means is set; and

first timing circuit means responsive to the operation of the compressor for setting said first bistable circuit means a predetermined first time interval after the completion of a compressor pumping operation.

2. The apparatus defined in claim 1 wherein said first timing circuit means comprises means responsive to said second bistable circuit means for setting said first bistable circuit means said predetermined first time interval after the resetting of said second bistable circuit means.

3. The apparatus defined in claim 1 wherein said control circuit further comprises reset circuit means responsive to the initiation of a compressor pumping operation for resetting said first bistable circuit means.

4. The apparatus defined in claim 1 wherein said control circuit further comprises reset circuit means responsive to the setting of said second bistable circuit means for resetting said first bistable circuit means.

5. The apparatus defined in claim 1 wherein said second bistable circuit means includes means for delaying the initiation of a compressor pumping operation for a predetermined second time interval after actuation of the compressor control means.

6. The apparatus defined in claim 1 wherein said second bistable circuit means comprises:

third bistable circuit means responsive to the compressor control means and having stable set and reset states for enabling the compressor when set, said third bistable circuit means being reset when said compressor control means is not actuated; and

second timing circuit means responsive to the compressor control means and said first bistable circuit means for setting said third bistable circuit means a predetermined second time interval after actuation of the compressor control means, said second timing circuit means being enabled when said first bistable circuit means is set.

7. The apparatus defined in claim 1 wherein said control circuit further comprises means responsive to said first bistable circuit means for enabling the valve control means when said first bistable circuit means is set.

8. The apparatus defined in claim 3 wherein said control circuit further comprises:

first means responsive to said first bistable circuit means for enabling the valve control means when said first bistable circuit means is set; and

second means responsive to said second bistable circuit means for enabling the valve control means when said second bistable circuit means is set.

9. The apparatus defined in claim 1 wherein said control circuit further comprises a source of control current and wherein said first timing circuit means comprises:

a first capacitor;

a first resistor connected in series between said first capacitor and said source of control current for charging said first capacitor with said control current; and

first threshold detection means responsive to the potential across said first capacitor for producing an output signal for setting said first bistable circuit means when the potential across said first capacitor reaches a predetermined threshold level.

10. The apparatus defined in claim 9 wherein said first timing circuit means further comprises means responsive to the operation of the compressor for discharging said first capacitor while the compressor is operating.

11. The apparatus defined in claim 2 wherein said control circuit further comprises a source of control current and wherein said first timing circuit means comprises:

a first capacitor;

a first resistor connected in series between said first capacitor and said source of control current for charging said first capacitor with said control current; and

first threshold detection means responsive to the potential across said first capacitor for producing an output signal for setting said first bistable circuit means when the potential across said first capacitor reaches a predetermined threshold level.

12. The apparatus defined in claim 11 wherein said first timing circuit means further comprises means responsive to said second bistable circuit means for discharging said first capacitor while said second bistable circuit means is set.

13. The apparatus defined in claim 6 wherein said control circuit further comprises a source of control current and wherein said second timing circuit means comprises:

a second capacitor;

a second resistor connected in series between said second capacitor and said source of control current for charging said second capacitor with said control current; and

second threshold detection means responsive to the potential across said second capacitor for producing an output signal for setting said second bistable circuit means when the potential across said second capacitor reaches a predetermined threshold level.

14. The apparatus defined in claim 13 wherein said second timing circuit means further comprises means responsive to said first bistable circuit means for discharging said second capacitor while said first bistable circuit means is reset.

15. In a recycling pumpdown cooling system including an evaporator, a condenser, a compressor for selectively pumping refrigerant from the evaporator to the condensor to recirculate refrigerant through the system during cooling cycles and to keep refrigerant from accumulating in the evaporator between cooling cycles, a refrigerant line including a normally closed valve for returning refrigerant from the condenser to the evaporator when the valve is open, compressor control means actuated by refrigerant pressure in said system for controlling the pumping operation of the compressor, and valve control means for opening the normally closed valve in response to a signal indicating that cooling is required, a control circuit for preventing operation of said system for a predetermined interval of time following completion of a compressor pumping operation comprising:

first bistable circuit means having stable set and reset states for enabling the valve control means when set, said first bistable circuit being initially reset;

second bistable circuit means responsive to the compressor control means and having stable set and reset states for enabling the valve control means and the compressor when set, said second bistable circuit means being reset when the compressor control means is not actuated;

first timing circuit means responsive to said second bistable circuit means for setting said first bistable circuit means a first predetermined time interval after the resetting of said second bistable circuit means;

second timing circuit means responsive to the compressor control means and said first bistable circuit means for setting said second bistable circuit means a second predetermined time interval after actuation of the compressor control means when enabled by said first bistable circuit means, said second timing circuit means being enabled when said first bistable circuit is set; and

reset circuit means responsive to the setting of said second bistable circuit means for resetting said first bistable circuit means.

16. The apparatus defined in claim 15 wherein said control circuit further comprises a source of control current and wherein said first bistable circuit means comprises a first relay having normally open contacts for enabling the valve control means when closed and a first silicon controlled rectifier device, the coil of said first relay being connected in series between said source of control current and the anode of said first silicon controlled rectifier device and the cathode of said first silicon controlled rectifier device being connected to ground.

17. The apparatus defined in claim 16 wherein the compressor control means includes a second relay having normally open contacts for enabling the compressor when closed and a switch having first and second terminals and contacts for completing a circuit between the first and second terminals when the compressor control means is actuated and wherein said second bistable circuit means comprises a second silicon controlled rectifier device, the coil of the relay being connected in series between said source of control current and the first terminal of the switch, the second terminal of the switch being connected to the anode of said second silicon controlled rectifier device, and the cathode of said second silicon controlled rectifier device being connected to ground.

18. The apparatus defined in claim 17 wherein said first timing circuit comprises:

a first capacitor;

a first resistor connected in series between said first capacitor and the coil of said first relay for charging said first capacitor with current flowing from the source of control current through the coil of said first relay; and

first threshold detection means responsive to the potential across said first capacitor for producing an output signal applied to the gate of said first silicon controlled rectifier device for causing said first silicon controlled rectifier device to conduct when the potential across said first capacitor reaches a predetermined threshold level.

19. The apparatus defined in claim 18 wherein said first threshold detection means comprises:

first and second serially connected voltage dividing resistors connected in parallel with said first resistor and said first capacitor; and

a first programmable unijunction transistor device connected in parallel with said first capacitor by its anode and cathode, the gate of said first programmable unijunction transistor device being connected between said first and second voltage dividing resistors and the cathode of said first programmable unijunction transistor device being connected to the gate of said first silicon controlled rectifier device.

20. The apparatus defined in claim 19 wherein said first timing circuit means further comprises first means responsive to said second bistable circuit means for discharging said first capacitor while said second bistable circuit means is set.

21. The apparatus defined in claim 20 wherein said second timing circuit comprises:

a second capacitor;

a second resistor connected in series between said second capacitor and the second terminal of the switch for charging said second capacitor with current flowing from the source of control current through the contacts of the switch; and

second threshold detection means responsive to the potential across said second capacitor for producing an output signal applied to the gate of said second silicon controlled rectifier device for causing said second silicon controlled rectifier device to conduct when the potential across said second capacitor reaches a predetermined threshold level.

22. The apparatus defined in claim 21 wherein said second threshold detection means comprises:

third and fourth serially connected voltage dividing resistors connected in parallel with said second resistor and said second capacitor; and

a second programmable unijunction transistor device connected in parallel with said second capacitor by its anode and cathode, the gate of said second programmable unijunction transistor device being connected between said third and fourth voltage dividing resistors and the cathode of said second programmable unijunction transistor device being connected to the gate of said second silicon controlled rectifier device.

23. The apparatus defined in claim 22 wherein said second timing circuit means further comprises second means responsive to said first bistable circuit means for discharging said second capacitor while said first bistable circuit means is reset.

24. The apparatus defined in claim 23 wherein said second bistable circuit means further comprises a diode connected between the coil of said first relay and the first terminal of the switch for permitting current to flow from the coil to the switch.

25. The apparatus defined in claim 24 wherein said reset circuit means comprises a capacitor connected in series between the anode of said first silicon controlled rectifier device and the anode of said second silicon controlled rectifier device.

26. The apparatus defined in claim 25 wherein said control circuit further comprises a first flyback diode connected in parallel with the coils of said first and second relays, the cathode of said first flyback diode being connected to said source of control current.