HK1161624B - Compressor discharge control on a transport refrigeration system - Google Patents

Description

Compressor discharge control on transport refrigeration system

Cross Reference to Related Applications

This application, to which reference is made and from which priority is claimed for U.S. provisional application No. 61/100,445 entitled "COMPOSITRESSORDISCHHARGENICOTROLONATRANSPORTREFRIGERATION SYSTEM" filed on 26.9.2008, the entire contents of which are incorporated herein by reference.

Technical Field

The present disclosure relates generally to transport refrigeration units and more particularly to controlling compressor discharge superheat without a quench valve (quench).

Background

Transport refrigeration systems for controlling enclosed areas, such as insulated boxes used on trucks, trailers, containers, or similar intermodal units, function by absorbing heat from the enclosed area and releasing the heat out of the box into the environment. Transport refrigeration systems typically include a compressor to pressurize refrigerant vapor and a condenser to cool the pressurized vapor from the compressor to change the state of the refrigerant from a gaseous state to a liquid state. Ambient air may be blown over the refrigerant coils in the condenser to effect heat exchange. The transport refrigeration system also includes an evaporator for drawing heat out of the tank by drawing or pushing return air over a coil containing refrigerant within the evaporator. This step evaporates any remaining liquid refrigerant flowing through the evaporator, which may then be drawn through a Suction Modulation Valve (SMV) and back into the compressor to complete the cycle. The system may include a thermostatic expansion valve (TXV) in the refrigerant line upstream of the evaporator that is responsive to superheat (superheat being defined as the difference between the sensed vapor temperature and the saturation temperature at the same pressure) generated in the evaporator. The transport refrigeration system also typically includes an electrical generator adapted to generate an AC current of a selected voltage and frequency to operate the compressor drive motor to drive the refrigeration compressor.

Some refrigeration systems, including transport refrigeration, need to operate with reduced capacity to maintain products within a very narrow temperature range. Suction modulation is employed in some cases to reduce and adjust capacity. This affects the suction and discharge temperatures. When suction regulation occurs at high ambient temperatures, the refrigerant supplied to the compressor may be too hot, lacking some corrective measures, resulting in a too high compressor discharge temperature.

In addition, refrigeration systems operating at low suction density and low mass flow conditions and high compression ratios require additional compression temperature control. In other refrigeration systems, such as mobile container systems used in tropical climates, high ambient temperatures have a detrimental effect on refrigerant temperature, particularly compressor discharge temperature. If the discharge temperature is not prevented from becoming too high, the compressor lubricant may decompose and eventually lead to compressor failure.

A typical method for controlling the discharge temperature of the compressor includes injecting liquid refrigerant via an economizer/vapor injection port on the compressor using a liquid injection circuit. One method of injecting liquid refrigerant is to use a solenoid operated valve, commonly referred to as a quench valve. The quench valve bypasses the evaporator, i.e. the liquid line branches upstream of the evaporator and dumps at the compressor suction.

Unfortunately, refrigeration systems employing quench valves increase complexity, which increases cost. The increased complexity also makes system packaging more difficult in the confined space of transport refrigeration systems. In addition, additional control parameters must be designed and implemented into the system controller.

Another drawback of systems using quench valves is that liquid refrigerant bypasses the evaporator, thereby reducing system efficiency. Also, solenoid valve control of compressor superheat is made more difficult because a large amount of liquid is dumped into the compressor suction. Too much liquid refrigerant can also cause backflow to the compressor and can ultimately lead to compressor failure.

Disclosure of Invention

A stable system and procedure is provided to control the degree of compressor superheat without the use of a quench valve.

In a refrigeration system having a compressor, a condenser, an evaporator and a controller for controlling an expansion valve, a program is provided for controlling compressor discharge during a cooling cycle, comprising the steps of: monitoring a compressor discharge parameter, comparing the compressor discharge parameter to a set point stored in a controller memory, and selectively operating the expansion valve upstream of the evaporator in response to a difference between the compressor discharge parameter and the set point. The compressor discharge parameter may be temperature, wherein the set point value may be about 132 degrees celsius.

The program may further include the steps of: an ambient temperature and a return air temperature are monitored and the ambient temperature, the return air temperature, and the compressed discharge parameter are compared to a first predetermined limit stored in the controller memory. Initializing a program for controlling compressor discharge during a cooling cycle only when the ambient temperature, the air return temperature, and the compressor discharge parameter comply with the first predetermined limit. The first predetermined limit may be the ambient temperature being greater than about 43 degrees celsius, the return air temperature being less than about minus 18 degrees celsius, and the compressor discharge temperature being greater than about 118 degrees celsius.

The program may further include the steps of: the program is stopped if the program parameters meet the second predetermined limit. The program parameter may be a return air temperature or an ambient air temperature, and the second predetermined limit may be greater than about 18 degrees celsius, and less than about 38 degrees celsius, respectively.

The step of operating the expansion valve may include operating the expansion valve without separately injecting liquid refrigerant at a location between the compressor inlet and the evaporator outlet.

Drawings

For a further understanding of the invention, reference will be made to the following detailed description of the invention which is to be read in connection with the accompanying drawings, wherein:

FIG. 1 schematically illustrates a prior art refrigeration system;

FIG. 2 schematically illustrates an exemplary embodiment of a refrigeration system according to the present invention; and

FIG. 3 is a block diagram generally representing a flow diagram illustrating an exemplary embodiment of a routine for controlling compressor superheat during operation of a refrigeration system.

Detailed Description

Fig. 1 shows a schematic diagram of an exemplary embodiment of a refrigerant vapor compression system 10, such as a conventional prior art transport refrigeration system. Such a system 10 generally includes a compressor 12, such as a reciprocating compressor, driven by a motor 14 to compress a refrigerant. In this compressor, the refrigerant is compressed to a higher temperature and pressure. The refrigerant then moves to a condenser 16, which may be an air cooled condenser. The condenser 16 includes a plurality of condenser coiled fins and tubes 18 that receive air that is typically blown by a condenser fan (not shown). By removing latent heat in this step, the refrigerant condenses into a high pressure/high temperature liquid and flows to the receiver 20, which provides storage for excess liquid refrigerant during low temperature operation. From the receiver 20, the refrigerant flows through a subcooler unit 22, then to a filter-drier 24 that keeps the refrigerant clean and dry, and then to a heat exchanger 26 that increases the subcooling of the refrigerant. Eventually, the refrigerant reenters the compressor 12 after passing through the evaporator 28. The flow rate of refrigerant through the evaporator 28 in this prior art technique will be regulated by a mechanical thermostatic expansion valve ("TXV") 30 in response to feedback from the evaporator through an expansion valve bulb (bulb) 32. The expansion valve 30 adjusts the amount of refrigerant delivered to the evaporator 28 to establish a predetermined superheat, hereinafter referred to as Evaporator Superheat (ESH) 33, at the evaporator outlet. As the liquid refrigerant passes through the opening of the expansion valve 30, at least a portion thereof evaporates. The refrigerant then flows through the tubes or coils 34 of the evaporator 28, and the tubes or coils 34 absorb heat from the return air (i.e., air returning from the tank) and in so doing evaporate the remaining liquid refrigerant. The return air is preferably drawn or forced across the tubes or coils 34 by at least one evaporator fan (not shown). The refrigerant vapor is then drawn from the evaporator 28 back into the compressor 12 through a suction modulation valve ("SMV") 36.

The prior art refrigerant vapor compression system 10 also includes a liquid injection valve ("LIV") 38, or quench valve, which liquid injection valve 38 connects the liquid line from the receiver 20 to the suction line at a point between the suction modulation valve 36 and the compressor 12. LIV36 has a sensing bulb 40 located on the compressor discharge line. In operation, the LIV36 is controlled in response to superheat measured at the compressor discharge. If the superheat sensed by ball 40 is above a predetermined value, LIV36 opens to allow liquid refrigerant to enter the compressor suction. Once the ball 40 senses overheating within predetermined limits, the LIV36 closes.

Referring to fig. 2, an exemplary embodiment of a refrigerant vapor compression system 100 according to the present disclosure is schematically illustrated, wherein like reference numbers refer to like elements from fig. 1. The refrigerant, which in the disclosed embodiment is R134A, is used to cool the tank air (i.e., the air within a container or trailer or truck) of the refrigerant vapor compression system 100. In the illustrated embodiment, the compressor 112 is a scroll compressor, but may be other compressors such as reciprocating compressors or screw compressors without limiting the scope of the present disclosure. The motor 114 may be an integral electric drive motor driven by a synchronous generator (not shown) operating at low (e.g., 45 Hz) or high (e.g., 65 Hz) speeds. However, another embodiment of the present disclosure specifies the motor 114 as a diesel engine, such as a four cylinder, 2200cc displacement diesel engine operating at high speed (about 1950 RPM) or low speed (about 1350 RPM).

The high temperature, high pressure refrigerant vapor leaving the compressor 112 then moves to an air cooled condenser 116. the air cooled condenser 116 includes a plurality of condenser coiled fins or tubes 114. the coiled fins or tubes 114 receive air that is typically blown by a condenser fan 146. By removing latent heat through this step, the refrigerant condenses into a high pressure/high temperature liquid and flows to the receiver 120, which stores excess liquid refrigerant during low temperature operation. From the receiver 120, the refrigerant flows to the filter-dryer 124, which keeps the refrigerant clean and dry, and then through the economizer heat exchanger 148, which increases the subcooling of the refrigerant.

From the economizer heat exchanger 148, the refrigerant flows to an electronic expansion valve ("EXV") 150. As the liquid refrigerant flows through the openings of the EXV, at least a portion thereof evaporates. The refrigerant then flows through the tubes or coils 152 of the evaporator 128, and the tubes or coils 152 absorb heat from the return air 154 (i.e., air returning from the tank) and in so doing evaporate the remaining liquid refrigerant. The return air is preferably drawn or forced across the tubes or coils 152 by at least one evaporator fan 156. The refrigerant vapor is then drawn from the evaporator 128 back into the compressor through a suction service valve 137.

The system 100 also includes an economizer circuit 158. When the circuit is active, valve 160 opens to allow refrigerant to pass through an auxiliary expansion valve 162, the auxiliary expansion valve 162 having a sensing bulb 164 upstream of a mid-section inlet 167 of the compressor 112. The valve 162 is controlled in response to the temperature measured at the ball 164 and serves to expand and cool the refrigerant advancing into the economizer reverse flow heat exchanger 148, thereby subcooling the liquid refrigerant advancing to the EXV 150.

The system 100 also includes a digital unloader valve 166 that connects the discharge of the compressor 112 to the suction inlet. When the system 100 creates an excessive pressure differential or amperage (amperadraww), the unloader valve 166 opens and balances the pressure between discharge and suction, causing the scroll device (scrollet) to separate and stop the refrigerant flow.

Many points within the refrigerant vapor compression system 100 are monitored and controlled by the controller 550. Controller 550 includes a microprocessor 552 and its associated memory 554. The memory 554 of the controller 550 may include operator or owner preselected, desired values for various operating parameters within the system 100, including, but not limited to, temperature set points at various locations within the system 100 or tank, pressure limits, current limits, engine speed limits, and any of a variety of other desired operating parameters or limits within the system 100. In the disclosed embodiment, the controller 550 includes a microprocessor board 556 including a microprocessor 552 and a memory 556, an input/output (I/O) board 558 including an analog-to-digital converter 560, the analog-to-digital converter 560 receiving temperature and pressure inputs, AC current inputs, DC current inputs, voltage inputs, and humidity level inputs from various points in the system. In addition, the I/O board 558 includes drive circuits or field effect transistors ("FETs") and relays that receive signals or current from the controller 550 and, in turn, control various external or peripheral devices within the system 100, such as the EXV 150.

The specific sensors and transducers monitored by the controller 550 include: a Return Air Temperature (RAT) sensor 168 that inputs a variable resistance value into microprocessor 552 as a function of evaporator return air temperature; an Ambient Air Temperature (AAT) sensor 170 which inputs a variable resistance value into microprocessor 552 according to the ambient air temperature read before condenser 116; a Compressor Suction Temperature (CST) sensor 172 which inputs a variable resistance value to the microprocessor according to the compressor suction temperature; a Compressor Discharge Temperature (CDT) sensor 174 that inputs a resistance value to microprocessor 552 based on the compressor discharge temperature inside the dome (dome) of compressor 112; an evaporator outlet temperature (EVOT) sensor 176 that inputs a variable resistance value to microprocessor 552 according to the outlet temperature of evaporator 128; a Compressor Suction Pressure (CSP) transducer 178 that inputs a variable voltage to microprocessor 552 according to the compressor suction value of compressor 112; a Compressor Discharge Pressure (CDP) transducer 180 which inputs a variable voltage to microprocessor 552 according to the compressor discharge value of compressor 112; an evaporator outlet pressure (EVOP) transducer 182 that inputs a variable voltage to microprocessor 552 according to the evaporator outlet pressure or evaporator 128; dc sensor 186 and ac sensor 188 (CT 1 and CT2, respectively) input variable voltage values to microprocessor 552 corresponding to the current drawn by system 100.

One of the improvements of the present invention is the elimination of the filling valve (LIV) and associated plumbing and control elements. Whereas prior art refrigeration systems rely heavily on the injection of liquid refrigerant into the inlet of the compressor stage to control the extent of compressor superheat, the present disclosure presents a unique procedure to control compressor superheat without relying on an LIV, as described in detail below.

In the basic implementation of the disclosed embodiment, microprocessor 552 uses inputs from EVOP sensor 182 and EVOT sensor 176 to calculate evaporator coil evaporator superheat and stores the calculation in memory module 133, using algorithms known to those of ordinary skill in the art. Microprocessor 552 then compares the calculated evaporator superheat value to a preselected desired superheat value or set point stored in memory 556. Microprocessor 552 is programmed to actuate EXV150 in accordance with the difference between actual and desired superheat in order to maintain the desired superheat setting (i.e., minimum superheat, thereby maximizing unit capacity). Microprocessor 552 can be programmed to maintain a minimum setting for superheat that will remain in control and still not result in backflow (i.e., liquid refrigerant spills into the compressor). This value will vary depending on the capacity and the particular configuration of the system and can be determined experimentally by one of ordinary skill in the art. This minimum level of superheat may then be used as a "base" setting from which superheat excursions are made at various operating and/or ambient conditions.

It has been appreciated in the basic embodiment discussed above that the attendant superheat generated within the compressor 112 exceeds safety limits in some operating conditions. One example of such a condition is when the ambient temperature is greater than 43.3 ℃ (110 ° f), the return air temperature is less than-18 ℃ (0 ° f), and the compressor discharge temperature is greater than 118 ℃ (224.4 ° f). The inventors have found that conventional control techniques (i.e., controlling evaporator superheat) are ineffective at preventing compressor discharge superheat if the quench valve is removed from the system and the above conditions are met. In the basic embodiment, the compressor discharge temperature continues to rise. To address this, the evaporator superheat setpoint is continually lowered in an effort to add more liquid refrigerant to the compressor 112. However, even when the evaporator superheat set point is 1.5 ℃, there is insufficient liquid refrigerant delivered to the compressor 112 to maintain the discharge temperature at acceptable operating limits. In addition, lowering the set point results in zero superheat, meaning that the refrigerant is within the dome of the PH map. This condition renders the expansion valve 150 unstable since the liquid/vapor composition (mass) is not determinable at operating temperature and pressure. A control algorithm different from the basic embodiment is required in order to control the superheat generated in the compressor.

Referring to fig. 2 and 3, a routine 200 for controlling compressor discharge superheat during a cooling cycle is shown. The routine 200 includes a step 210 of operating in the basic implementation mode, wherein, in the disclosed example, control of the EXV150 is responsive to the evaporator 128 overheating. At step 212, the RAT sensor 168, AAT sensor 170, and CDT sensor 174 are monitored. At step 214, the monitored value is compared to a first predetermined limit stored within the controller 550. If the first predetermined limit is not met in step 216, control of the system 100 remains in the base implementation. In the disclosed embodiment, the first predetermined limit is: the ambient air temperature is greater than 43.3 ℃ (110 ° f), the return air temperature is less than-18 ℃ (0 ° f), and the compressor discharge temperature is greater than 118 ℃ (244.4 ° f). If the first predetermined limit is met, control of the EXV150 is selected to be responsive to compressor discharge parameters.

At step 218, the set point for microprocessor 552 to control EXV50 is changed from the evaporator superheat set point to a compressor discharge parameter. Within the disclosed embodiment, the compressor discharge parameter is the compressor discharge temperature as sensed by the CDT 174. In yet another embodiment, the compressor discharge parameter is compressor superheat as calculated using the CDT sensor 174 and CDP sensor 180, which will be discussed below. This set point is initialized with a value equal to the then-existing reading from the CDT sensor 174. This initialization procedure results in substantially zero error between the setpoint and the location of the EXV150 and is free of large initialization errors of the EXV.

After initialization, the final set point for the compressor discharge parameter is input to the microprocessor along with instructions to reach the set point within a predetermined time period, step 220. In the disclosed example, the set point is the compressor discharge temperature equal to 132.2 ℃ (270 ° f) for a period of 90 seconds. As can be seen from the above example, the control algorithm is initialized when the compressor discharge temperature is below the set point. The inventors have discovered that if the routine 200 is initiated before the compressor discharge temperature rises to the desired set point, the system 100 is easier to control and the set point is also easier to achieve. If the routine 200 is initialized when the compressor discharge temperature is above the set point, the system 100 is less accessible for control.

In one example, routine 200 employs a Proportional Integral Derivative (PID) controller to correct for errors between a measured compressor discharge parameter and a desired set point. The PID calculates and then outputs a corrective action that can adjust the EXV150 to bring the compressor discharge temperature closer to the set point. The proportional value determines the reaction to the current error, the integral value determines the reaction based on the sum of recent errors, and the differential value determines the reaction to the rate of change of error. Taken together, the weighted sum of these three values is used to adjust the compressor discharge parameter via the location of the EXV 150. In the disclosed example, the set point values for the PIDs are changed as disclosed herein, while the proportional, integral and differential values remain the same as those employed in prior art systems.

The process 200 continues until the return air temperature or ambient temperature meets a second predetermined limit, or an alarm condition is encountered. At step 222, various system diagnostic monitoring checks are performed, and if any alarm conditions are encountered, the procedure 200 is terminated and the system 100 is shut down or remedial action is taken. In one example, if the compressor discharge temperature (as measured by CDT 174) is approximately equal to the ambient temperature (as measured by AAT 170) within ten minutes, an alarm code is issued indicating that the discharge temperature sensor has failed.

At step 224, a check is performed to determine if the conditions warrant a return to the base implementation mode of operation. If the program parameters meet the second predetermined limit, the control algorithm returns to the basic implementation mode at step 226 and the process 200 resumes at step 210. In one example, the program parameter is the return air temperature as sensed by the RAT sensor 168, and the second predetermined limit is greater than-17.8 ℃ (0 ° f). In another example, the program parameter is the outside air temperature as sensed by the AAT sensor 170, and the second predetermined limit is less than 37.8 ℃ (100 ° f). In other exemplary embodiments, a second predetermined limit on program parameters may cause program 200 to become substantially the basic implementation.

As discussed above, in another embodiment of the present disclosure, the routine for controlling compressor discharge superheat is controlled by different compressor discharge parameters (e.g., compressor superheat as calculated by the CDT sensor 174 and the CDP sensor 180). In this embodiment, at step 212, the compressor discharge pressure as sensed by the CDP sensor 180 is also monitored. At step 217, microprocessor 552 calculates a Compressor Discharge Superheat (CDSH) value 192 and stores this value in memory 554. The CDSH value 192 is determined by first calculating the compressor discharge saturation temperature using the value sensed by the CDP sensor 180 and known algorithms, and then subtracting the compressor discharge saturation temperature from the sensed compressor discharge temperature. This set point is initialized at initialization step 218 with a value equal to the then-existing CDSH value 192. At step 220, the compressor superheat setpoint is entered at 22.8 ℃ (73 ° f) and the time period to reach the setpoint is 90 seconds.

One advantage of the disclosed system 100 is its low complexity. The elimination of the liquid quench valve and associated plumbing and control elements simplifies the design and reduces manufacturing costs.

Another advantage of the disclosed system 100 and process 200 is that it is more efficient. As can be seen with reference to fig. 1, the liquid injection valve 138 and associated plumbing substantially bypass the evaporator 128. When the LIV138 is open, the system 100 efficiency decreases due to the amount of refrigerant bypassed decreasing the capacity of the evaporator 128.

Another advantage is improved stability of the system 100. In the prior art system shown in FIG. 1, LIV138 is a solenoid valve. Due to its design, the valve either opens or closes, which results in a large amount of liquid refrigerant being dumped into the compressor 112 suction port. The large amount of liquid may cause instability in the compressor 112. Removing the LIV138 also removes a source of instability.

Claims (19)

1. In a refrigerant vapor compression system having a compressor, a condenser, an evaporator, and a controller for controlling an expansion valve, a method for controlling compressor discharge during a cooling cycle, comprising the steps of:

actuating the expansion valve in a base mode in dependence on a difference between an actual superheat and a desired superheat of the evaporator;

monitoring compressor discharge parameters, ambient temperature and return air temperature;

comparing the ambient temperature, the return air temperature, and the compressor discharge parameter to a first predetermined limit stored in the controller memory;

beginning only when the ambient temperature, the return air temperature, and the compressor discharge parameter meet the first predetermined limit:

-comparing the compressor discharge parameter to a set point stored in a controller memory; and

-selectively operating the expansion valve upstream of the evaporator in response to a difference between the compressor discharge parameter and the set point.

2. The method of claim 1, wherein operating the expansion valve is performed without separately injecting liquid refrigerant at a location between an inlet of the compressor and an outlet of the evaporator.

3. The method of claim 1, wherein the compressor discharge parameter is a compressor discharge temperature.

4. The method of claim 3, wherein the set point value is greater than the compressor discharge temperature.

5. The method of claim 4, wherein the set point is approximately 132 degrees Celsius.

6. The method of claim 1, wherein the compressor discharge parameter is superheat.

7. The method of claim 3, wherein the first predetermined limit is:

the outside temperature is more than 43 ℃;

the return air temperature is less than minus 18 degrees Celsius; and is

The compressor discharge temperature is greater than 118 degrees celsius.

8. The method of claim 1, further comprising the steps of: the method is stopped if the process parameters, including the return air temperature and the outside air temperature, meet second predetermined limits.

9. The method of claim 8 wherein the process parameter is return air temperature and the second predetermined limit is greater than minus 18 degrees celsius.

10. The method of claim 8, wherein the process parameter is ambient air temperature and the second predetermined limit is less than 38 degrees celsius.

11. The method of claim 1, wherein the expansion valve is an electronic expansion valve.

12. A refrigerant vapor compression system comprising:

a compressor for compressing a refrigerant, the compressor having a suction port, a discharge port, and a compressor discharge sensor operatively coupled to the discharge port, the compressor discharge sensor configured to provide a compressor discharge parameter;

an air-cooled heat exchanger operatively coupled to the discharge of the compressor;

an evaporator heat exchanger operatively coupled to the air cooling heat exchanger and the suction inlet of the compressor, and at least one of an evaporator outlet pressure sensor or an evaporator outlet temperature sensor operatively coupled to the evaporator;

an expansion valve coupled to an inlet of the evaporator for at least partially evaporating the refrigerant entering the evaporator;

an outside air temperature sensor for monitoring an outside air temperature;

a return air temperature sensor for monitoring a return air temperature; and

a controller operatively associated with the expansion valve, the controller configured to:

-actuating the expansion valve in a basic manner depending on the difference between the actual superheat and the desired superheat of the evaporator;

-monitoring the ambient air temperature sensor, the return air temperature sensor and the compressor discharge sensor;

-comparing the ambient air temperature, the return air temperature, and the compressor discharge parameter to a first predetermined limit stored in the controller memory; and

-controlling the expansion valve in response to a difference between a set point and the compressor discharge parameter only when the ambient air temperature, the return air temperature and the compressor discharge parameter comply with the first predetermined limit.

13. A refrigerant vapor compression system as recited in claim 12 wherein said controller comprises a proportional integral derivative controller.

14. A refrigerant vapor compression system as recited in claim 12 wherein said expansion valve is an electronic expansion valve.

15. The refrigerant vapor compression system as recited in claim 12 wherein the compressor discharge sensor is at least one of the compressor discharge temperature sensor or the compressor discharge pressure sensor.

16. The refrigerant vapor compression system as recited in claim 15 wherein the compressor discharge parameter is a compressor discharge temperature, the set point is 118 ℃, and the controller is configured to control the expansion valve when the compressor discharge temperature is greater than the set point.

17. The refrigerant vapor compression system as recited in claim 16 wherein the controller is further configured to control the expansion valve when the return air temperature sensor reads less than-18 ℃.

18. The refrigerant vapor compression system as recited in claim 16 wherein the controller is further configured to control the expansion valve when the outside air temperature sensor reads greater than 43 ℃.

19. The refrigerant vapor compression system as recited in claim 12 wherein the discharge parameter is superheat.