Abstract:
In vapor compression refrigeration systems a mechanism and method are provided for protecting a compressor from failures related to lack of superheat, loss of lubricating oil and other system malfunctions. Also provided is a mean of monitoring system conditions and providing service personnel with a quick manner of diagnosing problems.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to refrigeration, air conditioning and heat pumps and more particularly to a method and apparatus for monitoring and controlling system pressures, temperatures, and superheat and subcooling.  
         [0002]     In the operation of refrigeration and air conditioning systems, the cooling effect is provided by the change in state of the refrigerant from a liquid to a gas in the evaporator of the system. The gaseous refrigerant is compressed by a compressor and is condensed to a liquid state in a condenser before passing through an expansion valve upon being returned to the evaporator.  
         [0003]     The failure of a compressor is usually very costly. Most compressor failures can be traced back to one of the following system conditions: “refrigerant floodback”, “flooded starts”, “slugging”, excessively high discharge temperature or loss of lubricating oil.  
         [0004]     “Refrigerant floodback” results when liquid refrigerant returns to the compressor during the running cycle. The lubricant oil becomes mixed with refrigerant to the point that it cannot properly lubricate the load bearing surfaces. This situation can usually be prevented by 1) maintaining proper evaporator and compressor superheat, 2) correcting abnormally low load conditions, and/or 3) installing accumulators to stop uncontrolled liquid return.  
         [0005]     “Flooded starts” are the result of liquid refrigerant vapor migrating to the crankcase oil during the off cycle. When the compressor starts, the diluted oil cannot properly lubricate the load bearing surfaces causing erratic wear. This situation can be prevented by 1) locating the compressor in a warm ambient location or installing a continuous pump down system, and/or 2) installing a crankcase heater.  
         [0006]     “Slugging” is the result of trying to compress liquid refrigerant and/or oil in the compressor cylinders. Slugging is an extreme floodback condition. This situation can be prevented by 1) maintaining proper evaporator and compressor superheat, 2) correcting abnormally low load conditions, 3) installing accumulators to stop uncontrolled liquid return, and/or 4) locating the compressor in warm ambient location or installing a continuous pump down system.  
         [0007]     In the case of excessively high discharge temperature the compressor head and cylinders become so hot that the oil loses its ability to lubricate properly. This causes rings, pistons and cylinders to wear resulting in “blow by”, leaking valves and metal debris in the oil. Excessively high discharge temperature can be corrected by 1) correcting abnormally high load conditions, 2) correcting high discharge pressure conditions, 3) providing proper compressor cooling, and/or 4) providing proper compressor cooling.  
         [0008]     Loss of oil is the result of insufficient oil in the crankcase to properly lubricate the load bearing surfaces. When there is not enough refrigerant mass flow in the system to return oil to the compressor as fast as it is pumped out, there will be a uniform wearing or scoring of all load bearing surfaces. To protect against loss of oil 1) check oil failure control operation, 2) check system refrigerant charge, and/or 3) correct abnormally low load conditions or short cycling.  
         [0009]     Superheat is defined as the temperature of vapor above the boiling point temperature of its liquid at that pressure. It is calculated in vapor compression refrigeration systems by 1) converting the suction side line pressure to a saturated vapor temperature, using a temperature/pressure chart for the specified refrigerant, 2) measuring the suction side line temperature six inches from the inlet to the compressor, and 3) subtracting the pressure to temperature conversion from the suction line temperature. The result is the system superheat.  
         [0010]     Subcooling is a measure of the heat being dissipated to the atmosphere at the exterior heat exchange coils. It is calculated in vapor compression refrigeration systems by 1) converting the discharge line pressure to temperature, using a temperature/pressure chart for the specified refrigerant, 2) measuring the line temperature at the liquid line service port, and 3) subtracting the pressure to temperature conversion from the line temperature. The result is the system subcooling temperature.  
         [0011]     In the prior art U.S. Pat. No. 4,563,878 describes a method for compressor protection from low superheat conditions by system shutdown, but does not indicate what caused the failure conditions. Investigation would have to begin, usually involving the connection of refrigerant pressure gauges. Each time that such industry standard gauges are connected to service ports on the refrigerant lines refrigerant is allowed to escape to the atmosphere. Repeated connections and disconnections allow a significant enough volume of refrigerant to escape and make up refrigerant must be supplied to maintain a proper system charge. Another cause for concern is introduction of foreign materials from the gauge set into the system.  
         [0012]     Other prior art addresses the monitoring of superheat and initiate compressor shutdown. U.S. Pat. No. 5,209,076 details a microprocessor based control device which will shut a compressor down if a low superheat state is entered. This device does not monitor or display other system parameters such as temperature and pressure. Further it does not provide simple system status indicators for the actual condition of superheat.  
         [0013]     U.S. Pat. No. 5,209,076 does not include in its functionality the options for energizing external subcooling or fan cycling relays. This device also relies on analog to digital converters for refrigerant pressure sensing, a method with inherent inaccuracies that would not provide the level of accurate control required with a refrigeration system under heavy load conditions.  
         [0014]     Other examples of prior art have been found, such as U.S. Pat. No. 4,038,061 and U.S. Pat. No. 4,545,212 that monitor system conditions and initiate some action towards compressor protection. These devices, however, only address one or two dangerous systems conditions and fail to provide and adequate level of compressor protection.  
       SUMMARY OF THE INVENTION  
       [0015]     Accordingly it is the object of the present invention to protect a refrigeration system or air conditioner compressor against adverse operating conditions such as floodback, slugging, excessively high discharge temperature and loss of oil. The present invention will also protect the system against high refrigerant line pressure and low refrigerant line pressure.  
         [0016]     This and other objects of the present invention are attained by monitoring and controlling the system pressures (high and low), temperatures (both system and ambient), superheat and subcooling. The present invention uses a microprocessor to sense pressure inputs using pressure transducers (example: 4-20 mili-amp, 0-10 Volt DC, or resistance) for high side and low side pressures. Temperature sensors are connected to the liquid line, the suction line and positioned to sense outdoor ambient temperature. This information is conveyed to the microprocessor. With this information the firmware installed in the microprocessor can calculate the superheat, subcooling, discharge temperature, high side pressure, low side pressure and outdoor temperature. Through the use of control relays the microprocessor can protect the compressor from mechanical failure from the previously mentioned conditions.  
         [0017]     The present invention will also prove useful on initial start up of a refrigeration system or air conditioner by checking the system&#39;s refrigerant charge and superheat with the manufacturer&#39;s specifications for both.  
         [0018]     It is also the purpose of this invention to assist a service technician in determining the present state of the refrigeration system and in rapidly determining what fault condition(s) may have occurred. This is done through the use of a bank of LED indicators showing the present superheat temperature status (“OK”, “WARNING” or “FAILURE”), the low pressure condition, high pressure condition, discharge temp condition, and sensor status. This bank of LEDs represents the first line of diagnosis, allowing even moderately skilled individuals to quickly determine the system&#39;s condition and/or reasons for failure. In the past this would have entailed the connection of a set of test gauges to service ports on the refrigeration system, allowing some refrigerant to escape to the environment as well as possibly allowing non-condensables to enter the system.  
         [0019]     Further detailed values can be displayed on an LCD digital display which can show all data collected and calculated by the microprocessor.  
         [0020]     The present invention is designed to work in conjunction with most sensing and metering devices presently used on refrigeration systems. Examples of these devices include capillary tube, thermostatic expansion valves, fixed orifice and electronic expansion valves.  
         [0021]     The invention also allows for condenser fan cycling (either vari-speed or simply ON/OFF) to maintain a head pressure range in low ambient conditions. Under high ambient conditions when proper subcooling is difficult to maintain, the invention can energize an auxiliary subcooling device.  
         [0022]     In its preferred embodiment the apparatus of the present invention is installed as a stand alone unitary controller but can easily be connected to building automation controls through the built in communication port. Further, this communication port can be configured to energize a telephone access module to alert service personnel of fault conditions.  
         [0023]     Also, the present invention can be configured for different refrigerants (example: R-22 or R410-A) through the use of preprogrammed pressure to temperature charts in the firmware.  
         [0024]     In addition to system protection from low superheat, the present invention uses the collected data to further increase system efficiency by providing a method of increasing subcooling under heavy load conditions or decreasing subcooling when needed.  
         [0025]     An electrical device monitors the mechanical aspects of standard refrigeration and air conditioning systems. The device protects these systems by calculating temperature, pressure, superheat, sub-cooling, ambient temperature and controlling system components. The device provides an easy, graphic representation of system conditions and faults for rapid verification of satisfactory operating parameters. Further detailed system conditions are provided for more extensive examination through use of digital read-outs. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]     Novel features and advantages of the present invention in addition to those noted above will become apparent to persons of ordinary skill in the art from reading the following detailed description in conjunction with the accompanying drawings wherein similar reference characters refer to similar parts and in which:  
         [0027]      FIG. 1  is a diagrammatic view of a refrigeration system and a protection system, according to the present invention;  
         [0028]      FIG. 2  is a functional block diagram of the protection system of the present invention;  
         [0029]      FIG. 3  is the main logic flow chart of the software in the microprocessor controller according to the present invention;  
         [0030]      FIG. 4  is a logic flow chart of the software in the sensor polling subroutine, according to the present invention;  
         [0031]      FIG. 5  is a logic flow chart of the software in the sensor validity check subroutine, according to the present invention;  
         [0032]      FIG. 6  is a logic flow chart of the software in the fault logging subroutine, according to the present invention;  
         [0033]      FIG. 7  is an illustration of the device display panel with detachable diagnostic tool, according to the present invention; and  
         [0034]      FIG. 8  is an illustration of an alternative device display panel with LED and LCD displays, according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]     Referring in more particularity to the drawings,  FIG. 1  illustrates a diagram of a standard refrigeration system which includes a compressor  26  driven by an electric motor in a conventional manner. The discharge side of the compressor  26  connects with a discharge line  42  which delivers the compressed refrigerant in a gaseous state to a condenser  10  or in some systems multiple condensers. Near the outlet of the compressor  26  a discharge temperature sensor  36  is connected with the discharge line  42 . The gaseous refrigerant condenses into a liquid state in the condenser  10 . Located at the inlet air side of the condenser  10  is an outdoor temperature sensor  28 .  
         [0036]     Exiting the condenser  10 , the liquid refrigerant travels in the liquid line  44  to a receiver  12  which stores excess refrigerant during low load conditions.  
         [0037]     Exiting the receiver  12  the refrigerant travels through the liquid line  46 . Located on the supply line  46  near the exit of the receiver  12  are a high pressure transducer  30  and a subcooling temperature sensor  32 . The liquid supply line  46  typically travels through a filter-dryer  14 , then through a sight glass  16  and a solenoid valve  18  before entering an expansion valve  20  where the liquid refrigerant changes state back to a gas.  
         [0038]     Gaseous refrigerant enters an evaporator  22  where heat is exchanged with the building or refrigerated enclosure. Refrigerant vapor leaves the evaporator  22  and travels through a suction line  48  into a suction accumulator  24  and finally back to the compressor  26 . Situated on the suction line  48  after the suction accumulator  24  is a low pressure transducer  40 . Situated on the suction line  48  after the suction accumulator  24  but before the compressor  26  is a suction temperature sensor  34 . Both the low pressure transducer  40  and the suction temperature sensor  34  are located from  6 ″ to  18 ″ from the compressor.  
         [0039]      FIG. 2  is a functional block diagram of the present invention in which a device  102  includes a conventional microprocessor  104  which receives power from a power supply and isolation circuits  152  connected to the power interface  128  which receives power from external AC supply  154 . The device  102  is factory programmed but can be modified in the field with dip switches  134  on a circuit board. A liquid crystal display (LCD) module  162  may be attached to the device  102 .  
         [0040]     A detachable module  162  includes an LCD  168  for monitoring of system conditions and a key pad  170  for scrolling through the various system readings. The detachable module  162  has a suitable interface  164  with the microprocessor  104 , and the keypad  170  has an interface  166  with the microprocessor  104 . The detachable module  162  communicates with the microprocessor  104  through the keypad and display control interface  136 .  
         [0041]     A real time clock  150  connects to the microprocessor  104  through a suitable interface  126 . A reset key  172  is provided on the device  102  and permits the device to be reset by a service technician after a system lockout.  
         [0042]     A light emitting diode (LED) display bank  160  has a number of LEDs to show system status at all times. These individual LEDs include, but are not limited to, “System OK”, “Low Pressure Failure”, “High Pressure Failure”, “Discharge Temperature Failure”, “Sensor Failure”, “Superheat OK”, “Superheat Warning”, “Superheat Failure”. The self diagnosis LED interface  132  on the microprocessor  104  sends a voltage output to the display bank  160  based on inputs from external sensors and calculations performed in the microprocessor  104 .  
         [0043]     There are six external sensor inputs: outdoor temperature sensor  106 , suction temperature sensor  108 , discharge temperature sensor  110 , liquid line temperature sensor  112 , low pressure sensor  114  and high pressure sensor  116 . All the external sensors communicate with the microprocessor  104  through a sensor signal interface  118 .  
         [0044]     The device  102  is provided with memory circuits that connect with the microprocessor  104  through a memory interface  124 . The memory circuits includes random access memory (RAM)  146  having a battery backup  148 , programmable read only memory (PROM)  144  and an electrically erasable read only memory (EEPROM)  142 .  
         [0045]     An equipment cut off and alarm relay control  130  on the microprocessor  104  communicates with a normally open relay  158  which controls power to the compressor  26 .  
         [0046]     A subcooling relay interface  120  on the microprocessor  104  is a normally open contact that would be closed based on calculations performed in the microprocessor  104 . Voltage output would energize a subcooling relay  138  which would operate an external device to maintain proper subcooling.  
         [0047]     A fan cycling relay interface  122  on the microprocessor  104  is also a normally open contact that would be closed based on calculations performed in the microprocessor  104 . Voltage output would de-energize a fan motor relay  140 , shutting off the condenser fan. This will help to maintain a constant refrigerant head pressure during low ambient temperature conditions. The condenser fan would be allowed to resume operation when system conditions demand.  
         [0048]     A communications port interface  174  sends information to the communications port  156  so the information from the microprocessor  104  can be shared with existing building control systems.  
         [0049]      FIG. 3  is a logic flow chart of the software in the microprocessor controller  104 . From a start block  202 , block  204  is entered to get readings from all sensors, including refrigerant pressure before the compressor, refrigerant pressure after the compressor, refrigerant discharge temperature, refrigerant suction line temperature, refrigerant temperature at the receiver and ambient temperature. These values are stored for future use.  
         [0050]     In block  206  the values returned from block  204  are evaluated for plausibility. If they are zero or outside the possible high limit it is determined that there is a sensor failure. Block  208  evaluates the result of the sensor check performed in block  206 . If it is determined that a sensor failure has occurred we proceed to block  234 , a fault and logging subroutine, from there to block  242  for a system shut down, and then to block  244  to terminate the logic loop.  
         [0051]     If all sensors respond and pass plausibility test in block  208 , block  210  is entered and a two minute start up delay is initiated. This delay is to prevent an operator from initiating repeated system starts in rapid succession. If a system shutdown were to occur, a two minute delay may be enough time for minor system difficulties to stabilize enough for a successful startup and to provide protection against compressor short cycling.  
         [0052]     After the two minute startup delay, block  212  is entered wherein the compressor start relay is energized causing the compressor to start.  
         [0053]     Block  214  polls all sensors as in block  204 .  
         [0054]     Block  216  tests for sensor validity as in block  206 .  
         [0055]     Block  218  evaluates the response from block  216 . If there is a sensor failure then block  236 , a fault and logging subroutine, is entered.  
         [0056]     If all sensors respond and pass plausibility in block  218  then block  220  is entered, wherein the discharge side refrigerant temperature is evaluated. If the refrigerant temperature is found to be within acceptable limits, block  222  is entered. Block  222  evaluates the liquid refrigerant pressure, and if the pressure is found to be within acceptable limits then block  224  is entered. Block  224  checks for refrigerant low pressure. If the refrigerant pressure is found to be above the low limit then block  226  is entered. Block  226  evaluates the calculated value for superheat to determine if it is within preset limits. If superheat is acceptable then block  228  is entered to determine if external system subcooling is required. If external subcooling is required block  230  is entered and an external subcooling relay is energized. After block  230  or if subcooling was not required, block  232  is entered where “system OK” LED is energized and all other system warning LEDs are turned off.  
         [0057]     Block  234  is then entered where a  0 . 5  second logic delay is entered. This delay is built in to reduce the number of sensor polling events to a reasonable number. Polling 20 or 30 times a second is not required. After the logic delay in block  234 , block  214  is reentered, all sensors are polled and the logic loop continues.  
         [0058]     In the first logical pass through blocks  220 ,  222 ,  224  and  226 , the dectections are as follows.  
         [0059]     If block  220  detects a value higher than the compressor manufacturer specified maximum for discharge temperature then a system fault has occurred.  
         [0060]     If block  222  detects a value higher than the compressor manufacturer specified maximum for refrigerant pressure then a system fault has occurred.  
         [0061]     If block  224  detects a value lower than the compressor manufacturer specified minimum for refrigerant pressure then a system fault has occurred.  
         [0062]     If block  226  detects a value lower than the compressor manufacturer specified minimum for superheat (typically 3 degrees) then a system fault has occurred.  
         [0063]     If a system fault is determined in blocks  220 ,  222 ,  224  or  226  then a fault and logging subroutine  236  is entered.  
         [0064]     After returning from block  236 , block  238  determines if the system has faulted more than three times for the same reason. If three sequential faults have occurred then a system shut down  242  is initialed. If less than three sequential faults have occurred then block  240  is entered initiating a  90  second system delay before attempting a compressor re-start.  
         [0065]      FIG. 4  is a logic flow chart of the software in the sensor polling subroutine  204 . Entering in block  302 , block  304  communicates with the discharge temperature sensor  36 . This value is stored in memory.  
         [0066]     Block  306  communicates with the high pressure transducer  30 . This value is stored in memory.  
         [0067]     Block  308  communicates with the low pressure transducer  40 . This value is stored in memory.  
         [0068]     Block  310  communicates with the suction line temperature sensor  34 . This value is stored in memory.  
         [0069]     Block  312  communicates with the subcooling temperature sensor  32 . This value is stored in memory.  
         [0070]     Block  314  communicates with the outdoor temperature sensor  28 . This value is stored in memory.  
         [0071]     Block  316  consults the temperature/pressure table for the selected refrigerant (example: R-22) stored in the programmable read only memory (PROM)  160  and converts the value returned by the low pressure transducer to the low side saturated temperature value.  
         [0072]     Block  318  calculates system superheat. Superheat is the low side saturated temperature minus the actual suction line temperature. The result is expressed in degrees Fahrenheit and stored in memory.  
         [0073]     Block  320  evaluates the calculated superheat value. If superheat is greater than twenty degrees it is determined to be acceptable and block  328  is entered wherein the “Superheat OK” LED is energized. If block  320  determines that superheat is less than twenty degrees then block  322  is entered.  
         [0074]     Block  322  evaluates the calculated superheat value. If superheat is greater than three degrees then block  326  is entered wherein the “Superheat Warning” LED is energized. If block  322  determines that superheat is less than three degrees then block  324  is entered.  
         [0075]     Block  324  energizes the “Superheat Failure” LED. After one of the three superheat indicator LEDs are energized block  330  is entered.  
         [0076]     Block  330  consults the temperature/pressure table for the selected refrigerant (example: R-22) stored in the programmable read only memory (PROM)  160  and converts the value returned by the high pressure transducer to the high side saturated temperature value.  
         [0077]     Block  332  calculates system subcooling. Subcooling is the high side saturated temperature minus the actual liquid line temperature. The result is expressed in degrees Fahrenheit and stored in memory.  
         [0078]     Block  334  returns logic flow to the parent program.  
         [0079]      FIG. 5  is a logic flow chart of the software in the sensor validity check subroutine  206 . Entering in block  402 , block  404  checks the value returned by the discharge temperature sensor  144  to see if it is greater than zero.  
         [0080]     Block  406  checks the value returned by the high pressure transducer  30  to see if it is greater than zero.  
         [0081]     Block  408  checks the value returned by the low pressure transducer  40  to see if it is greater than zero.  
         [0082]     Block  410  checks the value returned by the suction line temperature sensor  34  to see if it is greater than zero.  
         [0083]     Block  412  checks the value returned by the subcooling temperature sensor  32  to see if it is greater than zero.  
         [0084]     Block  414  checks the value returned by the outdoor temperature sensor  28  to see if it is greater than zero.  
         [0085]     If any of the above values are determined to be zero then the variable SENSORS_RESPOND_OK=NO is determined in block  418 . Otherwise if all the values are determined to be greater than zero then the variable SENSORS_RESPOND_OK=YES is determined in block  416 .  
         [0086]     Block  420  returns logic flow to the parent program.  
         [0087]      FIG. 6  is a logic flow chart of the software in the fault logging subroutine  234 .  
         [0088]     Entering in block  502 , block  504  enters the senor check subroutine  206  (see  FIG. 5 ).  
         [0089]     In block  506  the result of the sensor check performed in block  504  is evaluated. If it is determined that a sensor failure has occurred we proceed to block  534  and set the variable SYSTEM_SHUTDOWN=YES. Block  536  then performs any event logging specified by the system configuration (example: output to printer, output to message display). Block  538  energizes a remote dial module if the system is equipped with one.  
         [0090]     Block  540  energizes the appropriated LEDs to indicate which system fault has occurred while de-energizing the LEDs that indicate positive system status.  
         [0091]     Block  542  returns logic flow to the parent program.  
         [0092]     If in block  506  it is determined that the sensors have responded properly, then block  508  evaluates the discharge side refrigerant temperature. If the refrigerant temperature is found to be within acceptable limits, block  510  is entered. Block  510  evaluates the liquid refrigerant pressure, and if the pressure is found to be within acceptable limits then block  512  is entered. Block  512  checks for refrigerant low pressure. If the refrigerant pressure is found to be above the low limit then block  514  is entered. Block  514  evaluates the calculated value for superheat to determine if it is within preset limits.  
         [0093]     If the four system conditions evaluated in blocks  508 ,  510 ,  512  and  514  are all found to be within acceptable limits then the variable SYSTEM_SHUTDOWN=NO in block  532  is determined.  
         [0094]     If in any of the four block  508 ,  510 ,  512  or  514  it is determined that the value is outside the acceptable range then a counter for that value is incremented by one (blocks  516 ,  518 ,  520  and  522 ). In blocks  524 ,  526 ,  528  and  530  each of the four fault counters are evaluated to see if any one of the four system values has faulted more than three times. If any one of the four system values has faulted more than three times the variable SYSTEM_SHUTDOWN=YES in block  534  is determined, otherwise the variable SYSTEM_SHUTDOWN=NO in block  532  is determined.  
         [0095]     Logic then flows through blocks  536 ,  538 ,  540  and  542  as described previously.  
         [0096]      FIG. 7  illustrates a display panel of the present invention.  
         [0097]     This configuration consists of a main display and control module  602  and a separate diagnostic tool  624 . The main display and control module  602  consists of a case  604  which houses the microprocessor  104  and other component parts. This case would typically be attached to the exterior of the refrigeration unit that it is protecting and controlling.  
         [0098]     The LED display bank  160  can be seen on the face of the case  604 . These LEDs indicate system conditions: “System OK”  606 , “High Pressure Failure”  608 , “Low Pressure Failure”  610 , “Discharge Temperature Failure”  612 , “Sensor Failure”  614 , “Superheat OK”  616 , “Superheat Warning”  618 , “Superheat Failure”  620 .  
         [0099]     The hand held diagnostic tool  624  is connected to the main display and control module  602  through the use of a female modular connection  622  mounted on the case  604  and a male modular connection  628  connected to the hand held diagnostic tool  624  with an appropriate cable connection  630 .  
         [0100]     The hand held diagnostic tool  624  consists of a case  626  on which is mounted an LCD display  632  for quantitative viewing of system conditions monitored by the microprocessor  104 . This diagnostic tool  626  is equipped with buttons  634  and  636  for selecting the system readings to be displayed on the LCD screen  632 .  
         [0101]     A manual reset button  638  is used by service personnel to reset the system after a lockout has occurred.  
         [0102]      FIG. 8  is an illustration of an alternative device display panel with LED and LCD displays.  
         [0103]     This configuration of the present invention consists of a main display and control module  702  without a separate diagnostic tool as in  FIG. 7 . The main display and control module  702  consists of a case  704  which houses the microprocessor  104  and other component parts. This case would typically be attached to the exterior of the refrigeration unit that it is protecting and controlling.  
         [0104]     The LED display bank  160  can be seen on the face of the case  704 . These LEDs indicate system conditions: “System OK”  706 , “High Pressure Failure”  708 , “Low Pressure Failure”  710 , “Discharge Temperature Failure”  712 , “Sensor Failure”  714 , “Superheat OK”  716 , “Superheat Warning”  718 , “Superheat Failure”  720 .  
         [0105]     An LCD display  722  is mounted on the case  704  for quantitative viewing of system conditions monitored by the microprocessor  104 . Buttons  724  and  726  are used to for select the system readings to be displayed on the LCD screen  722 .  
         [0106]     A manual reset button  728  is used by service personnel to reset the system after a lockout has occurred.  
         [0107]     U.S. Pat. No. 6,318,108, granted Nov. 20, 2001, is incorporated herein by reference in its entirety for all useful purposes. As explained therein a water spray onto the heat exchange coil of a condenser is utilized to wash and clean the condenser coil for more efficient operation.