Abstract:
A method of controlling a heating cycle of a refrigeration system is provided that includes a refrigerant circuit. The refrigerant circuit includes a compressor having a suction port and an outlet having a discharge port with a hot gas compressor discharge line, a condenser for condensing the refrigerant, an evaporator for evaporating the refrigerant and an expansion valve. The method includes using refrigerant from the hot gas compressor discharge line to heat the evaporator during a heating cycle, detecting periodically a discharge superheat of the refrigerant leaving the outlet of the compressor, producing a control signal representing a difference between the detected discharge superheat and a minimum discharge superheat setpoint, adjusting the flow rate of the refrigerant to the suction port of the compressor according to the control signal so as to maintain the discharge superheat of the refrigerant at the outlet of the compressor substantially at the minimum discharge superheat setpoint.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to control methods for operating a refrigeration system which maintains a temperature set point by heating and cooling cycles, and more specifically to methods for enhancing the heating cycles of such systems. 
   Refrigeration systems capable of operating in a heating and defrosting mode are known in the art. Exemplary patents in this regard are commonly assigned U.S. Pat. Nos. 4,850,197; 5,228,301; 5,408,836; 5,410,889; 5,465,586; 5,465,587; 5,477,695; and 5,598,718, the disclosures of which are incorporated by reference herein. Such refrigeration systems generally employ a refrigerant compressor that is typically driven by an internal combustion engine in transport refrigeration systems. The compressor is connected to a refrigeration circuit that generally comprises a condenser coil for condensing gaseous refrigerant into a liquid, and an evaporator assembly that includes an expansion valve for converting the liquid refrigerant back into a gas, and an evaporator coil that is thermally connected to a conditioned space, which may be a truck trailer. 
   To achieve heating and defrosting, these systems typically incorporate a three-way mode valve to divert hot, gaseous refrigerant around the expansion valve of the evaporator assembly and directly into the evaporator coil. This converts the evaporator coil into a heat radiating condenser for either defrosting or heating applications. Such systems employ heat exchangers for transferring additional heat to the gaseous refrigerant to enhance the efficiency of the heating cycle. This additional heat may be provided from sources such as the hot liquid coolant of the radiator system of the internal combustion engine used to drive the compressor. 
   The foregoing illustrates existing refrigeration systems. It would be advantageous to provide an alternative refrigeration system having enhanced heat outputs during heating cycles including the features more fully disclosed hereinafter. 
   SUMMARY OF THE INVENTION 
   According to the present invention, a method of controlling a heating cycle of a refrigeration system is provided that includes a refrigerant circuit. The refrigerant circuit includes a compressor having a suction port and an outlet having a discharge port with a hot gas compressor discharge line, a condenser for condensing the refrigerant, an evaporator for evaporating the refrigerant and an expansion valve. The method includes using refrigerant from the hot gas compressor discharge line to heat the evaporator during a heating cycle, detecting periodically a discharge superheat of the refrigerant leaving the outlet of the compressor, producing a control signal representing a difference between the detected discharge superheat and a minimum discharge superheat setpoint, adjusting the flow rate of the refrigerant to the suction port of the compressor according to the control signal so as to maintain the discharge superheat of the refrigerant at the outlet of the compressor substantially at the minimum discharge superheat setpoint. 
   The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with accompanying drawing figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a refrigeration system utilizing a control method according to the present invention; and 
       FIGS. 2 and 3  are flow charts of a control method according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   It is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
   According to the present invention, a method for operating a refrigeration system is provided. More specifically, the method provided optimizes the heat output of a refrigeration system during heating cycles by introducing refrigerant into the compressor suction to force more refrigerant into these cycles. Although the heating capacity of a conventional refrigeration system typically decreases, for example, at low ambient temperatures (generally, below zero degrees Celsius) and is also highly dependent on the superheat setting of the economizer expansion valve, the control method of the present invention improves heating capacity to address such ambient and refrigerated space conditions. Responsive to these and other factors, the control method automatically increases or decreases the amount of liquid refrigerant injected via the liquid injection valve to maintain the heating capacity at a maximum level. 
   Referring now to the drawings, and to  FIG. 1  in particular, there is shown an exemplary refrigeration system  80  having a control method according to the present invention. Refrigeration system  80 , for example, may be a transport refrigeration system suitable for conditioning the air in a cargo space of a truck, trailer, or container. In general, refrigeration system  80  is of the type which maintains a temperature set point of a served space by heating and cooling cycles, both of which utilize the hot gas discharged from the discharge port of a refrigerant compressor. Defrosting of the evaporator section of such a refrigeration system may also be accomplished by using the hot gas compressor discharge. 
   More specifically, refrigeration system  80  includes a refrigerant circuit  82  comprising a compressor  14  driven by a prime mover  15 , a condenser  16 , check valves  18  and  19 , a receiver  20 , an evaporator  22 , and an expansion valve  24  for evaporator  22 . Downstream of evaporator  22  is an electronic throttle valve (ETV)  72  that controls the gaseous refrigerant flow entering suction port S to prevent the pressure from becoming high enough to overload the prime mover  15  that drives the compressor  14 . Compressor  14  is of the type having a suction port S, an intermediate pressure port IP, and a discharge port D, and two loading valves LV 1  and LV 2  described in detail below. A hot gas compressor discharge line  26  connects the discharge port D of compressor  14 , to condenser  16  via a three-way valve  28 , or its equivalent in two separate coordinated valves. A receiver outlet conduit  21  and a liquid line  30  interconnect receiver  20  and evaporator expansion valve  24 , and a suction line  32  interconnects evaporator  22  and the suction port S of compressor  14 . 
   A heat exchanger  34 , which will be referred to as an economizer heat exchanger, has first, second and third flow paths  36 ,  38 , and  40 , respectively. The first flow path  36  is connected in the liquid line  30 . The second flow path  38  is disposed about the first and third flow paths,  36  and  40 , respectively, includes an inlet  44  and an outlet  46 . The third flow path  40  is connected to a controllable source  50  of heat, with the control, for example, being in the form of a solenoid controlled valve  52 . The heat source  50  is outside refrigerant circuit  82 , and is preferably a fluid that is heated by operation of the compressor prime mover  15 . For example, prime mover  15  may be an internal combustion engine, such as a Diesel engine, and the heat source  50  may be liquid radiator coolant, or exhaust gas. 
   Receiver outlet conduit  21  is diverted via a tee  54  through economizer expansion valve  56  where it is expanded. The expanded refrigerant is then introduced into the second flow path  38  of economizer heat exchanger  34 . The expanded refrigerant is in heat exchange relation with the first flow path  36 , to cool refrigerant in the first flow path  36  during a cooling cycle of refrigeration system  80 , to enhance the cooling cycle. 
   As is common with compressors which have an intermediate pressure port IP, a normally closed first loading valve (LV 1 )  84 , called an economizer by-pass valve, is connected between the suction and intermediate pressure ports S and IP, respectively, of compressor  14 . A second loading valve (LV 2 )  86  is similarly connected between the suction port S and a higher pressure, intermediate point within compressor  14 . The first loading valve (LV 1 )  84  and second loading valve (LV 2 )  86  are solenoid-operated valves that are internally located within compressor  14  and controlled to open during heating and defrost cycles. These loading valves can be like those disclosed in commonly assigned U.S. Pat. Nos. 6,467,287 and 6,494,699, the disclosures of which are incorporated by reference herein. During heating and defrost cycles the normal flow to suction port S is closed. If the compressor pumps only through the limited economizer port, the pumping capability may be limited. 
   When heat is required by a served space to maintain the temperature set point, and also when heat is required in order to defrost evaporator  22 , three-way valve  28  is operated to divert the hot gas in hot gas line  26  to perform an evaporator heating function. In  FIG. 1 , evaporator  22  is heated by a heating element  58  disposed in heat exchange relation with evaporator  22 , such as by a separate set of tubes in the evaporator tube bundle. Refrigerant leaving evaporator heating element  58 , which is functioning as a condenser, is led via a second or alternate path or line  60  through an open check valve  19  directly into the receiver  20 . Check valve  18  is closed such that none of the liquid refrigerant enters the condenser  16 . The liquid refrigerant that collects in the receiver  20  then exits via receiver outlet conduit  21 . During a heating or defrost cycle, a liquid line solenoid valve (LLSV)  64  in liquid line  30  is closed to ensure that the refrigerant returns to compressor  14  via the economizer expansion valve  56  and the second flow path  38  of economizer heat exchanger  34  and to stop the flow of refrigerant to the evaporator  22  to stop the cooling of the conditioned space. 
   Also, during heating and defrosting cycles, solenoid valve  52  is opened to allow hot fluid from heat source  50  to circulate through the third flow path  40 , adding heat to refrigerant in the second flow path  38 , to enhance the heating and defrosting cycles. Thus, during heating and defrosting cycles, the economizer heat exchanger  34  functions as an evaporator, adding heat from a source  50  outside refrigerant circuit  82  to the refrigerant, to get more heat into the heating and defrosting functions. The heat added to refrigerant in the second flow path  38  by heat source  50  vaporizes any liquid refrigerant  48  that may have accumulated in the second flow path  38 , with outlet  46  only allowing vaporized refrigerant to be drawn into the intermediate pressure port IP of compressor  14 . 
   The system  80  includes a controller  100 , which may be implemented as a single controller or a plurality of controllers working in concert. As is known in the art, the controller  100  may be operably connected to control operation of the compressor  14 ; solenoid valve  52 ; three-way valve  28 ; liquid line solenoid valve (LLSV)  64 ; electronic throttle valve (ETV)  72 ; first loading valve (LV 1 )  84 ; second loading valve (LV 2 )  86 ; and liquid injection valve (LIV)  105  via electrical lines  13 ,  53 ,  29 ,  65 ,  73 ,  85 ,  87 , and  104 , respectively, as shown. 
   The present invention, includes a control method that improves the system capacity of a refrigeration unit in a heating mode by maximizing the heat output of a refrigeration unit while also protecting the compressor of the unit from lubrication loss during a heating cycle. The control method utilizes a control algorithm in the software of microprocessor controller  100  to control a liquid injection valve (LIV)  105  that fluidly connects receiver  20  to the suction port S of compressor  14 . An electrical line  104  provides command signals from controller  100  to liquid injection valve  105 . Controller  100  is also connected via an electrical line  108  to a compressor discharge temperature sensor  109  that is in contact with the compressor lubricant/refrigerant mixture so as to sense the compressor discharge temperature (CTemp). An electrical line  106  is also provided that connects controller  100  to a discharge pressure transducer (DPT)  107  that reads the saturated discharge pressure of the refrigerant. As described in detail below, the saturated discharge pressure is converted by controller  100  to the saturated compressor discharge temperature (DTemp SAT ), which is compared to the measured compressor discharge temperature (CTemp) to derive the compressor discharge superheat (CDSH). 
   The software algorithm monitors the compressor discharge superheat and controls the liquid injection valve in the refrigeration unit to inject a maximum amount of liquid refrigerant into the compressor to provide maximum heating capacity without injecting too much liquid refrigerant, thereby minimizing the washing out of lubricating oil from the compressor. If a calculated compressor discharge superheat is high, liquid injection valve  105  is controlled by controller  100  via electrical line  104  to inject refrigerant into suction port S. This increases mass flow of the refrigerant which maximizes the heat output during heating. If the calculated compressor discharge superheat is below a minimum setpoint, liquid refrigerant injection through liquid injection valve  105  is disabled by controller  100  thereby minimizing lubricant loss from compressor  14 . 
   Referring to  FIGS. 2 and 3 , the control algorithm is shown which calculates and controls the compressor discharge superheat beginning with Step  110  in which Liquid Line Solenoid Valve (LLSV)  64  is energized to close and three-way valve  28  is shifted to direct refrigerant to heating element  58  for beginning a heat/defrost cycle. The electronic throttle valve (ETV)  72  is initially set at 30 percent open. 
   INITIALIZATION 
   An initialization step  120  sets the values for the algorithm variables including maximum and minimum setpoint temperature values of the compressor discharge superheat at which the liquid injection valve is opened (DSON) and is closed (DSOF), respectively. These values are read from a global data table (GDT) of the microprocessor controller  100  and can be modified by an operator. If other than the startup cycle, also read is the calculated value of the compressor discharge superheat value (CDSH). 
   SENSOR READINGS AND FAILURE CHECK 
   The algorithm in Steps  130  and  160  reads the compressor discharge pressure from discharge pressure transducer (DPT)  107  and the compressor discharge temperature (CTemp) from temperature sensor  109 , respectively, and provide alarm signals in the event of their failure. If after initiating the heat mode both the pressure transducer and the temperature sensor are determined to be functioning and no alarm signals present, then a five minute wait period is provided in Step  170  to allow the compressor discharge pressure and temperature to stabilize in the heat mode. This step is performed only during the first startup cycle. The global data table value for the compressor discharge superheat (CDSH) is set to zero during this five minute wait period. 
   If either the pressure transducer or the temperature sensor are not functioning, then backup control is provided in Step  140  in which a backup heat/defrost mode is performed which continually loops to check whether the alarm signals have been cleared in Step  150 . If the unit has been running in heat after an alarm signal has been cleared, the controlled LIV operation based on discharge superheat described below is immediately enabled and the global data table value for the compressor discharge superheat (CDSH) is set to zero. 
   CONTROLLED LIQUID INJECTION VALVE (LIV) OPERATION BASED ON DISCHARGE SUPERHEAT 
   The algorithm proceeds to Step  180  in which the discharge saturation temperature (DTemp SAT ) is calculated from the compressor discharge pressure value from the formula:
 
 DTemp   SAT =[−5.4*( DPT +14.7)*( DPT +14.7)+5745*( DPT +14.7)−96839]/10000
 
   The compressor discharge superheat (CDSH) is then calculated in Step  190 , which is the difference between the compressor discharge temperature (CTemp) and the discharge saturation temperature (DTemp SAT ). In Steps  200 – 240 , the value of the on time for the liquid injection valve (LIV ontime ) is calculated as a percentage of a six-second cycle using pulse-width modulation. As shown in Step  200 , the formula for calculating LIV ontime  is:
 
 LIV   ontime =6* ( CDSH−DSOF )/( DSON−DSOF )
 
   The calculated LIV ontime  is then checked in Steps  210  and  230  and, if greater than six, reassigned a value of six seconds (Step  220 ) and, if less than zero, reassigned a value of zero seconds (Step  240 ). 
   DISCHARGE SUPERHEAT CONTROL BYPASS 
   Before proceeding with injecting liquid refrigerant to compressor  14  via liquid injection valve  105 , various parameters of the refrigeration system are first checked to determine whether discharge superheat control using the LIV ontime  from Steps  200  to  240  is to be bypassed. This is accomplished in Steps  250  to  310 , which check to see whether:
         1) the defrost mode is active (Step  250 );   2) an ambient temperature sensor (not shown) outside of the conditioned space is working (Step  260 ) and, if so, whether the ambient temperature is moderate, i.e., greater than or equal to zero Celsius (Step  270 ) and there is an adequate temperature differential (TD) between the discharge air temperature (DA) and the return air temperature (RA) of the conditioned space, i.e., greater than 7.2° F. (4° C.) (Step  280 ); or   3) if the discharge pressure of the compressor (DPT) is high, i.e., greater than or equal to 350 psig if LV 1  alone is energized or greater than 400 psig if LV 2  is also energized (Steps  290 – 310 ).       

   In the event that any of the three conditions above are true, and if the compressor discharge superheat (CDSH) is greater than the minimum compressor discharge superheat setpoint (DSOF) as determined by Step  320 , then discharge superheat control using the LIV ontime  from Steps  200  to  240  is bypassed. In this case, the liquid injection valve (LIV) is energized, however, the LIV ontime  is not based on discharge superheat control of the present invention. In this instance, the LIV ontime  may be based on other parameter(s) such as the compressor temperature and using other algorithms as will be recognized by those skilled in the art. 
   LIQUID INJECTION BASED ON DISCHARGE SUPERHEAT 
   If the unit is not in defrost mode (Step  250 ), the ambient temperature is not detected (Step  260 ) or is low (Step  270 ), and the discharge pressure is low (Steps  290 – 310 ), then the algorithm evaluates the compressor discharge superheat in Step  340 . If the compressor discharge superheat (CDSH) is greater than the minimum compressor discharge superheat set point (DSOF), then discharge superheat control is performed in Step  350  using the maximum LIV ontime  calculated in Steps  200 – 240 . 
   LIQUID INJECTION VALVE DISABLE 
   If in either Steps  320  or  340  the compressor discharge superheat (CDSH) is less than or equal to the minimum compressor discharge superheat set point (DSOF), then the liquid refrigerant injection is disabled in Step  360  to prevent overfeeding of refrigerant into the compressor by the liquid injection valve. In both cases, the liquid injection valve (LIV) is energized, however, the LIV ontime  is not based on discharge superheat control of the present invention. In these instances, the LIV ontime  may be based on other parameter(s) such as the compressor temperature and/or the ratio of the discharge pressure to the suction pressure, while using other algorithms as will be recognized by those skilled in the art. 
   From Steps  330 ,  350 , and  360 , the algorithm repeats beginning with taking sensor readings in Step  130 . 
   While embodiments and applications of this invention have been shown and described, it will be apparent to those skilled in the art that many more modifications are possible without departing from the inventive concepts herein described. It is understood, therefore, that the invention is capable of modification and therefore is not to be limited to the precise details set forth. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims without departing from the spirit of the invention.