Abstract:
A subcooling unit within a vapor compression refrigerant system contains a controller for controlling the rate of flow of refrigerant into the chamber of the subcooling unit. The controller receives a sensed temperature of the fluid entering a condensing unit within the chamber of the subcooling unit and computes a condensing pressure setpoint for the refrigerant flowing into the chamber of the subcooling unit. The controller is operative to compare the pressure in the subcooling chamber with the condensing pressure setpoint so as to determine whether to possibly increase or decrease the rate of flow of the refrigerant into the chamber of the subcooling unit.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to vapor compression refrigeration systems and, more particularly, to a subcooler within such systems for subcooling refrigerant. 
     Subcoolers have heretofore been used in vapor compression refrigeration systems to subcool refrigerant flowing from the condenser to the evaporator. Hot liquid refrigerant from the condenser typically passes through one or more orifices or nozzles located in the subcooler. These orifices or nozzles define a pressure drop between the condenser and the chamber of the subcooler. This pressure drop causes a portion of the liquid refrigerant to flash to vapor as it leaves the orifices or nozzles. The vapor refrigerant absorbs heat from the remaining liquid refrigerant passing into the chamber of the subcooler. The subcooler chamber may also include a condensing coil which circulates fluid having a temperature that recondenses the flashed vapor refrigerant. The recondensed refrigerant and the subcooled refrigerant exit the subcooler chamber for circulation through the evaporator. The above vapor compressor system is disclosed in U.S. Pat. No. 4,207,749 issuing to William J. Lavigne, Jr., on Jun. 17, 1980. 
     The orifices or nozzles of the aforementioned system are sized for a specific refrigerant flow that will create a particular pressure drop from the condenser into the subcooler chamber. The refrigerant flow is usually assumed to be the flow occurring at a full load condition for the vapor compression refrigeration system. This full load condition also assumes a particular entering condenser water temperature for the water circulating through the coil within the subcooler. The refrigerant flow to the orifices or nozzles will however drop as the full load condition on the refrigeration system drops. This drop in refrigerant flow will reduce the ability of the orifice or nozzle to produce the pressure drop needed to flash the refrigerant vapor in the subcooler chamber. This reduces the amount of cooling of refrigerant that may be provided by the subcooler. This in turn affects the overall efficiency and operating range of the refrigeration system. 
     It is an object of this invention to provide the necessary pressure drop through an orifice or nozzle within a subcooler so as to introduce sufficient flashed refrigerant vapor into a subcooler under a variety of operating conditions. 
     SUMMARY OF THE INVENTION 
     The above and other objects of the invention are achieved by an electronically controlled flash subcooler system that automatically adjusts to varying amounts of refrigerant flow from the condenser. The flash subcooler system preferably includes a metering device in the form of a valve upstream of the orifices or nozzles. The variable metering device is adjusted by a microprocessor control, which receives temperature of water entering the condenser coil within the flash subcooler chamber as well as pressure from a pressure sensor within the flash subcooler. The temperature of the water entering the condenser coil is used to determine a desired pressure setpoint within the flash subcooler chamber. The sensed pressure from the pressure sensor within the subcooler is fed back to the microprocessor controller in order to determine if the pressure in the flash subcooler chamber is within a predefined range of the desired pressure setpoint. Any difference in the sensed pressure value with respect to the predefined range of pressure from the desired pressure setpoint is used by the controller to determine the magnitude and direction of change to the valve opening in the metering device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention and its objects and advantages will be better understood by reference to the accompanying drawings, in which: 
     FIG. 1 illustrates a vapor compression refrigeration system having a subcooler therein wherein the subcooler has an associated control system for controlling a variable metering device associated with the subcooler; and 
     FIG. 2 illustrates a flow chart depicting a program resident within the controller of FIG. 1 for controlling the variable metering device of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a vapor compression refrigeration system is seen to include a compressor  10  having a motor  12  and a single stage of compression indicated by an impeller  14 . Vanes such as  16  control the amount of refrigerant entering the single stage of compression from an inlet  18 . The compressed refrigerant exits the compressor  10  at an outlet  20  which is connected to the inlet side of a condenser  22 . Water entering through an inlet  24  typically flows through a shell and tube-type heat exchanger (not shown) within the condenser  22  before exiting at an outlet  26 . The gaseous refrigerant changes to a liquid refrigerant state as it flows over the shell and tube-type heat exchanger within the condenser  22 . The condensed liquid refrigerant flows out of the condenser  22  through a conduit  28  to a flow metering device  30 . The flow metering device  30  meters the rate of flow of the refrigerant from the condenser to a series of orifices such as  32  and  34  formed in a refrigerant flow pipe  36 . The refrigerant exits from the flow pipe  36  through the orifices such as  32  and  34 . The refrigerant partially flashes into a gaseous vapor after passing through the metering device  30  and then again when exiting the orifices  32  and  34 . The liquid refrigerant collects in the bottom of a subcooler chamber  38 . The subcooler chamber  38  is maintained at a pressure less than the saturation pressure of the sprayed refrigerant from the orifices  32  and  34  in a manner which will be explained hereinafter. This assures that a sufficient amount of the refrigerant exiting the orifices  32  and  34  will always change to a gaseous state. The gaseous refrigerant absorbs heat from the liquid refrigerant before being condensed by water circulating through a condenser coil  40  within the subcooler chamber  38 . Liquid refrigerant is collected in the bottom of the subcooler chamber  38  until the liquid refrigerant rises to the level of a float  42 , which opens an outlet  44  so as to allow liquid refrigerant to pass through a conduit  46  to an evaporator/cooler  48 . Water enters the evaporator  48  via a water inlet  50  and preferably flows through a shell and tube type heat exchanger (not shown) within the evaporator  48  and exits through an outlet  52 . The liquid refrigerant entering from the flash subcooler chamber  38  flows over the tubes in the shell and tube type heat exchanger and absorbs heat from the water circulating through the tubes. Chilled water exits the evaporator at the outlet  52 . The resulting gaseous refrigerant is withdrawn from the evaporator  48  into the compressor  10  through the compressor inlet  18 . 
     Referring to the condenser coil  40 , it is to be noted that this coil receives water from the water inlet  24  to the condenser  22 . It is to be appreciated that this is preferably water from a source such as a cooling tower or tap water source having a sufficiently low temperature to remove heat from the hot refrigerant in the condenser  22  or the flashed refrigerant in the subcooler chamber  38 . It is furthermore to be appreciated that the water source for the condensing coil  40  need not necessarily be the same as the source for the condenser  22 . In any event, a temperature sensor  54  is mounted to the inlet side of the condenser coil  40  so as to thereby sense the temperature of the water entering the subcooler chamber  38 . The sensed temperature is noted by a controller  56 , which preferably is a programmed microprocessor but could however be hardwired discrete logic. The controller  56  also receives a sensed pressure of the liquid refrigerant within the subcooler chamber from a pressure sensor  58 . The controller  56  sends a control signal device to the flow metering  30  so as to preferably control a valve position of the flow metering device. The controlled valve position allows more or less refrigerant to flow to the flow pipe  36 . It is to be appreciated that flow metering by the device  30  could be replaced by one or more variable orifices controlled by the controller  56 . 
     Referring to FIG. 2, the control process implemented by a microprocessor version of the controller  56  is seen to begin with step  60  wherein a value for the flow opening “F” of the metering device  30  is initially set. It is to be understood that this particular flow opening has been predetermined based on the full load design conditions for the refrigerant system of FIG.  1 . This particular flow opening will normally produce the amount of refrigerant flow through the flow metering device  30  so as to cause the appropriate amount of vaporization of refrigerant through the orifices  32  and  34 . This will in turn produce the amount of subcooling of the refrigerant within the subcooling chamber  38 . The initial flow opening value “F” is sent to the flow metering device  30  in a step  62 . The microprocessor controller  56  next proceeds to inquire as to whether the compressor motor  12  associated with the compressor  10  is on. This will normally be a known state within the controller  56  if it controls the motor  12 . If it does not, then the controller  56  will simply receive a signal from the motor controller. The microprocessor controller simply awaits an indication that the motor  12  is on so as to cause refrigerant to flow within the system of FIG.  1 . The microprocessor controller proceeds at this time to a step  66  and sets an initial time period of “t”. The microprocessor controller will proceed to a step  68  and begin decrementing the initial time period “t”. The microprocessor controller will next proceed to a step  70  and read the temperature sensor  54  and set the read value equal to a water temperature “T w ”. It is to be appreciated that the temperature sensor  54  will be sensing the temperature of the incoming water to the subcooler chamber  38 . The microprocessor controller will next proceed in a step  72  to obtain an equivalent saturated refrigerant pressure “P e ” that would cause the condensation of the flashed refrigerant in the subcooler chamber  38  to occur. This equivalent saturated refrigerant pressure “P e ” is preferably obtained by going to a table of equivalent saturated refrigerant pressures for specific refrigerant temperatures corresponding to T w . 
     It is to be understood that there may not be a refrigerant temperature in the table precisely equal to T w . In this case, a linear interpolation is performed using the closest refrigerant temperature and corresponding equivalent saturated refrigerant pressure to the temperature T w . It is also to be understood that an equivalent saturated pressure “P e ” could be computed using a mathematical function which defines the relationship between equivalent saturated refrigerant pressure and inlet water temperature. In either case, an equivalent pressure “P e ” will be produced by the controller in step  72 . The microprocessor controller proceeds in a step  74  to read pressure sensor  58  and set the read value equal to the subcooler pressure “P s ”. The thus read pressure will reflect the pressure in the subcooler chamber  38  at a point underneath the orifices  32  and  34 . 
     The microprocessor proceeds in a step  76  to inquire as to whether the subcooler pressure “P s ” is greater than the equivalent saturated refrigerant pressure “P e ” plus an incremental pressure value of “ΔP 1 ”. The value of “ΔP 1 ” is chosen so as to ensure that all flash gas leaving the orifices  32  and  34  is condensed in the subcooler chamber. This “ΔP 1 ” value is preferably a differential pressure slightly above the equivalent saturated refrigerant pressure, “P e ” The value of “ΔP 1 ” used in step  76  is preferably determined by adding a small incremental amount of temperature to the water temperature, “T w ” and finding the equivalent saturated refrigerant pressure, “P e   1 ” for this elevated temperature in the stored table of data used in step  72 . This elevated temperature could for example be three to four degrees Fahrenheit above the water temperature “T w ”. Alternatively, the equivalent saturated refrigerant pressure, “P e   1 ” could be obtained by an algorithmic calculation. The value of “ΔP 1 ” would be the difference between “P e   1 ” and “P e ”. 
     The microprocessor controller will proceed to a step  78  in the event that the subcooler pressure “P s ” is greater than “P e ” plus “ΔP 1 ”. The microprocessor will decrease the flow opening, “F”, by an amount “ΔF” in step  78 . It is to be understood that “ΔF” may be either a small predefined amount of commanded flow opening or “ΔF” can be computed as a function of the error between the desired pressure “P e ” plus “ΔP 1 ” and the actual pressure “P s ”. “ΔF” is preferably the small predefined amount of commanded flow opening if the time “t” between successive executions of the logic of FIG. 2 is set low. The computation of “ΔF” would be more appropriate if the time between successive executions of the logic of FIG. 2 was such as to impact the condensing of the flashed refrigerant in the subcooler chamber. In either case, the resulting flow opening value determined in step  78  is sent to the flow metering device  30  in a step  80 . 
     The flow metering device  30  preferably includes a local device control system which will respond to commanded flow opening “F”. This local control will compare the commanded flow opening with a minimum flow opening position for the particular flow metering device. The commanded flow opening will be implemented to the extent that it exceeds the minimum flow opening position. It is to be appreciated that the above logic could also be included in the microprocessor within the controller  56 . 
     Referring again to step  76 , in the event that the microprocessor controller determines that “P s ” is not greater than the equivalent saturated refrigerant pressure “P e ” plus the differential pressure value “ΔP 1 ”, then the microprocessor controller will proceed to a step  82  and inquire as to whether the sensed pressure “P s ” is less than the equivalent saturated refrigerant pressure “P e ” plus a differential pressure value of “ΔP 2 ”. The differential pressure value “ΔP 2 ” is preferably a differential pressure value that will prevent excessive modulation of the metering device  30 . In other words, no repositioning of the flow opening “F” will occur if the sensed subcooler pressure “P s ” is within a range of pressure values defined by the difference between “ΔP 1 ” and “ΔP 2 ”. The value of “ΔP 2  may vary anywhere from zero to one hundred percent of the value of “ΔP 1 ” so as to thereby allow for less or more modulation of the flow metering device  30 . Referring again to step  82 , in the event that the subcooler pressure “P s ” is less than the equivalent pressure plus the differential pressure “ΔP 2 ”, then the microprocessor controller will proceed to a step  84  and increase the flow opening, “F” by the amount “ΔF”. The amount “ΔF” would be computed or defined in the same manner as previously discussed with respect to step  78 . The processor will proceed from step  84  to step  80  wherein the new flow opening value “F” is sent to the metering device  30 . 
     As has been previously discussed, the flow metering device  30  preferably includes a local device control system which will respond to commanded flow opening “F”. This local control will compare the increased commanded flow opening with a maximum flow opening position for the particular flow metering device. The commanded flow opening will be implemented to the extent that it is less than the maximum flow opening position. It is to be appreciated that the above local control logic could also be included in the logic of FIG. 2 if necessary. 
     Referring again to step  82 , as has been previously discussed, if the sensed pressure value is not less than the equivalent pressure plus the differential pressure “ΔP 2 ”, then no change in the flow opening “F” is computed and tile microprocessor simply maintains the same flow opening commanded value for the metering device  30  in step  80 . The microprocessor controller proceeds out of step  80  to a step  86  and inquires as to whether the initial time period “t” has expired. When this time period has expired, the microprocessor controller will again cycle back to step  64  and inquire as to whether the compressor motor  12  is on. In the event that the compressor motor  12  is off, the microprocessor controller will again proceed through steps  66 - 84  to determine whether or not further adjustment in the flow opening of the metering device  30  is necessary. This will continue to occur until such time as the compressor motor  12  is turned off. At such time, the microprocessor controller will simply await the next indication that the compressor motor has been turned on whereupon the steps  66 - 86  will again take place. It is to be appreciated that a method and apparatus has been disclosed for optimally controlling the flow of refrigerant through the flow metering device  30  so as to thereby produce the required pressure drop in the refrigerant exiting the orifices  32  and  34 . 
     It is also to be appreciated that the control process, as implemented by the microprocessor controller  56 , could be implemented in hard wired logic. In such a case, the various portions of logic would appear as discrete elements. It is to be furthermore appreciated that the flow metering device  30  and the orifice pipe  36  and orifices  32  and  34  could be replaced by an alternative means for spraying liquid refrigerant into the subcooler chamber  38 . For example, the flow metering device  30  might be replaced by one or more variable orifices, which would each have an opening that could be varied by an prescribed amount which would be computed and commanded in accordance with the logical steps described for the microprocessor controller  56  or in hard wired logic. 
     It will be appreciated by those skilled in the art that further changes could be made to the above described invention without departing from the scope of the invention. Accordingly, the foregoing description is by way of example only and the invention is to be limited only by the following claims and equivalents thereto.