GB2186399A - Refrigeration control - Google Patents

Abstract

A microcomputer system 16-19 receives electrical input signals indicative of an operator selected temperature setpoint for water entering evaporator 5 and refrigeration system operating parameters indicative of water temperature entering the evaporator at 15 and of water temperature leaving the evaporator at 13. The microcomputer processes these input signals to generate a leaving water temperature control point which is a function of the entering water temperature setpoint and the temperature drop across the evaporator. The capacity of the system is thereby controlled by adjustment of vanes 12 to match the refrigeration load. <IMAGE>

Description

SPECIFICATION Automatic chilled water setpoint temperature control using return chilled water temperature Background of invention The present invention relates to methods of operating and control systems for refrigeration systems and, more particularly, to methods of operating and control systems for capacity control devices, such as com pressor inlet guide vanes, in centrifugal vapor compression refrigeration systems whereby a predetermined entering setpointtemperature is maintained at the chiller.

Generally, refrigeration systems include an evaporator or cooler/chiller, a compressor, and a condenser.

Usually, a heattransferfluid is circulated through tubing in the evaporator thereby forming a heattransfer coil in the evaporator to transfer heat from the heat transfer fluid flowing through the tubing to refrigerant in the evaporator. The heat transfer fluid chilled in the tubing in the evaporator is normally water orglycol which is circulated to a remote location to satisfy a refrigeration load. The refrigerant in the evaporator evaporates as it absorbs heat from the heattransferfluid flowing through the tubing in the evaporator, and the compressor operates to extract this refrigerantvaporfrom the evaporator, to compress this refrigerant vapor, and to discharge the compressed vapor to the condenser.In the condenser, the refrigerantvapor is condensed and delivered back to the evaporatorwherethe refrigeration cycle begins again.

To maximize operating efficiency, it is desirable to match the amount of work done by the compressorto the work needed to satisfy the refrigeration load placed on the refrigeration system. Commonly, this is done by capacity control means which adjustthe amount of refrigerant vaporflowing through the compressor.

The capacity control means may be a device such as guide vanes which are positioned between the compressor and the evaporatorwhich move between a fully open and a fully closed position in responsetothe temperature of the chilled heattransferfluid leaving the coil in the evaporator. When the evaporator chilled heattransferfluid temperature falls, indicating a reduction in refrigeration load on the refrigeration system, the guide vanes move toward their closed position decreasing the amount of refrigerantvaporflowing through the compressor. This decreases the amount of workthat must be done by the compressorthereby decreasing the amount of energy needed to operate the refrigeration system. Atthe same time, this has the effect of increasing the temperature of the chilled heattransferfluid leaving the evaporator.In contrast,when the temperature of the leaving chilled heat transfer fluid rises, indicating an increase in load on the refrigeration system, the guide vanes move toward their fully open position. This increases the amount of vapor flowing through the compressor and the compressor does more work thereby decreasing the temperature of the chilled heattransferfluid leaving the evaporator and allowing the refrigeration system to respond to the increased refrigeration load. In this manner, the compressor operates to maintain the temperature ofthe chilled heat transfer fluid leaving the evaporator at, orwithin a certain range of, a set pointtemperature.

Many different capacity control systems are known for controlling a refrigeration system in the manner described above. For example, one such control system, a model CP-8142-024 Electronic Chiller Control available from the Barber-Colman Company having a place of business in Rockford, Illinois, adjusts a capacity control device in a refrigeration system as a function ofthe deviation of leaving evaporator chilledwater temperature from a desired set point temperature. When the evaporator chilled water temperature deviates from the selected set pointtemperature by a predetermined amount the capacity control device is con tinuously adjusted by an actuator which is continuously energized by a stream of electrical pulses supplied to the actuator.

Certain energy management systems make it desirable to maintain a constant entering chilled watertemperature and ietthe leaving chilled watertemperature "float" or seek its own equilibrium temperature as the load varies. This is the reverse of conventional prior art chilled watertemperature control systems. However, controlling chillercapacity based solely on a deviation of entering chilled watertemperaturefrom afixed entering chilled watertemperature setpoint, in a manner analogous to that used in prior leaving chilled water temperature control systems causes exaggerated vane movements with little stability in the system, because a substantial time delay exists between capacity changes made at the chiller and the resulting temperature changes sensed in the water entering the chiller.This time lag in the water loop causes the control to overcompensate and the result is control instability and excessive temperature oscillation.

Thus, there exists a need to develop capacity control techniques for chillers which maintain a constant entering chilled watertemperature and which minimizethe disadvantages of controlling chiller capacity in responseto leaving chilled watertemperature or deviation of entering chilled water temperature from a fixed setpoint.

Summary ofthe invention Therefore, it is an object of the present invention to provide a simple, efficient, and effective microcomputersystem for controlling the capacity of a refrigeration system in responseto entering water setpointtemperature.

It is another object of the present invention to provide an easily programmable microcomputer system for controlling the capacity of a refrigeration system by generating a leaving chilled water control point so asto provide a desired entering chilled water setpointtemperature.

It is still another objectof the present invention to provide a control scheme for controlling the capacity of a refrigeration system that is impervious to changes in chillerwaterflow rate.

These and other objects of the present invention are attained by a capacity control system for a refrigeration system comprising means for generating a setpoint signal corresponding to a selected setpointtem peraturefotthe hat transfer medium entering the evaporator (the setpointtemperature is a value selected by an operator and inputted into the microprocessor), means for generating a first control signal which is a function of the temperature of the heat transfer medium entering the evaporator, means for generating a second control signal which is a function of the temperature of the heat transfer medium leaving the evaporator, and processor means for receiving said setpoint signal, and said first and second control signalsfor processing the received signals according to preprogrammed procedures to determine a control pointtemperature for the heat transfer medium leaving the evaporator (the control point temperature is a value generated bythe microprocessor and is used to control the load, e.g. by positioning the guide vanes), and for generating an output control signal for controlling the capacity ofthe chiller in response to the output control signal.

The processor means, a microcomputer, determines the leaving chilled water temperature control point using the entering chilled watersetpoint and the temperature drop across the evaporator. This control feature is stated mathematically asfollows: LWCi' = EWSP - (AT) where AT=T-Th and where; LWCP is the leaving chilledwatertemperature control point, EWSP is the entering chilled water temperature setpoint, Ti is the water temperature entering the evaporator, and To is the water temperature leaving the evaporator.

By selecting a desired setpoint for the entering chilled water, operation of the capacity control device may be easily, efficiently, and effectively tailored to meet specific job requirements of a particular job application for the refrigeration system by establishing the leaving chilled watertemperature control point.

Briefdescription ofthe drawings Still other objects and advantages ofthe present invention will be apparent from the following detailed description of the present invention in conjunction with the accompanying drawings, in which the reference numerals designate like or corresponding parts throughout the same, in which: Figure lisa schematic illustration of a centrifugal vapor compression refrigeration system with a control system for varying the capacity of the refrigeration system according to the principles ofthe present invention, and Figure2 is a graph of entering and leaving chilled water temperature as a function of load for atypical chiller system.

Description of the preferred embodiment Referring to Figure 1, a vapor compression refrigeration system 1 is shown having a centrifugal compressor 2 with a control system 3 for varying the capacity ofthe refrigeration system 1 according to the principles ofthe present invention. As shown in Figure 1, the refrigeration system 1 includes a condenser4, an evaporator 5 and a poppet valve 6. In operation, compressed gaseous refrigerant is discharged from the compressor 2 through compressor discharge line 7 to the condenser 4wherein the gaseous refrigerant is condensed by relatively cool condensing water flowing through tubing 8 in the condenser 4.The condensed liquid refrigerant from the condenser4 passes through the poppetvalve 6, which forms a liquid seal to keep condenservaporfrom entering the evaporator and to maintain the pressure difference between the condenser and the evaporator, in refrigerant line 9 to evaporator 5. The liquid refrigerant in the evaporator 5 is evaporated to cool a heattransferfluid, such aswaterorglycol, flowing through tubing 10 in the evaporator 5. This chilled heattransferfluid is used to cool a building or is used for other such purposes. The gaseous refrigerantfrom the evaporatorS flows through compressor suction line 11 backto compressor 2 underthe control of compressor inlet guide vanes 12. The gaseous refrigerant entering the compressor 2 through the guide vales 12 is compressed by the compressor 2 and discharged from the compressor 2 through the compressor discharge line 7 to complete the refrigeration cycle. This refrigeration cycle is continuously repeated during normal operation of the refrigeration system 1.

The compressor inlet guide vanes 12 are opened and closed by a guide vane actuator 14 controlled by the capacity control system 3which comprises a system interface board 16, a processor board 17, a set point and display board 18, and an analog/digital converter 19. Also, temperature sensor 13 for sensing thetem- perature ofthe heattransferfluid leaving the evaporator 5through the tubing 10 and temperature sensor 15 for sensing the temperature of the heattransferfluid entering the evaporator S through the tubing 10, are connected by electrical lines 20 and 22 directly to the A/D converter 19.

Preferably,thetemperature sensors 13 and 15 are temperature responsive resistance devices such as a thermistors having their sensing portions located in the heattransferfluid in the tubing 10 in the evaporator 5 with their resistances monitored by theA/D converter, as shown in Figure 1. Of course, as will be readily apparent to one of ordinary skill in the artto which the present invention pertains, the temperature sensors 13 and 15may be any of a variety of temperature sensors suitable for generating a signal indicative ofthe temperature of the heattransferfluid in thetubing 1 O in the evaporator 5 and forsupplying thesegenerated signalsto the ND converter 19.

The processor board 17 may be any device, or combination of devices, capable of receiving a plurality of input signals, processing the received input signals according to preprogrammed procedures, and producing desired output control signals in response to the received and processed input signals, in a manner according to the principles of the present invention. For example, the processor board 17 may comprise a microcomputer, such as a model 8031 microcomputer available from Intel Corporation which has a place of business at Santa Clara, California.

Also, preferably,theA/D converter 19 is a dual slope A/D converterwhich shall process all analog inputs and which is suitable for use with the processor board 17. Also, it should be noted that, although the no converter 19 is shown as a separate module in Figure 1 ,this AID converter 19may be physically part of the processor board 17 in an actual capacity control system 3.

Further, preferably, the set point and display board 18 comprises a visual display, including, for example, light emitting diodes (LED's) or liquid crystal display (LCD's) devices forming a multi-digit display which is under the control of the processor board 17. Also, the set point and display board 18 includes a device,such as a key pad which serves as a data entry port as well as a programming tool, for entering thetemperature setpoint of the chilled water entering the evaporator Sthrough the evaporator chilled watertubing 10.

Still further, preferably, the system interface board 16 includes at least one switching device, such as a model SC-140triac available from General Electric, Corp. which has a place of business at Au burn, NewYork, which is used as a switching element for controlling a supply of electrical power(notshown) through electrical lines 21 to the guide vane actuator 14. The triac switches on the system interface board 16 are controlled in response to control signals received by the triac switches from the processor board 17.In this manner, electrical power is supplied through the electrical lines 21 to the guide vane actuator 14 under control ofthe processor board 17 to operate the guide vane actuator 14 in the manner according to the principles ofthe present invention which is described in detail below. Of course, as will be readily apparent to one ofordinary skill in the artto which the present invention pertains, switching devices otherthan triac switches may be used in controlling power flow from the power supply (not shown) through the electrical lines 21 to the guide vane actuator 14 in responseto output control signals from the processor board 17.

The guide vane actuator 14 may be any device suitable for driving the guide vanes 12 toward eithertheir open or closed position in response to electrical power signals received via electrical lines 21. For example, the guide vane actuator 14 may be an electric motor, such as a model MC-351 motor available from the Barber-Colman Company having a place of business in Rockford, Illinois,fordriving the guide vanes 12 toward either their open or closed position depending on which one of two triac switches on the system interface board 16 is actuated in response to control signals received by the triac switches from the processor board 17.The guide vane actuator 14 drives the guide vanes 12 toward eithertheirfully open orfullyclosed position at a constant, fixed rate only during that portion of a selected basetime interval during which the appropriate traic switch on the system interface board 16 is actuated.

Referring now to Figure 2, a solid straight-line horizontal curve A is shown which represents the entering chilled watertemperature setpointthat is desired to be maintained by the refrigeration system of the present invention. This setpoint is arbitrary and is entered into the setpoint and display board by the operator byway of the key pad. By maintaining a fixed entering chilled watertemperature the operator may reduce energy consumption during certain load conditions, particularly at low loads. The leaving chilled water control point of the present invention, as shown by the solid sloped line curve B, represents the control point at which the leaving chilled water is operated during any load condition with the setpoint of curve A.The vertical axis of Figure 2 is the temperature of the chilled water entering or leaving the evaporator. The horizontal axis of Figure 2 is the load on the refrigeration system. An arbitrary value of 5.6"C (1 0 F) was chosen as evaporator.

AT at full load and corresponding values of AT at various loads are shown.

In Figure 2, the curve labeled A illustrates an arbitrary fixed setpointforthe entering chilled watertem- peratu re. Th us, in this example, the operator desires the refrigeration system to control the temperature of the entering chilled water at 12.8"C (55"F).

The dashed straight-line horizontal Curve C represents the fixed leaving chilled water temperature setpoint of the prior art, while the dashed sloped line Curve D represents the floating entering chilled watertemperature of the prior art.

Once a predetermined entering chilled watersetpoint is selected, the leaving chilled watertemperature control point is calculated by the microprocessor, and is periodically updated e.g. once every five seconds, to ensurethatthe leaving chilled watertemperature isthe correctvalueforthe present load. If however,the load changes, for example a load would, over a period of time, decrease from 50% to 10%, the leaving chilled water temperature control point would increase from 1 0 C (50"F) to 1 2.2"C (54 F). This increase will be done at a gradual rate such as 0.069"C/min (0.125 degrees F per minute).Thus, the actual leaving chilled watertemperature sensed by the sensor 13 would, instantaneously, still indicate 10"C (SO"F)whilethe leaving chilled water temperature control point would be calling for a leaving chilled water slightly above 1 0 C (50F). Thus, a deviation would exist between the actuai leaving chilled watertemperature and the leaving chilled water temperature control point, and the deviation is inputted to the system interface board which causes the guide vane actuator 1 4to move the guide vanes 12 toward their closed position until the actual temperature ofthe leaving chilled water is equal to the leaving chilled watertemperature control point.

In this mannerthe chiller will make a smooth transition from a given load level to another because the leaving chilled watertemperature sensor immediately detects the watertemperature change caused by the alteration in guide vane position. Thus through the use of the generated control point, the desired entering chilled water temperature is maintained during the transition.

Further, is should be noted that the chilled waterflow rate may vary without changing the control scheme, since the leaving chilled watertemperature control point is only a function ofthe entering chilled water temperature setpoint and the watertemperature entering and leaving the evaporator.

Whilethis invention has been described with reference to a particularembodimentdisclosed herein, itis not confined to the details setforth herein and this application is intended to cover any modifications or changes as may come within the scope of the invention.

Claims (6)

1. A capacity control system for a refrigeration system of the type which includes an evaporator wherein a refrigerant absorbs heat from a heattransfer medium passing therethrough, comprising: means for generating a setpoint signal corresponding to a selected fixed setpointtemperature forthe heat transfer medium entering the evaporator; means for generating a first control signal which is a function of the temperature of the heattransfer medium entering the evaporator; means for generating a second control signal which is a function of the temperature ofthe heattransfer medium leaving the evaporator; and processor means for receiving said setpoint signal, and said first and second control signals for processing the received signals according to preprogrammed procedures to determine a control point temperature for the heat transfer medium leaving the evaporator, and for generating a control pointtemperature signal for controlling the load on the evaporator.

2. A capacity control system as set forth in claim 1 wherein said processor means determines said control pointtemperature in proportion to the difference between said selected fixed setpointtemperature and a difference in temperature of the heat transfer medium across the evaporator.

3. A method of generating a leaving chilled fluid temperature control point for refrigeration system having an evaporatorwherein a refrigerant absorbs heat from the chilled fluid passing therethrough,which comprises: generating a temperature setpointsignal corresponding to a desired entering chilled fluid temperature; generating a first temperature signal corresponding to actual entering chilled fluid temperature; generating a second temperature signal corresponding to actual leaving chilled fluid temperature; and generating a leaving chilled fluid temperature control point as a function of said setpoint, saidfirsttem- perature, and said second temperature signals.

4. A method as set forth in claim 3 wherein said generated leaving chilled fluid temperature control point is in proportion to the difference of said temperature setpoint signal and, the difference between said first and said second temperature signals.

5. In a refrigeration system having a centrifugal compressor, an evaporator, a liquid heat exchanger in the evaporator, and a condenser, a method of controlling the liquid entering the heat exchanger at a pred etermined setpointtemperaturecomprising the steps of: sensing the temperature of the liquid entering the heat exchanger; sensing the temperature of the liquid leaving the heat exchanger; producing a control poipttemperature signal for the liquid leaving the heat exchanger that is a function of the predetermined setpointtemperature and said temperatures of the liquid entering and leaving the heat exchanger; and varying the capacity ofthe liquid heat exchanger in response to the deviation between the temperature of the liquid leaving the heat exchanger and the produced control pointtemperature.

6. A method of controlling the liquid entering the heat exchanger at a predetermined setpointtem perature as set forth in claim 5 wherei n the step of producing a control point temperature signal includesthe step of producing said control pointtemperature signal in proportion to the difference between (a) the pred etermined setpointtemperature of the Liquid entering the heat exchanger, and (b) the difference between the sensed temperature ofthe entering the heat exchanger and the sensed temperature of the liquid leaving the heat exchanger.