GB2191021A - Automatic reset of chilled water setpoint temperature control - Google Patents

Abstract

A microcomputer processes sensed and entered data to generate a reset for the set point temperature of water leaving a chiller. The microcomputer receives two data pairs entered by an operator corresponding to the amount of reset of the set point at one load and another amount of reset at another load, and receives refrigeration system operating parameters indicative of the temperature of water entering and leaving the chiller.

Description

SPECIFICATION Automatic reset of chilled water setpoint temperature control Background of the 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 compressor inlet guide vanes, in centrifugal vapor compression refrigeration systems whereby the temperature of the chilled water leaving the chiller is automatically raised as the cooling load decreases.

Generally, refrigeration systems include an evaporator or chiller, a compressor, and a condenser. Usually, a heat transfer fluid is circulated through tubing in the evaporator thereby forming a heat transfer 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 or glycol which is circulated to a remote location to satisfy a refrigeration load.

The refrigerant in the evaporator evaporates as it absorbs heat from the water flowing through the tubing in the evaporator, and the compressor operates to extract this refrigerant vapor from the evaporator, to compress this refrigerant vapor, and to discharge the compressed vapor to the condenser. In the condenser, the refrigerant vapor is condensed and delivered back to the evaporator where the refrigeration cycle begins again.

To maximize operating efficiency, it is desirable to match the amount of work done by the compressor to the work needed to satisfy the refrigeration load placed on the refrigeration system. Commoniy, this is done by capacity control means which adjust the amount of refrigerant vapor flowing through the compressor. The capacity control means may be a device such as guide vanes which are positioned between the compressor and the evaporator which move between a fully open and a fully closed position in response to the temperature of the chilled water leaving the chilled water coil in the evaporator. When the evaporator chilled water temperature falls, indicating a reduction in refrigeration load on the refrigeration system, the guide vanes move toward their closed position, decreasing the amount of refrigerant vapor flowing through the compressor.This decreases the amount of work that must be done by the compressor thereby decreasing the amount of energy needed to operate the refrigeration system. At the same time, this has the effect of increasing the temperature of the chilled water leaving the evaporator. In contrast, when the temperature of the leaving chilled water 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 water 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 of the chilled water leaving the evaporator at, or within a certain range of, a set point temperature.The leaving chilled water temperature setpoint may usually be adjusted at the operator's panel and once set will control the temperature of the leaving chilled water at the selected setpoint regardless of the machine load.

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 availabie 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 of the deviation of evaporator chilled water temperature from a desired set point temperature. When the leaving chilled water temperature deviates from the selected set point temperature by a predetermined amount the capacity control device is continuously adjusted by an actuator which is continuously energized by a stream of electrical pulses supplied to the actuator.The predetermined amount of temperature deviation before the actuator is continuously energized provides a temperature deadband in which the capacity control device is not adjusted. The pulse rate of the stream of electrical pulses supplied to the actuator determines the overall rate of adjustment of the capacity control device. This pulse rate may be set at either a minimum, middle, or maximum value thereby providing a limited capability for tailoring operation of the control system to meet specific job requirements of a particular job application for the refrigeration system.However, As with most capacity control systems, once the leaving chilled water temperature setpoint is selected, the leaving chilled water temperature will be controlled at the selected setpoint temperature from zero load to full load, while the entering chilled water temperature will deviate from the leaving chilled water temperature in a linear fashion from zero AT at zero load to design AT at full load.

Operating a capacity control system, however, with a fixed leaving chilled water setpoint is not very energy efficient at low loads, because the refrigeration system is stili maintaining a leaving chilled water temperature that is lower than that which is actually needed to ensure comfort in the space being cooled.

Thus there exists a need to develop capacity control techniques for chillers which can reduce energy consumption at lower loads by raising the temperature of the chilled water leaving the chiller as the cooling load decreases.

Summary of the Invention Therefore, it is an object of the present invention to provide a simple, efficient, and effective microcomputer system for controlling the capacity of a refrigeration system.

It is another object of the present invention to provide an easily programmable microcomputer system for controlling the setpoint of the chilled water leaving the chiller directly in response to changes in the cooling load.

These and other objects of the present invention are attained by a capacity control system for a refrigeration system comprising a capacity control device for controlling refrigerant flow in the refrigeration system, a microcomputer for resetting the leaving chilled water temperature setpoint in accordance with selected parameters, and means for generating first and second signals indicative of a sensed temperature of the heat transfer fluid entering the chiller and a sensed temperature of the heat transfer fluid leaving the chiller, respectively. The first and second signals are supplied to the microcomputer which determines the schedule of reset for the leaving chilled water temperature setpoint in accordance with two data pairs programmed by the system operator into the microcomputer.Each data pair consists of a temperature difference and a desired value of reset at the temperature difference. Thus the leaving chilled water temperature setpoint can vary with load, either from minimum to maximum load or between any two intermediate load values. The capacity control device is adjusted to control refrigerant flow in the refrigeration system in response to the control signal generated by the microcomputer. By selecting different data pairs 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 a reset schedule for the leaving chilled water temperature setpoint based on the ioad.

Brief Description of the Drawings Still other objects and advantages of the 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 iike or corresponding parts throughout the same, in which: Figure 1 is a 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 of the present invention; and Figure 2 is a graph of chilled water temperature as a function of load.

Description of the Preferred Embodiment Referring to Fig. 1, a vapor compression refrigeration system 1 is shown having a centrifugal compressor 2 with a control system 3 for varying the capacity of the refrigeration system 1 according to the principles of the present invention. As shown in Fig. 1, the refrigeration system 1 includes a condenser 4, 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 4 wherein the gaseous refrigerant is condensed by relatively cool condensing water flowing through tubing 8 in the condenser 4.

The condensed liquid refrigerant from the condenser 4 passes through the poppet valve 6 in refrigerant line 9 to evaporator 5. The liquid refrigerant in the evaporator 5 is evaporated to cool a heat transfer fluid, such as water or glycol, flowing through tubing 10 in the evaporator 5. This chilled heat transfer fluid is used to cool a building or is used for other such purposes. The gaseous refrigerant from the evaporator 5 flows through compressor suction line 11 back to compressor 2 under the control of compressor inlet guide vanes 12. The gaseous refrigerant entering the compressor 2 through the guide vanes 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 3 which 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 the temperature of the heat transfer fluid leaving the evaporator 5 through the tubing 10 and temperature sensor 15 for sensing the temperature of the heat transfer fluid entering the evaporator 5 through the tubing 10, is connected by electrical lines 20 and 22 directly to the A/D converter 19.

Preferably, the temperature sensors 13 and 15 are a temperature responsive resistance devices such as a thermistors having their sensing portions located in the heat transfer fluid in the tubing 10 in the evaporator 5 with their resistance monitored by the A/D converter 19, as shown in Fig. 1. Of course, as will be readily apparent to one of ordinary skill in the art to which the present invention pertains, the temperature sensors 13 and 15 may be any of a variety of temperature sensors suitable for generating a signal indicative of the temperature of the heat transfer fluid in the tubing 10 in the evaporator 5 and for supplying these generated signals to the A/D 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, the A/D converter 19 is a dual slope A/D converter which shall process all analog inputs into digital outputs and which is suitable for use with the processor board 17. Also, it should be noted that, although the A/D converter is shown as a separate module in Fig. 1, this A/D converter 19 may 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 the data pairs to establish a reset schedule for the chilled water leaving the evaporator 5 through the evaporator chilled water tubing 10.

Still further, preferably, the system interface board 16 includes at least one switching device, such as a model SC-140 triac available from General Electric, Corp. which has a place of business at Auburn, New York, which is used as a switching element for controlling a supply of electrical power (not shown) 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 line 21 to the guide vane actuator 14 under control of the processor board 17 to operate the guide vane actuator 14 in the manner according to the principles of the present invention which is described in detail below.

Of course, as will be readily apparent to one of ordinary skill in the art to which the present invention pertains, switching devices other than triac switches may be used in controlling power flow from the power supply (not shown) through the electrical line 21 to the guide vane actuator 14 in response to 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 either their open or closed position in response to electrical power signals received via electrical line 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, for driving 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 either their fully open or fully closed position at a constant, fixed rate only during that portion of a selected base time interval during which the appropriate triac switch on the system interface board 16 is actuated.

Referring now to Fig. 2, a dashed straightline curve A, is shown which represents the leaving chilled water temperature which a prior art control system without reset would produce. Also, a solid sloped line curve B is shown which represents the leaving chilled water temperature setpoint of the present invention with reset. These setpoints are arbitrary. The amount of reset that is to be applied is computed by the microprocessor after two data pairs are entered into the setpoint and display board by the operator by way of the key pad. By having the microprocessor automtically adjust the chilled water setpoint as a function of load, the operator may reduce energy consumption during certain load conditions.The entering chilled water temperature corresponding to curve A without reset, is shown by dashed sloped curve C while curve D, represents the entering chilled water temperature during any load condition with the desired reset of curve B. The vertical axis of Fig. 2 is the temperature of the chilled water entering and leaving the evaporator. The horizontal axis of Fig. 2 is the load on the refrigeration system. Also shown on the horizontal axis is a typical set of AT values. The value of 5.6"C (10"F), chosen for evaporator AT at full load, was an arbitrary choice.

In the example given in Fig. 2, the curve labelled A illustrates an arbitrary setpoint for the leaving chilled water temperature and the curve labeled B illustrates the corresponding leaving chilled water temperature with the desired amount of reset applied. Thus, in the example, assuming a design AT of 5.6"C (10"F) at 100% load, the operator desires the refrigeration system to control the temperature of the leaving chilled water at 7.2"C (45"F) when the load is 100% and 10 C (50"F) when the load is 0%. Thus the operator would input two data pairs into the microcomputer, the first data pair would be a reset of 2.8"C (5"F) at AT=0 C (0 F) and the second data pair would be a reset of 0 C (0 F) at AT=5.6 C (10F). Accordingly, with the data pairs in the example inputted into the setpoint and display board 18, as the cooling load changes, the microprocessor will calculate a new leaving chilled water temperature setpoint to ensure that the leaving chilled water temperature is the correct value for the load conditions.If for example the load decreased from 100% to 20%, with the above data pairs inputted, the leaving chilled water temperature setpoint would increase from 7.2"C (45"F) to 9.4"C (49"F, whereas without setpoint reset the leaving chilled water temperature setpoint would remain at 7.2"C (45"F). Thus, energy consumption is reduced as the chilled water leaving the chiller is raised as the cooling loading decreases.

Any number of data pairs can be inputted into the setpoint and display board depending upon the desired setpoint.

While this invention has been described with reference to a particular embodiment disclosed herein, it is not confined to the details set forth herein and this application is intended to cover any modifications or changes as may come within the scope of the invention.

Claims (5)

1. A control system for controlling the reset of a leaving chilled fluid temperature setpoint in a refrigeration system of the type which includes a microprocessor system and an evaporator wherein a refrigerant absorbs heat from a heat transfer fluid passing therethrough, comprising: means for entering at least two data pairs into the microcomputer and for generating a signal corresponding to each of said entered data pairs wherein the first data pair corresponds to the amount of reset from a fixed leaving chilled fluid temperature setpoint at a first load and the second data pair corresponds to the amount of reset from said fixed leaving chilled fluid temperature setpoint at a second load; means for generating a first control signal which is a function of the temperature of the heat transfer fluid entering the evaporator;; means for generating a second control signal which is a function of the temperature of the heat transfer fluid leaving the evaporator; and processor means for receiving said signal corresponding to each of said entered data pairs, and said first and second control signals, for processing the received signals according to preprogrammed procedures to determine a setpoint temperature for the heat transfer fluid leaving the evaporator for the actual load on the refrigeration system.

2. A control system as set forth in claim 1 wherein said processor means determines said setpoint temperature in proportion to the entered data pairs and the difference between entering and leaving temperature of the heat transfer medium.

3. A method of resetting a leaving chilled liquid temperature setpoint of a refrigeration system, having a microcomputer system and an evaporator wherein a refrigerant absorbs heat from the chilled liquid passing therethrough, which comprises: generating a first setpoint signal corresponding to a first data point entered into the microprocessor; generating a second setpoint signal corresponding to a second data point entered into the microprocessor; generating a first temperature signal corresponding to actual entering chilled liquid temperature; generating a second temperature signal corresponding to actual leaving chilled liquid temperature; and generating an output signal in proportion with a predetermined relationship of said first and second setpoint signals, and said first temperature and said second temperature signals.

4. A method as set forth in claims 3 wherein said first entered data point corresponds to a data pair corresponding to the amount of reset from a fixed leaving chilled liquid temperature setpoint at a first load condition, and said second entered data point corresponds to a data pair corresponding to the amount of reset from said fixed leaving chilled liquid temperature setpoint at a second load condition.

5. A method as set forth in claim 4 wherein said output signal is generated in proportion to the sum of said setpoint signal and the difference between said first and said second temperature signals.