Method and Control System for Limiting the Load Placed on a Refrigeration System Upon a Recycle Start

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 controlling recycle starts of a compressor in a refrigeration system.

Generally, refrigeration systems include an evaporator or cooler, 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 evapbrator. The heat transfer fluid chilled in the tubing in the evaporator is normally water 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. Commonly, this is done by capacity control means which adjusts 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 and 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. Under certain operating conditions, such as low load conditions, the refrigeration system may provide excess capacity for satisfying the load placed on the refrigeration system even though the guide vanes are at their fully closed position which corresponds to a minimum operating capacity for the compressor. Under these conditions, it is customary to turn off the refrigeration system compressor to prevent undesirable excess cooling of the water flowing through the heat transfer tubes in the evaporator which, if unchecked, could result in freezing of this water. Then, when a new, increased load on the refrigeration system is detected, the compressor is restarted and the guide vanes are again used to adjust refrigeration system capacity to match the load placed on the refrigeration system. A restart of the refrigeration system compressor under the foregoing conditions is known as a recycle start. Recycle starts are not particularly desirable since they produce wear and tear on the mechanical and electrical systems of the refrigeration system and may reduce the operating life and decrease the reliability of the overall refrigeration system.

Therefore, it is an object of the present invention to prolong the operating life of a refrigeration system and to improve the reliability of the refrigeration system by reducing the number of recycle starts made by the refrigeration system.

This and other objects of the present invention are attained by a method of operating and control system for a refrigeration system which limits the load placed on the refrigeration system upon a recycle start. This is accomplished according to the present invention with a programmable electronic control system for the refrigeration system, such as a microcomputer control system, by programming the electronic control system to provide a preselected, relatively gradual increase in load placed on the refrigeration system, which is followed only during a recycle start. When starting the refrigeration system for other reasons, such as daily operation, safety trip, etc., the refrigeration system is controlled to respond to the actual load placed on the refrigeration system.

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:

FIG. 1 is a schematic illustration of a centrifugal vapor compression refrigeration system with a control system for operating the refrigeration system according to the principles of the present invention.

FIG. 2 is a graph illustrating the principles of operation of the control system shown in FIG. 1.

Referring to FIG. 1, a centrifugal vapor compression refrigeration system 1 is shown having a control system 3 for operating the refrigeration system 1 according to the principles of the present invention. As shown in FIG. 1, the refrigeration system 1 includes a compressor 2, a condenser 4, an evaporator 5, and an expansion device 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 refrigerant line 9 and expansion device 6 to the evaporator 5. The liquid refrigerant in the evaporator 5 is evaporated to cool a heat transfer fluid, such as water, flowing through tubing 10 in the evaporator 5. This cool 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 the 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.

Also, as shown in FIG. 1, the centrifugal compressor 2 of the refrigeration system 1 includes an electric motor 25 for driving the compressor 2 which is under the control of the control system 3. Also, it may be seen that the compressor inlet guide vanes 12 are opened and closed by a guide vane actuator 14 controlled by the control system 3.

The control system 3 includes a compressor motor starter 22, a power supply 23, a system interface board 16, a processor board 17, and a set point and display board 18. Also, a temperature sensor 13 for sensing the temperature of the heat transfer fluid leaving the evaporator 5 through the tubing 10, is connected by electrical lines 20 directly to the processor board 17.

Preferably, the temperature sensor 13 is a temperature responsive resistance device such as a thermistor having its sensing portion located in the heat transfer fluid leaving the evaporator 5 with its resistance monitored by the processor board 17. Of course, as will be readily apparent to one of ordinary skill in the art to which the present invention pertains, the temperature sensor 13 may be any of a variety of temperature sensors suitable for generating a signal indicative of the temperature of the heat transfer fluid leaving the evaporator 5 and for supplying this generated signal to the processor board 17.

The processor board 17 may be any device or combination of devices, for receiving a plurality of input signals, for processing the received input signals according to preprogrammed procedures, and for 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, Calif.

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, preferably, the set point and display board 18 includes a device, such as a set point potentiometer model AW5403 available from CTS, Inc. which has a place of business at Skyland, N.C., which is adjustable to output a signal to the processor board 17 indicative of a selected set point temperature for the heat transfer fluid leaving the evaporator 5 through the tubing 10.

The system interface board 16 includes a plurality of switching devices for controlling the flow of electrical power from the power supply 23 through the system interface board 16 to the guide vane actuator 14 and the motor 25 for driving the compressor 2. Each of the switching devices may be a model SC-140 triac available from General Electric Company which has a place of business at Auburn, N.Y. However, as will be readily apparent to one of ordinary skill in the art to which the present invention pertains, switches other than triac switches may be used as the switching devices.

The switching devices on the system interface board 16 are controlled in response to control signals received by the switching devices from the processor board 17. In this manner, the guide vane actuator 14 and the motor 25 driving the compressor 2 are controlled by the processor board 17.

The guide vane actuator 14 may be any device suitable for driving the guide vanes 12 toward either their fully open or fully 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-Coleman Company having a place of business in Rockford, Ill., for driving the guide vanes 12 toward either their fully open or fully closed position depending on which one of two switching devices on the system interface board 16 is actuated in response to control signals received by the switching devices from the processor board 17. The guide vane actuator 14 may be controlled to drive the guide vanes 14 toward their fully open or fully closed position according to any one of a variety of control schemes designed to control the capacity of the refrigeration system 1 to match the load placed on the refrigeration system 1.

The compressor motor starter 22 is a device for supplying electrical power from the power supply 23 to the electric motor 25 of the compressor 2 to start up and run the motor 25. For example, the compressor motor starter 22 may be a conventional wye-delta (Y-.DELTA.) contactor type motor starter. Of course, as will be readily apparent to one of ordinary skill in the art to which the present invention pertains, the compressor motor starter 22 may be any one of a variety of systems for supplying electrical power from the power supply 23 to the electric motor 25 of the compressor 2 to start and run the motor 25.

In operation, the temperature sensor 13 senses the temperature of the heat transfer fluid in tubing 10 leaving the evaporator 5 and a signal indicative of this sensed temperature is supplied to the processor board 17 of the control system 3. Also, a signal indicative of a set point temperature is supplied from the set point and display board 18 to the processor board 17. This set point temperature is an operator selected temperature to which the heat transfer fluid leaving the evaporator 5 through the tubing 10 is to be cooled by operation of the refrigeration system 1. Thus, the temperature sensed by the temperature sensor 13 relative to the set point temperature setting of the set point and display board 18 represents a refrigeration load to be satisfied by operation of the refrigeration system 1.

The processor board 17 is programmed to compare the temperature sensed by the temperature sensor 13 to the selected set point temperature setting of the set point and display board 18. If the sensed temperature sensed by the temperature sensor 13 exceeds the set point temperature setting of the set point and display board 18 by a predetermined amount, the processor board 17 generates control signals to turn on the refrigeration system 1. As part of turning on the refrigeration system 1, the processor board 17 supplies electrical control signals to the system interface board 16 to close certain switching devices on the system interface board 16. This results in electrical power flow from the power supply 23 through the system interface board 16 to the compressor motor starter 22 which starts and runs the electric motor 25 of the compressor 2 in the refrigeration system 1. Also, electrical power flows from the power supply 23 through the system interface board 16 and the electrical lines 21 to the guide vane actuator 14 under control of the processor board 17 so that the guide vanes 12 may be controlled by the processor board 17 to match the load placed on the refrigeration system 1. Thus, in the foregoing manner, the processor board 17 turns on the refrigeration system 1, including the refrigeration system compressor 2, when the processor board 17 detects a load to be satisfied by operation of the refrigeration system 1.

After the refrigeration system 1 is turned on by the processor board 17, the refrigeration system 1 continuously operates to satisfy the refrigeration load. The processor board 17 adjusts the capacity of the refrigeration system 1 to match the load by controlling the guide vane actuator 14 to move the compressor inlet guide vanes 12 between their fully open and fully closed positions in response to detected changes in the load on the refrigeration system 1. However, if the processor board 17 determines that the load has been satisfied and that the refrigeration system 1 is providing excess cooling capacity for satisfying the load even though the guide vanes 12 are positioned at their fully closed position corresponding to the minimum operating capacity for the compressor 2, the processor board 17 generates a control signal to open the appropriate switching device on the system interface board 16 to discontinue the power flow from the power supply 23 through the compressor motor starter 22 to the electric motor 25 of the compressor 2 of the refrigeration system 1. This effectively turns off the refrigeration system compressor 2 while otherwise maintaining the refrigeration system 1 ready for operation.

According to the present invention, when the compressor 2 is turned off by the processor board 17 due to excess cooling capacity, this information is stored in the memory of the processor board 17. Then, when it is desired to again turn on the refrigeration system compressor 2 to operate the refrigeration system 1 to satisfy a new, increased load on the refrigeration system 1, the processor board 17 controls The refrigeration system 1 in a special way to reduce the likelihood that another recycle start will be required in the near future. Specifically, upon a recycle start, the processor board 17, through control of the appropriate switching devices on the system interface board 16, controls the guide vane actuator 14 and thus the guide vanes 12 to greatly reduce the rate of decrease in the temperature of the heat transfer fluid cooled in the evaporator 5 compared to the normal, relatively fast rate at which the temperature of the heat transfer fluid is usually decreased to directly match the detected load placed on the refrigeration system 1. This control strategy is followed until the temperature of the heat transfer fluid cooled in the evaporator 5 is decreased to the set point temperature setting of the set point and display board 18. Then, control of the guide vanes 12 by the processor board 17 is carried out directly in response to the detected, actual load requirements on the refrigeration system 1. By controlling the refrigeration system 1 in this manner to reduce the temperature of the heat transfer fluid in the evaporator 5 at this relatively slow rate upon a recycle start, the refrigeration system 1 is prevented from quickly satisfying the new, increased load placed on the refrigeration system 1 after which the refrigeration system compressor 2 will again have to be turned off thereby necessitating another recycle start of the compressor 2. Thus, fewer recycle starts are made thereby reducing wear and tear on the mechanical and electrical systems of the refrigeration system 1 to prolong the operating life and to improve the reliability of the refrigeration system 1.

The foregoing described operation of the refrigeration system 1 according to the principles of the present invention is best understood by referring to FIG. 2 which is a purely illustrative graph showing evaporator 5 leaving heat transfer fluid temperature as a function of time after a recycle start of the refrigeration system 1. The curve labeled "A" represents a typical, normal, relatively fast rate of decrease in the evaporator 5 leaving heat transfer fluid temperature as a function of time after a recycle start when the capacity of the compressor 2 is controlled by the processor board 17 directly in response to the load placed on the refrigeration system. The curve labeled "B" represents a special, relatively slow rate of decrease in the evaporator 5 leaving heat transfer fluid temperature as a function of time after a recycle start when the capacity of the compressor 2 is controlled by the processor board 17 according to the principles of the present invention.

As shown in FIG. 2, temperature T.sub.S represents the desired set point temperature for the heat transfer fluid leaving the evaporator 5 as set by the potentiometer on the set point and display board 18. Temperature T.sub.L represents the temperature at which the compressor 2 is turned off due to excess cooling capacity being provided by the refrigeration system. For example, if a set point temperature T.sub.S of 44.degree. F. is selected then the temperature T.sub.L may be 39.degree. F. Temperature T.sub.H represents the temperature at which a recycle start of the refrigeration system compressor 2 occurs after the compressor 2 has been turned off due to excess cooling capacity. For example, if T.sub.S is 44.degree. F. and T.sub.L is 39.degree. F. then T.sub.H may be 49.degree. F.

As shown in FIG. 2, if the rate of decrease in the evaporator 5 leaving heat transfer fluid temperature follows the curve labeled "A" then the temperature of the heat transfer fluid leaving the evaporator 5 relatively quickly reaches, at time T.sub.1, the desired set point temperature T.sub.S. For example, T.sub.1 may be on the order of 5 minutes. Then, if the refrigeration system 1 is providing excess cooling capacity for satisfying the load placed on the refrigeration system 1, the temperature of the heat transfer fluid leaving the evaporator 5 will relatively quickly decrease to the temperature T.sub.L at time T.sub.2 thereby resulting in a subsequent, relatively quick recycle start.

However, also as shown in FIG. 2, if the rate of decrease in the evaporator 5 heat transfer fluid temperature follows the curve labeled "B" then the temperature of the heat transfer fluid leaving the evaporator 5 is much more slowly decreased to the desired set point temperature T.sub.S in a time period T.sub.3, which may be, for example, on the order of 15 minutes, which is a significantly longer time period than the time period T.sub.1 necessary to reach the desired set point temperature T.sub.S when following the curve labeled "A". This is accomplished by the processor board 17 generating pseudo set point temperatures in response to which the capacity of the compressor 2 is controlled by operation of the guide vanes 12 upon a recycle start. For example, initially upon a recycle start the processor board 17 may generate a pseudo set point temperature approximately equal or just slightly less than T.sub.H. Then over a preprogrammed time interval, the pseudo set point is incrementally decreased to the actual, desired set point temperature T.sub.S. Throughout the preprogrammed time interval the capacity of the compressor 2 is controlled in response to the pseudo set point temperature which is greater than the actual, desired set point temperature thereby resulting in a relatively gradual decrease in the temperature of the heat transfer fluid leaving the evaporator 5. After the pseudo set point temperature is decremented to equal the actual, desired set point temperature, then control of the capacity of the compressor 2 by the processor board 17 is carried out directly in response to the actual load placed on the refrigeration system 1. Thus, if the refrigeration system 1 is providing excess cooling capacity with the guide vanes 12 at their fully closed position, the temperature of the heat transfer fluid leaving the evaporator 5 will still decrease to the temperature T.sub.L at which the compressor 2 is turned off due to excess cooling capacity thereby requiring a subsequent recycle start. However, time T.sub.4 at which this occurs is a significantly longer time period than the time period T.sub.2 at which a recycle start would otherwise be required. Thus, the overall number of recycle starts is reduced when the refrigeration system 1 is operated according to the principles of the present invention.

It should be noted that the curves labeled "A" and "B" in FIG. 2 are not intended to be representative of actual rates of decrease in evaporator 5 leaving heat transfer fluid temperature which may occur in an actual refrigeration system 1. These curves "A" and "B" are provided only for purposes of facilitating understanding of the principles of the present invention. As will be readily apparent to one of ordinary skill in the art to which the present invention pertains, actual operating curves followed in a real refrigeration system 1 may have any of a variety of forms including forms which do not comprise straight lines.

Of course, the foregoing description is directed to a particular embodiment of the present invention and various modifications and other embodiments of the present invention will be readily apparent to one of ordinary skill in the art to which the present invention pertains. Therefore, while the present invention has been described in conjunction with a particular embodiment, it is to be understood that various modifications and other embodiments of the present invention may be made without departing from the scope of the invention as described herein and as claimed in the appended claims.