Refrigeration – With indicator or tester – Condition sensing
Reexamination Certificate
2003-07-15
2004-08-10
Tanner, Harry B. (Department: 3744)
Refrigeration
With indicator or tester
Condition sensing
C062S175000, C062S217000, C062S228500, C062S230000, C062S510000
Reexamination Certificate
active
06772599
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a control system for compressors operating in parallel. Specifically, the present invention relates to a control system that re-establishes the stability of dual centrifugal compressors operating in parallel upon one of the centrifugal compressors entering into an unstable operating condition such as a surge condition.
To obtain increased capacity in a refrigeration system, two compressors can be connected in parallel to a common refrigerant circuit. Frequently, for capacity control, one of the compressors is designated as a “lead” compressor and the other compressor is designated as a “lag” compressor. The capacity of the refrigeration system, and of each compressor, can be controlled by the use of adjustable pre-rotation vanes or inlet guide vanes incorporated in or adjacent to the suction inlet of each compressor. Depending on the particular capacity requirements of the system, the pre-rotation vanes of each compressor can be positioned to control the flow of refrigerant through the compressors and thereby control the capacity of the system. The positions of the pre-rotation vanes can range from a completely open position to a completely closed position. The pre-rotation vanes for a compressor can be positioned in a more open position to increase the flow of refrigerant through the compressor and thereby increase the capacity of the system or the pre-rotation vanes of a compressor can be positioned in a more closed position to decrease the flow of refrigerant through the compressor and thereby decrease the capacity of the system.
One frequently used method to control the capacity of a refrigeration system is to control the position of the pre-rotation vanes of a compressor in response to a deviation from a desired set point of the leaving chilled water temperature in the evaporator. For a system with two parallel compressors, the pre-rotation vanes of the lead compressor are controlled based on the leaving chilled water temperature and the pre-rotation vanes of the lag compressor are controlled to follow the capacity of the lead compressor. In one technique, to follow the capacity of the lead compressor, the pre-rotation vanes of the lag compressor are positioned to obtain the same percentage of full-load motor current in the lag compressor that is present in the lead compressor.
During the operation of centrifugal compressors, a compressor instability or surge can occur in a centrifugal compressor. Surge or surging is an unstable condition that may occur when compressors, such as centrifugal compressors, are operated at light loads and high pressure ratios. Surge is a transient phenomenon having high frequency oscillations in pressures and flow, and, in some cases, the occurrence of a complete flow reversal through the compressor. Surging, if uncontrolled, can cause excessive vibrations in both the rotating and stationary components of the compressor, and may result in permanent compressor damage. During a surge condition there can exist a momentary reduction in flow and pressure developed across the compressor. Furthermore, there can be a reduction in the net torque and mechanical power at the driving shaft of the compressor. In the case where the drive device of the compressor is an electric motor, the oscillations in torque and power caused by a surge condition can result in oscillations in motor current and excessive electrical power consumption.
As discussed above, a surge condition in a centrifugal compressor can result in a reduction in motor current or load on the compressor or a reduction in discharge pressure or temperature from the compressor. Thus, the presence of a surge condition can be detected by measuring the motor current or load on the compressor or the discharge pressure or temperature from the compressor and checking for the appropriate reduction in the measured amount. It is to be understood that other operational parameters, in addition to the ones discussed above, can be used to detect the presence of a surge condition.
When a surge or lack of pumping condition occurs on one compressor in dual compressor applications, the compressor which does not surge has an increase in refrigerant flow. The increase in refrigerant flow to the non-surging compressor makes it more difficult for the surging compressor to overcome the instability. One technique for overcoming a surge condition in a dual compressor configuration is disclosed in U.S. Pat. No. 4,646,530, hereafter referred to as the U.S. Pat. No. '530 . The U.S. Pat. No. '530 is directed to the operation of a refrigeration system having a pair of centrifugal compressors connected in parallel. During a surge condition in the lag compressor, the control operation of the compressors is changed from the normal control operation to a surge control operation. In the U.S. Pat. No. '530 , a surge condition is detected when the motor current of the lag compressor is more than a selected percentage below the lead compressor motor current. If a surge condition is detected to be present for a predetermined period of time, the inlet guide vanes to the lead compressor are closed for another predetermined period of time to increase the flow of refrigerant and current in the lag compressor. If the current in the lag compressor increases above the selected percentage, after the predetermined time period for the closing of the vanes of the lead compressor, normal control operation of the compressors is resumed. One drawback of this technique is that it can only detect and correct a surge condition in the lag compressor and does not address a surge condition in the lead compressor. Another drawback of this technique is that a predetermined time has to elapse before a response to the surge condition is provided.
Another technique for controlling surge in a dual compressor arrangement is disclosed in U.S. Pat. No. 5,845,509 hereafter referred to as the U.S. Pat. No. '509 . The U.S. Pat. No. '509 is directed to a refrigeration system using a plurality of centrifugal compressors operated in parallel. To avoid surge in a two compressor system, the lag compressor is initially shut off in a reduced load situation to thereby increase the rotational speed of the other compressor and avoid a surge condition. However, if load conditions continue to decrease and the surge condition has not been avoided, the lag compressor is re-started and the lead compressor is shut down to attempt to avoid the surge condition. One drawback of this technique is that the compressors can be cycled on and off several times in attempting to avoid surge conditions thereby resulting in significant power consumption.
Therefore, what is needed is a control system and method for dual centrifugal compressors operated in parallel that can detect a surge condition in either the “lead” compressor or the “lag” compressor and can correct the surge condition in the compressor without a complex procedure or repeated on-off cycling of compressors.
SUMMARY OF THE INVENTION
One embodiment of the present invention is directed to a method for detecting compressor instability in a multiple compressor refrigeration system. The method includes the steps of determining an operating parameter from both a first compressor of a multiple compressor refrigeration system and a second compressor of the multiple compressor refrigeration system. The operating parameter of the first compressor is then compared to the operating parameter of the second compressor. Next, an inlet vane position for both the first compressor and the second compressor is determined. Finally, the inlet vane position of the first compressor is compared to the inlet vane position of the second compressor and a compressor instability is determined in one of the compressors in response to that compressor having both a lower operating parameter and a more open inlet vane position than the other compressor.
Another embodiment of the present invention is directed to a computer program product embodied on a computer read
Bodell, II Mark Robinson
Miller Wanda Jean
McNees Wallace & Nurick LLC
Sattizahn Brian T.
Tanner Harry B.
York International Corporation
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