Refrigeration – Automatic control – Refrigeration producer
Reexamination Certificate
1999-01-15
2001-03-20
Tanner, Harry B. (Department: 3744)
Refrigeration
Automatic control
Refrigeration producer
C062S201000, C062S209000
Reexamination Certificate
active
06202431
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to refrigerating systems or chilling systems, and more particularly, to an apparatus and method for controlling a hot gas bypass valve to eliminate or minimize surge in centrifugal liquid chilling systems.
2. Description of the Related Art
As is generally known, 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. It is a transient phenomenon characterized by high frequency oscillations in pressures and flow, and, in some cases, a complete flow reversal through the compressor. Such surging, if uncontrolled, causes excessive vibrations and may result in permanent compressor damage. Further, surging causes excessive electrical power consumption if the drive device is an electric motor.
It is generally known that a hot gas bypass flow helps avoid surging of the compressor during low-load or partial load conditions. As the cooling load decreases, the requirement for hot gas bypass flow increases. The amount of hot gas bypass flow at a certain load condition is dependent on a number of parameters, including the desired head pressure of the centrifugal compressor. Thus, it is desirable to provide a control system for the hot gas bypass flow that provides optimum control and is responsive to the characteristic of a given centrifugal chiller system.
An hot gas bypass valve control in the prior art is an analog electronic circuit described in U.S. Pat. No. 4,248,055. This prior art control provides as its output a DC voltage signal that is proportional to the required amount of opening of the valve. This prior art method requires calibration at two different chiller operating points at which the compressor just begins to surge. As a consequence of this, a good deal of time is consumed performing the calibration and it requires the assistance of a service technician at the chiller site. Further, variation of flow is necessary for many applications, and therefore, repeated calibration of the control is required. Another disadvantage of the prior art method is that it makes the false assumption that the surge boundary is a straight line. Instead, it is often characterized by a curve that may deviate significantly from a straight line at various operating conditions. As a consequence of this straight line assumption, the hot gas bypass valve may open too much or too little. Opening the valve too much may result in inefficient operation, and opening it too little may result in a surge condition.
SUMMARY OF THE INVENTION
The advantages and purpose of the invention are set forth in part in the description that follows, and in part is obvious from the description, or may be learned by practice of the invention. The advantages and purpose of the invention is realized and attained by means of the elements and combinations particularly pointed out in the claims.
To attain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, systems and methods consistent with this invention automatically calibrate a surge control of a refrigeration system including a centriftigal compressor, a condenser, pre-rotational vanes, a load, and an evaporator through which a chilled liquid refrigerant is circulated. The system or method comprises a number of elements. First, systems or methods consistent with this invention sense a presence of a surge condition, sense a head parameter representative of the head of the compressor, and sense a load parameter representative of the load. Second, systems or methods consistent with this invention store the head parameter and the load parameter when the surge condition is sensed as calibration data to be used by the control of the refrigeration system.
To attain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, systems and methods consistent with this invention control a hot gas bypass valve in a refrigeration system including a centrifugal compressor, a condenser, pre-rotational vanes, and an evaporator through which a chilled liquid refrigerant is circulated. The system or method comprises a number of elements. First, systems or methods consistent with this invention sense a current pressure representative of the current pressure of the liquid refrigerant in the condenser, sense a current pressure representative of the current pressure of the liquid refrigerant in the evaporator, and sense a current position representative of the current position of the pre-rotational vanes. Second, systems or methods consistent with this invention control the operation of a hot gas bypass valve so as to avoid surging in the compressor in response to a comparison of the current condenser pressure, the current evaporator pressure, and the current vane position, or functions thereof, to stored calibration data.
The summary and the following detailed description should not restrict the scope of the claimed invention. Both provide examples and explanations to enable others to practice the invention. The accompanying drawings, which form part of the detailed description, show one embodiment of the invention and, together with the description, explain the principles of the invention.
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PCT International Search Report, International Application No. PCT/US00/00729, Apr. 21, 2000, 5 pages.
Beaverson Gregory K.
Deltz Dennis L.
Ginder Harold B.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Tanner Harry B.
York International Corporation
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