Refrigeration – Processes – Compressing – condensing and evaporating
Reexamination Certificate
2000-01-10
2001-07-31
Wayner, William (Department: 3744)
Refrigeration
Processes
Compressing, condensing and evaporating
C062S188000, C137S392000
Reexamination Certificate
active
06266964
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to heating, ventilating and air conditioning (HVAC) systems, to refrigeration systems, and to chiller systems which modulate an expansion valve to maintain a system condition such as superheat, refrigerant liquid level, or chilled water temperature. The present invention proposes to also modulate the expansion valve to maintain minimum lubricant flow to the compressor or compressors. For purposes of this application, chiller systems is defined to also include HVAC systems and refrigeration systems.
Certain systems use the differential pressure across the compressor to return lubricant to the compressor. The lubricant is used in the compressor to lubricate bearings or the like and to seal the gap between the compressor's rotors, wraps or other compressing elements.
In some systems, the expansion valve is modulated to maintain refrigerant liquid level control in one of the system heat exchangers. The condensing heat exchanger can be cooled by a chilled water loop provided by, for example, a cooling tower and determined by a cooling water temperature. The evaporating heat exchanger can provide chilled water for use as a heat transfer medium and the expansion valve can be modulated to maintain the chilled water temperature of the fluid provided by the evaporating heat exchanger. If the evaporating heat exchanger is a falling film type evaporator, the expansion valve is modulated to maintain a liquid level in the evaporating heat exchanger.
With such liquid level control, the differential pressure across the compressor is determined by the difference between the cooling water temperature and the chilled water temperature. If the difference between the cooling water temperature and the chilled water temperature is small or inverted, the differential pressure will be too small to pump lubricant back to the compressor. The chiller system will shutdown on a low oil flow diagnostic or a loss of oil diagnostic. The conditions causing this are typical of those which occur when a system is started with a low cooling tower temperature and warm chilled water temperature.
More specifically, under normal running conditions, the liquid level controller maintains a pool of liquid in the bottom of the evaporating heat exchanger. A liquid level sensor measures the depth of the pool and a PID algorithm in the controller maintains a desired level by modulating an electronic expansion valve to change its position and affect the rate of refrigerant flow into the evaporator. The liquid level controller maintains a mass balance between the flow of refrigerant vapor removed from the evaporator by the compressor and the flow of liquid refrigerant returned from the condenser to the electronic expansion valve. When the electronic expansion valve is opened, the flow of refrigerant into the evaporator increases and at some point will exceed the flow out of the evaporator. This causes the condenser to drain to the point that the vapor will flow from the condenser to the evaporator rather than liquid refrigerant. Mass balance will then be re-established because of the refrigerant vapors lower density. However, the flow of refrigerant vapor reduces the chiller system efficiency because the vapor is eventually pumped back to the condenser without providing effective cooling.
On the other hand, when the expansion valve is closed, refrigerant flow out of the evaporator is such that it is less than the flow in. This causes the evaporator pool to fall and eventually dry out. Because the compressor is removing more refrigerant from the evaporator than the electronic expansion valve is allowing to enter the evaporator, the evaporator pressure will fall. As this evaporator pressure falls, the differential pressure across the compressor increases. The higher differential pressure reduces the compressor efficiency and flow through the compressor falls such that the mass flow balance is re-established but the chiller efficiency is again reduced.
It would be advantageous that the expansion valve could be controlled to both maintain the liquid level and to maintain the compressor pressure differential at or above a desired minimum threshold.
SUMMARY OF THE INVENTION
It is an object, feature and advantage of the present invention to solve the problems in the prior art expansion valve controllers.
It is an object, feature and advantage of the present invention to control an expansion valve to maintain a minimum compressor pressure differential.
It is an object, feature and advantage of the present invention to control an expansion valve to maintain a system criteria such as liquid level, superheat, or chilled water temperature as a primary criteria.
It is a further object, feature and advantage of the present invention to use the expansion valve to maintain a secondary criteria such as a minimum compressor pressure differential.
It is an object, feature and advantage of the present invention to establish lubricant flow to the compressor in inverted start conditions.
It is an object, feature and advantage of the present invention to establish and/or maintain oil flow to the compressor in system starts where there are low system differential temperatures or pressures.
It is an object, feature and advantage of the present invention to increase a chiller systems operating envelope.
It is an object, feature and advantage of the present invention to use an electronic expansion valve to assist in building and controlling system differential pressures.
The present invention provides a method of controlling an expansion valve including the steps of: measuring a primary system condition; determining an error in the primary system condition; measuring a secondary system condition; determining an error in the secondary system condition; and modulating the expansion valve based upon the smaller of the first or second error.
The present invention also provides a method of controlling an expansion valve including the steps of: measuring a refrigerant liquid level; comparing the measured refrigerant liquid level with a desired refrigerant liquid level to establish a refrigerant level error; measuring a system pressure differential; comparing the measured system pressure differential with a minimum required system pressure differential to determine a system differential pressure error; comparing the liquid level error to the differential pressure error to determine the smaller error; and modulating the expansion valve to control the smaller error. Smaller means smallest positive or largest negative which will cause the smallest opening or biggest close.
The present invention further provides a method of controlling liquid level in an HVAC system. The method comprises the steps of: physically calibrating a liquid level sensor to a desired level; calculating an offset from a selected point of the liquid level sensor to a lower end; measuring a liquid level; subtracting the calculated offset from the measured liquid level; comparing the subtracted result to zero to determine an error; and controlling the liquid level to minimize the error.
The present invention still further provides a method of maintaining a minimum differential pressure across a compressor. The method comprises the steps of: operating a compressor to compress a fluid and thereby creating a pressure differential between a compressor input and a compressor output; measuring the pressure differential, comparing the measured differential to a desired pressure differential, and determining a pressure differential error; and controlling an expansion valve, responsive to the pressure differential error, to maintain a minimum pressure differential across the compressor.
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patent: 3224638 (1965-12-01), Harrell, Jr.
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patent: 4843832 (1989-07-01), Yamada et al.
patent: 5000009 (1991-03-01), Clanin
patent: 5011112 (1991-04-01), Glamm
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patent: 5419146 (19
Meyer Jonathan M.
Sibik Lee L.
Smith Sean A.
American Standard International Inc.
Beres William J.
Ferguson Peter D.
O'Driscoll William
Wayner William
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