Refrigeration – Automatic control – Refrigeration producer
Reexamination Certificate
1999-03-26
2001-10-16
Wayner, William (Department: 3744)
Refrigeration
Automatic control
Refrigeration producer
C062S217000, C062S228300
Reexamination Certificate
active
06301911
ABSTRACT:
I. FIELD OF THE INVENTION
The field of the present invention relates to control systems for transport refrigeration systems. More specifically, the present invention is directed towards implementing a control algorithm for manipulating the suction modulation valve (the “SMV”), the compressor unloaders and the engine speed solenoid to precisely maintain the compressor of the transport refrigeration unit within its design envelope under any normal operating condition.
II. DESCRIPTION OF THE PRIOR ART
A transport refrigeration system used to control enclosed areas, such as the box used on trucks, trailers, containers, or similar intermodal units, functions by absorbing heat from the enclosed area and releasing heat outside of the box into the environment. A number of transport refrigeration units, including units currently sold by assignee, employ a reciprocating compressor to pressurize refrigerant to enable the removal of heat from the box. Reciprocal compressors used in such applications commonly include a suction inlet and a discharge which are connected, respectively, to the evaporator and condenser of the transport refrigeration system. It is axiomatic that in order to ensure the reliability of the reciprocating compressor, the compressor should operate within the limits of the suction and discharge pressures for which it was designed. The ranges and ratios of suction and discharge pressures designed to be handled by a reciprocating compressor at various stages of operation is known as an operating envelope. The failure to operate within the compressor operating envelope will result in unnecessary wear and tear, and ultimately will bring about the premature failure of the compressor, thus creating unacceptable costs of money and time to the operator.
Transport refrigeration systems currently employ a variety of controls to manipulate the operating envelope of a reciprocating compressor. As can be shown by U.S. Pat. Nos. 5,626,027 and 5,577,390, both assigned to the assignee of the present invention, compressors can be operated in a multi-stage mode or in single stage modes depending upon operating temperature. Such dislosure further discuss generally the use of suction modulation for capacity control. However, currently available commercial designs, including those sold by assignee, do not offer the combination of discharge pressure control and suction pressure control to maintain a reciprocating compressor within its designed operating envelope.
The applicants have found that, in order to optimize maintenance of the reciprocating compressor of a transport refrigeration system within its design operating envelope, it is desirable to use a controller within the transport refrigeration system which closes the suction modulation valve (the “SMV”) and/or unloads compressor cylinder banks in response to discharge pressures in excess of a preselected limit calculated or compared to a set value by the controller. An unloader, when energized has the effect of removing one or more cylinders from operation within compressor, thus effectively reducing the displaced volume within the compressor. In addition, the applicants have found that maintaining the reciprocating compressor within its design operating envelope is best maintained by selectively unloading compressor cylinder banks in the event that suction pressure is below the minimum suction pressure allowed, as determined the controller.
III. SUMMARY OF THE INVENTION
The control process and system of this invention provides a refrigeration unit for a transport system having a controller for selectively maintaining the reciprocating compressor of the system within its design operating envelope. One of the control parameters for the design operating envelope is the maximum allowable compressor discharge pressure. The maximum allowable compressor discharge pressure is calculated as a function of the suction pressure when the suction pressure is below the transition point. Above the transition point, the maximum discharge pressure is a constant. In situations where the suction pressure is above the transition point, and the discharge pressure is a above the maximum allowable for the operating envelope, the controller of the present invention issues a control signal so as to gradually close the SMV based upon a proportional, integral and derivative (a “PID”) control loop to reduce the pressure to within the design operating envelope. If the suction pressure is below the transition point, in addition to the PID control for the SMV, the controller also issues control signals to the compressor cylinder bank unloaders. If the suction pressure is below the transition point and the discharge pressure is above the envelope maximum, the compressor rear cylinder bank is unloaded. If, after a preselected time limit, the discharge pressure is still above the envelope, the front cylinder bank is also unloaded. Once the discharge pressure falls below a predetermined limit within the envelope, the controller issues signals reloading the front and rear cylinder banks.
The controller of the present invention also works to correct conditions wherein the suction pressure is below the minimum allowed for the design operating envelope. If the suction pressure is below the minimum suction pressure allowed for more than a predetermined time limit, the controller issues a control signal to unload the rear cylinder bank. If the suction pressure remains below the minimum allowed by the operating envelope for an additional predetermined time limit, the front cylinder bank is likewise unloaded. If both front and rear banks have been unloaded and the suction pressure is still below the operating minimum for the design operating envelope for a predetermined time limit, then the diesel engine is force into low speed for a preselected period of time. If the suction pressure remains below its design operating envelope minimum after this point, then a proportional-integral (a PI″) control is used to gradually open the SMV position. This PI control is disabled if the system current draw is equal to or greater than the maximum allowable current draw for the system. If the suction pressure rises above the minimum required for the design operating envelope plus a hysteresis band value, then the front and rear cylinder banks are loaded.
If suction pressure is greater than the maximum allowable suction pressure, the SMV is gradually closed until the suction pressure drops below the maximum allowable design operating envelope value minus a hysteresis band.
Accordingly, one object of the present invention is to provide a microprocessor control for the regulation of both compressor discharge pressure and suction pressure values.
It is a further object of the invention to provide a microprocessor control for selectively activating and deactivating both the compressor cylinder banks and the suction modulation valve in order to maintain the compressor within its design operating envelope.
It is yet another object of the present invention to provide a controller for using the SMV in combination with the compressor unloaders and engine speed solenoid in order to precisely maintain the compressor within its design envelope under any normal operating conditions.
These and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, and as illustrated in the accompanying drawings.
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de Andrade Joao Eduardo Navarro
Reason John Robert
Carrier Corporation
Niro Scavone Haller & Niro
Wayner William
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