Refrigeration – Automatic control – Refrigeration producer
Reexamination Certificate
2000-01-10
2001-07-17
Wayner, William (Department: 3744)
Refrigeration
Automatic control
Refrigeration producer
C062S503000
Reexamination Certificate
active
06260368
ABSTRACT:
REFERENCES
1. Finn, D. P., and Doyle, C. J.; “A BEMS-Integrated Electronic Expansion Valve For Real-Time Optimization of Refrigeration Evaporation”. 20th International Congress of Refrigeration, IIR/IIFT, Sydney Australia, 1999.
2. U.S. Pat. No. 4,878,355
TECHNICAL FIELD
This invention relates to vapor compression refrigeration having evaporator superheat regulation by a closed loop control whose input is superheat temperature and whose output is a control signal which causes an electronic expansion valve to increase refrigerant flow in response to increased superheat. Specifically the invention is concerned with stabilizing the superheat control loop.
BACKGROUND ART
Vapor compression refrigerators achieve maximum efficiency when the evaporator, in which liquid refrigerant is vaporized by heat absorbed from the refrigerated space, is supplied at its inlet with an optimum mass flow of liquid refrigerant that is just sufficient so that vaporization is complete at the evaporator outlet. Flow in excess of the optimum results in liquid refrigerant leaving the evaporator outlet, thereby sacrificing its refrigeration capability.
Flow less than optimum results in complete vaporization occurring within the evaporator. Between the point of complete vaporization and the evaporator outlet, vapor is “superheat”, as used in reference to vapor compression refrigeration, means the difference between the temperature of vapor at some point in the suction live downstream of the evaporator and the temperature of the liquid-vapor mixture at the evaporator inlet. High superheat is a source of inefficiency because only part of the evaporator is available to absorb heat by efficient heat transfer from the refrigerated medium to boiling liquid refrigerant. The remaining part transfers heat inefficiently from the refrigerated medium to refrigerant vapor. The result is that superheat causes the evaporator to operate at lower than optimum temperature and pressure, requiring more compressor work per unit of refrigeration.
Nearly optimum flow of refrigerant has been achieved in prior art with electronically controlled expansion valves (EEVs). Some prior art EEVs regulate refrigerant flow with an electromechanically adjustable flow resistor such as a needle valve. In others, an electromechanical valve periodically opens to admit flow to a fixed orifice for a controllable time interval.
In prior art, an EEV is part of a closed loop feedback control in which superheat is sensed by temperature sensors, and a superheat signal controls an EEV so as to increase refrigerant flow when superheat temperature increases above a preset value and reduce refrigerant flow when superheat falls below the preset value. Since increased flow reduces superheat, the system has negative feedback and will, if it is stable, settle at or near the preset superheat. The value of preset superheat is typically below 7 degrees Centigrade, which is low enough so that most of the evaporator is used efficiently.
In an EEV control loop, a step increase in flow rate at the evaporator input generates a corresponding step increase in flow rate at the evaporator output after a delay equal to the time required for refrigerant to transit the evaporator. This delay is typically about 10 seconds, and has serious implications for control loop stability, as may be seen from the following sequence of events in a “proportional only” EEV control in which change in flow rate is simply proportional to change in superheat.
Suppose that a “proportional only” system has been running with preset superheat, and at time=0, a disturbance such as a momentary interruption of power, causes superheat to increase well above its preset value. Then, at time=0, the EEV will automatically be stepped to high flow rate in an attempt to restore preset superheat. Assuming a delay time of 10 seconds, the step increase in flow results in liquid refrigerant reaching the output temperature sensor at time=10 seconds. In a short time interval prior to and after the arrival of liquid at the output temperature sensor, the sensor temperature and consequently the superheat signal both decrease, and the controller reacts with an abrupt decrease in flow rate at the evaporator input. However, this decrease does not reach the output temperature sensor until time=20 seconds, at which time the superheat signal abruptly increases and the foregoing sequence begins to repeat itself.
In prior art, EEV controls have been stabilized electronically by empirical adjustment of a “PID” (proportional-integral-differential) controller (Ref. 1, FIG.
2
)., which typically results in slow controller response and low margins of stability. Also, the cost of a PID controller precludes its use in many applications.
Accordingly, the purpose of the present invention is to provide inexpensive stabilization an EEV control loop so as to achieve a high margin of stability and relatively fast controller response with “proportional only” control.
BRIEF DISCLOSURE OF THE INVENTION
In a refrigerator system using the invention, all superheat takes place downstream of the evaporator, and liquid is prevented from reaching the location of the sensor that measures temperature of the superheated vapor, thereby eliminating abrupt, delayed changes in temperature of that sensor which, as previously described herein, cause severe instability. Eliminating the source of instability enables the use of simple, inexpensive “proportional only” EEV control whereby an EEV control signal is simply proportional to a superheat temperature signal.
The basic invention is a metal cavity installed downstream of the evaporator and inside the refrigerated space. The cavity performs two functions; separation of liquid from vapor and superheating of the separated vapor. Separated vapor is superheated within the cavity by heat transferred from the refrigerated space through the cavity walls, the amount of superheat being substantially a preset value. The sensor that measures superheated vapor temperature is located downstream of the cavity.
A combined form of the invention is a cavity as described above combined with “proportional only” EEV control. This combination results in a stable system, while “proportional only” control without a cavity according to the invention is unstable.
REFERENCES:
patent: 4523435 (1985-06-01), Lord
patent: 4527399 (1985-07-01), Lord
patent: 4878355 (1989-11-01), Beckey et al.
patent: 5505060 (1996-04-01), Kozinski
Finn, D.P. & Doyle, C.J. . A BEMS-Integrated Electronic Expansion Valve for Real Time Optimization of Refrigeration. 20th International Congress of Refrigeration, IIR/IIF. Sydney, Australia, 1999.
Foster Frank H.
Kremblas, Foster Phillips & Pollick
Wayner William
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