Refrigeration – Processes – Compressing – condensing and evaporating
Patent
1999-05-19
2000-12-26
Tapolcai, William E.
Refrigeration
Processes
Compressing, condensing and evaporating
62224, 236 92B, F25B 4104
Patent
active
061640816
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for controlling a refrigeration system, to a refrigeration system and to an expansion valve for such a refrigeration system.
From WO 82/04142 is known a refrigeration system having in series a compressor, a condenser, an expansion valve and an evaporator. This system is controlled by the expansion valve having as actuator a diaphragm or a bellows and can be acted upon by a heat supply from a heating element. One side of the actuator is biased by the vapour pressure of a liquid-vapour filled sensor system, whose sensor temperature is determined by heat supply. The superheat is measured on the outlet side of the evaporator, and the heat supply is controlled in dependence of the measured value. The heatable sensor is mounted on the outlet side refrigerant line of the evaporator, where superheated refrigerant vapour is already available. Therefore, the heat dissipation is relatively low and changes concurrently with the superheat temperature.
From DE 40 05 728 A1 is known a refrigeration system, which is controlled in dependence on superheat at the evaporator outlet. For that purpose, the expansion valve has an actuator in the form of a diaphragm, one side of which is biased by the refrigerant pressure at the evaporator outlet and the other side of which is biased by a pressure corresponding to the refrigerant temperature at the evaporator outlet. This control requires either the intake line leading to the compressor or a measuring line in the form, for example, of a capillary tube, to be run right up to the expansion valve. This frequently leads to restrictions in the design of the refrigeration system. In addition, the control is often very unstable, with wildly fluctuating superheat.
In the known case, this superheat control has an additional influence imposed on it, derived from the temperature in the line between compressor and condenser. For that purpose, one of the two pressure chambers of the diaphragm capsule is filled with a control medium, which through the diaphragm capsule is heated by heat-exchange with the superheated refrigerant at the outlet of the evaporator and is additionally heated by a heating element, for example, a PTC resistor.
From U.S. Pat. No. 3,313,121 is known a refrigeration system and a method for controlling a refrigeration system by means of an expansion valve having as actuator a diaphragm, one side of the diaphragm being acted upon by the refrigerant pressure on the outlet side of the expansion valve, the other side being acted upon by the pressure of a sensor mounted on the superheat path of the evaporator.
The invention is based on the problem of improving control of a refrigeration system using simple and inexpensive means.
In this practical form the sensor is in continuous thermal contact with liquid refrigerant, which gives a good heat transmission at substantially constant temperature conditions. The opening degree of the valve is determined substantially by the heat supply by means of the heating element, since it is by the heating that the vapour pressure in the sensor system is increased, as the heating increases the pressure in the sensor system. The filling, whose pressure is temperature dependent, can either be a liquid-vapour filling or an adsorption filling. Here, the vapour pressure is a function of the temperature and increases with increasing temperature. The greater is the power supplied to the heating element, the greater is the opening degree of the valve. Proportionality is practically achieved by the following relationship:
This relationship applies regardless of how high saturation pressure and saturation temperature of the refrigerant at the valve outlet are at the time. The opening degree of the valve is therefore independent of the evaporator pressure. Any compensation by means of the heating element is not necessary.
Since the heat supply is controlled, that is, is pre-set by a controller, all automatic control engineering options can be applied to improve the control, for example a PI controller can be used. Moreo
REFERENCES:
patent: 4879879 (1989-11-01), Marsala et al.
patent: 5148978 (1992-09-01), Stapelbroek
patent: 5195331 (1993-03-01), Zimmern et al.
Jensen Kenn S.o slashed.nder
Schmidt Frede
Danfoss A/S
Tapolcai William E.
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