Refrigeration – Automatic control – Refrigeration producer
Patent
1997-04-15
1998-07-21
Bennett, Henry A.
Refrigeration
Automatic control
Refrigeration producer
236 78D, F25B 4104
Patent
active
057821038
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a control arrangement for the superheat temperature of at least one evaporator of a refrigeration system having a refrigerant circulation, in which at least one compressor, a condenser, an electronically controlled expansion valve and the evaporator are arranged in series, a measuring device being connected to the evaporator, which device produces a measurement signal that is a measure of the superheat temperature of the refrigerant in the evaporator, the arrangement having a comparator to which the measurement signal and a desired superheat temperature signal are arranged to be supplied, and having a PID controller, arranged between the output of the comparator and the control input of the expansion valve, to which the output signal of the comparator is supplied and which controls the expansion valve.
In a known control arrangement of that kind, as a measure of the superheat temperature of the refrigerant in the evaporator, the difference between the input and output temperatures is measured and compared with a desired superheat temperature. The control difference is supplied by way of the PID controller to the expansion valve, which controls the quantity of refrigerant injected into the evaporator.
If the load of the evaporator changes abruptly, for example, because of a sudden change in the ambient temperature of the evaporator or as a consequence of compressors of a group of compressors being switched on and off in steps, the evaporation temperature, which corresponds to the temperature measured at the input of the evaporator or to the evaporation pressure taken as a measure of the evaporation temperature, also changes correspondingly. The superheat temperature consequently also changes, and may fluctuate wildly around the desired value until it is adjusted by the PID controller by way of the expansion valve. It is especially critical if the superheat temperature falls too far below the desired value, because there is then a danger that the refrigerant will still not have evaporated at the output of the evaporator and will be taken up by the compressor.
SUMMARY OF THE INVENTION
The invention is based on the problem of providing a control arrangement of the kind mentioned in the introduction, in which too great a departure of the superheat temperature from the desired value because of a change in load is avoided.
According to the invention, this problem is solved in that a control signal proportional to the evaporation temperature of the refrigerant is arranged to be supplied additionally to the PID controller.
That solution is based on the correlation between the refrigeration output or load Q and the temperature difference between the air temperature T.sub.a at the evaporator and the evaporation temperature T.sub.o, that is,
If the evaporation temperature changes, the refrigeration output also changes in accordance with the equation controller. The proportionality factor K can here be between 1 and 5.
The solution according to the invention enables an evaporation temperature change .DELTA.T.sub.o to effect a corresponding change of level in the degree of opening of the expansion valve, which corresponds to the change in the refrigeration output. The controller therefore counteracts a change in the evaporation temperature initially to a correspondingly great extent, so that the superheat temperature is not able to change wildly either. As soon as the evaporation temperature is no longer changing, its influence by way of the D-element of the PID controller also becomes ineffective, so that only a change in the superheat temperature controls the expansion valve by way of the PID controller.
Preferably, provision is made for the proportionality factor between evaporation temperature and control signal to be selected to be the same as the ratio of the load on the evaporator to the difference between the air temperature at the evaporator and the evaporation temperature.
It is then advantageous for a summation element to which the additional control si
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patent: 4571951 (1986-02-01), Szymaszek
patent: 4807445 (1989-02-01), Matsuoka et al.
patent: 5157934 (1992-10-01), Uchida
patent: 5222371 (1993-06-01), Doyama et al.
patent: 5259210 (1993-11-01), Ohya et al.
Bennett Henry A.
Danfoss A/S
Tinker Susanne C.
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