In a conventional refrigerant circuit such as shown in FIGS. 8 and 9, operation of the compressor is controlled by magnetic clutch 17. Activation of magnetic clutch 17 is controlled by serially connected main switch 12 and air conditioner switch 13. Main switch 12 may also be the ignition switch of an automobile. In order to protect the compressor from inadvertent damage, a number of safety devices are provided including thermostat switch 14, low pressure cutout switch 19 and high pressure cutout switch 15. As shown in FIG. 8, each of these safety devices is serially connected with one another between air conditioner switch 13 and magnetic clutch 17. Thermostat switch 14 is disposed on the air outlet side of an evaporator (not shown) to detect the outlet air temperature. Low pressure cutout switch 19 is disposed between the high pressure chamber of the compressor and an expansion valve (not shown) to detect refrigerant pressure, i.e., to detect the amount of refrigerant enclosed in the refrigerant circuit. High pressure cutout switch 15 is disposed at the high pressure side of the compressor, e.g., in the discharge chamber of the compressor to detect dangerous high pressure levels. Thus, the operation of the compressor may be interrupted upon the occurence of abnormally high temperature or abnormally low or high pressure.
FIG. 9 illustrates another embodiment of the arrangement of conventional control devices for a compressor. In this embodiment, magnetic clutch 17 is controlled by main switch 12 through relay 16. The coil of relay 16 is serially connected with compressor switch 13 and thermostat switch 14, high pressure cutout switch 15 and low pressure cutout switch 19.
In the above-mentioned embodiments of safety devices for a refrigerant compressor, a pressure cutout switch functions as a refrigerant leakage detecting device. Since the pressure cutout switch detects the saturation pressure of the refrigerant, which changes in response to outside air temperature, the pressure cutout switch cannot reliably operate to detect refrigerant leakage.
Also, even though as much as fifty percent of the liquid refrigerant may remain in a refrigerant circuit, the compressor still cannot operate efficiently. Accordingly, the temperature of the discharged gas from the compressor is increased due to a lack of refrigerant. In addition, the lubricating oil for the compressor cannot be circulated properly when the refrigerant level is low.
One solution to the above-mentioned deficiencies in refrigerant systems known in the art is to increase the minimum pressure at which low pressure cutout switch 19 operates to stop the operation of the compressor. Thus, the operation of the compressor may be prevented even if there is liquid refrigerant remaining in the refrigerating circuit. However, when the external temperature is below a certain level, low pressure cutout switch 19 has a tendency to operate even if there is no leakage of refrigerant. Thus, the minimum pressure at which the low pressure cutout switch operates cannot be readily increased.