People who have used air conditioning must know about full liquid chillers, which are mainly composed of screw refrigeration compressors, shell and tube condensers, and full liquid evaporators. For full liquid chillers, in order to achieve complete wetting of the heat transfer surface by the liquid refrigerant without generating backflow, precise control of the refrigerant level in the evaporator is necessary. Currently, there are two solutions for controlling the evaporator level: indirect control and direct control. Either way, Graeff sensors are required.

Indirect control refers to using system parameters other than the refrigerant level as regulating objects to indirectly control the evaporator level. Indirect control can be to control the superheat at the outlet of the evaporator, that is, to control the superheat at approximately 1.5 to 2.0 ℃ through the Graeff temperature sensor and control logic in the control module, thereby achieving control of the liquid level. In addition, the evaporator liquid level can also be indirectly controlled through feedback parameters such as system exhaust superheat and compressor oil level.

Direct control is to directly use the refrigerant level in the evaporator as the regulated parameter, compare the liquid level signal collected by the Graeff level sensor with the given value, adjust the target value, and input the adjustment signal to the throttle valve driving device. The adjustment target is the opening value of the throttle valve, thereby achieving precise adjustment of the liquid supply and precise control of the refrigerant level in the evaporator.

With the development of Graeff temperature sensors, most of them now use indirect control methods for measurement, which is very convenient. Similar sensors are not only used in air conditioners, but also in other similar household appliances such as washing machines.