The differential pressure switch with adjustable set point is installed at the inlet and outlet of the heat exchanger of the central air conditioning chiller. It can be used to detect the water flow of the heat exchanger. It is an accurate flow control method. It is based on the resistance and flow of the HVAC equipment. The curve design, we know that the heat exchanger, water filter, water pump and valve of HVAC equipment have performance curves of resistance and flow. We detect the difference between the inlet and outlet water pressure at both ends of the device and set it in advance with the device. The fixed value is compared to accurately control the flow. As a flow control switch, it has the characteristics of accurate flow control, no additional resistance to the system, no requirements on the pipe diameter and no disturbance of water flow disturbance. It can replace any form of target flow switch as the flow control of HVAC water system. Compared with the target type flow switch, it can avoid the false flow caused by the pump cavitation (the actual flow rate is large, but the water is mixed with air and the actual flow rate is not large), so it can be widely used in the use of brazed plate heat exchangers, casing Large and medium-sized air-cooled or water-cooled chillers with heat exchangers and shell-and-tube heat exchangers are used for water flow protection of the condenser and evaporator and also have the function of partial antifreeze protection..