Heat is received in high temperature fluid and heat is released to low temperature fluid in low temperature fluid heat exchanger. 4. Direct contact heat exchanger: direct contact heat exchanger is a device for direct contact of two kinds of fluid, such as cold water tower, gas condenser, etc（ 2） Heat exchanger is classified according to the purpose 1. Heater: heater is to heat the fluid to the necessary temperature, but the heating fluid has no phase change. 2. Preheater: pre heating fluid of preheater to provide standard process parameters for operation of process. 3. Superheater: superheater is used to heat fluid (process gas or steam) to superheat state. 4. Evaporator: evaporator is used to heat fluid to reach the temperature above boiling point, so that its fluid evaporates, generally has phase change（ 3） According to the structure classification of heat exchanger, it can be divided into floating head heat exchanger, fixed tube plate heat exchanger, U-tube plate heat exchanger, plate heat exchanger, etc. 3、 Heat exchanger is a general equipment in chemical industry, petroleum, power, food and many other industrial sectors. It plays an important role in production. In chemical production, heat exchanger can be used as heater, cooler, condenser, evaporator and reboiler, and it is widely used. There are many kinds of heat exchangers, but according to the principle and mode of heat exchange of cold and heat flow bodies, there are three types: wall type, mixed type and heat storage type.