A heat exchanger is a device used to transfer heat between two or more fluids at different temperatures. Its main purpose is to transfer heat from one fluid to another, causing one fluid to be heated while the other is cooled.
1. Working Principle
The heat transfer in a heat exchanger is based on three fundamental heat transfer methods: conduction, convection, and radiation. However, in most industrial heat exchangers, conduction and convection play the major roles.
Conduction:
- When two fluids at different temperatures are on either side of a solid wall (such as the tube wall or plate wall of a heat exchanger), heat is transferred from the high – temperature side to the low – temperature side through the solid wall. For example, in a shell – and – tube heat exchanger, heat is conducted from the high – temperature fluid inside the tubes through the tube wall to the low – temperature fluid outside the tubes.
Convection:
- Due to temperature differences within a fluid, density differences occur, causing natural convection. Additionally, forced convection can be generated by external power sources (such as pumps and fans). During convection, heat is transferred within the fluid and between the fluid and the solid wall. For instance, in a plate heat exchanger, as the fluid flows between the plates, heat is continuously transferred to the plate wall through convection, and then conducted to the fluid on the other side of the plate by the plate wall.
2. Basic Structural Components (Taking the Common Shell – and – Tube Heat Exchanger as an Example)
Shell:
- It is an external container, usually cylindrical in shape. It encloses the tube bundle, provides a flow space for the shell – side fluid, and can withstand the internal pressure. The material of the shell can be selected according to operating conditions (such as temperature, pressure, and fluid corrosiveness), and common materials include carbon steel, stainless steel, alloy steel, etc.
Tube Bundle:
- It consists of many parallel tubes. These tubes are the key channels for heat transfer, with one fluid flowing inside them. The tubes in the tube bundle can be arranged in various patterns, such as equilateral triangle and square arrangements. Different arrangements affect the flow characteristics of the fluid and the heat transfer efficiency.
Tube Sheet:
- Both ends of the tube bundle are fixed to the tube sheet through welding, expansion, or other methods. The tube sheet serves to separate the tube – side and shell – side fluids and also supports the weight of the tube bundle.
Head:
- Located at both ends of the shell, it is used to seal the shell. At the same time, it can be connected to pipelines to enable the smooth entry and exit of fluids into and out of the heat exchanger.
3. Application Fields
Industrial Sectors
Petrochemical Industry:
In the process of oil refining, heat exchangers are widely used for pre – heating crude oil, cooling and heating various fractions, etc. For example, in a crude oil atmospheric and vacuum distillation unit, before the crude oil enters the distillation tower, it needs to be pre – heated by using the heat of other high – temperature fractions through a heat exchanger, which can reduce energy consumption.Chemical Industry:
- During chemical reactions, it is necessary to heat or cool the reaction materials to control the reaction temperature. For example, in the ammonia synthesis process, the raw gas entering the synthesis tower needs to be pre – heated to an appropriate temperature through a heat exchanger to ensure efficient reaction. At the same time, the high – temperature gas after the reaction also needs to be cooled through a heat exchanger to recover heat and for subsequent separation and treatment.
Power Industry:
- In thermal power plants, heat exchangers are used for steam condensation and water heating. For example, the high – temperature steam discharged from the steam turbine is cooled into water in the condenser (a special type of heat exchanger), and the heat released in this process is carried away by the cooling water. Meanwhile, in the boiler system, water is heated into high – temperature and high – pressure steam through a heat exchanger to drive the steam turbine for power generation.
HVAC (Heating, Ventilation, and Air – Conditioning) Field
Heating Systems:
- In a centralized heating system, heat exchangers are used to transfer the heat of the high – temperature hot water or steam generated by thermal power plants or boiler houses to the circulating water on the user side, thereby providing heating for buildings. For example, in a heat exchange station, the high – temperature hot water on the primary side transfers heat to the low – temperature hot water on the secondary side through a heat exchanger, and then the hot water on the secondary side is transported to the radiators in each building through pipelines for heating.
Air – Conditioning Systems:
- Heat exchangers play a crucial role in the cooling and heating processes of air – conditioners. During cooling, the indoor evaporator (a type of heat exchanger) absorbs the heat from the indoor air, reducing the indoor air temperature. The outdoor condenser (another type of heat exchanger) releases the heat absorbed by the refrigerant into the outdoor environment. During heating, the flow direction of the refrigerant is changed by a four – way reversing valve, swapping the functions of the condenser and the evaporator, thus absorbing the heat from the outside and transferring it indoors.
Food and Beverage Industry
Food Processing:
- In food processing operations such as sterilization, disinfection, concentration, and drying, heat exchangers are often used to control the temperature. For example, in the pasteurization process of milk, the milk needs to be heated to a specific temperature and held for a certain period through a heat exchanger, and then rapidly cooled through a heat exchanger to kill harmful microorganisms and maintain the quality of the milk.
Beverage Production:
- In the production of beverages such as fruit juice and beer, heat exchangers are used for operations such as heating, cooling, or evaporative concentration of the raw liquid. For example, in the concentration process of fruit juice, the fruit juice is heated through a heat exchanger to evaporate the water and increase the concentration of the fruit juice.