A heat exchanger is an energy-saving device that transfers heat between two or more fluids at different temperatures. For large-area heating, the existence of a heat exchanger is essential.
According to the heat transfer method of the heat exchanger, the heat exchanger can be divided into three categories:
A direct contact heat exchanger, also called a hybrid heat exchanger, is a device in which hot and cold fluids are in direct contact and exchange heat. Usually, the two fluids in direct contact are gas and liquid with lower vaporization pressure;
The working principle of the energy storage heat exchanger is to use the thermal conductivity of solid substances. Specifically, the heat medium first heats the solid substance to a certain temperature, and the cold medium then obtains heat from the solid substance. Through this process, the heat transfer can be realized. ;
The partition wall heat exchanger also utilizes the heat conduction of the intermediary, and the cold and hot media are separated by a solid partition wall, and exchange heat through the partition wall. For heating enterprises, partition wall heat exchangers are the most widely used. According to different structures, it can also be divided into tube heat exchangers, plate heat exchangers and heat pipe heat exchangers.
1
Shell and tube heat exchanger
Shell and tube heat exchangers are also called shell and tube heat exchangers. It is a partitioned wall heat exchanger that uses the wall of the tube bundle enclosed in the shell as the heat transfer surface. This kind of heat exchanger has a relatively simple structure and reliable operation. It can be made of various structural materials (mainly metal materials), and can be used under high temperature and high pressure. It is the most widely used type at present.
Shell and tube heat exchanger
According to the compensation measures adopted, shell-and-tube heat exchangers can be divided into four types: fixed tube-sheet heat exchangers, floating head heat exchangers, U-tube heat exchangers, and stuffing box heat exchangers.
2
Fixed Tube Sheet Heat Exchanger
The fixed tube-sheet heat exchanger is a type of shell-and-tube heat exchanger. The tube sheets at both ends of the fixed tube sheet heat exchanger are connected to the shell by welding, and are mainly composed of shell, tube sheet, tube bundle, top cover (head) and other components.
Fixed Tube Sheet Heat Exchanger
The advantages of fixed tube-sheet heat exchangers are:
◆Simple structure;
◆In the same shell diameter, the number of pipes is the largest and the bypass is the least;
◆Each heat exchange tube can be replaced, and the inside of the tube is easy to clean.
The disadvantages of fixed tube-sheet heat exchangers are:
◆The shell side cannot be cleaned mechanically;
◆When the temperature difference between the heat exchange tube and the shell is large (greater than 50°C), the temperature difference stress will be generated. The solution is to install an expansion joint on the shell, so the pressure on the shell side cannot be too high due to the limitation of the strength of the expansion joint;
◆It is only suitable for working occasions where the fluid is clean and not easy to scale, and the temperature difference between the two fluids is small or the temperature difference is large but the shell side pressure is not high.
3
floating head heat exchanger
The floating head heat exchanger is a kind of shell-and-tube heat exchanger. It has one end of the tube plate that is not connected to the shell and can float freely in the axial direction. It is also called a floating head. The floating head is composed of a floating tube plate, a hook ring and a floating head end cover, which is a detachable connection, and the tube bundle can be pulled out from the shell.
floating head heat exchanger
The advantages of floating head heat exchangers are:
◆If there is a temperature difference between the heat exchange tube and the shell, that is, when the shell or the heat exchange tube expands, there will be no temperature difference stress;
◆The tube bundle can be pulled out from the casing, which is convenient for cleaning inside and between the tubes.
The disadvantages of floating head heat exchangers are:
◆Complicated structure, large amount of materials and high cost;
◆If the seal between the floating head cover and the floating tube sheet is not tight, internal leakage will occur, resulting in the mixing of the two media.
4
U-tube heat exchanger
The U-shaped tube heat exchanger is a kind of shell-and-tube heat exchanger, which is composed of tube sheets, shells, tube bundles and other components. Each tube of the U-tube heat exchanger is bent into a U shape, and the inlet and outlet are respectively installed on both sides of the same tube sheet. The head is divided into two chambers by a partition, so that each tube can be freely expanded and contracted. , regardless of other tubes and housings.
U-tube heat exchanger
The advantages of U-tube heat exchangers are:
◆The tube bundle can float freely without considering the stress of temperature difference, and can be used in occasions with large temperature difference;
◆It has only one tube plate, less flanges, fewer leakage points and simple structure;
◆U-shaped tube heat exchanger is reliable in operation and low in cost.
The disadvantages of U-tube heat exchangers are:
◆It is difficult to clean inside the tube. Since the tube needs to have a certain bending radius, the utilization rate of the tube sheet is low;
◆The distance between the tubes in the innermost layer of the tube bundle is large, and the shell side is easy to short circuit. When the flow velocity in the pipe is too high, it will cause serious erosion to the U-shaped bend pipe section and affect its service life;
◆If the inner tube is damaged, it cannot be replaced, so the scrap rate is high.
5
Spray heat exchanger
The spray heat exchanger is a kind of tubular heat exchanger, which fixes the heat exchange tubes on the steel frame in rows, the hot fluid flows in the tubes, and the cooling water is evenly poured down from the upper spray device, so it is also called Spray cooler. The outside of the tube of the spray heat exchanger is a layer of liquid film with a high degree of turbulence, and the heat transfer coefficient outside the tube is much larger than that of the submerged heat exchanger. In addition, the spray heat exchanger is mostly placed in the place where the air circulates, and the evaporation of the cooling water also takes away part of the heat, which can reduce the temperature of the cooling water and increase the driving force of heat transfer. Compared with the immersion heat exchanger, the heat transfer effect of the spray heat exchanger is greatly improved.
Spray heat exchanger
The advantages of spray heat exchangers are:
◆Simple structure and low cost;
◆It can reduce the temperature of cooling water and increase the driving force of heat transfer;
◆Can withstand high pressure;
◆Easy for maintenance and cleaning, low water quality requirements.
The disadvantages of spray heat exchangers are:
◆Uneven spraying of cooling water will affect the heat transfer effect;
◆It can only be installed outdoors.
6
Tube heat exchanger
The casing heat exchanger is a kind of tube heat exchanger, which is a concentric circular casing connected by two standard tubes of different sizes. The outer one is called the shell side and the inner one is called the tube side. Two different media can flow in reverse (or in the same direction) in the shell side and the tube side to achieve the effect of heat exchange. Tube-and-tube heat exchangers usually consist of a shell (including an inner shell and an outer shell), a U-shaped elbow, a stuffing box, and the like.
Tube heat exchanger
The advantages of casing heat exchangers are:
◆Simple structure, able to withstand high pressure;
◆The heat transfer area can be increased or decreased according to the needs, which is convenient for application.
The disadvantages of casing heat exchangers are:
◆There are many joints between pipes, which are easy to leak;
◆Large floor area and large metal consumption per unit heat transfer surface.
7
Jacketed heat exchanger
The jacketed heat exchanger is a kind of plate heat exchanger, which is made by installing a jacket on the outer wall of the container, and has a simple structure; but its heating surface is limited by the wall of the container, and the heat transfer coefficient is not high. In order to improve the heat transfer coefficient and make the liquid in the kettle evenly heated, a stirrer can be installed in the kettle. When cooling water or non-phase-change heating agent is passed into the jacket, spiral baffles or other measures to increase turbulence can also be set in the jacket to increase the heat transfer coefficient on one side of the jacket. In order to make up for the lack of heat transfer surface, coils can also be installed inside the kettle. Jacketed heat exchangers are widely used for heating and cooling of reaction processes.
Jacketed heat exchanger
The advantages of jacketed heat exchangers are:
◆Simple structure;
◆Easy to process.
The disadvantages of jacketed heat exchangers are:
◆Small heat transfer area and low heat transfer efficiency.
8
Spiral plate heat exchanger
Spiral plate heat exchanger is a kind of plate heat exchanger, which is made of two parallel metal plates, forming two concentric spiral channels inside. A partition is set in the center of the heat exchanger to separate the spiral channels, and a distance column is welded between the two plates to maintain the channel spacing.
Spiral plate heat exchanger
The advantages of spiral plate heat exchangers are:
◆High heat transfer coefficient;
◆Not easy to scale and block;
◆Able to utilize heat source with lower temperature;
◆Compact structure.
The disadvantages of spiral plate heat exchangers are:
◆The operating pressure and temperature should not be too high;
◆It is not easy to overhaul.
9
heat pipe heat exchanger
The heat pipe is formed by filling a certain amount of certain working liquid in a sealed metal tube that removes non-condensable gas. The working liquid absorbs heat at the hot end and boils and vaporizes. The generated steam flows to the cold end to condense and release latent heat. The condensate returns to the hot end and boils and vaporizes again. This cycle is repeated, and the heat is continuously transferred from the hot end to the cold end.
heat pipe heat exchanger
The advantages of heat pipe heat exchangers are:
◆Simple structure, long service life and reliable operation;
◆Extremely high thermal conductivity and good isothermal property;
◆The heat transfer area on both sides of the cold and hot sides can be changed arbitrarily, it can transfer heat over a long distance, and can control the temperature.
Its disadvantages are:
◆Poor oxidation resistance and high temperature resistance.
Precautions for use of heat exchanger:
◆Keep the pipe network clean. Whether it is before work or after work is completed, the pipe network must be cleaned to avoid blockage of the heat exchanger. In addition, attention should be paid to the cleaning of the decontamination device and filter, so that the entire heat exchange work can be completed smoothly.
◆Strictly control the softened water. Before processing the heating medium, check the water quality in the system and in the softening tank, and inject it after confirming that the water quality is qualified.
◆ Test the new system. For the new system, it cannot be used alternately with the heat exchanger immediately, but the new system must be operated within a specified period of time, and the heat exchanger can only be incorporated into the system after the operation test is completed. The purpose of this is to prevent impurities in the pipe network from damaging the internal equipment of the heat exchanger
