Energy is one of the most important issues facing humanity. Energy development and conversion have become an important issue in all countries. Heat exchangers are indispensable equipment in the process of energy utilization. They are used in almost all industrial fields, chemical, metallurgical, The power, transportation, aerospace and aerospace sectors are particularly widely used. In recent years, due to the development of new technologies and the development and utilization of new energy, various types of heat exchangers have received more and more attention from the industry, and heat exchangers are more critical equipment in energy conservation measures. Therefore, whether from industrial Development, or from the effective use of energy, the rational design, manufacture, selection and operation of heat exchangers are of great significance.

1. Classification of heat exchangers

1. 1. Direct heat transfer heat exchanger. A heat exchanger that does not require a heat transfer wall and is in direct contact with a hot fluid for heat exchange. Such heat exchangers are commonly used in industrial production.

1. 2. Wall heat transfer heat exchanger. The heat exchanger for heat transfer operation of heat exchange between the cold and hot fluids through the wall of the pipe, plate, etc. is a common and commonly used heat exchanger. The cold and hot fluids are all fluids, which may be air, smoke, Steam, water. This is the type of heat exchanger that is discussed in this article.

1. 3. Regenerative heat exchanger. Intermittent heat transfer is a practical and effective way to recover waste heat in the waste heat regenerator. It is often used as a means of regulating the waste heat of the combustion gas and the uneven amount of steam.

2. Characteristics and use of several heat exchangers

In the actual design selection, it is often known that both the high temperature fluid and the low temperature fluid on both sides of the inlet and outlet temperature, in the process design selection, it is necessary to consider the heat transfer area as small as possible, as large as possible Heat exchange rate, as well as lower equipment cost and construction costs. In addition, considering the design and selection of the heat exchanger under the premise of convenient operation and maintenance and cleaning.

Basic equation for heat transfer:
Q = UAΔt Kcal/ h ;
Where: U is the heat transfer coefficient, Kcal / m2 . h. °C; A is the heat transfer area, m2; Δt is the average temperature through the boundary layer of the two fluids.
The heat transfer coefficient h, Kcal/m2.h. °C of the heat exchanger and the flow rate u, m/s have the following relationship:
Tube heat gain coefficient ht:
Laminar flow zone (Re ≤ 2100) ht∝ u0. 33t
Transitional flow zone (2100 ≤ Re ≤ 10000) ht∝ u0. 33～0. 8t
Turbulent zone (Re ≥ 10000) ht∝ u0. 8t
Shell process heat coefficient hg:
The shell-side fluid flows vertically through the tube bundle, so the flow pattern is disordered, laminar,
There is no obvious difference in the turbulent zone: ht∝ u0. 55g
The corresponding pressure drop ΔP, kg / cm2, tube and shell side are roughly the same:
Laminar flow, transitional flow zone ΔP ∝ u1. 0t
Turbulent zone ΔP ∝ u1. 8t

It can be seen from the above formula that at a certain flow rate, the larger the Reynolds number, the larger the heat transfer coefficient and the greater the pressure drop.

2. 1. Shell and tube heat exchanger. Shell-and-tube heat exchangers are common structures commonly used, including: fixed tube plate heat exchangers, U-tube shell-and-tube heat exchangers, with expansion joint heat exchangers, floating-head heat exchangers, and sectional replacement Heaters, casing heat exchangers, etc.

The fixed tube plate heat exchanger has the advantages of simple structure, light weight and low cost; the disadvantage is that the tube is bent due to thermal expansion. U-tube-and-tube heat exchanger overcomes this shortcoming and makes the tube into a "U" shape. The one end is fixed at the other end, so that the heat exchanger is not affected by the expansion. The structure is simple and light, and the disadvantage is that it cannot be mechanically cleaned. Inconvenient to replace the pipe, low heat transfer per unit capacity and unit mass, suitable for occasions where the temperature difference is large and the fluid medium in the pipe is relatively clean.

The expansion joint heat exchanger can solve the expansion problem. With the structure of the expansion joint, it is suitable for fluids with high temperature difference and high pressure fluid. Because the joint can be removed for cleaning, it can handle the fouling fluid, while for low pressure gas. Not suitable, but its disadvantage is that it is complicated to manufacture.

The floating head shell heat exchanger has a floating head which is not connected to the outer casing and can be freely expanded and contracted, so that the problem of thermal expansion is solved, and the cleaning is convenient, and the core can be taken out during maintenance.

For fixed tube, tube and tube heat exchangers with a volume of 1 m3 per housing, the heat transfer area is approximately 30 to 40 m3. For U-tube and shell heat exchangers and floating-head heat exchangers, the heat transfer area is about 70m2 for each case with a volume of 1 m3.

2. 2, plate heat exchanger. Since the heat transfer surface of the plate heat exchanger can press the concave-convex liquid discharge groove, the turbulent flow state can be generated under the condition of lower Reynolds number, so the heat transfer coefficient is high, generally up to 3,000-5 000 Kcal/ M2 . h. °C, compared with the shell-and-tube heat exchanger at the same flow rate, this value is about 3 to 5 times that of the shell-and-tube heat exchanger, although the resistance of the plate heat exchanger at this time It will be larger, as compared to the case of the same power consumption, the heat release coefficient of the plate heat exchanger is about twice as high as that of the shell-and-tube type.

Due to the compact structure and small gap of the plate heat exchanger, the heat transfer area per unit volume is increased, and the installation area is about 1/2 to 1/3 of the shell-and-tube type, which can save floor space and construction cost. When the volume of the outer casing is 1 m3, the heat transfer area is about 80 m2. In addition, the plate heat exchanger is easy to increase or decrease the heat exchange area. For the shell-and-tube heat exchanger, when the liquid processing capacity needs to be increased, the original heat exchanger The heat transfer area is almost impossible to increase, but the heat transfer area of the plate heat exchanger is easily increased, thereby increasing the processing capacity. In addition, the plate heat exchanger only exposes the outer plate of the heat transfer plate to the atmosphere, thus, heat loss is lost. Negligible and no insulation measures are required.

One of the characteristics of the plate heat exchanger in terms of operation and maintenance is that it is convenient to assemble and disassemble, and it is not necessary to completely disassemble it. Only the loosening bolt can be loosened to remove the plate to clean, replace the gasket, and even replace the plate. This is especially important for materials that are prone to deposits in the heat exchange medium.