Design, selection and use of heat exchanger

Energy is one of the important issues facing mankind at present. Energy development and conversion and utilization have become important topics in various countries. Heat exchangers are essential equipment in the process of energy utilization. They are used in almost all industrial fields, such as chemical industry, metallurgy, Power, transportation, aviation 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. The development is still from the effective use of energy, and the reasonable design, manufacturing, selection and operation of heat exchangers are of very important significance.　　

1. Classification of heat exchanger　　

1. 1. Direct heat transfer type heat exchanger. A heat exchanger that does not require a heat transfer wall, and is operated by cold fluid and hot fluid in direct contact with heat exchange. This type of heat exchanger is often used in industrial production.　　

1. 2. Partition wall heat transfer type heat exchanger. The heat exchanger in the heat transfer operation process in which cold and hot fluids exchange heat through the walls of tubes, plates, etc., is the most common and most commonly used heat exchanger. Both cold and hot fluids are fluids, which can be air, flue gas, Steam, water. This is the type of heat exchanger that this article focuses on.　　

1. 3. Regenerative heat exchanger. Intermittent heat transfer is a practical and effective way to recover waste heat in a waste heat regenerator. It is often used to recover waste heat from combustion gas and steam as an adjustment method when the consumption is uneven.

2. The characteristics and use of several heat exchangers　　

In the actual design and selection, the inlet and outlet temperatures of the high-temperature fluid and the low-temperature fluid are often known. When making process design selection, what needs to be considered is to have the smallest possible heat exchange area and the largest possible Heat exchange rate, and lower equipment cost and construction cost. In addition, consider the design and selection of heat exchangers on the premise that operation, operation, maintenance and cleaning are more convenient. Basic heat transfer equation:
Q = UAΔt 　　　　　Kcal/ h;
In the formula: U is the heat transfer coefficient, Kcal/m2. H. ℃; 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 of the heat exchanger, Kcal/m2. h. ℃ and the flow velocity u, m/s have the following relationship:
Tube side heat transfer coefficient ht:
Laminar flow area　(Re ≤2100) 　　　　　ht∝ u0. 33t
Transition flow area (2100 ≤Re ≤10000) ht∝ u0. 33～0.8t
Turbulence area　(Re ≥10000) ht∝ u0. 8t
Shell side heat transfer coefficient hg:
The shell side fluid flows vertically through the tube bundle, so the flow pattern is chaotic, laminar flow,
There is no obvious difference in the turbulence zone: ht∝ u0. 55g
The corresponding pressure drop ΔP, kg/cm2, the tube side and the shell side are roughly the same:
Laminar flow, transitional flow area　　　　　　　　ΔP ∝ u1. 0t
Turbulent flow 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 at the same time, the larger the pressure drop.
 
　　2. 1. Shell and tube heat exchanger. The shell and tube heat exchanger is the most commonly used common structure, which includes: fixed tube sheet heat exchanger, U-shaped shell and tube heat exchanger, heat exchanger with expansion joints, floating head heat exchanger, segmented heat exchanger Heater, double-pipe heat exchanger, etc.　　

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

The heat exchanger with expansion joints can solve the problem of expansion. The structure of expansion joints is suitable for fluids with large temperature differences and high-pressure fluids. Because the joints can be removed for cleaning, it can handle fouling fluids, while for low-pressure gases Not suitable, but its disadvantage is that it is complicated to manufacture.　　

The floating head type shell-and-tube heat exchanger has a floating head that is not connected with the shell and can be freely expanded and contracted. This not only solves the problem of thermal expansion, but also facilitates cleaning. The tube core can be extracted during maintenance.　　

When the volume of each shell of the fixed tube sheet, tube and double tube heat exchanger is 1 m3, the heat transfer area is about 30-40 m3. When the volume of each shell of U-shaped shell and tube heat exchanger and floating head heat exchanger is 1 m3, the heat transfer area is about 70 m2.
 
　　 2. 2. Plate heat exchanger. Since the heat transfer surface of the plate heat exchanger can be extruded with concave and convex drainage grooves, turbulent flow can occur under the condition of lower Reynolds number, so the heat transfer coefficient is higher, generally up to 3 000 ~ 5 000 Kcal/ m2. h. ℃, compared with the shell-and-tube heat exchanger at the same flow rate, this value is about 3 to 5 times the heat transfer coefficient of the shell-and-tube heat exchanger, although the resistance of the plate heat exchanger at this time Will be larger, such as under the same power consumption conditions, 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. When the shell volume is 1 m3, its 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 It is almost impossible to increase the heat transfer area of ​​the plate heat exchanger, but the heat transfer area of ​​the plate heat exchanger is easily increased, thereby increasing the processing capacity.In addition, only the shell plate of the heat transfer plate is exposed to the atmosphere in the plate heat exchanger, therefore, the heat dissipation loss It can be ignored, and no insulation measures are required.　　

One of the features of the plate heat exchanger in terms of operation and maintenance is that it is relatively easy to assemble and disassemble, and it does not even need to be completely disassembled. Only by loosening the compression bolts, the plates can be drawn out for cleaning, replacement of gaskets, and even replacement of plates. This is particularly important for materials that are prone to deposits in the heat exchange medium.