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Common basic knowledge of heat exchangers

March 11, 2024

Latest company news about Common basic knowledge of heat exchangers

1. How to classify heat exchange equipment?

Answer: According to the "Equipment Classification Catalog of Petrochemical Corporation", it can be divided into:

(1) Shell and tube heat exchanger

(2) Casing heat exchanger

(3) Water immersed heat exchanger

(4) Spray heat exchanger

(5) Rotary (snake tube) heat exchanger

(6) Plate heat exchanger

(7) Plate fin heat exchanger

(8) Tube-fin heat exchanger

(9) Waste heat boiler

(10) Others

2. How does a heat exchanger transfer heat?

Answer: In the most common dividing wall heat exchanger, there are two main heat transfer methods: conduction and convection. The hot fluid first transfers heat to one side of the tube wall by convection, and then transfers heat from one side of the tube wall to the other side by conduction. Finally, the other side of the tube wall transfers heat by convection. The heat is transferred to the cold fluid, thus completing the heat transfer process of the heat exchanger.

3. What effect does the medium flow rate have on the heat transfer effect?

Answer: The greater the flow rate of the medium in the heat exchanger, the greater its heat transfer coefficient. Therefore, increasing the flow rate of the medium in the heat exchanger can greatly improve the heat exchange effect, but the negative impact of increasing the flow rate is to increase the pressure drop through the heat exchanger and increase the energy consumption of the pump, so there must be certain appropriate scope.

4. What effect does the surface structure of the heat exchange tube have on the heat transfer effect?

Answer: The use of specially designed surface structures of heat exchange tubes, such as fin tubes, nail head tubes, threaded tubes, etc., on the one hand increases the heat transfer area, and on the other hand the turbulence of the special surface greatly increases the flow of fluid outside the tube. The degree of turbulence, both aspects can improve the overall heat transfer effect of the heat exchanger, so these surface structures have better performance than the surface of the light pipe.

5. What are the commonly used methods for descaling the surface of heat exchange tubes?

Answer: Common methods for descaling the surface of heat exchange tubes include:

Mechanical descaling: manual descaling with steel drill, pressure water descaling

chemical descaling

6. What are the methods to prevent scale on the surface of heat exchange tubes?

Answer: (1) Nickel phosphorus plating

(2) Chemical coating, 847 coating

7. What are the common methods to enhance heat transfer in heat exchange equipment?

Answer: The main methods for enhancing heat transfer in heat exchange equipment

One is to adopt a structure that increases the heat transfer surface, such as

1 Use fin tubes, nail head tubes, threaded tubes, bellows, etc.

2. Mechanical processing of pipe surface: spiral ring pipe, spiral groove pipe, threaded pipe, etc.

3. Using small diameter pipes can increase the number of pipes on the same tube plate area and increase the heat transfer area.

The second is to increase the flow rate of the fluid in the heat exchanger, which can greatly improve its heat transfer coefficient, such as:

1 Add spoilers, such as inserting spiral strips into the tube, setting baffles outside the tube, false tubes, etc.

2. Increase the number of tube passes or shell passes.

In addition, using materials with good thermal conductivity to manufacture heat exchangers, taking anti-corrosion and anti-scaling measures for the heat exchangers, and timely cleaning are all ways to improve the heat transfer effect.

8. When overhauling the tube-and-tube heat exchanger, what are the requirements for the number of blocked tubes?

Answer: Corrosion holes in individual tubes of the tube bundle are allowed to be blocked with processed metal plungers with a cone of 3 to 5°. Generally, in the same pipe process, the number of blocked pipes does not exceed 10% of the total number of pipes, but it can be increased appropriately according to process requirements.

9. Why must the gaskets on both sides of the tube sheet be made of the same material?

Answer: Because the flange fastening bolts on both sides of the tube plate are the same bolts, the specific pressure applied to the gaskets on both sides of the tube plate is the same.

If different materials are selected for the gaskets on both sides, the specific pressure of the gasket on one side will not be enough, causing sealing failure, or the gasket on the other side will have too high specific pressure, causing sealing failure. Therefore, the gaskets on both sides of the tube sheet must be Use the same material.

10. Why does the cooling water heat exchanger produce scale?

Answer: Scale is formed by the crystallization of dissolved salts in the water and adhesion to the heat exchanger tube wall. It is characterized by being dense and hard, with strong adhesion and difficult to remove.

A large number of suspended particles in the water can become crystal seeds. Other impurity ions, bacteria, rough metal surfaces, etc. all have a strong catalytic effect on the crystallization process, greatly reducing the supersaturation required for crystallization. Therefore, cooling water heat exchangers It is easy to produce scale.

11. What are the main components of a floating head heat exchanger?

Answer: The main components are: tube bundle, baffle, anti-collision plate, tie rod, distance tube, shell, tube box, tube sheet, inlet flange, outlet flange, floating tube sheet, floating head flange, floating head cover, Floating head hook ring, floating head gasket, outer head cover flange, outer head cover side flange, outer head cover, outer head cover gasket, air vent, drain port, pipe box flange, pipe box side flange, pipe box gasket, Pipe box side gasket, fixed saddle, movable saddle.

12.What are the main components of fixed tube plate heat exchanger?

Answer: The main components are: tube bundle, baffle, tie rod, distance tube, shell, tube box (top cover), tube plate, inlet flange, outlet flange, tube box flange, tube box gasket, fixed Saddles, movable saddles, ear supports, expansion joints.

13.What are the main components of U-tube heat exchanger?

Answer: The main components are: U-shaped tube bundle, baffle, anti-collision plate (inner guide tube), tie rod, distance tube, shell, pipe box, tube plate, inlet flange, outlet flange, pipe box method Flange, pipe box side flange, pipe box gasket, pipe box side gasket, fixed saddle, movable saddle.

14. What are the main components of a jacket-and-tube heat exchanger?

Answer: The main components of the sleeve-and-tube heat exchanger are: inner tube, outer tube, return elbow

15.What are the main components of a water-immersed heat exchanger?

Answer: The main components of the water-immersed heat exchanger are: inlet pipe, outlet pipe, collecting pipe, coiled pipe, and cooling water tank.

16.What are the main components of a spray heat exchanger?

Answer: The main components of the spray heat exchanger are: tube bundle, fan, water nozzle, drainage pipe, and water supply pump.

17. What are the characteristics of fixed tube plate heat exchangers, U-shaped tube heat exchangers, and floating head heat exchangers?

Answer: The fixed tube plate heat exchanger is characterized by compact structure, simplicity, low cost, the largest number of tubes in the same shell diameter, easy replacement and maintenance of a single tube, and convenient cleaning inside the tube, but it is difficult to clean outside the tube, and the tube and shell The temperature difference causes great stress.

The U-shaped tube heat exchanger is characterized by a relatively simple structure, no temperature difference stress problem, large fluid flow rate, low metal consumption, and is suitable for high temperature and high pressure fluids. The tube bundle can be extracted for easy cleaning of the shell side and between tubes, but the elbows in the tubes are not easy to clean. , the number of tubes on the tube plate is small, the distance between tubes is large, there is a gap in the center of the tube bundle, and the fluid outside the tube is easy to short-circuit.

The characteristics of the floating head heat exchanger are that the tube bundle can move freely without temperature difference stress problems. The tube bundle can be freely extracted, which is convenient for cleaning the outside of the tube and the tube bundle. However, the floating head has a complex structure and high cost. The sealing requirements at the floating head are strict, and the floating head is easy to operate during operation. Leakage and difficult to detect.

18. Where are fixed tube plate heat exchangers suitable for use?

Answer: The fixed tube plate heat exchanger is suitable for occasions where the shell-side medium is clean, not prone to scaling, and the medium temperature difference is relatively small.

19.Where are U-shaped tube heat exchangers suitable for use?

Answer: The U-shaped tube heat exchanger is suitable for high-temperature and high-pressure situations where the temperature difference between the tube and shell walls is large and the clean medium flows in the tube.

20. Where are floating head heat exchangers suitable for use?

Answer: The floating head heat exchanger is suitable for occasions where the temperature difference between the tube and shell is large, the medium is not clean, and frequent cleaning is required.

21. The arrangement of tube and tube heat exchangers includes triangular arrangement and square arrangement with a 45° angle. Why?

Answer: Triangular arrangement and square arrangement at a 45° angle each have their own advantages and disadvantages. The advantages of the triangular arrangement are compactness and high heat transfer efficiency. It has the largest number of tubes in the same tube plate area, about 15% more than the square arrangement. However, it is not easy to clean the outer surface of the tubes; while the square arrangement is turned at a 45° angle to clean the outer surface of the tubes. It is more convenient, but the number of pipes is much less than that of the triangular arrangement.

22. What are the commonly used materials for tubes in tube-and-tube heat exchangers?

Answer: Commonly used tube materials include: 10#, 20#, 12CrMo, 15CrMo, 0Cr13, 1Cr13, 1Cr5Mo, 0Cr18Ni9Ti, 1Cr18Ni9Ti, titanium tube, 410, 321, etc.

23. In the tube-type heat exchanger, why are the tube diameters φ32, φ25, φ19, and φ16 chosen respectively?

Answer: The size of the tube diameter will directly affect the performance of the heat exchanger. The pipe diameter is small, the heat transfer coefficient is large, and the effective heat transfer area is also large within the same volume. This can make the structure compact and save materials. However, if the pipe diameter is too small, it will also have adverse effects. For fluids with the same flow rate, the smaller the pipe diameter, the greater the resistance to flow, and the pressure loss will also increase. In addition, too thin pipes are easily blocked by dirt. This makes cleaning difficult, so the tube diameter of the heat exchanger is generally 16 mm to 32 mm.

24. Why are the bolt holes of the heat exchanger support either round or long?

Answer: The bolt holes on the fixed support are round in order to firmly fix the shell to the foundation. The bolt holes on the movable support are long and circular. The purpose is to allow the shell to expand and contract freely when it is subject to temperature changes, so as to avoid large stress and protect the equipment.

25.What are the commonly used heat exchanger gaskets?

Answer: Commonly used heat exchanger gaskets include oil-resistant asbestos gaskets, iron-clad gaskets, corrugated tooth gaskets, and metal gaskets.

26. What issues should be paid attention to when selecting small floating head bolts for floating head heat exchangers?

(1) Length

(2) Wet H2S stress corrosion

(3)Temperature

27. What is the function of the baffles (baffle rods) in the tube-and-tube heat exchanger?

Answer: The baffles (baffle rods) in the heat exchanger can change the flow direction of the fluid in the shell side, increase the flow rate of the fluid in the shell side, increase the turbulence of the medium, improve the heat transfer efficiency and support the tube bundle.

28. Why do tube and tube heat exchangers have single-tube pass, two-tube pass, four-tube pass, six-tube pass, and eight-tube pass?

Answer: When the total number of tubes in the heat exchanger is the same, increasing the number of tube passes can increase the flow rate in each tube, thus increasing the heat transfer coefficient and reducing the required heat transfer area. However, it also increases the pressure drop and prevents the fluid from fully exchanging heat in a counter-current manner, and the heat exchanger structure becomes more complex. Therefore, the number of tube passes generally used is no less than 2 and no more than 8. The specific selection should be based on the actual process requirements.

29. What are the causes of internal leakage in tube-and-tube heat exchangers?

Answer: Internal leakage of the heat exchanger may be due to the following reasons:

The heat exchange tube is corroded, perforated, or broken.

Corrosion and thinning of pipe mouth causing leakage

The expansion joint between the heat exchange tube and the tube plate is loose

Cracks, pores or corrosion perforations occur at the welding between the heat exchange tube and the tube sheet

The small floating head bolt is loose or broken

The small floating head gasket is damaged

Small floating head or floating tube sheet seal is damaged

30. Why should the water pressure be tested after the heat exchanger is overhauled?

Answer: The purpose of testing the water pressure after the heat exchanger is overhauled is to check whether the heat exchanger has the ability to safely withstand the design pressure (i.e. pressure strength), tightness, the quality of the interface or joint, the welding quality and the tightness of the sealing structure. degree. In addition, the residual deformation of the base metal welds of containers and pipes can be observed after pressure, and problems with the materials can be discovered in a timely manner.

31. Why are some tube heat exchangers installed vertically and some horizontally (horizontal)?

Answer: Some tube-type heat exchangers are vertical and some are horizontal, mainly considering the following aspects:

① Production process requirements: For example, some reboilers require a certain medium liquid level. If a horizontal heat exchanger is used, the liquid level height requirement cannot be met, so a vertical heat exchanger must be selected;

② Large-scale: If the heat exchange area of ​​a process unit requires thousands of square meters, if you choose a horizontal heat exchanger with a heat pipe length of 6 meters, you may need several heat exchangers, which will occupy a large area and is not conducive to effective spatial arrangement of the device. , if you choose a vertical heat exchanger with a heat exchange tube length of 12 meters, one unit can solve the problem;

③ Reduce pressure drop: Some production processes require minimizing the pressure drop during the medium transportation process. Vertical heat exchangers are selected and arranged side by side with the tower to shorten the connecting pipeline to the tower and reduce the pressure drop.

32. Why do some places use sleeve-and-tube heat exchangers and water-immersed heat exchangers, while other places use tube-and-tube heat exchangers?

Answer: Currently, most of the heat exchange equipment selected for oil refining and chemical production units are tubular heat exchangers. However, in some production units, there are still a small number of sleeve-type heat exchangers and water-immersed heat exchangers. Although the tube-and-tube heat exchanger has a compact structure and high heat exchange efficiency, due to the small heat exchange tubes, it is easy to cause blockage if used in media containing solid particles. Therefore, in places where the medium contains solid particles, sleeve-type heat exchangers or water-immersed heat exchangers are generally used.

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