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Technical question and answer of heat exchanger

What is a heat exchanger?

Table of Contents

Heat exchanger is a kind of energy-saving equipment which can realize heat transfer between two or more fluids at different temperatures. It can transfer heat from the fluid with higher temperature to the fluid with lower temperature, so that the fluid temperature can meet the requirements of process conditions, and it is also one of the main equipment to improve energy utilization.

20201119230547 33680 - Technical question and answer of heat exchanger

How to classify heat exchange equipment?

Answer: according to the equipment classification catalogue of Sinopec, it can be divided into:

  • (1) Shell and tube heat exchanger;
  • (2) Tubular heat exchanger;
  • (3) Water immersed heat exchanger;
  • (4) Spray type heat exchanger;
  • (5) Rotary (serpentine) heat exchanger;
  • (6) Plate heat exchanger;
  • (7) Plate fin heat exchanger;
  • (8) Tube fin heat exchanger;
  • (9) Waste heat boiler;
  • (10) Others.

How does the heat exchanger conduct heat?

Answer: in the most common wall heat exchanger, there are two heat transfer modes: conduction and convection. The heat flow body first transfers heat to one side of the tube wall by convection, and then transfers heat from one side of the pipe wall to the other side in the way of conduction. Finally, the other side of the tube wall transfers heat to the cold fluid in the way of convection, thus completing the heat transfer process of the heat exchanger.

What is the effect of medium velocity on heat transfer?

Answer: the greater the flow rate of the medium in the heat exchanger, the greater the heat transfer coefficient. Therefore, increasing the flow rate of the medium in the heat exchanger can greatly improve the heat transfer effect, but the negative effect 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 should be a certain appropriate range.

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

Answer: the surface structure of heat exchange tube with special design, such as finned tube, nail head tube, threaded tube, etc., on the one hand, increases the heat transfer area, on the other hand, the turbulence effect of the special surface greatly increases the turbulence degree of the fluid outside the tube, both of which can improve the overall heat transfer effect of the heat exchanger, so the performance of these surface structures is better than that of the smooth tube surface.

What methods are commonly used to remove scale on the surface of heat exchange tube?

Answer: the common methods for removing scale on the surface of heat exchange tubes are as follows:
Mechanical descaling: manual cleaning with steel drill rod and cleaning with pressure water

What are the methods to prevent scale on the surface of heat exchange tube?

Answer:

  • (1) nickel phosphorus plating;
  • (2) Chemical coating, 847 coating.

What are the common methods to enhance heat transfer of heat exchanger?

Answer: main methods of heat transfer enhancement for heat exchanger

One is to adopt the structure of increasing heat transfer surface, such as:

  1. Use finned tube, nail head tube, threaded pipe, bellows, etc.
  2. Pipe surface machining: spiral ring pipe, spiral groove pipe, screw pipe, etc.
  3. Using small diameter tubes can increase the number of tubes in the same tubesheet area and increase the heat transfer area.

Second, the heat transfer coefficient can be greatly improved by increasing the flow rate of fluid in the heat exchanger.

  1. Add spoiler, such as inserting spiral tape into the tube, setting baffle plate and false tube outside the tube.
  2. Increase the number of tube side or shell side.

In addition, the use of materials with good thermal conductivity to manufacture heat exchanger, good anti-corrosion and anti scaling measures of heat exchanger, and timely cleaning are the means to improve the heat transfer effect.

What are the requirements for the number of blocked tubes in the maintenance of tubular heat exchanger?

Answer: the corrosion perforation of individual tube bundle is allowed to be blocked by metal plunger with processed taper of 3-5 degrees. In general, the number of dead pipes in the same pass can not be increased by more than 10% according to the process requirements.

Why do gaskets on both sides of tubesheet have to be made of the same material?

Answer: because the flange fastening bolts on both sides of the tube sheet are the same bolt, the specific pressure applied to the gaskets on both sides of the tubesheet is the same. If different materials are selected for the gaskets on both sides of the tube sheet, the sealing failure will be caused by insufficient specific pressure of one gasket, or the sealing failure will be caused by the excessive specific pressure of the gasket on the other side. Therefore, the gasket on both sides of the tubesheet must be made of the same material.

Why does the cooling water heat exchanger produce scale?

Answer: scale is formed by precipitation of dissolved salt crystals in water and attached to the tube wall of heat exchanger. It is characterized by dense and hard, firm adhesion and difficult removal. A large number of suspended particles in water can become crystal seeds. Other impurity ions, bacteria and rough metal surface have strong catalytic effect on the crystallization process, which greatly reduces the supersaturation required for crystallization precipitation. Therefore, the cooling water heat exchanger is easy to produce scale.

What are the main components of floating head heat exchanger?

Answer: the main components are: tube bundle, baffle, anti impact plate, tie rod, distance pipe, 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 gasket, air vent, drain port, pipe box flange, tube box side flange, tube box gasket, tube box side gasket, fixed saddle, movable saddle.

What are the main components of fixed tubesheet heat exchanger?

Answer: the main components are: tube bundle, baffle, tie rod, distance pipe, shell, tube box (top cover), tube sheet, inlet flange, outlet flange, pipe box flange, box gasket, fixed saddle, movable saddle, ear support, expansion joint.

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

Answer: the main components are: U-shaped tube bundle, baffle plate, impact plate (inner guide tube), tie rod, distance pipe, shell, tube box, tube sheet, inlet flange, outlet flange, tube box flange, tube box side flange, tube box gasket, tube box side gasket, fixed saddle and movable saddle.

What are the main components of the tubular heat exchanger?

Answer: the main components of the tubular heat exchanger are: inner tube, outer tube, return elbow

What are the main components of the water immersed heat exchanger?

Answer: the main parts of the water immersed heat exchanger are: inlet pipe, outlet pipe, collecting pipe, snake tube and cooling water tank.

What are the main components of spray heat exchanger?

Answer: the main components of spray heat exchanger are: tube bundle, fan, water nozzle, drain pipe, feed pump.

What are the characteristics of fixed tubesheet heat exchanger, U-tube heat exchanger and floating head heat exchanger?

Answer: the fixed tubesheet heat exchanger is characterized by compact structure, simple structure, low cost, the largest number of tubes in the same shell diameter, convenient replacement and maintenance of single tube, and convenient cleaning in the tube. However, it is difficult to clean the outside of the tube, and the temperature difference between the tube and the shell is large. The U-tube heat exchanger is characterized by simple structure, no thermal stress problem, high fluid velocity and low metal consumption. It is suitable for high temperature and high pressure fluid. The tube bundle can be drawn out, which is easy to clean the shell side and between the tubes. However, the elbow in the tube is not easy to clean, the number of tubes on the tube sheet is small, the tube spacing is large, there is a gap in the center of the tube bundle, and the fluid outside the tube is easy to short circuit. Floating head heat exchanger is characterized by free movement of tube bundle, no temperature difference stress problem, free extraction of tube bundle, easy cleaning of tube outside and tube bundle. However, floating head has complex structure, high cost, strict sealing requirements at floating head, easy leakage at floating head during operation and difficult to detect.

Where is the fixed tubesheet heat exchanger suitable for?

Answer: the fixed tubesheet heat exchanger is suitable for medium cleaning in shell side, not easy to scale, and medium temperature difference is small.

Where is the U-tube heat exchanger suitable for?

Answer: U-tube heat exchanger is suitable for high temperature and high pressure occasions where the temperature difference between the shell and tube is large and the cleaning medium is in the tube.

Where is floating head heat exchanger suitable for?

Answer: floating head heat exchanger is suitable for the occasion where the temperature difference between tube and shell is large, the medium is not clean and needs to be cleaned frequently.

The arrangement of tubular heat exchanger has triangle arrangement and square turn 45 ° angle arrangement. Why?

Answer: triangle arrangement and square turn 45 ° angle arrangement have their own advantages and disadvantages. The advantages of triangular arrangement are compact and high heat transfer efficiency. The number of tubes arranged in the same area of tube sheet is the most, which is about 15% more than that of square arrangement, but it is not easy to clean the outer surface of tubes; while the square arrangement with 45 ° angle is more convenient for cleaning the external surface of tubes, but the number of tubes arranged in triangular arrangement is much less.

What materials are commonly used in tubular heat exchanger?

Answer: the commonly used materials of the tubes are: 10 × 10, 20 × 10, 12CrMo, 15CrMo, 0Cr13, 1Cr13, 1Cr5Mo, 0Cr18Ni9Ti, 1Cr18Ni9Ti, titanium tube, 410321, etc.

In the tubular heat exchanger, why are the diameters of φ 32, 25, 19 and 16 selected respectively?

Answer: the size of pipe diameter will directly affect the performance of heat exchanger. Small diameter, large heat transfer coefficient and large effective heat transfer area in the same volume. In this way, the structure is compact and the material is saved. However, too small pipe diameter will also bring adverse effects. For the fluid with the same flow rate, the smaller the pipe diameter, the greater the resistance when flowing, and the pressure loss will also increase. In addition, if the pipe is too thin, it is easy to be blocked by dirt, which makes cleaning difficult. Therefore, the diameter of heat exchanger is generally 16 mm to 32 mm.

Why are the bolt holes of heat exchanger support round and strip round?

Answer: the bolt hole on the fixed support is round, so that the shell can be firmly fixed on the foundation. The bolt hole on the movable support is long and round, so that the shell can freely expand and expand when the temperature changes, so as to avoid large stress, so as to protect the equipment.

What are the commonly used heat exchanger gaskets?

Answer: asbestos gasket, metal gasket.

What problems should be paid attention to in the selection of small floating head bolts of floating head heat exchanger?

  • (1) Length
  • (2) Wet H2S stress corrosion
  • (3) Temperature

What is the function of baffle (baffle rod) in tubular heat exchanger?

Answer: baffle plate (baffle rod) in heat exchanger can change the flow direction of fluid in shell side, increase fluid velocity in shell side, increase turbulence degree of medium, improve heat transfer efficiency and support tube bundle.

Why does tubular heat exchanger have single tube pass, two tube pass, four tube pass, six tube pass and eight tube pass?

Answer: when the number of main pipes in the heat exchanger is the same, increasing the number of tube passes can increase the flow rate in the tubes in each pass, thus increasing the heat transfer coefficient and reducing the required heat transfer area. But at the same time, it also increases the pressure drop and makes the heat transfer of fluid not all in the counter flow mode, and the structure of the heat exchanger is more complex. Therefore, the number of tube passes generally used is no less than 2 and no more than 8, which should be selected according to the actual process requirements.

What are the causes of inner leakage of tubular heat exchanger?

Answer: the internal leakage of heat exchanger may be caused by the following reasons:

  • a. Corrosion perforation or fracture of heat exchange tube;
  • b. Leakage caused by corrosion thinning of nozzle;
  • c. Loose expansion joint between heat exchange tube and tube sheet;
  • d. Cracks, blowholes or corrosion perforation are found at the welding joint between heat exchange tube and tube sheet;
  • e. Loose or broken small floating head bolt;
  • f. Damage of small floating head gasket;
  • g. The seal of small floating head or floating tubesheet is damaged.

Why do you need to test the water pressure after the maintenance of the heat exchanger?

Answer: the purpose of water pressure test after maintenance of heat exchanger is to check whether the heat exchanger has the ability to withstand the design pressure safely (i.e. pressure strength), tightness, quality of interface or joint, welding quality and tightness of sealing structure. In addition, the residual deformation of the base metal weld of the vessel and pipe can be observed after the pressure, and the problems of the material can be found in time.

Why are some tube and tube heat exchangers installed vertically and horizontally?

Answer: some of the tubular heat exchangers are vertical and some are horizontal, which are mainly considered from the following aspects:

  • ① production process requirements: for example, some reboilers need a certain height of medium liquid level, if horizontal displacement heat exchanger is used, it can not meet the requirements of liquid level height, so vertical heat exchanger must be selected;
  • ② large scale: if the heat exchange area of a process unit needs thousands of square meters If the horizontal displacement heat exchanger with heat pipe length of 6m is selected, several heat exchangers may be needed, which is not conducive to the effective space layout of the device. If the vertical displacement heat exchanger with the length of 12 m is selected, one can solve the problem;
  • ③ reduce the pressure drop: some production processes require that the pressure drop in the process of medium transportation be minimized, and the vertical displacement heat exchanger is selected and arranged in parallel with the tower To shorten the connecting pipeline with the tower and reduce the pressure drop.

Why do some places choose tubular heat exchanger and water immersed heat exchanger, while others choose tubular heat exchanger?

Answer: at present, most of the heat exchange equipment selected for oil refining and chemical production plants are tubular heat exchangers, but in some production units, there are still a small number of tubular heat exchangers and water immersed heat exchangers. Although the tubular heat exchanger has a compact structure and high heat transfer efficiency, it is easy to cause blockage if it is used in the medium containing solid particles due to its small size. Therefore, in the medium containing solid particles, the tubular heat exchanger or water immersed heat exchanger is generally selected.

Source: China Tube sheet Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

If you want to have more information about the article or you want to share your opinion with us, contact us at sales@epowermetals.com

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