What is the general problems and solutions for shell and tube heat exchangers?

- Aug 24, 2018-

Generally, the general general problem of shell-and-tube heat exchangers is that the use of shell-and-tube heat exchangers in daily life is relatively common. What are the solutions to the conventional problems of shell-and-tube heat exchangers? Some information is given today as follows.

1. For shell-and-tube heat exchangers whose tube and shell process design pressures are internal pressure, under what circumstances can the pressure-receiving component be designed to be poorly pressed? What problems should be considered?

For components that are simultaneously controlled by the tube and the internal pressure of the shell, the differential design can be pressed only when the tube and shell are simultaneously raised and depressurized. The value of the differential pressure should also be considered during the pressure test. The pressure difference, while the designer should propose a step test for the pressure test.

2. Explain the difference between the design temperature of the tube and shell side and the temperature of the tube wall and shell wall in the shell-and-tube heat exchanger.

The design temperature of the tube and shell is the design temperature of the tube and shell shell respectively, which is the temperature of the tube and shell pressure component corresponding to the tube and shell design pressure. The high or low value of the average value. It is used to determine the allowable stress of the component material.

The temperature of the tube wall and the shell wall are respectively the average temperature of the heat exchange tube and the shell cylinder metal along the length, which are the average values of the temperature of the heat exchange tube and the shell cylinder metal formed in the heat transfer process along the length direction. Calculate the stress caused by the difference between the thermal expansion of the shell cylinder and the heat exchange tube in the tube sheet, heat exchange tube and shell cylinder.

These two sets of temperature are not only different in definition, nature and function, but also have large differences in numerical values. Therefore, it must be paid attention to during calculation and cannot be mixed.

3. How to determine the design temperature of components in the shell-and-tube heat exchanger that are simultaneously subjected to tube and shell temperature?

The design temperature of the components in the shell-and-tube heat exchanger that are simultaneously subjected to the tube and shell temperature can be determined according to the metal temperature, and the design temperature of the higher side can also be taken.

4. What are the considerations for the corrosion margin of the main components of the shell-and-tube heat exchanger?

Principles for consideration of corrosion margins of main components of shell-and-tube heat exchangers:

a) corrosion allowance should be considered on both sides of the tube sheet, floating head flange, spherical crown head and hook and loop;

b) the inner surface of the flat cover, convex head, pipe box and cylinder should take into account the corrosion allowance;

c) When the tube plate and the flat cover are grooved, the metal above the bottom of the partition groove can be used as a corrosion margin, but when the corrosion margin is greater than the groove depth, the difference between the two should be added;

d) corrosion allowance should be considered on the inner diameter surface of the pressure vessel flange and pipe flange;

e) the heat exchange tube does not consider the corrosion margin;

f) Non-stressed components such as tie rods, spacers, baffles and support plates, generally do not consider corrosion margin.

5. What are the special requirements for the design of the tube (or interface) of the shell-and-tube heat exchanger compared to the general container?

a) the connecting pipe should be flush with the inner surface of the casing;

b) the nozzle should be placed along the radial or axial direction of the heat exchanger as much as possible;

c) When the design temperature is higher than or equal to 300 °C, the butt welding flange shall be used;

d) If necessary, the thermometer interface, pressure gauge interface and liquid level meter interface should be set;

e) For heat exchangers that cannot use the take-over (or interface) for venting and draining, the venting port should be set at the high point of the tube and shell, and the drain port should be set at the low point, the small nominal diameter is 20mm;

f) The vertical heat exchanger can be set with an overflow.

6. How to determine the effective thickness of the integral tube sheet of the shell-and-tube heat exchanger?

1). The effective thickness of the integral tube sheet is equal to the tube sheet thickness at the bottom of the tube section partitioning groove minus the sum of the following two thicknesses:

a) the portion of the tube process corrosion margin exceeding the depth of the tube partition groove;

b) the larger value of the shell-side corrosion margin and the structural groove depth of the tube sheet on the shell side.

2) When the tube plate and the heat exchange tube are welded, the small thickness of the tube sheet should meet the requirements of structural design and manufacturing, and not less than 12mm.

3). Small thickness of composite tube sheet and corresponding requirements:

a) The composite tube sheet welded and connected between the tube sheet and the heat exchange tube shall have a thickness of not less than 3 mm. For the layer with corrosion resistance requirements, the chemical composition of the layer should not be less than 2 mm from the surface of the layer. And metallographic organization meets the requirements of the composite material standard;

b) The composite pipe sheet with the account connection shall have a small thickness of not less than 10 mm, and shall ensure that the chemical composition and metallographic structure of the composite layer with a depth of not less than 8 mm from the surface of the composite layer meet the requirements of the composite material standard.

7. For multi-tube tube-and-tube heat exchangers, what should be considered when designing the split-pass?

When designing the split-tube heat exchanger, the following should be considered:

a) The number of heat transfer tubes in each tube should be approximately equal as much as possible.

b) The layered partition groove has a simple shape and a short sealing surface length.

8. How is the small inside depth of the tube-and-tube heat exchanger tube box determined?

The small inside depth of the shell-and-tube heat exchanger tube box is determined according to the following requirements:

a) single-tube tube box with axial opening, the small depth at the center of the opening should be not less than 1/3 of the inner diameter of the connecting tube;

b) The inside depth of the multi-pass pipe box shall ensure that the small flow area between the two passes is not less than 1.3 times the flow area of the heat transfer pipe per pass; when the operation permits, it may also be equal to the flow area of the heat transfer pipe per pass.

9. What are the main types of connection between tube and tube heat exchanger tube sheets and heat exchange tubes?

The connection between the tube-and-tube heat exchanger tube sheet and the heat exchange tube mainly includes welding, expansion joint, expansion welding and the like.

10. What is the applicable scope and requirements for the strength expansion joint between the heat transfer tube and the tube sheet of the shell-and-tube heat exchanger?

1). Scope of application:

a) the design pressure is less than or equal to 4 MPa;

b) the design temperature is less than or equal to 300 ° C;

c) There is no severe vibration during operation, no excessive temperature change and no obvious stress corrosion.

2). General requirements:

a) the hardness value of the heat exchange tube material generally must be lower than the hardness value of the tube sheet material;

b) When there is stress corrosion, the local annealing of the pipe end should not be used to reduce the hardness of the heat exchange tube.

11. Under what circumstances should the tube-and-tube heat exchanger be connected and welded?

The connection between the tube sheet and the heat exchange tube should be expanded and used in the following cases:

1). Where the sealing performance requirements are high;

2). Where the vibration or fatigue load is applied.

3). Where there is crevice corrosion;

12. Under what circumstances should the expansion tube of the shell-and-tube cylinder of the fixed tube-plate heat exchanger be set?

In the calculation of the fixed tube sheet, the axial stress of the shell, the axial stress of the heat exchange tube, and the pull-off force q between the heat exchange tube and the tube sheet are calculated according to various working conditions with temperature difference, one of which cannot be satisfied. In the case of strength (or stable) conditions, it is necessary to set the expansion joint. In the calculation of the fixed tube sheet strength check, when the tube sheet thickness is determined, when the expansion joint is not provided, sometimes the tube sheet strength is insufficient, and after the expansion joint is set, the tube is set. The thickness of the plate may meet the requirements. At this time, the expansion joint may also be provided to thin the tube sheet, but it is determined by comprehensive evaluation of material consumption, manufacturing difficulty, safety and economic effects.

13. The forms of baffles and support plates commonly used in shell-and-tube heat exchangers come in several forms as follows

The baffle plate and the support plate commonly used in the shell-and-tube heat exchanger are in the form of a bow baffle, a disc baffle, and a circular baffle. The bow baffle has a single bow, a double bow and a triple bow. Kind.

14, the shell-and-tube heat exchanger baffle layout must follow the principles including several aspects as follows

The principle of the baffles in the shell-and-tube heat exchanger is as follows:

1). The baffle arrangement must meet the requirements of the process optimization design conditions. Especially the form of the baffle, the spacing of the baffles, the position of the baffle near the inlet and outlet of the shell material must meet the process design conditions as much as possible.

2). In the case that the process design conditions are not specifically required, the baffles should generally be arranged at equal intervals, and the baffles at both ends of the bundle are as close as possible to the shell inlet and outlet.

3). When the shell side of the horizontal heat exchanger is a single-phase cleaning fluid, the baffle notch should be horizontally arranged up and down. If the gas contains a small amount of liquid, the liquid should be opened at the lower part of the baffle with the notch facing upward. If the liquid contains a small amount of gas, the vent should be opened at the height of the baffle with the notch facing down.

4). When the shell-side medium of the horizontal heat exchanger, condenser and reboiler is gas or liquid phase coexisting or the solid material is contained in the liquid, the baffle notch should be arranged vertically to the left and right, and open at the lower part of the baffle. Liquid port.

15. The small spacing value of the baffle of the shell-and-tube heat exchanger

Under special circumstances, the small spacing of the baffle of the shell-and-tube heat exchanger can be appropriately taken as small value; under normal circumstances, the small spacing of the baffle of the shell-and-tube heat exchanger can not be less than 50mm, and generally not less than the shell and tube round. One-fifth of the inner diameter of the cylinder.