Selection of heat exchanger type and fouling treatment method

1. When the temperature difference is large, floating head heat exchangers, U-shaped tube heat exchangers, stuffing box heat exchangers and sliding tube plate heat exchangers can be used. When the shell side is often mechanically cleaned, the structure with extractable tube bundles can be selected. For high temperature and high pressure, U-shaped tube heat exchanger can be used. When the shell side medium is flammable, explosive, toxic or volatile, and the working pressure and temperature are high, it is not suitable to use stuffing box heat exchanger. When the tube-side medium and the shell-side medium are not allowed to mix, a heat exchanger with a double-tube-sheet structure can be used.

2. Types and selection principles of shell and tube heat exchangers

(1) Shell and tube heat exchangers can be divided into the following main types:

① The tube sheets at both ends of the tube bundle of the tube-and-tube heat exchanger are integrated with the shell. The structure is simple, but it is only suitable for heat exchange operations when the temperature difference between the hot and cold fluids is not large and the shell side does not need mechanical cleaning. When the temperature difference is slightly large and the shell side pressure is not too high, an elastic compensation ring can be installed on the shell to reduce thermal stress.

② The tube sheet at one end of the tube bundle of the floating head heat exchanger can float freely, which completely eliminates thermal stress, and the whole tube bundle can be pulled out from the shell, which is convenient for mechanical cleaning and maintenance. Floating head heat exchangers are widely used, but the structure is more complicated and the cost is higher.

③ Each heat exchange tube of the tube-and-tube heat exchanger is bent into a U shape, and the two ends are respectively fixed on the upper and lower areas of the same tube plate. This kind of heat exchanger completely eliminates thermal stress, and its structure is simpler than that of the floating head type, but the tube side is not easy to clean. For the heat exchange of highly corrosive fluids in chemical production, non-metallic materials such as ceramics, glass, polytetrafluoroethylene, and graphite must be used to make shell-and-tube heat exchangers. This type of heat exchanger has poor heat transfer performance and is only used in occasions with low pressure, low vibration, and low temperature.

(2) For the hot and cold fluids for heat exchange, the flow channel shall be selected according to the following principles:

① Unclean and fouling fluids should go through the tube, because cleaning inside the tube is more convenient.

② Corrosive fluid should go through the tube side, so as to prevent the tube bundle and shell from being corroded at the same time.

③ The fluid with high pressure should go through the tube side to avoid the shell from being under pressure.

④ Saturated steam should go to the shell side, because the heat transfer coefficient of steam condensation has nothing to do with the flow rate, and the condensate is easy to discharge.

3. Precautions and working principle of fixed tube-sheet heat exchanger

Matters needing attention in the operation of fixed tube-sheet heat exchangers are as follows:

(1) After the heat exchanger is newly installed or overhauled, it must be tested before it can be used.

(2) When the heat exchanger is started, the cold flow should be passed first, and then the hot flow should be passed. When the heat exchanger is shut down, the heat flow should be stopped first, and then the cold flow should be stopped. To prevent leakage or damage caused by uneven thermal expansion and contraction.

(3) The fixed tube-sheet heat exchanger is not allowed to be heated in one direction, and the temperature difference between the two sides of the tube and shell of the floating heat exchanger is not allowed to be too large.

(4) During the start-up process, the exhaust valve should be kept open to discharge all the air, and should be closed after start-up.

(5) If hydrocarbons are used, the air in the heat exchanger shall be purged with inert gas before hydrocarbons are charged to avoid explosion.

(6) When shutting down for purging, the condensed water must be drained before steam induction, and the ventilation should be performed slowly to prevent water hammer. When ventilating on one side of the heat exchanger, the vent valve on the other side must be opened to avoid pressure damage. When closing the heat exchanger, the exhaust valve and drain valve should be opened to prevent vacuum damage to the equipment due to cooling.

(7) When using the air cooler, pay attention to the uniform flow of some parts to ensure the cooling effect.

(8) Always pay attention to monitoring to prevent leakage.

1. When the temperature difference is large, floating head heat exchangers, U-shaped tube heat exchangers, stuffing box heat exchangers and sliding tube plate heat exchangers can be used. When the shell side is often mechanically cleaned, the structure with extractable tube bundles can be selected. For high temperature and high pressure, U-shaped tube heat exchanger can be used. When the shell side medium is flammable, explosive, toxic or volatile, and the working pressure and temperature are high, it is not suitable to use stuffing box heat exchanger. When the tube-side medium and the shell-side medium are not allowed to mix, a heat exchanger with a double-tube-sheet structure can be used.

2. Types and selection principles of shell and tube heat exchangers

(1) Shell and tube heat exchangers can be divided into the following main types:

① The tube sheets at both ends of the tube bundle of the tube-and-tube heat exchanger are integrated with the shell. The structure is simple, but it is only suitable for heat exchange operations when the temperature difference between the hot and cold fluids is not large and the shell side does not need mechanical cleaning. When the temperature difference is slightly large and the shell side pressure is not too high, an elastic compensation ring can be installed on the shell to reduce thermal stress.

② The tube sheet at one end of the tube bundle of the floating head heat exchanger can float freely, which completely eliminates thermal stress, and the whole tube bundle can be pulled out from the shell, which is convenient for mechanical cleaning and maintenance. Floating head heat exchangers are widely used, but the structure is more complicated and the cost is higher.

③ Each heat exchange tube of the tube-and-tube heat exchanger is bent into a U shape, and the two ends are respectively fixed on the upper and lower areas of the same tube plate. This kind of heat exchanger completely eliminates thermal stress, and its structure is simpler than that of the floating head type, but the tube side is not easy to clean. For the heat exchange of highly corrosive fluids in chemical production, non-metallic materials such as ceramics, glass, polytetrafluoroethylene, and graphite must be used to make shell-and-tube heat exchangers. This type of heat exchanger has poor heat transfer performance and is only used in occasions with low pressure, low vibration, and low temperature.

(2) For the hot and cold fluids for heat exchange, the flow channel shall be selected according to the following principles:

① Unclean and fouling fluids should go through the tube, because cleaning inside the tube is more convenient.

② Corrosive fluid should go through the tube side, so as to prevent the tube bundle and shell from being corroded at the same time.

③ The fluid with high pressure should go through the tube side to avoid the shell from being under pressure.

④ Saturated steam should go to the shell side, because the heat transfer coefficient of steam condensation has nothing to do with the flow rate, and the condensate is easy to discharge.

3. Precautions and working principle of fixed tube-sheet heat exchanger

Matters needing attention in the operation of fixed tube-sheet heat exchangers are as follows:

(1) After the heat exchanger is newly installed or overhauled, it must be tested before it can be used.

(2) When the heat exchanger is started, the cold flow should be passed first, and then the hot flow should be passed. When the heat exchanger is shut down, the heat flow should be stopped first, and then the cold flow should be stopped. To prevent leakage or damage caused by uneven thermal expansion and contraction.

(3) The fixed tube-sheet heat exchanger is not allowed to be heated in one direction, and the temperature difference between the two sides of the tube and shell of the floating heat exchanger is not allowed to be too large.

(4) During the start-up process, the exhaust valve should be kept open to discharge all the air, and should be closed after start-up.

(5) If hydrocarbons are used, the air in the heat exchanger shall be purged with inert gas before hydrocarbons are charged to avoid explosion.

(6) When shutting down for purging, the condensed water must be drained before steam induction, and the ventilation should be performed slowly to prevent water hammer. When ventilating on one side of the heat exchanger, the vent valve on the other side must be opened to avoid pressure damage. When closing the heat exchanger, the exhaust valve and drain valve should be opened to prevent vacuum damage to the equipment due to cooling.

(7) When using the air cooler, pay attention to the uniform flow of some parts to ensure the cooling effect.

(8) Always pay attention to monitoring to prevent leakage.