For shell and tube heat exchangers, why is the temperature difference at the high temperature end not lower than 20°C?

For shell and tube heat exchangers, why is the temperature difference at the high temperature end not lower than 20°C?

Process conditions for the selection of shell and tube heat exchangers mainly include three aspects, namely the selection of temperature, pressure drop, and fluid space. These three aspects are described below.

1. Temperature

The outlet temperature of the cooling water should not be higher than 60°C to avoid serious scaling.

The temperature difference at the high temperature end should not be less than 20°C, and the temperature difference at the low temperature end should not be less than 5°C. When exchanging heat between two process fluids, the temperature difference at the low temperature end should not be less than 20°C.

When using multi-tube, single-shell shell-and-tube heat exchangers and using water as the coolant, the outlet temperature of the coolant should not be higher than the outlet temperature of the process stream.

When cooling or condensing the process stream, the inlet temperature of the coolant should be higher than the freezing point of the easy-freezing components in the process fluid, generally higher than 5°C.

When cooling the reactants, in order to control the reaction, the temperature difference between the reaction fluid and the coolant should be maintained not lower than 10°C. When condensing the process fluid with inert gas, the outlet temperature of the coolant should be lower than the dew point of the process fluid, generally 5°C lower.

The design temperature of the heat exchanger should be higher than the maximum operating temperature, generally 15°C higher.

2. Pressure drop

Increasing the flow rate of the process fluid can increase the heat transfer film coefficient, thereby increasing the overall heat transfer coefficient and making the heat exchanger compact, but increasing the flow rate will increase the pressure drop of the heat exchanger, which will cause abrasion and vibration of the heat exchanger Increased damage, etc. At the same time, the increase in pressure drop increases the power consumption of the heat exchanger during operation.

3. Selection of fluid space

For a heat exchanger to operate properly and efficiently, the flow space must be carefully selected.

(1) Temperature High-temperature fluid generally goes through the tube, because high temperature will reduce the allowable stress of the material, and the high-temperature fluid can save the insulation layer and reduce the thickness of the shell. Sometimes in order to facilitate the heat dissipation of the high-temperature fluid, the high-temperature fluid can also go Shell side, but in order to ensure the safety of operators, an insulation layer is required.

(2) Pressure Fluid with higher pressure goes through the tube side, which can reduce the thickness of the shell.

(3) Viscosity.

(4) Corrosiveness Strongly corrosive fluids should go through the tube side to save corrosion-resistant materials.

(5) Pressure drop.

(6) Cleanliness Dirty and easily scaled fluids should go through the tube side to facilitate cleaning and control scaling. If the shell side must be used, it should be arranged in a square, and a detachable (floating head type, stuffing box type, U-shaped tube type) heat exchanger should be used.

(7) Flow rate.

(8) Heat transfer film coefficient.