Selection and Design Calculation Steps of the Plate Heat Exchanger

The selection and design calculation steps of the plate heat exchanger are basically the same, and the basic steps are as follows:

The selection and design calculation steps of the shell-and-tube heat exchanger are basically consistent, and the basic steps are as follows:

1. Trial calculation and preliminary selection of equipment specifications

(1) Calculate the heat transfer rate according to the heat transfer task;

(2) Calculate the temperature difference for heat transfer, and determine the number of shell passes or adjust the final temperature of the heating or cooling medium based on the principle that the temperature difference correction factor should not be less than 0.8;

(3) Select the fluid channels in the heat exchanger;

(4) Determine the flow path of the fluid in the heat exchanger.

(5) Calculate the heat load Q according to the heat transfer task.

(6) Determine the fluid temperatures at both ends of the heat exchanger, select the type of shell-and-tube heat exchanger; calculate the qualitative temperature, and determine the properties of the fluid at the qualitative temperature.

(7) Calculate the mean temperature difference, and decide the number of shell passes based on the principle that the temperature correction factor should not be less than 0.8.

(8) Select the total heat transfer coefficient K value based on the empirical range of the overall heat transfer coefficient or according to actual production conditions.

(9) Estimate the heat transfer area according to the basic heat transfer equation, and determine the basic dimensions of the heat exchanger or select the specifications of the heat exchanger according to the series standard;

(10) Select the fluid velocity, and determine the number of tube passes and baffle spacing of the heat exchanger.

2. Calculate the pressure drop in the tube and shell passes. Based on the initially determined equipment specifications, calculate the fluid velocity and pressure drop in the tube and shell passes. Check whether the calculation results are reasonable or meet process requirements. If the pressure drop does not meet the requirements, adjust the velocity, then determine the number of tube passes or baffle spacing, or select another specification of equipment, and recalculate the pressure drop until the requirements are met.

3. Calculate the heat transfer coefficient and verify the heat transfer area. Calculate the convective heat transfer coefficients of the tube and shell passes, determine the fouling resistance, calculate the heat transfer coefficient and the required heat transfer area. Generally, the actual heat transfer area of the heat exchanger is selected to be 10% to 25% larger than the calculated required heat transfer area, if K'/K = 1.15 to 1.25; otherwise, set another overall heat transfer coefficient, select another heat exchanger, and return to the earliest step to re-verify the calculation.

Usually, when selecting or designing a heat exchanger, other issues should be considered after meeting the heat transfer requirements. These factors often conflict with each other. For example, if the designed heat exchanger has a larger overall heat transfer coefficient, it will lead to an increased pressure drop (resistance) of the fluid passing through the heat exchanger, thereby increasing power costs; increasing the heat exchanger surface area may reduce the overall heat transfer coefficient and pressure drop, but it will be limited by the allowable installation size of the heat exchanger and also increase the cost of the heat exchanger.