Basic Guideline to Shell and Tube Heat Exchanger Design

Our basic guide covers detailed steps involved in designing shell and tube heat exchangers, the basics of heat transfer, key equations like Q = U × A × ΔT, and the importance of the log mean temperature difference. Learn about heat duty, determining heat exchanger size, and estimating the overall heat transfer coefficient. Explore the iterative design process influenced by parameters such as tube layout, shell diameter, baffle configuration, and fluid properties. Understand the significance of inlet conditions, energy balance, and the prevention of temperature crossover.  Know the considerations for multi-pass exchangers, correction factors, and optimizing designs for cost-efficiency. Whether you're designing a new heat exchanger or evaluating an existing one, this guide provides the knowledge and insights you need for effective heat exchanger design and operation. 

In this blog, we discuss some basic design aspects of heat exchangers, focusing primarily on shell and tube heat exchangers.

For all heat exchanger designs, the basics of heat transfer remain consistent, governed by the equation   Q = UAΔT. Here, Q represents the heat transferred between the two fluids, U is the overall heat transfer coefficient, A is the heat transfer area, and ΔT is the temperature difference. The subscript M denotes that the temperature difference may vary throughout the heat exchanger, requiring an estimation of the mean temperature difference.

 

Two types of calculations may be necessary for heat exchangers. When the heat duty and temperature difference are known, the size of the heat exchanger and the overall heat transfer coefficient must be determined. For an existing heat exchanger, you may need to re-estimate the overall heat transfer coefficient due to fouling, determining if it’s acceptable or requires cleaning.

 

Heat exchanger design is iterative due to the dependence of U and A on various parameters like shell and tube diameter, tube layout, length, baffle type, and the number of shell and tube passes. These parameters affect both the heat transfer area and the coefficient by altering the flow properties of the fluids.

 

Typically, designs start with given conditions: inlet temperatures, pressures, compositions, flow rates, and phase conditions of the streams, defining ΔT and heat duty. When selecting a heating or cooling utility, inlet and exit temperatures are chosen from standard tables, considering environmental restrictions.

 

Design steps begin with the allocation of streams. For a double pipe heat exchanger, one stream goes to the annulus and the other to the inner pipe. For shell and tube exchangers, one stream goes to the shell side and the other to the tube side, based on stream properties. After stream allocation, an overall energy balance calculates the heat duty and stream conditions.

 

It’s crucial to check for temperature crossover, ensuring compliance with the second law of thermodynamics. For counter-current heat exchangers, the exit temperature of the cold stream can be higher than that of the hot stream, unlike parallel flow.

 

Initial design assumptions include a heat transfer coefficient from standard values for the fluid pair. With this and the log mean temperature difference, a preliminary heat transfer area estimate is made. If the area exceeds around 8000 square feet, multiple heat exchangers are used. For multi-pass exchangers, a correction factor for the log mean temperature difference is applied, with a desirable value above 0.85.

 

The design then estimates individual heat transfer coefficients for the tube and shell sides. Tube velocity typically ranges from 1 to 10 feet per second, with 4 feet per second as a common choice. The required inside tube cross-sectional area is determined by the flow rate and desired fluid velocity, dictating tube size and number.

 

Shell side heat transfer coefficient calculations depend on shell dimensions and baffle configuration. Minimum baffle spacing is 20% of the shell diameter, with a 25% baffle cut common maximum of 45% baffle cut can be taken. After determining flow areas and velocities, shell side heat transfer coefficients are calculated.

 

Combining tube and shell side heat transfer coefficients with material properties and fouling factors provides the overall heat transfer coefficient. If this matches the initial assumption, the design is complete; otherwise, the process repeats iteratively.

 

In summary, shell and tube heat exchanger design involves fluid allocation, overall energy balance, checking for temperature crossover, and iterative calculations of heat transfer coefficients and areas. Future sessions will detail specific calculations for tube and shell side heat transfer coefficients, overall heat transfer coefficients, and the impact of design adjustments on delta P and cost optimization.

 

Thank you for reading!