Shell and tube heat exchangers are widely used in many industries to exchange heat between two fluids, one internal and the other external to a bundle of tubes. They are highly valued for their efficiency, durability, and flexibility and are used in almost all industries, including petrochemicals, power generation, food processing, HVAC, etc.
What Is a Shell and Tube Heat Exchanger?
A shell and tube heat exchanger comprises several tubes placed in a cylindrical casing. One fluid moves through the tubes while the other circulates outside the tubes, known as the tube and shell side respectively. These fluids flow in circuits when heat from the hot fluid is used to heat or cool the other fluid, depending on the system’s needs. These exchangers are suitable for high pressure applications and can be ordered to meet operational conditions and the type of fluids, pressures, and temperatures needed.
Types of Shell and Tube Heat Exchangers
Fixed Tube-Sheet Heat Exchanger
In a fixed tube-sheet exchanger, the tube sheets are fastened to the shell, and there is no flexibility at all. This type is cheaper and appropriate for applications where the variation in temperature between the shell and the tube side is negligible. However, they cannot be bent, so it is difficult to negotiate thermal expansion and contraction.
Floating Head Heat Exchanger
In floating head exchangers, one end of the tube is free to move, referred to as the floating head. This design is applicable when there is a large temperature variation between the fluids since it has low thermal stress on the tubes and easy accessibility for cleaning and maintenance.
U-Tube Heat Exchanger
U-tube heat exchangers are used, and each tube is bent into the shape of the letter U to expand and contract freely. The U-tube design has minimum expansion problems and is typically used in systems where temperature fluctuations are common.
Kettle-Type Heat Exchanger
This type is intended for use in applications where phase separation of vapour and liquid phases is necessary. The shell side has a large-volume chamber for vapour-liquid disengagement, employed in steam generation and reboiler services.
The Heat Exchange Process
Fluid Flow Arrangement
The fluids circulate through the shell and tubes in either parallel or counterflow. In parallel flow, both fluids flow in and out of a given system from the same direction, while in counterflow, the fluids flow in opposite directions. Counterflow arrangements are generally more efficient because they offer a wider range of temperature differences between the two fluids.
Heat Transfer Mechanism
One fluid passes through the tubes while the other flows around them in a structure called the shell, through which heat transfer occurs by conduction and convection between the two fluids. Heat transfer can also vary depending on the temperature difference between two objects, the total surface area of the object, and the type of materials used.
Thermal Expansion Management
Fluctuations in the fluids’ temperatures can result in the expansion or contraction of tubes, possibly inducing actual stress or damage. Designs such as the floating head and U-tube exchangers counter this; these allow some tube movement to offset the structural stress.
Conclusion
Shell and tube heat exchangers are widely used in many industries because of their flexibility, effectiveness and ability to work under high pressure. Fixed tube-sheet, floating head, U-tube, and kettle types have special characteristics to meet specific operating conditions so it is important to decide which type is best in terms of temperature range, types of fluids to be used, and ease of maintenance. Despite their heavier construction and their capacity to support main operations, shell and tube heat exchangers still form an integral part in industrial heat exchange.
