Shell and Tube Heat Exchanger

A shell and tube heat exchanger is a type of heat exchanger where a process stream is used to heat or cool another process stream. It consists of two separate, non-moving parts. The shell is a pressure vessel which contains one process fluid. The secondary process stream flows through the tubes which are inserted inside the shell. This type of heat exchanger uses conduction between the fluids to transfer heat to the cooler fluid. By controlling one or both process streams, a precise temperature can be met.

Shell and Tube Heat Exchanger Types

Shell and Tube heat exchangers have multiple configurations. The Tubular Exchanger Manufacturers Association, TEMA, has developed a standard nomenclature to describe a heat exchanger using three separate letters. Additionally, there are different classifications of heat exchangers, according to TEMA. They are listed below from the lease to the most conservative:

TEMA Type C - General Service
TEMA Type B - Chemical Service
TEMA Type R - Refinery Service.

Applicable codes and standards are:

ASME Section VIII Boiler and Pressure Pressure Code.

Standards of Tubular Exchanger Manufacturers Association (TEMA Standards) - used to supplement the ASME BPV Code.

API 660 - Shell and Tube Heat Exchangers for General Refinery Services - used to supplement ASME BPV Code and TEMA Standards.

Notes

This article addresses the specification of and usage of shell and tube heat exchangers. It does not address specific design criteria of the heat exchanger, proper. Certain design criteria to mitigate tube vibration, vortex shedding, pressure fluctuations are discipline specific and and require detailed analysis of the unit.
Large Diameter and High Pressure heads are designated usually BxM (Bonnet (Integral Cover) with a Fixed Tubesheet - for a Bonnet)'
The most common S&T heat exchangers are designated as TEMA A_L. (Channel and Removable Cover with a Fixed Tubesheet Stationary Head)

Shell and Tube Heat Exchanger Advantages and Disadvantages
Advantages	Disadvantages
They provide efficient heat transfer due to the large surface area available for heat exchange. They can handle a wide range of temperatures and pressures. Suitable for various fluids, including liquids, gases, and even steam. They are available in different sizes and configurations to suit various industrial needs. Shell and tube heat exchangers are robust and can withstand high pressures and temperatures. The design allows for cleaning, inspection, and tube replacement, especially with removable tube bundles. Can be designed for specific requirements, such as counter-flow, cross-flow, or parallel-flow arrangements. Can handle multiple streams with different fluids (e.g., multi-pass design). For the level of heat transfer they provide, shell and tube heat exchangers are relatively compact.	Manufacturing and material costs can be high, especially for large or custom designs. The design and fabrication process can be complex compared to simpler heat exchangers (like plate heat exchangers). Despite accessibility, cleaning the inside of the tubes can be time-consuming, especially if fouling occurs frequently. Requires more installation space than some alternatives, such as plate heat exchangers, for the same heat transfer capacity. Can develop leaks between the shell and the tube side, especially under extreme operating conditions. Significant pressure drops may occur, especially in multi-pass configurations or with highly viscous fluids. Prone to vibration due to fluid flow, which can cause wear and tear over time, especially in long tubes. Depending on the materials used, the tubes and shell can be vulnerable to corrosion when exposed to aggressive fluids.

Shell and Tube Heat Exchanger Advantages and Disadvantages

Advantages

Disadvantages

They provide efficient heat transfer due to the large surface area available for heat exchange.
They can handle a wide range of temperatures and pressures.
Suitable for various fluids, including liquids, gases, and even steam.
They are available in different sizes and configurations to suit various industrial needs.
Shell and tube heat exchangers are robust and can withstand high pressures and temperatures.
The design allows for cleaning, inspection, and tube replacement, especially with removable tube bundles.
Can be designed for specific requirements, such as counter-flow, cross-flow, or parallel-flow arrangements.
Can handle multiple streams with different fluids (e.g., multi-pass design).
For the level of heat transfer they provide, shell and tube heat exchangers are relatively compact.

Manufacturing and material costs can be high, especially for large or custom designs.
The design and fabrication process can be complex compared to simpler heat exchangers (like plate heat exchangers).
Despite accessibility, cleaning the inside of the tubes can be time-consuming, especially if fouling occurs frequently.
Requires more installation space than some alternatives, such as plate heat exchangers, for the same heat transfer capacity.
Can develop leaks between the shell and the tube side, especially under extreme operating conditions.
Significant pressure drops may occur, especially in multi-pass configurations or with highly viscous fluids.
Prone to vibration due to fluid flow, which can cause wear and tear over time, especially in long tubes.
Depending on the materials used, the tubes and shell can be vulnerable to corrosion when exposed to aggressive fluids.

Floating Head

Advantages
- Ability to handle dirty fluids and high differential temperatures
- Both head and tubes can be cleaned.
- Individual tubes can be removed and replaced
Disadvantages
- Cost more than fixed tube heat exchangers
- More gaskets than fixed tube heat exchangers which can cause leakage.
Notes
- xTW (Cross Flow with a Pull Through Floating Head) are the least expensive but also have the highest possibility of leakage

U-Tube

Advantages
- Lower cost than a fixed tub or floating head type echanger
- Desisn inherently allows for thermal expansion
- Internal gasketed joint is eliminated.
- Tube bundle is removable and replaceable
Disadvantages
- Design does not allow for tubes to be mechanically cleaned
- The U Shaped tubes reduce the number of tubes that can be installed
- Individual tubes are not replaceable
Notes
- U-Tube heat exchangers are used for high differential temperatures
- Usually these exchangers are used for "clean" service

Fluid Location

Tubes

Fluids should be put in the tubes (rather than the shell) if the fluid is:

When pressures are above 300 psi, it is more economical to increase the wall thickness of the tubes than it is to increase the wall thickness of the shell.
The tubes have a higher pressure drop than the the shell.
Two Phase fluids or single phase gas
Fluid has a tendency to scale. This is because it is easier to mechanically clean tubes on the inside rather than the outside.

Shell

Fluids should be put in the shell if the process requires:

Low Pressure Drop
Viscous heat transfer fluids
Fluid is not sandy or fouling
Low pressures

Design Considerations

Shell and tube heat exchangers should be located to facilitate the ease of tube removal for cleaning and maintenance.

When the tube bundles are withdrawn, they should not project into an emergency escape route or any road with unrestricted vehicle access.

The spacing between heat exchanger shells must allow for sufficient, unobstructed clearance for access for the bundle withdrawal equipment and to permit access for shell flange gasket replacement (when applicable).

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