Shell and Tube Heat Exchanger

Tags: 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
Typically are least expensive type of heat exchangers Less Gaskets than other types Tubes are individually replaceable	Shell may not be able to be easily cleaned or inspected.  Maximum temperature difference between fluids is approximately 200oF with out the inclusion of an expansion joint.

 

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).