Number of tube passes in Shell and Tube Heat Exchanger

Number of tube passes in Shell and Tube Heat Exchanger

A pass is when liquid flows all the way across from one end to the other of the exchanger. We will count shell passes and tube passes.

•  An exchanger with one shell pass and two tube passes is a 1-2 exchanger. Almost always, the tube passes will be in multiples of two (1-2, 1-4, 2-4, etc.)

• Odd numbers of tube passes have more complicated mechanical stresses, etc. An exception: 1-1 exchangers are sometimes used for vaporizers and condensers.

A large number of tube passes are used to increase the tube side fluid velocity and heat transfer coefficient and minimize fouling.

 This can only be done when there is enough pumping power since the increased velocity and additional turns increases the pressure drop significantly.

The number of tube passes depends on the available pressure drop.

•  Higher velocities in the tube result in higher heat transfer coefficients,at the expense of increased pressure drop.

 Therefore, if a higher pressure drop is acceptable, it is desirable to have fewer but longer tubes (reduced flow area and increased flow length).

•  Long tubes are accommodated in a short shell exchanger by multiple tube passes.

 The number of tube passes in a shell generally range from 1 to 10

• The standard design has one, two, or four tube passes.
•  An odd number of passes is uncommon and may result in mechanical and thermal problems in fabrication and operation.