Lesson 9 - Heat Exchangers SAQ

*/** What does the term ‘multi-pass exchanger’ mean? How are multiple passes achieved in the exchangers specified below?

•  Plate heat exchangers

•  Shell and tube heat exchangers

How does the performance of a multi-pass exchanger compare with that of a single-pass exchanger?

Response:

A multi-pass heat exchanger is an exchanger where either one, or both, fluid(s) will pass through the length of the exchanger more than once.

•  In plate heat exchangers, multiple passes are achieved by blocking first the inlet distribution channel, and then the outlet, at one or multiple points inside the exchanger. The distribution and flow channel gaskets of the plate just before where the distribution channel is blocked are removed. This means that all the fluid in the distribution channel is forced to flow over the heat transfer surface creating an additional pass. The same method is used for either of the fluids in the exchanger.

•  In shell and tube heat exchangers, multi-pass flow is achieved in a number of ways:

•  In fixed tubesheet or floating head exchangers, the tube side fluid is directed back through the shell using baffles that are positioned in the head ends of the exchanger.

•  By the nature of their design, U-tube exchangers are multi-pass, as the shape of the tube bundle forces the tube side fluid to pass back through the exchanger.

•  On the shell side, multiple passes are achieved by installing a horizontal baffle in the shell of the exchanger. This forces the shell side fluid to run almost the full length of the shell, multiple times.

Compared with single-pass exchangers, we can make the following observations about the performance of multi-pass heat exchangers:

•  They have better heat transfer because the narrower flow path increases fluid velocity and turbulence.

•  Because of the increase in velocity, they have a higher pressure drop through the exchanger.

•  They have the potential to deliver a greater change in temperature within the multi-pass fluid.