I’ve been working on a shell and tube heat exchanger design for a university project and decided to put some thoughts down about baffles. I find the mechanisms behind heat exchangers quite interesting, so I might flow this up with a future post on heat exchanger design.
The baffles provide two main functions: they support the tubes and increase the heat transfer coefficient. Supporting the tubes reduces flow induced vibrations that are caused by eddies, it also helps in the positioning of the tubes when assembling and aids in maintenance when removing the tube bundles. The secondary objective of baffles is to increase the heat transfer coefficient. This is achieved by two mechanisms: increase in the cross flow velocity by guiding flow back and forth over the tubes, and by inducing turbulence through directed flow at these greater velocities.
The spacing between baffles is heavily dependant on the application of the heat exchanger. Some key things I picked up: When the allowable pressure drop is small, the baffle spacing and cut size should be large. The window flow velocity can be larger or smaller than for cross flow depending on the spacing and cut. Unequal baffle spacing requires a correction factor.
The most common baffle shape is a single segment. In order to ensure that adjacent baffle overlap is at least one full tube row, the segment must be less than half the diameter. Usually the maximum cut size will be used as this decreases pressure drop.
Check out the references below or have a look at Wolverine: Engineering Data Handbook III for an in-depth look into single-phase shell-side flows and heat transfer.
Edwards, J. (2008, Aug). Design and Rating of Shell and Tube Heat Exchangers. Retrieved May 2012, from Chemstations: www.chemstation.com/content/documents/technical_articles/shell.pdf
Wolverine. (n.d.). Construction of a Shell and Tube Heat Exchanger. Retrieved June 16, 2012, from wlv.com: http://www.wlv.com/products/databook/ch1_4.pdf
Wolverine. (n.d.). Single-phase Shell-side flows and heat transfer. Retrieved June 16, 2012, from wlv.com: http://www.wlv.com/products/databook/db3/data/db3ch3.pdf