Heat exchangers, baffles pressure drop

The baffle cut determines the fluid velocity between the baffle and the shell wall, and the baffle spacing determines the parallel and cross-flow velocities that affect heat transfer and pressure drop. Often the shell side of an exchanger is subject to low-pressure drop limitations, and the baffle patterns must be arranged to meet these specified conditions and at the same time provide maximum effectiveness for heat transfer. The plate material used for these supports and baffles should not be too thin and is usually minimum thick-... 

Test, P. L., AStudy of Heat Transfer and Pressure Drop Under Conditions of Laminar Plow in the Shell Side of Cross Baffled Heat Exchangers, Paper No. 57-HT-3, ASME-AlChE Heat Transfer Conference, ASME, New York, NY (1957). 

Twisted Tube construction (Figure 6.32) eliminates the need for baffles altogether, since the tubes can be oriented ( tuned ) to contact surrounding tubes periodically along the exchanger. The shell-side flow is longitudinal, and special correlations have been developed for both heat transfer and pressure drop on both sides of the surface. Laminar flow inside the tube is enhanced by the secondary flow induced by the twisted flow path. 

The closer the baffles are spaced the greater the turbulence and heat transfer. The pressure drop increases at close spacings but not in proportion to the increased transfer rate. However, spacings of less than 6 in. are seldom used because of the difficulty encountered in cleaning the outside of the tubes. Exchangers in use today have spacings of 2 to... 

There are many text books that describe the fundamental heat transfer relationships, but few discuss the complicated shell side characteristics. On the shell side of a shell and tube heat exchanger, the fluid flows across the outside of the tubes in complex patterns. Baffles are utilized to direct the fluid through the tube bundle and are designed and strategically placed to optimize heat transfer and minimize pressure drop. 

ESDU Engineering Sciences Data Unit Report 83038 Baffled shell and tube heat exchangers flow distribution, pressure drop and heat transfer on the shell side. (ESDU International, London 1983)... 

Baffle pitch, or distance between baffles, normally is 0.2-1.0 times the inside diameter of the shell. Both the heat transfer coefficient and the pressure drop depend on the baffle pitch, so that its selection is part of the optimization of the heat exchanger. The window of segmental baffles commonly is about 25%, but it also is a parameter in the thermal-hydraulic design of the equipment. 

The pressure drop due to friction when a fluid is flowing parallel to and outside of tubes can be calculated in the normal manner described in Chap. 14 by using a mean diameter equal to four times the hydraulic radius of the system and by including all frictional effects due to contraction and expansion. In heat exchangers, however, the fluid flow on the shell side is usually across the tubes, and many types and arrangements of baffles may be used. As a result, no single... 

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