Managing Pressure Drop in the Design of Shell and Tube Heat Exchangers

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DÜPERTHAL Sicherheitstechnik GmbH & Co. KG

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• Further pressure drop reduction can be achieved in TEMA H shell as it has double split flow and reduced velocity. Pressure drop is least in TEMA X shell which has cross flow, largest flow area and least velocity.

• Increasing the tube pitch ratio. Increase in tube pitch ratio reduces the cross flow velocity and, thereby, reduces the pressure drop. However, tube pitch ratio is generally 1.25, 1.33 or 1.50 and, therefore, can be varied in a limited way only.

• Increasing the nozzle size. If the pressure drop across the nozzles is excessive relative to total pressure drop, the nozzle size can be increased reasonably to lower the pressure drop.

• Using the shells in parallel. Multiple shells can be used in parallel so that total shell side flow is split and flow velocity is reduced. Consequently, pressure drop is reduced. However, it increases the cost due to increase in number of shells, tube sheets, channels/ bonets, nozzles, flanges, etc.

Tube Side

Tube side pressure drop can be lowered in following ways:

• Increasing the shell diameter. Increasing the shell diameter increases tube flow area due to increased number of tubes and, thereby, reduces tube flow velocity and, hence, reduces tube side pressure drop. Further, it also means reduced tube length which, too, leads to reduced pressure drop.

• Increasing tube diameter. Increase in tube diameter reduces tube velocity and, thereby, reduces pressure drop. However, tube diameters are standardized and standard outside diameters are limited as given in table 1. Further, tube outer diameter more than 1.0 inch is generally not desirable as higher tube diameter means higher shell diameter to accommodate required number of tubes due to increased tube pitch which, in turn, means higher cost.

• Increasing the nozzle size. If nozzles are too small in diameter, their diameter can be increased reasonably to lower the pressure drop.

• Using the shells in parallel. Multiple shells can be used in parallel so that total tube side flow is split and flow velocity is reduced. Consequently, pressure drop is reduced. However, it increases the cost due to the reasons as mentioned for shell side pressure drop.

Pressure Drop Surplus Situation

When the pressure drop is surplus, the objective of the thermal design of the exchanger should be to utilize the available pressure drop fully, i.e. increase the pressure drop till it reaches as near as possible the allowable pressure drop.

Shell Side

Shell side pressure drop can be increased in following ways:

• Reducing the shell diameter. This increases shell side flow velocity and, thereby, increases the shell side pressure drop. However, minimum shell inside diameter is generally limited to 6 inches. Further, it also leads to increased tube side pressure drop. Therefore, it can be done only if tube side pressure drop also allows.

• Reducing the baffle spacing. Reduction in baffle spacing increases the cross flow velocity and, therefore, increases the pressure drop. However, minimum baffle spacing is generally limited to one fifth of the shell inside diameter or 2", whichever is larger. Moreover, reduction in baffle spacing leads to increased fraction of shell side flow by-passing the tube bundle, thereby, lowering the heat transfer. Therefore, it can be done in a limited way only.

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