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

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Managing the Pressure Drop

The following lines describe how to manage the pressure drop during thermal design of a shell and tube exchanger starting with the above configuration.

Pressure Drop Limiting Situation

When the pressure drop is a limiting factor, the objective of the thermal design of the exchanger should be to reduce the calculated pressure drop. Ultimately, the calculated pressure drop should be as close as possible to the allowable pressure drop without exceeding it.

Heat Exchangers

Exchange Through the Fin: Achieving Higher Efficiency in Heat Transfer

Shell Side

Shell side pressure drop can be reduced in following ways:

• Changing the baffle type from segmental to double segmental. This reduces the shell side flow velocity and, thereby, lowers the shell side pressure drop.

• Increasing the baffle spacing. This increases the cross flow area and, hence, decreases cross flow velocity resulting into decreased pressure drop. However, this can be done in a limited way only as maximum unsupported tube span should be limited to TEMA recommended value (refer Table 1).

• Increasing the baffle cut. Increase in baffle cut increases the window flow area and, therefore, reduces window velocity, thereby, reducing the pressure drop. This, too, has limited impact as only window pressure drop is affected and contribution of window pressure drop is generally small to total pressure drop. Moreover, it can be varied in a limited range only and generally does not exceed 35 per cent of shell inside diameter.

• Increasing shell diameter (thereby reducing tube length). This increases flow area, reduces flow velocity, and, hence, lowers pressure drop. However, this lowers tube side heat transfer coefficient as well because of lower tube velocity. Moreover, higher shell diameter means higher shell thickness and larger number of tubes, hence, higher material cost.

• Using no-tubes-in-window (NTIW) baffles. With this type of baffle, baffle spacing can be increased as much as required by providing sufficient number of intermediate support plates, thus, limiting the unsupported tube span within TEMA requirement. However, a large fraction of shell volume remains unoccupied by the tubes with this type of baffle which results into lower heat transfer area to volume ratio and, hence, higher cost.

• Changing the type of shell. Other aspects remaining same, TEMA J shell yields less pressure drop than TEMA E shell as shell side flow is divided and flow velocity becomes half.

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