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

Related Vendors

Changing the shell type. If the pressure drop cannot be fully utilized with TEMA E shell, one should use TEMA F shell with vertical cut segmental baffle. This increases shell side flow velocity and, hence, pressure drop. However, for a pure counter current flow, TEMA F shell should be combined with two tube passes, which means increased tube side pressure drop, too.

Using shells in series. If the pressure drop cannot be fully utilized with a single shell, shells in series can be used. However, this increases tube side pressure drop as well and, therefore, can be done only when tube side pressure drop allows so. Further, it results into increased cost due to increased shells, channels/bonets, tube sheets, nozzles, flanges, etc.

Tube side pressure drop can be increased in following ways:

Increasing number of tube passes. If the tube side pressure drop in an exchanger with one tube pass cannot be fully utilized, the pressure drop can be increased by increasing number of tube passes which increases the tube flow velocity and, hence, pressure drop. However, with TEMA E shell, use of 2 or more tube passes lowers the EMTD which means higher required heat transfer area. Therefore, combined impact of increased heat transfer coefficient and reduced EMTD on required heat transfer area should be evaluated.

Reducing tube diameter. Reduction in tube diameter increases pressure drop as it increases tube velocity due to decreased flow area. However, lower tube diameter can create difficulty in mechanical cleaning of inside of tubes. It may also create flow induced vibration problem.

Reducing the shell diameter. Reduction in shell diameter increases tube velocity due to reduced flow area and, hence, increases tube side pressure drop. However, it also increases shell side pressure drop. Therefore, this option can be exercised only when shell side pressure drop also allows. Further, minimum shell inside diameter is generally limited to 6 inches.

Using shells in series. If the pressure drop remains unutilized with one shell, shells in series can be used. However, this also increases shell side pressure drop, and, therefore, can be done only when shell side pressure drop allows so. Further, it has cost impacts as described above for shell side pressure drop.

* The author is General Manager (Head of Department)-Process at Simon India

(ID:42937525)