Helical tube support baffles

Tube bundles with helical baffles are now produced by both ABB Lummus Heat Transfer and Koch-Glitsch. HTRI now has a rating for this type of design option in their software. 

Although the traditional shell-and-tube heat exchangers have segmental tube support baffles at 90° to the line of the tubes, the helically baffled tube bundle has tube support baffles sloped so as to induce a helical flow pattern on the shell side (see Fig. 27.6). 

The concept is to ensure effective bundle penetration and avoid dead zones so as to leave no stagnant areas where shell-side fouling could accumulate. Also, helical baffle plates can be spaced to reduce unsupported tube spans (without affecting pressure drop or heat-transfer characteristics) to mitigate flow-induced vibration. The designers also claim that the helicoidal baffle arrangement permits increased heat transfer. 

We have heard of helical baffle exchangers being used in oil refineries to preheat delayed coker feed with vacuum resid on the shell side (with the helical baffles). In this service, the helical baffles seem to work well, resulting in less fouling, slower rate of increase in 

Another example from oil refineries is in crude preheat service with vacuum resid again on the shell side (with the helical baffles). Once again, similar results are seen as described above, with less fouling, reduced rate of increase in pressure drop, and better maintenance of heat-transfer coefficient as compared to the conventional shell-and-tube exchanger design. Our more direct experience of this comes from current practice in the United States, but we have also seen evidence of similar applications in Australia, as discussed in a recent article on the subject of crude preheat exchanger train redesign.  

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Smooth high alloy tubes Low-finned tubes Sintered metal tubes Spiral heat exchanger Tube inserts Twisted tubes Helical tube support baffles... 

Figure 27.6 Schematic arrangement showing helical tube support baffles in relation to tube alignment.

For an equivalent shell-side AP, the helical baffle will be less fouling and will provide less film resistance, and less fouling resistance than will a conventional vertical tube support baffle. 

The only one of the above that is in widespread use, and used with success in the correct application, is the helical baffle exchanger (see Figure 27.6). The tube support baffles are set at an angle to induce a helical flow pattern on the shell side. The idea is to avoid the dead zones that occur in a conventional vertical tube support baffle. Thus, AP and fouling are both reduced on the shell side. 

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