What Strategies Prevent Flow Maldistribution in a PTFE Exchanger Operating at Low Shell-Side Flows?

In a PTFE exchanger, a low shell-side flow may simply trickle through the huge bypass gaps around the tube bundle, taking the path of least resistance and making little contact with the heat transfer surface. Avoiding this bypass is critical to achieve the maximum thermal performance from constrained flow situations.

Shell Side Flow Maldistribution Nature of
In a low flow PTFE exchanger with shell side flow maldistribution, the shell side fluid has several competing flow channels. The fluid is, ideally, driven across the tube bundle where the heat transfer takes place. In actuality, there are other low resistance paths:

Flow bypass between the shell wall and the bundle

Baffle-to-Shell Clearance Leakage

Short circuiting by tube-to-baffle gaps

Momentum helps to spread the fluid more evenly at higher flow rates. However, at low flow levels, the driving pressure is modest and the flow tends to focus on the easiest possible routes. Therefore, a large part of the fluid may bypass the tube bundle completely, leading to poor thermal utilisation and lower effectiveness of the exchanger.

Low Flow Maldistribution: Critical Mechanisms
The main problem at low shell-side velocities is the lack of pressure difference to overcome leakage resistances. The fluid preferentially passes through:

Shell to baffle distance

Peripheral tube bundle gaps

Zones of low flow resistance near intake areas

When the bypass flow dominates, the effective heat transfer area is greatly reduced even while the actual surface area of the heat exchanger remains similar.

Design Strategies for Shell-Side Flow
Baffle and Shell Clearance, Adjustment
The simplest way to control maldistribution is to reduce leakage routes by tightening mechanical tolerances. Standards like TEMA establish the acceptable clearances between:

Outer diameter, baffle, shell, wall inner

Outer limit of tube bundle and shell wall

Tube holes and baffle plates

Decreasing these clearances lowers bypass flow and drives more fluid through the tube bundle. However, the mechanical binding has to be avoided by considering the production tolerances and the thermal expansion.

Use of Sealing Strip
Sealing strips put along the periphery of the tube bundle are often employed to block bypass passages between the bundle and shell. These strips act as physical barriers to deflect shell side flow to the tube bundle region.

The design of sealing strips in PTFE exchangers must take into account the flexibility of the material and the thermal expansion properties of the material, to ensure that sealing properties are maintained across the operating temperatures.

Optimisation of Baffle Geometry
The baffle design is crucial for flow dispersion. Smaller cut sizes on the baffle increase the fraction of flow pressed across the tube bundle and not allowed to bypass axially. But if baffle cut is reduced too much, pressure drop will increase dramatically.

Full baffles can be employed (little or no cut, with controlled clearance gaps) to impose a primarily cross-flow arrangement for applications with very low flow rates.

Helical Baffle Arrangements
More sophisticated solutions are helical or spiral baffle systems. These features channel the shell-side fluid in a continuous helical pattern, enhancing plug-flow-like behaviour and minimising stationary zones.

Helical baffles are costlier and more technically complex, but they greatly lessen the susceptibility to leakage and maldistribution. This makes them particularly effective in low-flow PTFE systems where conventional segmental baffles have difficulty achieving an even distribution.

Design Rules for Low Shell-Side Flow Operation 
Some basic design ideas can be summarised to effectively mitigate low flow PTFE exchanger shell side flow maldistribution issues:

Minimise baffle to shell and bundle to shell clearances to acceptable limits

Seal peripheral bypass tubes with sealing strips

Increase cross-flow fraction across tube bundle by reducing baffle cut

Use of helical baffles to increase flow homogeneity at very low flow rates

Make sure inlet distribution is as uniform as possible to avoid localised jetting