It's all in THE MIX

Gareth James presents the case for adding static mixer elements to shell and tube-type heat exchangers.

There is no doubt that heat transfer to and from viscous fluids, especially cooling duties, is an extremely difficult task when using conventional empty-tube heat exchangers.

In laminar flow, there is no natural convection to assist the rate of heat transfer. The only radial heat transfer mechanism is thermal conduction, but this is very slow in laminar flow. This means that large temperature gradients occur in empty-tube heat exchangers, often resulting in products of unacceptable, non-uniform quality.

With laminar flow in an empty tube, one thing is certain - a sub-layer will form at the tube wall, creating a barrier to heat transfer. This significantly reduces the heat transfer rate and automatically sets up a requirement for an extremely large surface area.

All of these problems can be overcome by using static mixer heat exchangers. Static mixer elements, type KM, (shown in the photo) can be used for a number of in-line mixing duties. These elements are inserted into the heat exchanger tubes to promote turbulence and to increase heat transfer coefficients.

The fluid simply passes through the tubes, where it is radially interchanged between the centre of the tubes and the walls. This continuous mixing along the length of the tubes ensures product homogeneity, and eliminates temperature and viscosity gradients.

In addition to vastly improving product quality, the introduction of static mixer elements achieves heat transfer rates that are three to ten times greater than those of conventional empty-tube heat exchangers. This gives the distinct advantage of using much smaller heat transfer areas. A graph showing the difference in Nusselt numbers (relating to convective heat loss) between static mixers and empty-tube heat exchangers for identical Reynolds numbers (indicating turbulence) is shown below.

Freezing and scorching

In the tubes of a static mixer heat exchanger, elements of fluid are continuously renewed at the tube wall. For cooling duties, this means that fluids can be cooled to near their freezing points without plugging the tubes - and for heating applications, the possibility of scorching at the tube walls is completely eliminated. Hence, extra cooling or heating capacity can be achieved - something which would not be possible in an empty-tube unit.

To optimise the performance of static mixer heat exchangers, specially profiled low-volume heads can be incorporated into the design to eliminate dead spots and recirculation in the head. They also ensure uniform distribution of the process fluid across the tubes in multi-tube laminar heat exchangers.

Static mixer elements can be either edge sealed or non-edge seal. In the latter, the elements are inserted into the pipe and fixed at the ends, or retained at one end only to be removable.

Edge-seal elements are physically bonded to the tube wall so that they act as internal fins. This greatly improves the heat transfer between the static mixer element and the tube wall - and since there are no gaps between the tube wall and the elements, edge-sealed elements are immune to dead spots, product hang-up and fouling. Edge-seal elements also provide much greater mechanical strength than non-edge-seal units.

Some of the more common applications for static mixer heat exchangers include man-made fibre processes which require precise handling of fluids that exhibit significant temperature sensitivity. These polymers have properties such as viscosity that are very temperature-dependant, and the fluids themselves are generally susceptible to thermal degradation or fouling. This makes temperature control a very critical requirement throughout the process and the piping. Static mixer heat exchangers have been utilised in these processes to maintain temperature uniformity and control the absolute polymer temperature.

Other applications include thermal homogenisation; heat removal or addition during exo- or endothermic reactions with viscous materials; and heating or cooling materials to their optimal processing temperature.

Adding static mixer elements to empty-tube heat exchangers is one of the most straightforward pieces of process engineering - and with such a variety of benefits across so many heat transfer applications, an addition that cannot be ignored.

Mr James is with Chemineer, an engineering consultancy based in Derby

The simple addition of static mixer elements to a shell-and-tube heat exchanger can have startling effects

Adding mixer elements increases heat loss for a given degree of turbulence

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