Fluids with an average viscosity, meaning from 10 cP up to 100 cP, are typically hydraulic oils employed as lubricants. Using a tube exchanger with this kind of fluids, the risk is to work with a laminar flow, therefore with low thermal transfer rates and reduced efficiency. The most suitable solution is then again a plate heat exchanger, where the design of the channels can impress a turbulent regime also to low speed fluids, increasing the thermal transfer.
An alternative is a welded plate exchanger, or even a brazed plate one will be a valid solution as well in case of chemical incompatibility with the construction material of the gaskets. In addition, brazed plate exchangers offer the advantage of withstanding wide ranges of temperatures and working pressures, while ensuring high thermal efficiency.
High viscosity fluids are those having viscosity value higher than 100 cP: these involve almost every time a laminar flow, which requires a proper flow distribution of the fluid in order to obtain a useful thermal transfer. It can happen indeed that during the cooling cycle the fluid with higher viscosity in contact with the cold surfaces leads to an improper distribution, with some areas with slow speed that invalidate the performance of the exchanger. Plate heat exchangers can offer an ideal solution in this case as well, with the precaution to insert the viscuous fluid from the bottom side of the exchanger, in order to optimize its distribution.
If the fluid has extreme viscosity, higher than 50.000 cP, it is better to employ mono channel spiral exchangers (they afford a correct distribution which is impossible to achieve with multi channel types), that offer the advantage of a better resistance to pressure and don’t have the problem of chemical compatibility of gaskets with the process fluids. Also tube in tube exchangers can be a good solution, ideals for limited thermal capacities and lower loads, due to the fact that it’s a typology of exchangers with contained transfer coefficients and therefore requiring quite bulky dimensions.
A special study of the corrugation design of the plates can often help to adapt the use a plate heat exchanger in applications using dirty or high viscosity fluids.
In case of very dirty fluids, or clogging and not prone to be filtered (such as fluids full of particles in suspensions, fermentation broths, pulps, flocculus, mud, not limpid juices), it is possible to employ free-flow exchangers, or large channels exchangers, where plates have a peculiar pressing with no contact points, generating a unique large flowing channel.
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