Regardless on the type of heat exchanger of choice, it is fundamental during the selection and engineering of it to always evaluate a proper fouling factor. Indeed, very few exchangers work in fact with completely clean fluids. While in reality working fluids always tend to soil in many ways the thermal transfer surfaces, with sediments, scaling and other kind of deposits that can seriously damage and cause corrosion on the exchanger if they get not cleaned.
Almost the totality of fluids employed in heat exchangers contains some amounts of substances in solution or suspended: salts, crystalline carbonates, sand particles and muds, but also substances generated within the fluids as a result of the chemical processes during the thermal cycle, chlorides, oxides, algae and bacteria. When settled, these substances create clogs that obstruct the normal fluid flow, and scaling that reduces the thermal conductivity of the materials, creating an increasing resistance to the thermal transfer. Gaskets as well are exposed to the mechanical and chemical wear caused by these substances.
This is something not to be underestimated at all, if sector’s surveys state that only in the USA an annual cost of 4 billion dollars is related to increased energy consumption and downtimes caused by the reduced efficiency of exchangers. In the long term, the permanence of sediments inside the exchangers leads to local effects of punching, pitting and corrosion: just think about the fact that plates in heat exchangers have an average thickness of 0,5-0,6 mm, sometimes even thinner, and therefore the risk is really concrete.
TYPICAL FOULING FACTORS FOR THE MOST COMMONLY USED FLUIDS
In order to avoid this risk, the right selection of materials in compliance with the working fluids, offering the proper chemical resistance to corrosion, and the right sizing of the exchangers are essential factors. Generally, it is used to estimate the size of a heat exchanger exceeding by 10% the size required by the thermal duty, in order to compensate the expected fouling factor.
Also the speed of the fluids is affecting the expected fouling factor of an exchanger: at this purpose, the fouling factor of plate heat exchangers is largely lower compared to tube heat exchangers, thanks to the higher turbulence regime of the fluids within plate heat exchangers. With this in mind, and also in addition to the fact that thermal transfer rates of plate heat exchangers are very different from those of a plate exchanger (approximately 6.000 W/m2K vs. 1.500 W/m2K), the sizing of a plate heat exchanger employs different parameters compared to those employed for the sizing of tube exchangers, where fouling factor is much higher due to the lower speed flow within the channels.
Having an exchanger that can be inspected is therefore an optimal solution in order to undergo descaling operations that can be done at the plant location itself (cleaning in place, or CIP). In presence of fiber solid suspended particles and fluids that settle down insoluble substances (for example calcium carbonates coming from thermal decomposition), the best solution is a spiral exchanger because it has single backlashing channels where the inversion of the flowing direction autonomously provides the removal of big particles of dirt and of scaling materials. It is also suitable to equip the exchanger with proper filters, cartridge filters or self-cleaning, and chemical treatment processes or using biocides, in order to purify fluids as much as possible from potential substances that in the long term can seriously damage and compromise the performances of the heat exchanger.
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