An information we are often asked for is the size of hydraulic passages in plate heat exchangers, especially when working with dirty fluids or with particles in suspension.
The actual trend is to decrease the size of passages, in order to obtain a higher turbulence of fluids, even when working with low flow rates, therefore obtaining a more packed transfer network with multiple crossings and angles. The reason of it is to increase the thermal transfer rate, decreasing the required thermal exchange surface for a certain thermal duty, producing smaller and more efficient exchangers, with lower costs and more competitive.
But there are some limits, obviously, because when working with dirty waste water, a small passage can be clogged very quickly. And working in industrial applications, it happens very often, working with not very clean water, some times in open circuit, or even in closed circuits but polluted by the production processes that they serve.
Brazed plate exchangers have passage of approx. 2 mm, which is a very small section. But brazed plate exchangers are conceived ad maintenance-free exchangers, except for chemical washing, the so called cleaning-in-place, which is possible only if the exchanger is not completely clogged.
Plate heat exchangers that can be inspected have instead a variety of passage sections, based on the thermal design of the exchanger. For example, in our Tempco range of heat exchangers the average section starts from 2,5 mm and goes up to 4 mm, for applications with very dirty fluids or requiring very contained pressure drops. It is also true that pressure drops can be piloted choosing a proper Chevron angle of the plates, and having a larger passage expands the application range of the plate exchangers.
There is also a particular type of plate heat exchangers, called free-flow exchangers, which have no contact points and very large passages. These exchangers are employed in particular production processes, such as paper mill or the food production of fruit juices with big pulp particles in suspension, that could clog a traditional plate heat exchanger very quickly.
Free-flow heat exchangers have very large passages, that can measure 6 mm or even 12 mm. These exchangers don’t exist in all the possible sizes and design plates, and not having contact point they offer a lower resistance to pressure gaps compared to traditional exchangers. In addition, plates have to be constructed with higher thickness, involving higher costs.
How to select the kind of passage in a plate heat exchanger, at last? The selection is clearly based on the kind of application, which defines the type of plates, the size of passages and the depth of the plate corrugation.
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