What’s the difference between counter-flow and parallel-flow heat exchangers, and why and when the two typologies of thermal transfer are employed?

We’re talking about the flowing of fluids in heat exchangers, which means counter-flow and parallel-flow exchangers. The meaning is said in the word itself: in a counter-flow exchanger, the two fluids exchanging thermal energy each other flow within the exchanger in opposite directions.

This is easy to understand with plate heat exchangers, where counter-flow means that one fluid flows top-down and the other goes bottom-up. In one-pass shell and tube exchangers, there is a fluid flowing inside the shell and the other flowing inside the tubes in the opposite direction. A little more complicated to understand with multi-pass exchangers. In an air-cooled exchanger, a finned exchanger, finally, the air flows in the opposite direction compared to the water.

Parallel-flow is the exact contrary working principle, with both fluids flowing in the same direction.

The most relevant difference is that a counter-flow exchanger achieves higher thermal transfer rates, allowing to obtain output temperatures of one fluid very close to the inlet temperature of the other. And it works for both cooling and heating. In addition, in plate heat exchangers is it also possible to have temperature crossing, obtaining much higher efficiency and thermal transfer coefficients than comparable temperature crossing in other kind of exchangers.

Parallel-flow thermal transfer doesn’t therefore allow temperature crossing, because the temperatures of the two fluids will tend to be very close during the overall process, from start to finish. This is employed when a process requires heating, cooling or thermal transfer tasks that are less intense and invasive, and so more ‘soft’, involving a lower thermal impact upon the product which gets cooled or heated. For example, thermal transfers in the pharma sector or the food industry, where the product to be cooled/heated needs specific thermal schemes with no shocks, that otherwise could cause an alteration of the product itself compromising the quality of the final product.

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