Counterflow vs. crossflow heat exchangers
What do you need to know about recovery plate heat exchangers? On January 1, 2018, EU Directive No. 1253 entered into force, increasing the requirements for the minimum heat efficiency of plate heat exchangers to 73% (previously 67%). The SFP value must also correspond to a pressure drop of 340 Pa. This value is also stricter than the 480 Pa of the preceding standard (EN13053, 2017).
Crossflow heat exchangers have difficulties achieving these values, especially with flow rates of 1,000 to 4,000 m3/h. This problem can be easily solved by counterflow heat exchangers.
If the same plate distance is used, a counterflow heat exchanger can be substantially smaller than the crossflow equivalent. Considering the values of the permitted pressure drop, this means that the side of a crossflow heat exchanger must be at least 800 mm in length. Such solution corresponds to a flow rate of approx. 3,500 m3/h.
We are often asked why the heat-transfer area is not increased by also putting additional waves in the crossflow zone. The reason is that the air speed at the heat exchanger supply is high, whereas its speed at the exhaust is low. Thus such waves would only have a negligible effect on efficiency, whereas, on the other hand, the pressure drop would sharply increase due to high air speed and turbulence.
Therefore it is obvious that the crossflow part should be completely smooth. But this solution is not perfect either, because air must also be directed to the heat exchanger’s peripheries. Plate stiffness is similarly vital. If it were completely smooth, it would suffer from high pressure differences in the heat exchanger zone, inflating the supply part while closing the exhaust part. This would all be followed by a dramatic increase in pressure drop. See the illustration with the speed outlines.
Advantages and main applications of crossflow heat exchangers
The main applications for crossflow heat exchangers are flow rates above 10,000 m3/h.
Crossflow heat exchanger advantages
Which heat exchanger type to choose?
In our opinion, the future belongs to counterflow heat exchangers, especially in recovery units with flow rates up to 5,000 m3/h.
Constantly increasing energy costs means that efficiency is essential. With higher efficiency and greater savings in heat energy, we show respect to the environment.
The former disadvantages of counterflow heat exchangers, e.g. their price, are decreasing over time due to the optimization of their production process. Counterflow heat exchangers can also be used for higher flow rates as a part of COMBI solutions.
