Crossflow or counterflow?

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With the implementation of the Ecodesign Directive 2009/125/EC, minimum thermal efficiency values were required for the first time. These had a direct impact on the choice and design of heat recovery systems. In the meantime there are further standards and directives on this subject, whereby the European regulation EU 1253/2014 dominates. 

It stipulates a minimum thermal efficiency of 0.73 (previously 0.67) for plate heat exchangers as of 01.01.2018. On top of this, the SFP value also indirectly limits the pressure drop; when converted, this is approx. 340 Pa. This is considerably more restrictive than the maximum value of 480 Pa in standard EN 13053 (2017).
As of 2018, these new specifications must therefore be complied with, which is difficult to achieve with plate heat exchangers based on the cross-flow principle with low air outputs (approx. 1000 - 4000 m³/h). However, it is possible to resolve this with counterflows.

The NTU method is ideal for calculating the impact of the flow type in the plate heat exchanger (crossflow or counterflow).
To put it simply, using this method, you can see that a pure crossflow heat exchanger would need to be 1.57 times larger than a counterflow heat exchanger to achieve the same thermal efficiency of 0.73. This confirms the old wisdom that crossflow heat exchangers require long edge lengths and minimal plate spacing to achieve a high thermal efficiency. In practice, this means that a crossflow heat exchanger with the minimum permissible plate spacing generally requires an edge length of at least 0.8 m to achieve the necessary thermal efficiency of 0.73. With the usual widths for ventilation units and permissible pressure drops, this results in an air flow rate of approx. 3500 m³/h. With anything less than this, it is not possible to create an economical solution with a plate heat exchanger using the crossflow principle alone. However, tried-and-tested crossflow heat exchangers do bring major advantages for greater air flow rates of up to 100,000 m³/h.

So if counterflow heat exchangers are so powerful, why are they not used in general? This is down to the difficult flow control at the inlet and outlet, which makes the units complicated to design and produce. As a result, the counterflow heat exchanger is made up of two mixing zones (in and out) with the actual counterflow heat exchanger itself in-between. In the mixing zones, the flow corresponds more or less to the crossflow, whereas pure counterflow prevails in the middle part. The length of this middle part determines the output first and foremost. This means that it is always possible to achieve the required thermal efficiency of 0.73, but this comes at a price. Using a counterflow heat exchanger is therefore only deemed to be economical when a crossflow heat exchanger with the same output cannot be used. In practice, this means that an air flow rate of approx. 1000 to 4000 m³/h can be covered, with a bit of a grey zone.

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Thomas Richter