The Ecodesign Directive 2009/125/EC stipulates minimum thermal efficiency values. These have a direct impact on the choice and design of heat recovery systems. A minimum thermal efficiency of 0.73 has been in place since 2018. On top of this, the pressure drop is also indirectly limited by the specific fan power (SFP); when converted, this is approx. max. 280 Pa.
These values cannot been achieved with crossflow heat exchangers in small ventilation units with a flow of up to approx. 3500 m³/h. However, counterflow heat exchangers can comply with the minimum thermal efficiency in this air flow rate range. Using larger counterflow heat exchangers for air flow rates of more than 5000 m³/h is currently not advisable for production reasons.
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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 pure crossflow heat exchanger. However, crossflow plate 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. It makes these units very complicated to design and produce.
The air enters into or exits from two mixing zones. The actual counterflow heat exchanger works between these or in the middle. 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.