This becomes particularly clear in the plate heat exchanger, which is acted upon by both air streams and where two different pressures prevail at one location, separated only by a thin plate. The difference between these two pressures is called the differential pressure; it influences a possible leakage and may deform the plate due to the resulting surface force. This results in a change of the plate spacing and thus the pressure loss.
Plate heat exchangers in ventilation technology are usually made of thin aluminium plates with a thickness of only 0.05-0.15 mm. A special, manufacturer-specific
profiling ensures a certain stability, but this cannot prevent the plates from deforming at higher differential pressures. This reduces the cross-section of the
extract air and increases the pressure loss. At the same time, the cross-section on the fresh air is increased; the pressure loss decreases.
The change in pressure loss at a given differential pressure depends on the relative deformation and therefore on the plate spacing. The smaller the plate spacing, the more a deformation affects the pressure loss and vice versa. This insight is important for high-performance applications where small plate spacings are necessary. Therefore especially counter-flow heat exchangers should use stable plates.
The previous considerations assume that the plates deform elastically in the way of a metal spring. It is different when the deformation becomes too large at high differential pressure and plastic changes occur. If the differential pressure now decreases (or the fans are switched off), the deformation still remains: the heat exchanger is defective.
In order to specify the permissible differential pressure for a certain heat exchanger, the manufacturer must therefore observe two criteria:
- Within the permissible limit value, no plastic deformations are allowed to occur; a safety margin is necessary.
- For elastic deformation, it is necessary to estimate or measure the increase in pressure loss resulting from the differential pressure. The user can then decide on the application himself with the corresponding documentation.
Planning and execution of heat recovery requires documents dealing with the effects of differential pressure. In order
to be flexible in planning and execution, the permissible differential pressures should be at least 1500 Pa, or even better 2000 Pa.