Difference pressures

On rotary heat exchangers a complete separation of the outdoor- and outgoing air is guaranteed only if the pressure difference is right. To prevent a cross flow of outgoing air into the outdoor air the employment of a purge sector is necessary. This prevents the outgoing air that is in the heat storing matrix is transported by co-rotation into the outdoor air and thereby contaminates it. A small amount of outdoor air flows through the purge sector into the outgoing air and purges out the outgoing air remaining in the rotor. The purging air amount is slightly more than the amount of air that would be transferred by co-rotation. The selection of the right pressure potentials is of great importance in order to ensure the function of the purge sector.

The purge sector fulfils its function only then when the pressure potential of the outdoor air is greater than that of the outgoing air.

difference pressures

Difference pressures

0-200 Pa
effect of the purge sector not guaranteed,
use rotor without purge sector

200-500 Pa
Standard purge sector
2x5 degrees required

500-800 Pa
Purge sector 2x2.5 degrees required

800 Pa and more
Purge sector installation should be omitted

purge and split air amounts

In the diagram above the purge and split air amounts are specified. The data refer to a purge sector 2 x 5 degrees and an inflow rate of 3.5 m/s.

Dust and odour transfer

The double purge sector prevents the cross flow of outgoing air into the outdoor air. Test have shown that with a functioning double purge sector dusts, gases and so forth are transferred at a rate of 0.0013 % from the outgoing air into the outdoor air. At this rate non-water-soluble odours such as toilet smells are transferred.

Water-soluble odours such as kitchen smells can be transferred in the sorptions regenerator depending on the water solubility and the temperature efficiency. In the condensation rotor water-soluble odours can be transferred only when the when dropping below the condensation point on the outgoing air side in the heat storing matrix.

Basically a condensation rotor should always be used for heat recovery in case the air used is contaminated with water-soluble odours. The ventilation system has to be separated from the rest of the system according to the valid regulations.