Rotary heat exchangers are an example of regenerative energy recovery systems. The supply and expelled airflows are not hermetically separated.
As the wheel rotates, small amounts of exhaust air can be returned to the supply airstream. Most rotors are used without purge sectors because this slight mixing of exhaust air into the supply air is permitted.
However, if the supply air must be completely free from exhaust air, a purge sector can be used. This prevents exhaust air still in the rotor matrix from being carried round to the supply airstream. Some of the incoming air from outside is forwarded to the exhaust air side, where it rinses out any used air remaining in the rotor matrix. Consequently, only fresh air remains in the matrix as it is rotated into the supply airstream.
Purge Sectors Prevent Mixing of Airflows
The sector is only effective, however, if the correct pressure potential is present and the correct fan configuration is used. Most rotors operate without purge sectors since a small mixing of exhaust air into the supply air is permissible.
More detailed information on purge chamber: