Rotary heat exchangers are an example of regenerative energy recovery systems. The supply and expelled airflows are not hermetically separated.
And as the wheel rotates, small amounts of exhaust air can be returned to the supply airstream. Most rotors are used without purge chambers 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 chamber 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.