Abstract: The present arrangement provides for having the plenum divided into at least two sections. One section dumps quench gas from a first direction into the hot zone chamber during a first period and the other section dumps quench gas into the hot zone from an opposite direction during a second period whereby the cooling is relatively uniform. During the quenching operation the quenching gas is drawn from the hot zone chamber through a heat exchange and is returned therefrom to a loop through the hot zone chamber in touch with the workpiece. Direction flow gates control the alternate use of the two plenum.