The operation is controlled through the sequence programming by the energy saving system. When the dewpoint reaches at the specified temperature, a signal is fed back to step the reactivating process, and, several minutes before the dewpoint of outlet dry air becomes higher than the specified temperature, a signal is fed back again to restart the reactivation process. thus, the reactivation process suspending system considerable saves the purge air consumption incomparison with the usual continuous reactivating system.