line 76 to the heat exchanger 18 which forms a heat accumulator heat exchanger 18 provides for the preheating of the air being circulated and a counterflow heat exchange whenever its temperature is under that of the exhaust air the remaining portion of the exhaust from the turbine exhaust line 74 may be directed to the atmosphere through the discharge line 72 if desired as long as heat accumulator 18 is preheated by exhaust air the pilot valve 22 switches the working air current to bypass line 20 in order to prevent effective heat from being withdrawn from the working air during the preheating of the heat accumulator 18 after the exhaust air has been disconnected and valve 22 has been switched heat accumulator 18 is heated further by the hotter working air and serves to compensate for any brief temperature fluctuations of the working air current depending upon the heat capacity in certain other operating phases where the exhaust air is disconnected from heat accumulator 18 and the accumulator has already been heated by working air above the preheating temperature it may also be advisable to conduct the working air current by means of pilot valve 22 bridging heat accumulator 18 over bypass 20 this will be done when the temperature of.