of the subject matter of the invention the heat exchanger is formed from a plurality of cross flow heat exchanger elements it being advantageous that four cross flow heat exchanger elements are arranged one above the other in an essentially vertically running air shaft in this way an adequate temperature reduction can be achieved with a relatively small cross section of the air shaft as a result of which the separation of condensible harmful substances is achieved in addition this offers the advantage that the elements can be removed individually which greatly facilitates manipulation for the purpose of cleaning cooling air drawn in from the environment is preferably used as a cooling medium which cooling air since it is not exposed to any pollution from harmful substances can subsequently be discharged again into the environment the washing device conveniently comprises a pressure line a spray system and a basin arranged below the heat exchanger for collecting the washing liquid the washing liquid fed by means of the pressure line is sprayed into the exhaust air flow through the spray system as a result of which solid and liquid constituents of the exhaust air flow combine with the washing liquid and are then fed to the basin a preferred further development of the washing device will be seen from the fact that it is constructed as a closed liquid circuit in which washing liquid is drawn off from the basin by means of a pump and is.