first heat exchanger via a conduit 227 the by pass flow rejoins the flow through the heat exchanger 12 downstream of the valve 211 the temperature of the second heat transfer medium after mixing the by pass flow and the flow through the heat exchanger 12 corresponds to the situation when operating in the first mode of operation the flow of gas in this case corresponds to that shown in fig3 the heat exchanger 13 is now connected to the first heat transfer medium circuit i e the ht circuit 311 312 according to fig4 in order to compensate or offset the greater power obtained by the larger surface area of the heat exchanger 12 a part of the heat transfer medium of the lt circuit 301 302 may be guided to bypass the heat exchanger 12 through the conduit 227 the conduit 227 is advantageously provided with a closing or shut off valve 225 and control valve 226 by means of which the flow rate may be adjusted the arrangement is operated in the second mode of operation for the time required for cleaning of the heat exchanger 13 after which the mode of operation is changed again the foregoing disclosure is merely a certain possibility to implement the arrangement with heat transfer medium circuits and in addition to the previously disclosed the heat transfer medium circuits may be provided with separate loops operated by circulation pumps in this manner the inlet and outlet flows of the separate loops may be separately adjusted not shown a corresponding effect may also be achieved by guiding the gas.