PURPOSE: To permit the control of temperature of process gas, going out of a heat exchanger, by a method wherein the flow rate of the process gas, flowing through the inner bypass tube in the heat exchanger and the heat transfer tubes of the heat exchanger, is regulated in accordance with the operating load of the device.

CONSTITUTION: When the operating load of a device is low and the heat transfer area of a heat exchanger becomes excessive, the temperature of process gas, going out of a heat transfer tube 5, becomes lower than a predetermined temperature. In this case, the window of an inner tube 8 is opened to increase the flow rate of high-temperature process gas, passing through the inner bypass tube 2, and maintain the temperature of the process gas, going out of a gas outlet nozzle, at a predetermined temperature. When the operating load of the device is 100%, the inner tube 8 is pivoted to close the outlet port of the inner bypass tube 2 totally to maintain the temperature of the process gas, going out of the gas outlet nozzle, at the predetermined temperature. According to this method, the temperature of process gas, going out of the heat exchanger, can be controlled surely.