PURPOSE: To enable saving of a labor, by a method wherein the optimum control table of a total volume and a total consumption power is stored, and from a detecting temperature and a flow rate, an optimum control table under a nearmost condition is selected to produce a control signal.

CONSTITUTION: Output signals from a temperature detector 10 and flow rate detectors 11AW11C, located in a common feed water line 7, and temperature detectors 9AW9B situated in suction lines 2AW2C are inputted to a microcomputer 13. An optimum control table, according to which a relation between a total flow rate and a total consumption power is optimized, is stored in the microcomputer 13, and from the output signals, an optimum control table under a nearmost condition is selected. Based on the signal, suction airflow control valves 3AW3C are controlled. This constitution enables control of saving energy.