To accelerate indirect cooling of circulation cooling water flowing through each U-bent tube at each stage by feeding an outdoor air flow to the joint of a straight tube and the U-bent tube and spraying water.

Recesses 15 for holding the outside of a U-bent tube 12 in an enclosed heat exchanger B at each stage are made in the opposite inner surfaces of a pair of supporting members 14. Outside of the U-bent tube 12 is inserted in the recesses 15 from the inside thereof and the enclosed heat exchanger B is supported vertically is multistage by supporting side plates 13. Since the outside at the bend of each U-bent tube 12 is contained in each recess 15, water is also sprayed to each U-bent tube 12 and outdoor air flow touches that part thus cooling circulation cooling water flowing through the enclosed heat exchanger B indirectly even at that part.