To enhance heat transfer performance by a method wherein a spacer is attached between adjacent upper and lower heat exchanger tubes along the length thereof and a liquid accumulating part is formed near a joint part of the spacer and the lower heat exchanger tube to prevent the expansion of a dry part of a refrigerant liquid flowing down on the surface of the heat exchanger tubes.

A refrigerant liquid sprayed and dropped from above the top stage heat exchanger tube 31a by a refrigerant liquid spraying device flows down circumferentially on the surface thereof to reach a spacer 19. The refrigerant liquid flowing down on the spacer 19 runs down continually getting together in the longitudinal direction of the heat exchanger tube 31b though a part thereof is accumulated into a liquid accumulating part 19a formed at a joint part of the heat exchanger tube at the subsequent stage and the spacer 19. Even if a dry part exists at a corresponding position of the surface of the subsequent stage heat exchanger tube 31b, the part is narrowed by the refrigerant liquid flowing down from the liquid accumulating part 19a. This improves wettability of the heat exchanger tubes Thereby achieving a higher heat transfer performance.