PURPOSE: To enable to surely prevent the core from being clogged by a structure wherein a large number of vertical fins arranged parallel to one another with very small distances, are piercingly inserted with a plurality of horizontal tubes and the surface formed by the envelope of the leading edges of the fins is face toward the direction of the shaft of a suction fan and inclined so that the lower part of said surface is nearer to the fan than its upper part.

CONSTITUTION: Thin fins 1 are formed longitudinally in a slender, are longitudinally forwardly inclined on the surface at the top. The distance between the adjacent fins 1 is set so small as to prevent dusts carried by air flow from entering said distance. Furthermore, the horizontal flat tubes 2 are provided in such a manner that the flat surface of the tube 2 is perpendicular to the leading and trailing edges of the fin 1 by piercing through each fin 1. Owing to the structure as described above, because dusts carried from front by the air flow caused by means of the suction of the suction fan 4 go downward by gravitation and by downward air flow along the slope of the leading edges of the fins 1, the clogging of the core is eliminated.