PURPOSE: To prevent corrosion due to local electrolytic action to be generated through contact between a tube plate and a cooling plate from occurring, by insulating an external surface of an expanded pipe part of a cooling tube by coating it with a thermal contraction tube.

CONSTITUTION: Prevention of corrosion is done by insulating the whole circumference of a cooling tube 2 to be expanded and sticked closely to tube plate 1 by coating with an insulating thermal contraction tube 5 made of fluorine resin. Althoug there is, however, the possibility that an inlet of the cooling tube 2 is clogged with product 4 through corrosion to be generated due to corrosion of the tube plate 1 or a water flow is interrupted through intrusion and remaining of the product 4 within the tube 2 when the tip of the cooling tube 2 is fixed about flush with the surface of the tube plate 1, such evil effect is eliminated if the tip of the colling tube 2 is made to protrude by 3W10mm from the surface of the tube plate 1.