. . "exchanger tube for use in the watercooler, in which the tube is before its machining;FIG. 3 is a timing chart of various elements upon preparation of cold water in the watercooler;FIG. 4 is a cross-sectional view of a primary cooling unit in a watercooler according to a second embodiment of the present invention;FIG. 5 is a cross-sectional view of a primary cooling unit and its vicinity area in a" . .