PURPOSE: To easily deal merely by replacing a regulating grate in the case of change of a blow-down amount by providing the grate at an upper opening of an over-flow tube in a blow-down apparatus for a cooling tube in which a lower end of a water conducting tube extended from a sprinkling tank of a DC-AC type cooling tower is disposed at the opening of the over-flow tube.

CONSTITUTION: A blow-down apparatus for a cooling tower has a water conducting lower end 32 of a water conducting tube 3 connected from a water conducting upper end 31 connected to a bottom of a sprinkling tank 4 of a DC-AC type cooling tower A, and comprises a detachable regulating grate 2 provided at an upper end opening 11 of an over-flow tube 1. The grate 2 has a frame 22 at its peripheral edge in such a manner that a depth side of the frame 22, a diameter of a perforated part 21 formed at a bottom, number of holes are so previously set as to become desired blow-down amount. The grate 2 is selectively used in order to simultaneously hold a circulating water amount (b) constant and to obtain the desired blow-down amount (a) in response to a change of a cooling load or a change of a circulating water amount.