^{The helical construction uses the stator helical method that the screen lines of casing at the suction and discharge sides are cut to a helical shape, and the striangle suction and discharge port formed by a straight line of the rotor top is to be opened and closed and closed gradually. Therefore, the suction and discharge ports of this type is not opened or closed at moments, which makes these blowers have an only limited operation sound and almost free from pulsations from discharge.}