The windward side of the air cooler refers to the windward side of the bundle. The area of the windward side is the area within the inner wall of the outer frame of the bundle, which is approximately equal to the length times the width of the bundle. The speed of air passing through the windward side is called the windward speed for short.

When the air velocity on the windward side of the air cooler is too low, the heat transfer rate will be affected. Especially wet air cooler, if the wind speed is too low, water can not be sucked to the inside of the pipe surface. Most of the tube bundles are dry air coolers, which greatly reduces the cooling effect. Too much approaching wind speed affects air pressure drop and thus power consumption. Therefore, it is necessary to specify a certain range of windward speed, with the maximum not exceeding 3.1m/s and the minimum not less than 1.4m/s (standard state).