Most fans are designed for a maximum fan tip speed of 14,000 feet per minute. To calculate the tip speed of the fan, do not calculate the fan rpm, from the pulley size and driver speed. The belts may be slipping. Measure the fan speed directly with a tachometer. Then calculate the fan tip speed as follows:
F = Fan blade length, ft.
T.S. = Fan tip speed, ft./min.
If T.S. is less than 14;000 feet per minute, first check the tension of the fan belts. Next, for fans powered via a belt drive from a gas driven engine, determine if the fan speed corresponds correctly to the engine speed:
Fan RPM = Engine RPM X (PDE/PDF)
where PDE = Diameter of the fan pulley
PDF = Diameter of the engine pulley
The smaller the pulley (also called a sheave) the faster the fan speed. A number of standard size pulleys for fans are readily available. For example, if you decided more air flow was needed on a cooler, and the calculated fan tip speed was only 10,000 feet per minute, a smaller pulley could be placed on the fan. For instance, changing a 24″ pulley to a 20″ pulley (both are standard sizes) would increase the fan tip speed to 12,000 feet per minute. The end result of such a reduction in pulley size would then be:
• Air flow would increase by 20% (i.e., linear with fan speed.
• The pressure head developed by the fan should increase by 44% (i.e., fan speed squared).
• The engine horsepower consumed by the fan would increase by 73% (i.e., fan speed cubed).
As the horsepower absorbed by a fan is typically in the three to five percent range of total engine horsepower, the 73% increment to obtain an increase in cooling air flow of 20% is normally not prohibitive. Caution: It is good engineering practice to check with the fan manufacturer prior to reducing the size of the fan pulley.