Book - School of Science and Technology

Fan types and performance 475Total pressureStatic pressureVariableorifice‘A’Test ductFigure 17.1 Simplified diagram of fan test arrangementwhereH t ˆ total air power (W)V ˆ air volume handled (m 3 /s)P ˆ total pressure (Pa)The mechanical efficiency of a given fan will be the ratio between this air power and theactual mechanical power supplied to the fan shaft (fan shaft power): this will depend ondesign, type of fan, speed, and proportion of full discharge. If the static pressure is used,the efficiency derived will be static efficiency: if the total pressure is used the efficiency willbe total efficiency.The standard air* for testing fans is taken at a density of 1:2 kg/m 3 . Any fan at constantspeed will deliver a constant volume at any temperature; as the temperature varies thedensity will increase or decrease proportionately with the absolute temperature, hence thepower input will vary in the same ratio. With increase of temperature the power will bereduced and vice versa. Similarly, decrease of atmospheric pressure (as in the case of a fanworking at high altitude) will cause a reduction in power and conversely.If a fan running at a certain speed be rearranged to run at some higher speed, the systemto which it is connected remaining the same, the volume will increase directly as the speed;the total pressure will increase in the ratio of the speeds squared and the power input willincrease in the ratio of the speeds cubed.These relationships are known as the fan laws which, for practical purposes, may beexpressed for impeller diameter (d ), speed of rotation (n) and air density () as:Volume flow is proportional to d 3 and nFan pressure is proportional to d 2 , n 2 and Fan power is proportional to d 5 , n 3 and Characteristic curvesFans are of three main types, with sub-divisions as follows:. Centrifugal type:(a) multivane, forward bladed(b) multivane, radial bladed(c) multivane, backward bladed(d) paddle wheel.* For details of standard fan testing see BS 848: Part 1: 1992.