HVACR knowledge

Fan characteristics

Fan Performance

While the selection of a ventilation unit from the output tables to perform under free air conditions is a simple matter, it is useful to know the rudiments of fan performance against some resistance to airflow, such as ducting and filters, or even sufficient free area for the passage of replacement air into a room from which the unit is extracting.

Characteristic Curve

Any particular fan design has its own characteristic curve, which is a graph made by plotting a number of test points showing volume delivered against different resistances. Volumes are measured in cubic metres per second (m³/s), which for the sake of convenience in calculations in our class of work is converted to cubic metres per hour (m³/h, and pressures are measured in Pascals (Pa.).

Pressure

The total pressure produced by a fan is made up of the static pressure, that is the useful working pressure available for overcoming the resistance of a ventilating system and velocity pressure, which is the pressure due to the speed of the air. These fan pressures are of a very low order and that is the reason for using the unit of Pascals instead of kg/m² or g/cm².

Pascals

One Pascal is a pressure equal to a force of one Newton applied over an area of one square metre (= 1 N/m²). As one Newton approximately equals a force of 0.1 kg, one Pascal = 0.1 kg/m² or 0.0000 1 kg/cm² (=0.000 145 lb/in²). Other expressions used for resistance are inches water guage (1 in.wg = 249.1 pa) and mm waterguage, an intriguing combination of metric and imperial, 1mm wg = 9.8Pa.