The ratio between developed pressure at different temperatures can be expressed as:ĭp o / dp r = (273 + t o) / (273 + t r) (2)ĭp o = dp r (273 + t o) / (273 + t r) (2b)ĭp r = reference pressure developed (Pa) - in general at NTP conditions The fan must be selected for volume flow 1.2 m 3/s (and whatever the pressure loss in the system is). The required volume capacity in the fan at operating conditions can be calculated using 1b as The fan operates when the air is heated to 80 oC. T o = operating temperature ( oC) Example - Required Fan Volume CapacityĪ drying process requires 1 m 3/s air at normal conditions 20 oC. T r = reference temperature ( oC) - in general 20 oC NTP conditions

Q r = reference volume flow (m 3/s) - in general NTP conditions

If you have the required volume flow for a fan at a reference condition (typical NTP) - the operating volume volume flow for the fan at other operating temperatures can be calculated as The amount of air (mass) passing through the fan varies with the air density and the air density depends on the temperature (as long as the pressure is constant). Note! - the volume flow through a fan is constant as long as the physical dimensions and the speed of the fan are not changed. The examples below may clarify the procedures: Operating Volume Flow vs. The formulas below can be used to calculate the volume flow, pressure head and power consumption at NTP conditions if the operating conditions are known, or vice versa if the NTP conditions are known. When selecting a fan it is important to know if the specification of the system is based on operating conditions or NTP conditions.

equal speed and dimensions - the volume flow remains equal.

colder air and higher air density - more mass will be transported through the fan.

hotter air and lower air density - less mass will be transported through the fan.

Since the density of air varies with temperature and pressure the mass flow through a fan varies with temperature and pressure.

NTP - Normal Temperature and Pressure Conditions - 20 oC, 101.325 kN/m 2, 1.204 kg/m 3 (68 oF, 29.92 inches Hg, 0.075 pounds per cubic foot).Ī fan is a 'constant volume' device where the volume in the fan - and the transported air volume through the fan - is always the same (with same fan speed and size).Manufactured specifications for fans are in general based on the NTP - Normal Temperature and Pressure Conditions Density of air varies with temperature and pressure (or altitude or elevation above sea level) and a fan will not deliver according manufactured specifications if operating conditions are outside NTP - Normal Temperature and Pressure Conditions.

Effective airflow vs relative airflow