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In general, fans are selected for their ability to deliver a required air volume flow rate against an estimated system resistance to flow, or pressure. This defines an Operating Point and is usually expressed in m³/h at Pascals, or cfm at inches Water Gauge.

Volume flow rate is calculated from application data and may be based on a requirement to heat, cool, or ventilate air, ensure even air temperature distribution, or to contain air that has been contaminated.

Where building ventilation is concerned there is a need to extract stale air, fumes and heat at a rate that depends on the purpose of the building. The calculation is based on air changes per hour and may be as few as 1 for a church or 60 for a furnace room.

When extracting a known amount of heat from, for example, a power electronic enclosure, the minimum volume flow rate for a given permitted temperature rise may be calculated using a formula that takes account of the density and specific heat of air.

For a laboratory fume cupboard, or hood, the flow rate is calculated from knowledge of the minimum air speed required to contain fumes at the open sash of a fume cupboard, or through the inlet area of a hood.

Pressure loss is the resistance to air flow and is caused by molecular friction and surface friction as air passes through a system. It is expressed in Pascals (Newtons/m²) or inches Water Gauge and varies with the square of the air velocity. Thus doubling the flow rate through a given system has the effect of quadrupling the pressure, and halving it quarters the pressure.

The pressure loss within equipment containing complex air paths is difficult to estimate, so requires experience of similar systems. For ductwork it is a well researched subject with pressure charts and loss factors for each element in the system.

Static Pressure is the pressure exerted on duct walls and corresponds to potential energy. Velocity Pressure is the flow pressure and corresponds to kinetic energy. The sum of the two is Total Pressure and corresponds to the total energy increase provided by the fan.

Static Pressure loss is estimated by calculating the Velocity Pressure through each component in the ductwork system and multiplying it by a constant, known as a ‘k’ factor for that component, to provide an estimate of the component Static Pressure. The System Static Pressure is then the sum of all component Static Pressures, and the Velocity Pressure is calculated from the discharge air velocity.

By knowing the System Operating Point it is possible to plot a System Characteristic curve from which the performance of any fan on that system can be estimated.