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# Fan Capacity Diagrams

Pressure head as function of air flow volume may be expressed in a diagram like the one below:

• red lines are pressure heads as function of air volume flow and speed of the fan
• green lines marked - l - are throttle lines
• blue lines are power consumption

Operation of fans are often expressed in terms of pressure

pt = ps + pd = constant                                          (1)

where

pt = total pressure (Pa, N/m2)

ps = static pressure (Pa, N/m2)

pd = dynamic pressure (Pa, N/m2)

Dynamic pressure can be expressed as

pd = ρ v2 / 2                                                  (2)

where

ρ = density of air (kg/m3)

v = flow velocity (m/s)

Head developed can be expressed as

dp = ps2 - ps1 + (v22 - v12) ρ / 2                                                (3)

where

subscription 1 is the inlet of the fan and subscription 2 is the outlet

## Related Topics

### • Ventilation Systems

Design of systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.

## Related Documents

### Fan Affinity Laws

The affinity laws can be used to calculate resulting volume capacity, head or power consumption when speed or wheel diameters are changed.

### Fan Inlets - Air Density vs. Suction Pressure

High suction pressure at a fan inlet decreases the density of air - and should be corrected for proper selection of the fan.

### Fans - Capacity Control

How to modulate fans and their capacities.

### Fans - Efficiency and Power Consumption

Power consumption and typical efficiencies of fans

### Fans - Volume Flow, Pressure Head and Power Consumption vs. Air Temperature and Density

The temperature and density of the air influences on the volume flow, pressure head and power consumption in a fan.

### Types of Fans

Axial and propeller fans, centrifugal (radial) fans, mixed flow fans and cross flow fans.

### Types of Fans - Capacity Ranges

Centrifugal, axial and propeller fans and their capacity ranges.

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