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Energy provided by the ventilator

The absorptive energy by the ventilator breaks up:

Mechanical energy provided to the fluid

It is the communicated hydraulic power to the air at the time of its passage through the ventilator.

This mechanical power is given by the following formula:

Mechanical energy, fan, ventilator, calculation, formula


  • P = Power transmitted to the fluid by the ventilator in W.
  • Q = Flow rate in m3/s.
  • Hm = Head pressure loss in Pa.

Degraded energy expressed by the output of the ventilator

It is the power measured on the shaft of the ventilator.

The mechanical energy necessary to a ventilator is always higher than the energy transmitted to the fluid consequently to various frictions of the bodies of rotation.

Output, fan, ventilator, transmission, air, flow calculation, formula


  • Pmec = Mechanical power necessary to the ventilator.
  • Pfl = Power transmitted to the fluid.
  • Rv = Output of the ventilator.
  • Rt = Output of the transmission.

The generally allowed outputs are:

Ventilator type

- Centrifugal fan with blades bent backwards (6 to 16 units)


- Centrifugal fan with blades bent forwards (38 to 42 units) - (known as squirrel-cage)


- Propeller fan without diffuser but with rectifier


- Propeller fan with diffuser and rectifier


- Ventilator of wall


The only types of ventilators which are appropriate for the installations for constant pressure loss or variable volume of air are the centrifugal blades fans inclined backwards and the propeller fans (curved steeply sloping characteristics)

Flow-volume cannot be reduced generally to the 1/3, seldom below 50%.

In the contrary case, it is necessary to use engines with progressive control system, that is to say mobile blades or variable blades helicoidally ventilators.

Output of the transmission

The transmission of the energy of the ventilator engine is done with a certain loss, mainly in the case of a belt drive, because of the slip of the latter on the pulleys.

Mode of drive Losses

- Engine with direct drive (wheel of the ventilator directly fixed on the shaft of the engine)

2 to 5 %

- Drive by coupling

3 to 8 %

- Belt drive

P motor < 7.5 kW: 10 %
  7.5 kW < P. motor < 11 kW: 8 %
  11 kW < P. motor < 22 kW: 6 %
  22 kW < P. motor < 30 kW: 5 %
  30 kW < P. motor < 55 kW: 4 %
  55 kW < P. motor < 75 kW: 3 %
  75 kW < P. motor < 100 kW: 2.5 %


At the time of the choice of the engine, it is the absorptive power by the ventilator which determines the power delivered by the engine and thus also the absorptive power by the duct. It is necessary thus to take guard so that the engine has a sufficient power to satisfy all the situations of operation of the installation.

Let us take the case of a ventilator having an absorptive power of 8.5 kW. The engine will provide these 8.5 kW, independently owing to the fact that it is conceived for 7 kW or 10 kW. An engine of 7 kW, having to function at 40°C, would thus always be overloaded of 21.5 %.

The direct consequence of an overload of the engine is an increase in the temperature of winding. When it exceeds the limiting temperature envisaged according to the class of insulation, the lifespan of the insulation decreases. A going beyond of the limiting temperature of 8-10°C, decreases the lifespan of the insulation of approximately half. Goings beyond of 20°C means shortening of 75 %.

The engines of standard construction are planned for a use at maximum ambient temperature at 40°C (and a maximum altitude of the site of 1 000 m). Any variation requires a correction of the nominal output.

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