Performance geometrically similar, fans can be predicted quite accurately using the fan laws. For certain diameter, and at the inlet air density, fan laws can be summarized as follows:
the amount varies as speed
pressure varies in accordance with the (speed)2
power varies as (speed)3
Similarly, for a fixed speed:
the amount varies as (diameter)3
pressure changes as (diameter)2
power varies as (diameter)5
The laws apply only to same point in the work of the fan characteristic, so they cannot be used to predict other points on the curve fan. They are often used to calculate changes in flow, pressure and torque in the case when the volume or speed changes.
Airflow control is an essential element of most applications and fans can be wasteful of energy, if their work is not regulated effectively. There are several possible methods of management and the choice of method will depend on the effectiveness and size of equipment.
Cruise control using a fixed frequency is possible for small asynchronous motors with variable voltage regulator speed.
This is often used for single-phase motors. Frequency adjustment (inverter) the actuators can be used for motors with capacity of 150 watts up, and can reduce the speed of about 20%. These devices carry their own energy losses, both the device and in the engine where the electrical signal deviates from the true sine wave. Impact on the fan is given in Figure. 24.11.
Alternatives to control the speed of change of the system fan or geometry. Volume flow control for constant speed of rotation of the fan can be achieved with inlet vane angle change. By spinning the inlet air in the same direction as the high efficiency impeller can be preserved even at low fees (see Fig. 24.12) .
The interaction of the fan blades with air can be changed by changing the direction of air flow, as it enters or leaves an impeller. Change the blade angle can be a manual process. The fans, for which step the blades may vary while the impeller during the movement is becoming more complex and can be managed down to zero flow. An alternative way is to change the system geometry, volume control dampers, to increase the resistance of the system. This approach may not achieve worth saving energy.
Air-Conditioning 
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