General comments
Fans may be driven by a varied range of machines, as indicated in Section 13.1.
The most common are:
• fixed speed electric motors of the synchronous and induction types
• variable speed electric motors
• steam turbines
• internal combustion engines of the petrol, diesel oil or gas types
If a suitable supply of steam is available, for example where steam is produced in a power station or is a by-product of an industrial process, a steam turbine driver may well be the most appropriate choice. It has the advantage of being easily adjusted to a variable speed, resulting in a more efficient method of providing an output matched to demand.
If a suitable steam supply is not available e. g. domestic or commercial buildings, agriculture etc., etc., then the most reliable and economical form of driver is invariably an electric motor, provided of course that an adequate and sufficiently robust source of electricity is present.
The most reliable type of electric motor is generally accepted to be the induction design. This rotates at a little below synchronous speed which for a two pole machine running on a 50Hz AC supply limits the maximum speed to something just less than 3000 rev/min or 3600 rev/min on a 60Hz AC supply.
Some fans may need to operate at speeds in excess of this, in which case a speed increase belt drive or a step-up gearbox may be necessary. An alternative is to convert the supply to a much higher frequency e. g. 400 Hz when much higher speeds are possible.
The driving motor should in all cases be sized to provide the power demanded by the fan impeller plus any losses in bearings, vee belt drives etc. As far as the power supply is concerned, it will be necessary to provided for additional losses in the electric motor itself together with losses in the control gear.
The driver should also be sized to provided the power required by the fan, its bearings and transmission under all expected operating conditions with a suitable margin to cover:
• uncertainty or inaccuracy in the definition of the fan duty
• variation in the fan duty due to changes in air/gas density
• deterioration in the fan performance due to erosion, corrosion or dust build-up
• uncertainty in the measured performance
• variation between a prototype and a production machine due to manufacturing tolerances
• deterioration in performance of the driver such as gradual breakdown of electric motor insulation or fouling and erosion of a steam turbine
• variations in the energy source e. g. power supply voltage or steam pressure
The likely magnitude of this margin may need to be considered in detail. A minimum recommendation, which is a reasonable approximation for most centrifugal fans cases, is given in Table 13.1.
|
Power at start up = 75 x |
|
= 153.1 kW |
|
75 x |
|
105 |
|
1.2 x- |
|
Impeller type |
Width |
||
|
Narrow |
Medium |
Wide |
|
|
Backward inclined |
14% |
10% |
7% |
|
Backward curved |
8% |
7% |
5% |
|
Aerofoil |
8% |
6% |
5% |
|
Forward curved |
20% |
17% |
15% |
|
Shrouded radial |
14% |
12% |
12% |
|
Radial tipped |
16% |
14% |
12% |
|
Open paddle |
14% |
12% |
12% |
|
Backplate paddle |
14% |
12% |
12% |
|
Table 13.1 Approximate margins to be added to absorbed power |
Posted in Fans Ventilation A Practical Guide