Advantages and disadvantages
Apart from duty flexibility, there are many other considerations in the decision as to whether to incorporate a direct or indirect drive.
To take an extreme case, a requirement to produce a high volumetric flowrate at a low pressure will inevitably mean a large diameter fan running at a low speed, if multiple fans cannot be considered. If direct drive were to be specified, then, with an AC electric driving motor, this would require a large number of poles and a large frame size with a correspondingly high purchase price. It might also result in a somewhat lower efficiency motor with less starting torque available.
Conversely, with a belt or rope drive interposed between the fan and motor, it is possible to select a much cheaper motor at a better efficiency with improved starting characteristics. It is also possible to select fans running at greater than the two pole motor speed on an AC supply i. e. approximately 3,000 rev/min on 50Hz AC. All these advantages can more than offset the disadvantage of the transmission efficiency, which will of course be less than 100%.
There are many cases in industrial applications where the gas stream is at a temperature higher than ambient or contains corrosive/erosive/explosive constituents. Any indirect drive may then permit the driving motor to be positioned away from these dangers such that with minimal precautions, a relatively standard machine can be used. A disadvantage of rope and belt drives is the need for maintenance. Tension in the belt or rope(s) has to be correctly maintained to ensure that the power is transmitted without slip. This is especially important in multiple vee belts when each belt has to have an equal tension to ensure that it correctly transmits its share of the absorbed power. In the past matched sets of belts, in regard to length, were specified. Now, however, the manufacturers are able to guarantee, by improved manufacturing processes, that nominally identical ropes are equal in length to within very close tolerances.
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