Page 166 - Industrial Power Engineering and Applications Handbook

6/146 Industrial Power Engineering and Applications Handbook
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ventional method of throttle control through a vane or a their initial cost compared to static drives. The only case
damper causes a considerable waste of energy. To obtain against their use, despite the exorbitant costs of static
a variable speed and yet save on energy, one can use drives, is their recurring power losses, as mentioned in
either a static drive as discussed earlier or a variable Table 6.5 (items 19 and 20). It is a constant and recurring
speed fluid coupling (Chapter 8). The choice between drain on useful energy. A static drive, irrespective of its
the two will be a matter of system requirements and an cost, has a pay-off period of three to four years, depending
overall assessment of the ease of application, economy upon the size of drive, frequency and duration of speed
and accuracy of speed control in addition to the amount control and its accuracy. Thereafter it achieves regular
of energy it will be able to conserve. The application and high energy saving. This advantage is not possible
engineer would be a better judge to make a more in a fluid coupling due to higher power losses.
appropriate choice between the two, based on system Conservation of energy is therefore the main criterion in
requirements. We give a brier comparison between these the selection of static drives.
drives in Table 6.5. This comparison should help in making Until a few years ago, when static technology was still
a more judicious choice of drive. in its infancy, variable-speed fluid drives had very wide
Fluid couplings larger than, say, 400 kW can be application. With the advent of static technology, the
employed with ease in achieving the required speed trend is shifting in favour of static controls, particularly
control. They also provide a mechanical coupling between for drives that have to undergo frequent and wide speed
the drive and the load. A variable-speed fluid coupling variations during their normal course of operation, or
also results in energy saving along the lines as discussed which require very accurate speed control. For applications
in Example 6.1. In some countries this device is classified not needing very precise speed control or widc variations
as ‘energy saving’ and attracts subsidies from the state. in speed (e.g. high-capacity pumps or ID fans) variable-
They are simple to operate and are highly economical in speed fluid couplings are still the best choice.