Automatic controls

Improving automatic control, or fitting it where it did not exist before, is a classic way to save energy and it applies to all users from the largest to the smallest. Most of the published literature concerns the control of building service (see separate overview article) but in this article I will suggest a few other potentially fruitful lines of enquiry, particularly for the industrial energy user.

It might pay to look at how your ancillary equipment (air compressors, hydraulic packs, local extract ventilation) is controlled. If it is under manual on/off control, could it be interlocked to the equipment it serves, so that it will stand down when idle?

In engineering works, intermittent equipment like the fans in sandblasting cabinets may be prone to being left running. As well as wasting energy, this wears them out and contributes to the noise nuisance. Is there a way that they can turn off automatically, for example by using a pressure mat to detect the fact that the operator has left?

Improved motor control can be very cost-effective, especially with motors driving fans and pumps where the delivery rate needs to be varied. Here, a variable-speed control may yield savings compared with the use of dampers or valves to throttle the flow. Running a fan at half speed, when only half the delivery is required, takes only one-eighth of the power needed at full speed. This incidentally means that if you have a bank of, for example, cooling towers, then the sensible control strategy at low load is to run them all at low speed rather than fewer of them at full speed. Another point to make is that a multispeed motor could be the more economical choice if full speed modulation is not necessary.

Sometimes even in complex process plants, opportunities may arise. A case was reported to me where the operators of a distillation column had manual control of its 'reflux ratio' but on one occasion forgot to make the necessary adjustment when the still's feed rate changed. Automating the ratio control -- using the existing control system -- has prevented any chance of this occurring and has potentially prevented £200 per day being wasted. Plant-wide, the potential preventable loss is reported to be £12,000 to £15,000 per year “for the sake of ten minutes’ programming” on each of nine stills.

Very simple solutions, like door-switches to control the lights in walk-in storage cupboards, or timeswitches on non-refigerated vending machines, could have a part to play. Have a look around and see what you could automate to save power.

V.V. 20/11/05

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