According to a recent report, UK industry has been slow to take full advantage of the energy savings offered by variable speed drives. Colin Carter says now is the time to take stock of how your plant is running.

Look around any modern car and the drive count will usually be well into double figures, controlling everything from the seat position to the angle of your ipod screen. This ubiquity is mirrored in their use in the industry, where they are increasingly used to cut costs, by increasing automation levels, and offering higher energy efficiency processes.

With the well-publicised shortage of gas across


during the cold winter, and the non-renewable nature of most fuels we consume, the situation is likely to get worse before it gets better. but drives can make a contribution by ensuring that as little energy as possible is wasted.

One of the most effective examples of energy-saving technology comes from the use of variable speed drives to power pumps or fans. It's so effective that the government is offering tax breaks via 'Enhanced Capital Allowances' to be offset against equipment costs upon purchase.

It is obvious to any schoolboy scientist that a pump or fan working at full speed (and therefore torque) uses more energy than one at less than full capacity. In very simple terms the 'affinity laws' dictate that the throughput pressure is proportional to the square of the speed of the fan or pump (and to the cube of the torque required).

So making adjustments via variable speed drives and a suitable control system so that pumps and fans are not constantly running 'flat out' can result in great energy savings. For instance, a pump operating at half speed will only use 12.5 per cent of the energy compared to one operating at full capacity.

Where several pumps or fans are in use, the savings can be huge — especially as designers tend to over rather than under-design. A [non energy-efficient] key part of traditional process design was the incorporation of throttling or damping mechanisms — running pumps or fans at full speed and mechanically reducing the flow or liquid or gas. Clearly a wasteful and expensive solution.

There are other benefits of running pumps and fans at lower speeds, one being a lack of noise. Another of the 'affinity laws' states that the noise emitted is proportional to the speed to the power of five. So variable speed drives can allow for a more comfortable working environment as well as save money. These laws, although theoretical and not always realised 100 per cent in complex industrial systems, are backed up by many practical applications.

Diageo's Port Dundas distillery, for example, produces grain scotch whiskies for blending for famous brands such as


's, Black & White and Dimple. Whisky production uses vast amounts of water — Port Dundas needs over 500 million litres a year, for example — and this needs to be pumped to the required section of the plant for mashing the grain or cooling the distillate.

Until the system was replaced, this had been achieved using a system of fixed drives and valves to throttle the flow, but at that stage Diageo opted to use Danfoss variable speed drives for the pumps.

As a result the two 132kW drives were replaced by drives that operate at a typical 50kW. This saves close to 37,000 kWh per annum, recovering the project costs in approximately 18 months.

Richard Wood, Port Dundas business manager, explained: 'Energy is a significant cost in our process and we take the Climate Change Levy (CCL) very seriously and look at every and all opportunities to cut our energy costs.'

And there are more savings to come. Wood explained: 'We propose to experiment with lower pressure set-points which will increase our energy savings still further.'

Another energy-saving application comes from Mayflower, a producer of body panels for cars, including Aston Martin. Installation of ABB variable speed drives (with an associated pid control system) on the company's phosphate/alkali rinse system — an essential part of the pre-treatment of the end product — saved the company something like 80 per cent of its energy costs.

The 30kW pumps used in the pre-treatment process were running for 10 hours a day, with no facility to start and stop them quickly. As they were only required for about 25 per cent of the pre-treatment cycle, this was obviously wasting both energy and cash.

As an illustration of how much can be saved, Mayflower replaced some 17 drives, and expect energy savings of around £98,000 per year — with installation payback in about a year.

As an indication as to how much energy can be wasted, a recent Gambica report 'Making the most of the climate change levy package' contained the following facts:

  • Electric motors cost UK industry £3bn a year;
  • Motor running costs can exceed the purchase cost within one month;
  • A 90kW motor may cost up to £30,000 a year to run;
  • A 110kW machine will use £1m of electricity during its 20-year working life.

With large savings achievable by using energy-efficient drive technology, even a small dent in that £3bn figure will improve the competitiveness and green credentials of UK plc.

As a footnote, one of the report's authors, Eddie Kirk, added: 'I find it difficult to believe industry has been so slow to pick up on variable speed drives, especially as some 50 per cent of the energy used by industry is in rotating machinery, and of that some 60 per cent is used on centrifugal fans and pumps.

'It seems remarkable that UK industry has not taken advantage of the energy savings variable speed drives can offer many applications — perhaps rising energy costs will prompt action.'

Drives will have a potentially huge impact on the


energy market for some time to come, and legislation will drive the need for increased efficiency for many large-scale applications. So now is the time to look at how your plant is running…