Compressed air efficiency: up-to-date systems and good housekeeping vital

Compressed air accounts for about 10% of industrial electricity consumption in the UK, according to the Carbon Trust, and there’s no reason to suppose things are different in other markets.

What’s more, with a huge installed base, many users could be saving up to a quarter of compressed air-related energy through a combination of ‘good housekeeping’ measures and investing in up-to-date, efficient and correctly-sized equipment.

“It is difficult to provide any accurate indications as systems will vary significantly between applications,” says Marion Beaver, British Compressed Air Society (BCAS) technical officer.

“However, regular maintenance (to maintain low leak rates and reliability of equipment) and staff involvement are good starting points. Add to this a policy that specifies that more energy-efficient options are purchased when replacing all equipment – from a drain valve through to the compressor – along with the use of genuine spare parts, and you will realise significant savings in excess of 25%.”

A tiny leak of just three millimetres can cost more than £700 a year in wasted energy, but a simple, out-of-hours survey using ultrasonic leak detection equipment can identify leaks easily

Marion Beaver, British Compressed Air Society technical officer

Designing and maintaining a well-run system is crucial – and tackling leaks should be a top priority, according to the consensus.

Beaver suggests that typical leak rates in industrial systems can be between 20 and 40%. “A tiny leak of just three millimetres can cost more than £700 a year in wasted energy, but a simple, out-of-hours survey using ultrasonic leak detection equipment can identify leaks easily,” she says.

“First you must locate and fix leaks,” confirms Marcus Hamilton, product manager for industrial compressors with Gardner Denver. “This not only saves money on power, but could also possibly save on the size of any capital equipment in which you invest.”

Go with the flow

The next step is to improve the flow of air around the system, eliminating any dead-legs, incorrectly-sized pipes, or other restrictions that increase the pressure drop.

Completing these improvements first should enable any subsequent air audit or engineering mapping exercise to highlight the actual demand from consumers, rather than sizing the compressed air supply to deliver a higher-than-necessary pressure or feed leaks, explains Hamilton.

Of course, there needs to be an allowance for demand fluctuations and future expansions. “I allow around 10% extra because there’s some tolerance in the machines, although some people might argue for more,” he says, suggesting that some engineers allow up to 20% wriggle room when sizing a new compressor.

Other measures that essentially boil down to matching the supply to the demand include reducing the system pressure as far as it is practical and reviewing and optimising compressed air use.

“Reducing pressure by 10% can lead to as much as 5% savings in energy,” says Beaver.

Other measures that essentially boil down to matching the supply to the demand include reducing the system pressure as far as it is practical and reviewing and optimising compressed air use

She adds that some jobs – such as ventilation or drying – may be better served by more energy-efficient technologies and that even where compressed air is appropriate, it can sometimes be delivered more efficiently.

“For example, air knives may be open-ended pipes. Fitting a venturi-type nozzle can use 30% less compressed air,” she states.

Other potential energy-saving measures focus on equipment that uses purge air – mainly the dryers and drains that are essential to control moisture in the system. For instance, some drains lose compressed air each time they expel condensate but zero-loss models can eliminate this.

The situation is a little more complex for dryers. The dew point is a measure of how dry the air is; it’s the temperature below where there’s a risk of moisture dropping out of the air.

A fridge dryer can bring the dew point down to 3°C but this is not low enough in many cases. Desiccant dryers can reach -40 to -70°C but they can also consume a lot of compressed air. In a typical two-chamber system dry, compressed air is used to regenerate the saturated desiccant in one chamber while the second chamber dries untreated air.

Carl Sharpe, sales manager with BOGE explains: “You can use a timer to control the cycle but the timer is often set for a worst-case scenario of very warm, moist air. What you can do is use dew point control instead to give you savings of as much as 60 to 70% in a cold, dry atmosphere.”

Climate change

Desiccant dryers can reduce losses still further by using an electric heater to drive off moisture from the spent desiccant, or by installing a fridge dryer so that air feeding the desiccant dryer already has a dew point reduced to 3°C. Dryers can also work under vacuum to reduce purge air losses.

Users may also opt for compressors fitted with variable speed drives (VSDs), which can be combined with the most efficient (IE4 class) motors. VSDs enable the compressor to match demand precisely.

“It’s about pressure. Pressure is energy and energy is money. Every 1 bar increase in pressure costs an extra 7%,” says Paul Clark, business line manager for Atlas Copco Compressors’ Industrial Air Division. “The advantage of a VSD is that it’s infinitely variable to 0.1 bar increments.

“An ideal system, once you get above a certain size, would be one that incorporates a fixed-speed unit and a variable-speed unit. With a fixed-speed compressor, any idling is going to cost you energy for no benefit. A VSD will take up the difference between the average load and any peaks in demand,” he said.

An ideal system, once you get above a certain size, would be one that incorporates a fixed-speed unit and a variable-speed unit

Paul Clark, business line manager, Atlas Copco Compressors

“Probably the biggest advancement in compressor efficiency is the wide-scale use of variable-speed control with high-efficiency motors,” agrees Beaver. “If applied correctly, this technology enables operators to match factory air demand to generation, using a combination of fixed-speed compressors to provide the base load, with variable-speed machines coming on line to meet the peaks and troughs in demand.”

Adopting a system using multiple compressors also paves the way for using smart controllers. Each of the major suppliers has their own version.

“A master controller will link to all the compressors and select the best compressors to match demand and ensure they’re optimised automatically all the time,” says Hamilton.

“If you’ve got multiple compressors, smart control can ensure you’re not wasting energy as compressors turn on and off in response to demand events,” concurs Sharpe. “It’s a relatively cheap way of making big savings because it’s mainly about software and having the right algorithms.”

Under pressure

Hamilton adds that the location of the pressure transducer is also important, since positioning the transducer on the ring main, rather than in the compressor room, enables the master controller to maintain a more accurate factory pressure which enables it to be set lower than if it would be positioned back in the compressor room.

“If you take the pressure signal from the factory ring main rather than from the compressor room, the master controller knows the true requirement it needs to meet. You can then reduce and fine-tune the pressure to match, which also reduces leaks. By having the transducer for the controller in the ring main you can reduce leaks and save up to 10%, and also reduce power by only generating at a pressure to meet true factory requirement,” he says.

Substantial efficiency improvements can be achieved by implementing new processes and encouraging staff to use compressed air more efficiently

Since the physics of air heating while it is being compressed is unavoidable, no round-up of energy-saving in compressed air would be complete without reference to heat recovery. “The combination of energy recovery products can recover up to 90% of heat and use it for things like the boiler feed, hot water and space heating,” says Clark.

Finally, Beaver stresses that the impact of the human element should not be overlooked in any improvement programme. Many of the most helpful good housekeeping measures rely on well-trained personnel who are invested in driving down compressed air use and energy consumption: “Substantial efficiency improvements can be achieved by implementing new processes and encouraging staff to use compressed air more efficiently.”