Plugging the cost of compressed air leaks01 December 2006

New products are now available that step up the efficiency of compressed air systems - especially compressors - finding and attacking leaks, and improving the management of pneumatic systems, while slashing energy losses and costs by up to 50%.

Since pneumatic components are usually simpler, lighter and more robust than their electrically-driven equivalents, this once again makes pneumatic solutions a very attractive way of doing things. Adding up all the possible energy savings, a chart published on the Boge UK website concludes that it might be possible to reduce energy consumption by more than 72%. Certainly, it is true to say that substantial savings can be made, without investing an excessive amount of either effort or expense.

Starting at the air compressor end of things, Boge Compressors, based in Brighouse, estimates that using an inverter-driven variable-speed screw compressor will save about 25% of energy. Savings calculated by different manufacturers vary. Ingersoll Rand states in its brochure that its Nirvana compressor will save "at least 28% of the energy cost over its life", although later on more cautiously suggests "22% to 30% compared to a fixed-speed rotary air compressor". CompAir calculates that its L45SR oil lubricated rotary screw machine - the 45 refers to 45kW - is able to save up to 30% through the use of its switched reluctance drive. Atlas Copco tops this by claiming that its variable-speed drive compressors can reduce energy by an average of 35%. It has 24 model ranges, from 7kW to 900kW, including oil-free versions and full-feature workplace models, with integrated air and condensate treatment equipment.

While energy savings will vary greatly, according to prevailing service conditions and circumstances, variable-speed driven compressors should save substantial amounts of energy, because they only produce the amount of air that is required at any given time, instead of running at full speed and switching on and off.

Intermittently-run fixed speed compressors blow down when they stop, use substantial amounts of energy to get the motor and compressor elements up to speed (and the air up to pressure when they start again). And, when demand drops, they overshoot required system pressure until the excess pressure causes them to be shut down again.
As variable-speed driven compressors cost more than fixed-speed compressors, it is sensible to use fixed-speed units to meet the constant part of compressed air requirements, up to the known minimum demand level, and a variable-speed compressor to deliver the varying demand levels above that.

Modern fixed-speed compressors can also be more efficient than their predecessors. The CompAir L50 lubricated screw compressor is said to deliver energy savings of 7.2% above its predecessors. Innovations include: slower rotation speeds in the premium air end rotors, intelligent controls and use of a more energy-efficient motor. It is also quieter, producing only 67dB(A) noise, so that it is possible to have a conversation next to it. It does not have to be relegated to a distant compressor house, but can be placed adjacent to its points of use, reducing pipe runs with all their attendant pressure drops and leak possibilities. Ingersoll-Rand also quotes reduced noise levels - down to 59 dB(A) and 67 db(A), according to the model.

There are other control tweaks that can be introduced to reduce compressor energy consumption. All the major vendors offer various electronic control systems, but simple remedies that can save a lot of energy include replacing the clogged air filters that cause excessive pressure drops - and which require compressors to work at higher pressure to overcome them - and ensuring that air is only compressed to the pressure that is really required by the applications.

And then there are the losses incurred through leaks. According to Boge, a single 3mm diameter hole accounts for around 3kW of energy and more than £1,000 per annum, depending on energy prices and how efficiently the air was compressed in the first place. Based in Bromsgrove, Beko Technologies' ultrasonic Leak Detector can identify leaks from up to 15m away. The company's latest enhancement is to add an ultrasonic generator that can be used to test pipes or welded vessels, without having to pressurise them first. The generator is placed in the vessel and seams are scanned to detect ultrasound coming through. Beko says it is also considering combining the Leak Detector with a laser pointer that would visually mark the leak with a red spot of light, whose location could be captured using a digital camera. The Leak Detector weighs just 400g, is 170mm long, and is used in conjunction with headphones and a probe on a 1m long cable. Power is from a 9V battery, ensuring continuous usage for about 10 hours.

Leaks might be from small holes resulting from corrosion, from joints or from malfunctioning condensate drains on air receivers and filters. Replacing old, rusting, screwed steel pipes with a more modern system of pipework can easily pay for itself in a short period of time.

Dunlop Aerospace, which makes aircraft braking systems, has recently acquired two Boge S series fixed-speed screw compressors and a Boge SF frequency controlled screw compressor, following an audit by suppliers Direct Air & Pipework. Direct Air also identified a new location for the compressors that allowed a bespoke zoning system to be installed. Zoning the compressed air system allows the isolation of parts of the site where no air is required at certain times, which eliminates leaks in those areas.

Finally, it was proposed that the entire system should be controlled and monitored via an energy management system, enabling Direct Air continuously to monitor the site remotely. A post audit shows a reduction of 29% in the cost of compressed air energy and a 50% reduction in air consumption during low production periods.

Atlas Copco offers an aftermarket auditing and servicing service called AIRConnect, which features round-the-clock monitoring and data available online. Automatic warning notifications are sent via email. The service includes remote diagnosis, maintenance and online reports. Hardware monitors all devices in the compressor room and provides access to the Internet. The package is upgradeable to AIROptimiser, a hardware and software packaged solution designed to ensure optimum efficiency. Ingersoll-Rand also offers a remote monitoring service called Intelliguard. The company's main UK distributor is Air Solutions in Wigan.

Dryers are another area worthy of consideration. Boge offers zero purge desiccant dryers, with vacuum regeneration. Atlas Copco offers hollow fibre membrane dryers. A special coating on the insides of the fibres allows water molecules to pass through, but not air. Water vapour is collected in the space between the fibres. A small amount of dry air at the dryer outlet is directed to the space between the fibres, where it expands. This purge air flushes out the water vapour to the ambient atmosphere through openings in the assembly.

But the biggest energy loss is the waste heat inherently produced by compressing air - up to 94% of input energy, according to Atlas Copco. It suggests that the available heat from a water-cooled compressor should be recovered as hot water to be used on site for washrooms, space heating, water radiators, water-to-air heat exchanger, industrial process water and boiler-feed water. For an air-cooled compressor, the warmed air could be used for space heating.

Air solves a watery problem
Reliability has been improved in the operation of a large valve in a major UK water treatment plant by switching from electrical operation to pneumatic.

The Thames Water Ashford Common water treatment plant processes 690 million litres of water a day and supplies a quarter of London's entire potable water. The plant includes a 1600mm butterfly valve, both as a gravity bypass and to control flows on to the works, halfway down a 30m shaft.

Towards the end of 2005, Thames Water commissioned a replacement valve, as its existing valve was constantly oscillating under load conditions and required frequent servicing - costly and inconvenient, owing to its inaccessible location. Thames Water's main contractor Costain consequently tasked process valve manufacturer Erhard to come up with something better.

Erhard chose to use a pneumatic, rather than electrical, actuator, as that offered a much more robust control solution, with minimal maintenance requirements. The actuator is a custom-designed unit produced by Festo, directly coupled to the butterfly valve without gearing or spindles. The system is controlled by a remote Siemens Programmable Logic Controller, with local operator override facilities for routing inspection. Predicted operating life is more than 2 million cycles.

Aspects Engineering replaced the valve in 16 hours - eight hours less than the allocated time.

SOE

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