Taking a whole system approach

A compressed air system is just that; a system, and every element of it impacts on its energy consumption.

Identify wastage

An ideal place to start is to identify some of the key areas where compressed air can be wasted.  

For example, even if the most efficient compressor available is installed, but it is connected to a system with a 30 per cent leak rate, then all the benefits are lost.  Operators should aim to improve the overall system efficiency. Efficiency in the generation of compressed air is one aspect but targeting avoidable waste in the system is even more important.

Many actions will require elements of maintenance or equipment/system upgrade, but the human element should not be overlooked. Substantial efficiency improvements can be made by implementing new processes and encouraging staff to use compressed air more efficiently and safely.

Detect and fix leaks

Reducing air leaks can have the biggest impact on overall system efficiency. Leak rates in industrial systems are typically between 20 and 40 per cent, meaning the compressor has to work harder, and therefore consume more energy, to compensate for the pressure loss.

A tiny leak of just three millimetres can cost more than £700 a year in wasted energy, but an out-of-hours survey can identify leaks easily. Opeartors should walk the site listening for leaks and then confirm the location using an ultrasonic leak detector, a leak detection spray or event a soap solution brushed on to pipe fittings.

Once the leaks have been identified,  an improvement programme can be implemented. First, the leaks should be tagged and recorded on a site plan. Next, the leaks should be graded in priority order, fixing the largest leaks first and as soon as practicable.  Finally, it is important to make this a collaborative process by encouraging staff to report leaks as soon as they are discovered.

Once the leaks have been repaired, it is important to check the pressure drop from the compressor to each point of use, as it may be possible to reduce the generation pressure and save further energy.

Air leaks should equally be considered as part of any wider system adjustments. For example, turning down the compressor can have an impact on the performance of air treatment. It is worth taking advice from a reputable supplier or service agent to help determine the demand placed on the compressor to supply the system and therefore its energy consumption.

Switch off

Even when off-load, compressors can consume up to 70 per cent of their full load power, so units should be switched off overnight where there is no demand for air.  The time switch settings should be checked regularly to ensure that running hours are optimised, as this can also help to reduce maintenance costs.

Where appropriate, install energy management systems to turn compressors off when they are not being.

Recover heat

As much as 95 per cent of the energy consumed by a compressor is converted to heat and, unless captured, will be wasted to the atmosphere.  Many manufacturers offer heat recovery systems, which can often be retrofitted. This allows users to recycle this excess heat; re-directing the hot oil to an oil-to-water heat exchanger.

Reduce the pressure

Compressed air is often generated at around 8 bar, (116 psi) even if the point of use only requires 6.5 bar (94 psi).    Turning down the pressure can often help to save energy and therefore, cost.

Pressure drop can also contribute to inefficiencies. This should be less than ten per cent of the compressor’s discharge pressure, as measured from the compressor outlet to the point of use.  Thus at a pressure of 7 bar, the pressure drop should be less than 0.7 bar.

Review compressed air usage

Compressed air is energy intensive to run, and cheaper options exist for certain jobs. For example, there may be more energy-efficient alternatives for drying and ventilation. However, for applications where there are risks of explosion or electrical interference, compressed air remains the best option.

Train and involve staff

BCAS recommends simple awareness sessions to advise staff about the costs and safe use of compressed air. For example, not allowing benches or equipment to be cleaned down with compressed air will save a significant amount of air being vented into the atmosphere.  It is far safer to carry out such cleaning using a vacuum system to reduce the risk of injury.

Optimise compressed air use

If compressed air is appropriate for the job, could it be delivered more efficiently? If air knives feature open-ended pipes, fitting a venturi-type nozzle can use 30 per cent less compressed air. By making the operation much quieter, it will improve the working environment too.

Air distribution network - zoning

Not all parts of the network operate to the same hours or the same pressure, so it is worth separating the compressed air system into zones. At the same time, redundant pipework should be isolated. When replacing piping, consider all the alternatives to the usual galvanised steel.  Aluminium and plastic pipes do not corrode and also have a much smoother internal finish causing less pressure drop and thus saving energy.

Don't over treat air

Treating air to remove dirt, water and oil is necessary but can use a lot of energy. Many processes only need a proportion of the compressed air to be treated to a very high purity. In these cases, excellent savings are achievable be treating all the generated air to the minimum acceptable level and improving the purity (quality) to the desired level at the usage point.

Service and maintain

Low cost, regular maintenance will help retain low leak rates and reliability of equipment.  Users should consider a policy that specifies that energy efficient options are purchased when replacing all equipment – whether it is a basic drain valve through to the actual compressor unit itself.  

Finally, it is important to specify the manufacturer’s genuine spare parts and avoid the temptation of cheaper alternatives, realising significant savings in excess of 25 per cent.

Further reading

BCAS’s Reducing Energy Consumption from Compressed Air Usage’ best practice guide

BCAS’s ‘The Filtration and Drying of Compressed Air’ best practice guide

www.bcas.org.uk