Compressed Air
Key information
Almost all industrial SME's use compressed air within the production process and few
employ even simple controls to ensure that waste and misuse is kept to a minimum.
Approximately 10% of the electrical energy supplied to industry is used to compress
air and typically some 30% of this is energy is lost due to leaking systems. The
electrical losses each year due to air leaks, is equivalent to the entire output of a
medium size power station. Even worse, a much larger sum is wasted each year due
to compressed air misuse and general complacency. In particular most businesses
ignore the potential for energy recovery from their compressed air systems.
Major opportunities for savings
Good Housekeeping
Who carries the overall responsibility for the compressed air system on your site? The
quickest and simplest way to save energy is to adopt the following plan.
• Appoint a person to take overall responsibility for the system.
• Walk the system - outside normal production hours - listen for leaks, valves left
open and faulty fittings. If your production runs for 24 hours a day, consider
buying or hiring an ultrasonic leak detector to help you locate leaks. Repair all
leaks immediately.
• A loaded compressor has a different tone to an unloaded compressor. Run your
compressor out of production hours and listen to the changes in tone, you will
quickly establish which tone represents a loaded machine and which tone
represents an unloaded machine. Over a period of ten minutes, time how often
the compressor runs at the two different tones. If the compressor runs 'on-load'
for a total of say three minutes and 'off-load' for a total of seven minutes, this
means that 30% of the air being generated is supporting leaks and wastage -
because there is no production. Target those leak reductions.
• Make sure that compressors and associated equipment are turned off when they
are not in use. SWITCH IT OFF!!
• Fit isolating valves to air operated equipment. This may not cure any leaks in the
equipment but shutting the valve when the machine is not in use will stop the
leak just the same.
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• Ideally you should monitor how often the compressors run on a week-by-week
basis. Fit an electrical meter to the supply, or read the 'hours-run' meter fitted to
most modern compressors. Abnormal increases usually indicate new leaks.
Good Maintenance
Although it is a statutory requirement to maintain compressed air equipment in
accordance with a manufacturers recommendations, an equally good reason for
good maintenance is the increase in efficiency.
• Blocked intake filters reduce efficiency. Throttling the air going into a
compressor can increase power absorbed by 4%. Therefore saving a few
pence by not changing an intake filter will cost a few pounds in energy.
• Poor ventilation has exactly the same effect as a blocked intake filter as this
starves the compressor. Make sure that ventilation grills are kept clear from
obstruction.
• Blocked or dirty intercoolers will increase the power used by the compressor. The
dirtier the production process, the more regularly these items need to be
cleaned.
• Start a compressor maintenance log. Proactive maintenance is always cheaper
than reactive repairs and a well maintained compressor uses less energy.
Energy recovery
All compressors generate heat and many industries can benefit from some form of
heat recovery system. It is a simple task to obtain either hot water or hot air from a
compressor, irrespective of whether the machine is air-cooled or water-cooled. The
following list of suggestions would afford significant opportunities for energy
recovery.
Hot Air
• Full ducted space heating. Compressor cooling air can be 20 - 25 deg C warmer
than ambient air. If the flow is great enough, hot air can be ducted into a
workshop or warehouse to provide 'free' space heating.
• Air tempering. If the hot air flow is insufficient for full space heating, it can still
be ducted into the existing space heating system to provide pre-heating.
• Keeping product and packing materials dry. Simply placing a compressor in a
warehouse may provide sufficient heat to stop cardboard boxes from becoming
damp.
• Providing combustion air to boilers. If the cooling air is 22 deg C above ambient,
ducting the air to a boiler can increase boiler efficiency by as much as 1%.
Hot Water
• Sanitary water to washrooms, canteens & showers. Compressors can provide hot
water up to 80 deg C, quite warm enough for most sanitary applications.
• Central heating.
• Boiler feed water. An increase of 6 deg C in boiler feed water temperature can
increase boiler efficiency by as much as 1%.
• Hot water for washing product.
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Having established a use for the hot air or hot water (the heat sink) it becomes
perfectly possible to recover 80% of the electrical energy consumed by the
compressor as heat. A quick calculation based upon your electrical usage will
determine how much you can save, however most heat recovery systems offer a
payback within 18 months.
Storage
Most compressed air systems contain at least one air receiver and the purpose of the
receiver is to iron-out fluctuations in air demand. If you can hear fast and frequent
changes in tone from the compressor, this indicates that it is hunting between an 'on-
load' and 'off-load' condition. It is likely that the air receiver is too small. A system
with inadequate storage will also show the following symptoms:
• Standby compressors turn on when they are not necessary.
• Pressure losses increase down the distribution pipe so that the pressure at point
of use is much lower than at the compressor.
• Product spoilage may occur.
Installing additional air receiver(s) is a fairly low cost exercise, typically about 5%-
10% of the cost of the relevant new compressor.
Air Treatment
It is not possible to define an ideal level of treatment as this varies from site to site,
however in some extreme cases air treatment will reduce system efficiency instead
of increasing it.
• Always treat the compressed air to the lowest acceptable level. Contact the
suppliers of your production machines or tools and ask them what it the minimum
air quality the equipment needs.
• If only a few pieces of equipment require high quality air, spot treat the
compressed air at point of use instead of treating the whole air supply.
• Change in-line filter elements regularly.
• Remember that refrigerated air dryers use less energy than desiccant air dryers.
• Remember that heat-regenerated desiccant dryers use less energy than heatless
dryers.
Because air treatment systems are expensive, make sure you choose the right one
first time.
Distribution Systems
Although most air compressors eventually get replaced by bigger compressors, the
original pipework usually remains in place. Over the years a pipe system may end up
carrying two or three times the original design load, resulting in excessive pressure
drop and high energy bills.
• Keep air velocities in the main header system below 6 m/sec. If in doubt, ask
your compressor supplier to calculate the correct pipe size to suit your
compressor(s).
• Keep the system pressure drop below 0.2 bar. Measure the pressure at the
compressor house with a gauge, then using the same gauge measure the pressure
at the farthest point on the site. High pressure drops indicate that the air pipe is
too small, stifling the air flow.
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• Add to pipe systems to create 'ring mains' wherever possible.
• Replace rusty pipe.
• Use long radius bends instead of elbows.
System Control
One of the most obvious savings to be made is to generate compressed air at the
minimum pressure that a production process requires. To keep safely above the
minimum pressure requires an adequate control system than not only measures
system pressure accurately, but can also control the compressor(s) within the pre-set
pressure band.
• Determine the minimum pressure for the system. This can be accomplished by
asking equipment and tool manufacturers to specify the minimum air pressure
needed by their equipment.
• Install a controller that uses either a pressure transmitter or a pressure
transducer. These are more accurate than pressure switches and will save energy
by controlling the operating pressures of the compressor more closely.
• Set the lower limit marginally above the minimum required pressure.
• Set the upper limit to a point that does not result in the compressor(s) hunting.
This may require larger or additional air receivers to be installed, however the
costs will be quickly recouped by energy savings - typically within 18 months.
• Monitor the system to make sure that site pressure is acceptable for all of the
production processes. Modify the operating pressure if necessary.
Compressed air is a safe source of energy with a reputation for being expensive. Take
control of your system now and put in place some simple steps that will have a
positive effect upon your energy costs.
For further Action Energy support
• Further information and advice is available free from the Action Energy
Helpline – 0800 58 57 94 or visit the website at www.actionenergy.org.uk.
Action Energy is a programme run by Carbon Trust and funded by the Department for Environment, Food and Rural
Affairs, the Scottish Executive, Invest Northern Ireland and the National Assembly for Wales. It is designed to help
businesses and public sector organisations cut their energy costs through the provision of free, professional advice
and assistance. Action Energy is an integral part of the Government’s climate change programme, aimed at meeting
the UK's carbon reduction obligations under the Kyoto Protocol.
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