Produced in Association with
SERIES 21 / Module 07 Compressed Air
can be removed from the compressed air by filters, sometimes in several stages to arrive at the quality required. Odours from air that is used for breathing can be removed by carbon towers. For specialist duties the air system
pipework is manufactured from welded polished bore stainless steel or copper to prevent particles. The ISO8573.1 compressed air
quality standard should be used when specifying air quality for the above contaminants. This will ensure that the correct levels of treatment are applied for the duties the air is to be used on saving both capital and energy costs. The chart given below (Fig 6) shows
a summary of the standard and the classification of the contaminants and various levels. Another contamination that occurs
in compressed air systems is microbial; this must be avoided in pharmaceutical manufacturing and some beverages and food products for domestic use where shelf life can be reduced by microbes that live in the compressed air systems. Treatment is by the use of steam
sterilised filters and use of desiccant dryers, as microbes cannot breed in air at pressure dewpoints below -30°C.
Distribution networks Following treatment there is often a dry air receiver; then air is fed to the usage points by distribution networks. These should be sized with a maximum flowing velocity of 6 metres per second with the full output of the compressor station on line to avoid pressure losses. Ring mains are preferred to spur mains
and local air receivers can be beneficial close to points of high demand.
Use and misuse Because compressed air is expensive its use should be carefully considered. As an example it may be possible to use an electric tool rather than an air tool, or a centralised vacuum system rather than local vacuum ejectors on a production machine. Once it is established compressed air
is to be used, then the correct pressure for the duty should be applied. To avoid overpressure local regulators can be employed. When there is no production on a
line, in a department or a factory, the air should be turned off to avoid waste. Any essential users can be supplied by small compressors.
Leakage Leakage is one of the main sources of waste in air systems and can often be easily rectified. The optimum leakage rate should be less than 5% of the mean production air demand. This figure can usually be arrived at by taking timings of the loaded and off loaded times or by speeds of variable drive units or by flow metering during and in and out of production times. There are several methods of leak
EIBI | FEBRUARY 2024
detection such as by ear and soap and water solutions but the most effective method is by use of a good quality ultrasonic leak detector. Leaks that occur in the hard piping
before any system regulators will vary in air loss according the absolute system pressure ratio as the pressure rises and falls with compressors loading and unloading. This is known as artificial or unregulated demand. The leakage loss downstream of regulation is a constant amount.
Waste heat recovery The heat rejected by air and water cooled compressors in their cooling streams can be recovered and used if a suitable application can be found.
New developments The compressed air and using equipment supply industries are becoming much more aware of the energy costs associated with the service. With compressors the drive efficiencies have been improved by use of permanent magnet motors mounted directly on the compressor drive shaft. These have very high electrical efficiencies, eliminating the coupling or belts reduces the drive losses. This type of motor can be controlled by an invertor to provide variable speed drive. The most popular compressors are of
the oil injected single stage rotary screw configuration that have good efficiency
Fig 6.
at relatively low cost. To further improve compressor
efficiency it is now possible to obtain two stage oil injected machines where both the low pressure and high pressure elements are contained in the same casing. Two stage oil free screw compressors are now available with LP & HP air ends individually driven by its own permanent magnet motor thus eliminating the need for a gear box and oil that enables the machine to have very good efficiency over a wide range of flows. Latest designs of centrifugal flow
dynamic machines include permanent magnet motors with the high-speed pinions directly driven. The pinions can be supported on magnetic or air foil bearings again meaning that no gear box and lubricating oil is required, saving energy and maintenance. Energy saving developments in air
treatment systems include desiccant dryers using waste heat of compression to regenerate the towers, dewpoint sensing control and low pressure loss dryer and filtration systems.
Maintenance Making sure that all the components of a compressed air system are properly maintained is most important from the energy efficiency and reliability standpoint. It is recommended that the original
equipment manufacturer or their accredited agent are used to undertake
all service work and that it is conducted within the recommended hours run. It is a false economy to run machinery for longer than the recommended intervals because the additional energy costs far outweigh any advantages.
AIR SYSTEM OPTIMISING Optimising compressed air systems can enable excellent energy savings of around 30%, some of which can be achieved at little or no cost. Achievable savings can be generalised
with the first low cost 10% coming from reducing leakage, wastage and generation pressure and condensate trap losses, the next medium cost 10% that should provide a return on investment within one year from improving distribution networks, compressor control and air treatment and improved maintenance, with the final higher cost 10% coming from new compressors, variable speed drives, new efficient air treatment systems and point of use improvements. Energy savings areas are air
generation, air treatment, distribution networks, use and misuse, leakage and waste heat recovery. Each of these is discussed below: With air generation the minimum
pressure that is required at the usage points should be established then the potential saving by reducing the air generation pressure can be calculated based on a cost reduction of 6% per bar of pressure reduction. To enable pressure reduction restrictions in the air system should be identified and eliminated. With the air system on full load the
pressure loss across the air treatment system should not exceed 0.5 bar and from the exit of the treatment system to the far end of the distribution system the loss should not be greater than 0.2bar. Control systems that ensure only the
minimum number of machines to meet the duty are on line. Variable speed driven machines can be very beneficial but should not be run for extended periods at over 80% capacity due to invertor losses. Control systems should always be programmed to ensure that variable speed machines are always used as the control unit when there is a mix of variable and fixed speed units. Fixed speed machines should always be used as base load units. Treatment should be to the minimum
standard required; as an example air at a pressure dewpoint of +3°C is perfectly suitable for most factories but there may be a small use of high quality air at -40°C for instrumentation that should be treated by a local dryer to save energy costs. All desiccant dryers should be fitted
with dewpoint sensing controls that will avoid over regeneration of the towers. It is possible to obtain energy efficient zero air loss desiccant dryers that have external blowers or vacuum pumps with the regeneration heat source being waste heat from the compressors or steam. ■
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