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SERIES 21 / Module 07 Compressed Air


Saving the fourth utility Author E C Harding CEngMIMechE – Air Technology Ltd C


ompressed air, often known as the fourth utility, is used throughout industry and other sectors as a very convenient and


popular means of transmitting energy from the incoming source to usage points for many applications and processes at several pressure levels. If the system is designed and correctly operated compressed air is safe but it can be very dangerous if improperly applied. Recent developments from the


compressed air industry, such as improved compressor and drive motor efficiency, variable speed drives, electronic compressor control systems and online condition monitoring, have helped to make air systems more efficient and these give opportunities for energy and other cost of ownership savings that can provide good paybacks on capital expenditure. Applications for compressed air


go back into quite ancient history as a means to increase the intensity of fires for metal smelting using manually operated bellows at furnaces – a process still in use today in the steel industry and other industries using large electrically driven low pressure compressors. As industries turned from manual


labour to use of machinery, compressed air was used and still is in mining, shipbuilding, automotive manufacturing and general industry and its use has grown into many other fields such as waste water treatment, pharmaceuticals, chemical processes, microelectronics and a plethora of other applications. Many of the more recent applications demand very high levels of air purity, all of which add to the cost of air generation. Table 1 below gives typical


applications of compressed air, from ultra-high vacuum to the highest used pressures. The diagram above (Fig 1) shows


the components of a typical industrial compressed air system.


Pressure bar Ultra high vacuum Medium vacuum -10


For details on how to obtain your Energy Institute CPD Certificate, see ENTRY FORM and details on page 20


EIBI | FEBRUARY 2024 Low pressure Medium pressure


Medium to high pressure High pressure


Ultra high pressure >-010 9 -10 Up to +4 +7 to +10


+15 +40


+400


Fig 1. The components of a typical industrial compressed air system


Evaluating effi ciency What is not always understood is that compressing is an extremely costly method of transmitting energy. As an example, the energy cost to drive a pneumatic tool at 7 barg such as a drill is increased by a factor of 10 when compared with using an electrically driven drill. This is due to the waste heat that is rejected by a compressor, which can amount to 90%. Compressors are only needed


because the customer has a use for compressed air but as they are the beginning of the process it is logical to start with air generation. There are several ways of expressing


the efficiency of compressors such volumetric, isentropic and polytrophic but the only important measure of efficiency is the power input versus the air output at the specified pressure – this is known as the specific power consumption (SPC). The SPC depends on the size and


configuration of the machines. At 7 barg it should be around 11 to 13kW/100m3/h with the compressor on full load.


Table 1. Typical applications of compressed air Description


Applications


Surface spectrometers, particle accelerators and other scientific applications


Glass blowing, dearation, dewatering and evacuation


Waste water treatment and product and powder conveying


General industry, handyman and dentistry


Aerosol filling PET bottle blowing Specialist air bottle filling It is important to know the off load and


part load power consumptions as well as the full load, as very few compressors will be running at full load. There are many configurations


of compressors based on the flow and pressure requirements such as reciprocating, vane, diaphragm, toothed rotor, scroll, roots blowers, rotary screw and centrifugal machines with lots of subsets around cooling, pressure and air quality requirements. The most popular machines seen


in the field are rotary screw and piston positive displacement machines and centrifugal flow dynamic machines. The performance of positive


displacement types can best be described with a pressure volume diagram as shown in Fig 2 on page 18. This type of machine inhales and compresses a fixed volume of air. This would be for a typical single stage piston or rotary screw machine. Here it can be seen the air volume


inhaled is compressed to the terminal pressure according to the equation PVn = C where n is the gas constant (for air this is around 1.39). As the air is compressed its volume decreases with the amount being delivered into the system being in relationship with the absolute compression ratio which in the case shown will be 1/8th. Once the piston or open screw or


vane flute completes its air delivery the compressed air left trapped within the machine has to re-expand until atmospheric pressure is reached, at which time the machine can inhale more air. The compressor can only deliver the


amount of air that it inhales – this is known as the Free Air Delivered (FAD). The volumetric efficiency is the FAD divided by the swept volume. The area contained within the curves is proportionate to the work being done to


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