Improving the efficiency of compressed air systems
06 November 2017
Compressed air systems are energy intensive but it is possible to reduce a compressor’s energy consumption – and boost its productivity at the same time - by applying sensible control measures. BOGE’s Mark Whitmore explains.
Compressed air is not free. It is a site utility like electricity and gas, and may indeed account for a significant fraction of a site’s overall utility costs. However, there is huge potential to improve the efficiency of compressed air production and delivery. As well as saving energy, reducing compressed air wastage by tightening up on control measures not only cuts costs, but also improves reliability and boosts productivity. The efficiency of a compressed air system is thus largely determined by the effectiveness of its controls.
Effective compressed air system controls match the compressed air supply with system demand, and compressor manufacturers have developed a number of control measures to achieve this. The most basic method is to turn the compressor motor on or off in response to the discharge pressure of the machine - a simple pressure switch is all that’s needed to provide the necessary motor start/stop signal.
Further up the ‘effective controls’ ranking is load/unload (sometimes referred to as constant speed control), which allows the motor to run continuously, but unloads the compressor when the discharge pressure is adequate. Modulating controls are also useful for throttling the inlet control, varying the compressor output according to demand. A better method – and one that many compressor manufacturers are now adopting – is to use variable speed drives (VSDs), which provide closer control of motor speed (and therefore energy consumption) to deliver a more constant supply pressure; VSDs also prevent the large inrush currents experienced when starting up large AC motors.
But the control of compressed air systems is not the only issue affecting costs; an outage due to a mechanical or electrical fault can skew production schedules and ramp up costs. As well as efficiency, we must look at reliability; the reliability of a compressed air system will reduce with age and will certainly depend on how well it has been maintained. Depending on frequency of use and machine type, a typical compressor life cycle is ten to 15 years before it is no longer economically viable to repair and requires replacement.
Some oil-free models will run for 20 years or more, but the performance of all compressors will eventually degrade over time. A major overhaul involving a motor rewind and refurbished compressor element might keep things running, but the unit’s efficiency will be reduced, leading to higher running costs. A new compressor is likely to be more reliable and efficient, and the use of VSDs will ensure greater energy efficiency. A VSD controlled compressor can provide significant energy savings - up to 50 percent may be possible in some cases.
Modern compressors are more reliable and can run for extended hours between maintenance interventions. Moreover, they deliver cleaner air, thanks to advances in air filtration that will also extend the lifespan of downstream, air-powered equipment. Retaining older plant assets beyond their useful life does not make good economic sense, particularly at a time of soaring energy costs.
There are many ways to measure the performance of a compressed air system; for example, a relatively simple data acquisition system could be used to track the compressor and downstream delivery performance, including upcoming servicing requirements based on actual usage. This would provide an early warning of potential compressor or air delivery system problems, as well as a record of the unit’s energy consumption and an analysis of its energy efficiency over a period of time.
BOGE, for example, offers an air compressor monitoring system called ‘airstatus’, which includes a fault and maintenance indicator, remote data polling and server-based data retention for up to 24 months. Offering remote viewing via an internet connection, airstatus measurements include hours run, indicated faults, and both air and electrical power consumption. There’s also an app for IOS and Android operating systems that provides system status and alarm indications for the itinerant maintenance technician on his or her smartphone screen, via email and/or SMS.
Most compressor manufacturers and consultants offer a one-week audit service involving a site visit and installation of monitoring equipment to measure parameters such as air flow, air pressure and air quality, and provide a report at the end of the period. While this type of ‘one-off’ survey is available from most compressor manufacturers, some customers prefer to monitor their compressor performance on a continual basis. BOGE’s response to this is to provide a fixed, remote monitoring system, which becomes an integral part of the compressor installation.
Compressed air energy assessments should be conducted under the guidelines of ISO 11011:2013, an international standard, covering procedures and reporting, that considers the entire system, from the energy inputs to the work performed as a result of these inputs. The standard regards a compressed air system as having three functional subsystems:
• Supply, which includes the conversion of the primary energy resource to compressed air energy;
• Transmission, which includes movement of compressed air energy from where it is generated to where it is used;
• Demand, which includes the total of all compressed air consumers, including productive end-use applications and various forms of compressed air waste.
The standard also outlines requirements for analysing the data from the assessment, reporting and documentation of assessment findings, and identification of an estimate of energy saving resulting from the assessment process. It also identifies the roles and responsibilities of those involved in the assessment activity.
Unless it is periodically and properly maintained, a compressed air system will become unreliable and inefficient, and therefore more expensive to operate. Properly conducted periodic maintenance will ensure that the system continues to operate reliably and at peak efficiency. Poor maintenance, on the other hand, is likely to result in increased energy consumption due to lower compression efficiency, air leakage or pressure variability, and may also lead to higher operating temperatures, contaminated air streams and poor moisture control.
On a typical industrial site, air-powered production systems derive their energy from high pressure air buffered by accumulators and delivered to the points of use via an air distribution system; air compressors simply replenish this system as air is consumed. In essence, the energy needed to perform tasks involving compressed air comes from air already stored in the site’s air delivery pipework. The efficiency of a compressed air system is thus affected as much by how air escapes the system as by how it is generated in the compressor itself.
Matching supply with demand requires that both air generation and storage are properly maintained. That means fixing leaks in compressed air lines as well as servicing the compressor itself – a task that can be undertaken either in-house or carried out by qualified personnel.
The Chartered Institution of Building Services Engineers sums up the whole maintenance issue rather well: “It is a false economy to ignore maintenance on any type of compressor. It is recommended that manufacturers, or their accredited agents, are used for service work and that genuine spare parts are used.”
Working with a compressor specialist that has the expertise to maintain and service all makes of compressors, dryers and ancillary equipment, and can provide 24/7 support for critical installations can have a significant impact on your operations.
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