Rethink compressed air and save energy costs

Plant managers, maintenance engineers, facilities managers, production managers, energy managers - and anyone else with responsibility for improving plant efficiency - generally know that compressed air is one of the areas where savings can be made, but making changes is not always an easy or comfortable process. According to Peter Tomlins of compressor control specialist EnergAir, it is essential that people in this position challenge convention in order to make the right decision and ensure that any available savings are actually realised.

"First there are the old conventions: the majority of compressor houses still employ fixed-speed compressors and a cascaded pressure switch control system. This arrangement will be familiar to many, but it is outdated and inefficient, and needs to be changed. There is a comfort factor with familiar technology, which is exacerbated by constant time pressure due to other day-to-day responsibilities and a lack of information and good advice on what can be achieved.

"Secondly there are new conventions forming that are little better, and can be misleading: operators are receiving mixed messages from the compressor manufacturers themselves; while most branded air compressors are good machines, an incorrectly sized new compressor can have a negative effect on overall system efficiency. The current trend towards buying variable-speed drive (VSD) compressors does offer the potential to generate compressed air more efficiently, but a VSD compressor that is not controlled as part of a coherent compressor management system (eg connected to an EnergAir compressor management system) that can optimise its use alongside other compressors in a system is unlikely to deliver any energy savings whatsoever or, worse still, could contribute to a deterioration in overall system efficiency."

Challenge the conventions

These conventions, old and new, have to be challenged if the operator is going to see real benefits; an old cascade pressure switch control system does have to be replaced, and do not assume that a new compressor will solve your problems. Where new compressors are required, pay close attention to compressor sizing and, in particular, the relationship between individual compressor output and the variation this provides when taken together and compared to compressed air consumption patterns.

It is important not to assume that a VSD compressor alone will solve all your problems - the big energy saving claims of circa 30 per cent from the compressor manufacturers relate to the comparison of a single fixed-speed compressor with its VSD equivalent, and not how this compressor will perform in the typical customer's multi-compressor house environment. It has to be integrated with existing units under some form of management control in order to deliver significant savings. If you have enough compressors already, consider retrofitting a VSD to an existing compressor rather than making another compressor purchase. A retrofit VSD from a specialist such as EnergAir will be cheaper to buy, cheaper to install and make better use of the existing equipment.

Peter Tomlins adds: "Other conventions about sizing compressors can also have a big impact on system efficiency; air demand tends to be erratic, follow shift patterns and in some cases even seasonal trends. We have seen literally hundreds of sites where the demand for compressed air often falls in between the generating capacity of available compressors. This leads to increases in 'off load' running in the case of fixed-speed compressors, which is inefficient, and where VSD compressors are installed, some hunting or 'dead banding' can occur.

The importance of reaction time

When selecting a VSD compressor, users should pay close attention to its minimum speed as well as maximum speed. Its output span, efficiency across its output span and the inverter reaction time are also considerations. EnergAir sees a lot of examples of VSD compressors with relatively slow reaction times. The result is a compressor that is simply incapable of reacting effectively to fluctuations in demand patterns, ie its intended purpose (like pulling away from the lights in fourth gear).

Consider an example of a typical compressor system with three equally-sized compressors. A conventional lead, top-up and standby compressor arrangement and, in recent times, one of the compressors is likely to be a VSD compressor. Conventional thinking is that the VSD compressor will satisfy primary demand. When demand increases above one compressor, a fixed-speed compressor is loaded. The VSD reacts accordingly by reducing its output to satisfy demand above that which the fixed speed compressor is now delivering. However, if demand for compressed air falls between 100 per cent and 150 per cent then 'dead banding' will occur where the VSD compressor cannot regulate down to a speed proportional to demand. Further to that, what intelligence is going to make the decision about which compressor to run and when?

Old conventions are to choose multiple compressors of the same size in a compressor house. The new convention is to choose machines of the same size and make one a VSD compressor. To challenge convention would be to consider another example in which, instead of three equally-sized compressors, a fourth smaller compressor has been added. Like the new convention, one of the three equally-sized compressors is a VSD. However, dead banding is all but eliminated through the introduction of an alternative size of compressor. Here supply and demand can be satisfied across the entire demand spectrum. Orchestrating it would be some form of management technology, such as that offered by EnergAir.

Questions to ask your supplier

Rising costs are providing the stimulus to change, and so EnergAir believes that challenging convention should take the form of asking questions about plant equipment and of suppliers: Has the demand profile been mapped? Are the compressors on the site the right combination of sizes? Has energy use been recorded and benchmarked so that the impact of any changes made can be validated? Are multiple compressors being controlled by an effective management system? Is a new VSD compressor the right solution, and can its capability for energy savings actually be realised?

Finding answers to these questions will involve challenging convention, but it should ensure that the available savings for compressed air generation are actually realised. EnergAir has made the first step as simple as possible by producing a free online Compressed Air Energy Saving Estimator. Users need only spend a minute of their time using drop-down selection boxes to pick the compressors they have on site and enter the shift pattern; the calculator will then provide a fairly accurate estimate of the energy usage in kWh and the saving that can be achieved in a range of currencies.

Take three 150kW air compressors as an example: working two eight-hour shifts, five days a week uses approximately 1,728,000kWh of electricity per annum. Prior to the recent price rises, the annual running cost would be just over £77,760 per year (based on an average £0.045/kWh). At what is now a common tariff rate of £0.07/kWh, the cost has leapt up to £120,960. The cost differential of £43,200 will remove profit directly from the company's bottom line unless the facilities and operational staff are able to challenge conventional thinking and look for a more efficient way of managing the compressed air generation system.

By installing an intelligent compressor control and management system, the cost of running the compressors in this example could be reduced by £40,800, thereby recouping the vast majority of the money lost due to the price increases. Since companies such as EnergAir are providing the tools to accurately judge savings, without committing significant time and resources to it, there is even less of an excuse for companies not to challenge convention on compressed air. If any more incentive were needed, companies such as Britvic, Delphi, GSK and Walkers Crisps have already done so.