Recovery position

At Smurfit Kappa's corrugated packaging plant in Weston-super-Mare, an audit of the factory's compressed air use uncovered an alarming statistic. Having monitored the usage of compressed air for 165 hours, capturing critical data every five seconds, it discovered that the system was wasting over 30% of the energy it was consuming – a loss that added thousands of pounds to the plant's annual energy bill. Part of Ireland's Smurfit Kappa Group, a 40,000-employee paper-based packaging company operating in more than 30 countries, the Weston-super-Mare plant's audit had been commissioned by site engineering manager Andy Bale. Compressed air systems consume a lot of energy, he realised, as well as impacting the plant's carbon footprint. With manufacturing industry reeling from the twin blows of sky-high energy bills and the steepest recession in 50 years, it's not surprising that energy audits like Smurfit Kappa's are becoming increasingly common. What is surprising is the level of opportunity that such audits often throw up. At Smurfit, for example, it turned out that a combined investment in a more efficient compressor and a compressor control system (in this case, from EnergAir) could reduce compressed air energy bills by over 27%. "Very few companies have any real understanding of the cost of generating compressed air," says Andrew Fletcher, managing director of Market Drayton, Shropshire-based energy conservation consultants Carbon Control. "Compressed air systems are typically a very significant proportion of overall energy usage – and a lot of people just don't realise this." Government-backed climate change body The Carbon Trust is even more hard-hitting. It is up to ten times more expensive to run air tools than their electric equivalents, says the Trust – because most of the electricity that is used to compress air is converted into waste heat. As a result, its recommendation is forthright in its starkness: for manufacturers wanting to save money, as well as cut their carbon emissions, the use of compressed air should be restricted to where it is the only practical alternative. It's a recommendation that certainly doesn't sit comfortably with the British Compressed Air Society, the industry's mouthpiece. While the Society's executive director, Chris Dee, chooses his words carefully, he quickly makes it clear that he's unhappy with a statement that is such a bald condemnation of compressed air in industrial applications. The Society works very closely with organisations such as the Carbon Trust, he notes, and has long promoted energy efficiency on both cost and environmental grounds. Yet the facts can't be denied, as he accepts. "Physics is physics – and yes, it takes eight units of electricity to produce one unit of compressed air: there's no way round that," he concedes. "Nor would we disagree that from a pure power generation perspective, compressed air is expensive. Where we would disagree is the focus on air tools, which is only one part of the picture – you have to take a broader perspective." And taking that broader perspective highlights the true complexities surrounding what's often called 'the fourth utility'. The upshot: British manufacturers, the compressed air industry – and even the Carbon Trust itself – need to take a far more realistic view of when, where and why compressed air should be used, and what can be done to improve its efficiency. To start with, there's no denying that the process of generating compressed air is inherently wasteful, with a huge amount of the electrical power involved going to generate heat. The motor generates heat, the compressor pump generates heat, and the compressed air itself rises in temperature during compression."The energy used to produce compressed air can represent as much as 30% of an organisation's total energy usage," says Paul McKinney, spokesman for the Carbon Trust. "Annually, UK industry is estimated to use over 20TWh of electricity to compress air – equivalent to the output of four power stations – with associated carbon dioxide emissions of more than ten million tonnes." Clearly, though, power tools constitute only a relatively small proportion of that usage. Mainstream applications, such as spray painting, pneumatic controls and actuators, vacuum pumps, automated assembly stations and filling lines, and point-of-use dryers... these are where much compressed air is used in practice. Which isn't to say, of course, that such applications can't be made more efficient. Even the humble air nozzle can be improved on. Meech, for example, sells nozzles that employ the 'Colanda' effect, a natural phenomenon, to draw in ambient air outside the nozzle in much the same way that water follows the curve of a spoon, boosting the blast of air. "For every 1cc of compressed air, we're adding a further 25cc of ambient air that hasn't had to be compressed," explains international product manager Ian Cameron. And even when compressed air is used for power tools, it's often as a result of a carefully-thought through decision process. In addition to a superior power-weight ratio, which has obvious ergonomic advantages when used repetitively throughout a shift, air tools are often used when there is no safer alternative, points out David Burton, general manager at Boge Compressors. "Air tools will often have been selected for health and safety reasons, due to damp or humid conditions where the alternative, electric power tools, would be dangerous," he notes. Equally, there's no denying that compressor purchases are often poorly specified. The initial cost of a compressor is relatively low when compared to the lifetime costs of operating a compressed air system, says Geoff Brown, applications consultant in the drives and motors division of engineering giant ABB. But all too often, users focus on the initial cost of the compressor – despite the fact that the key to maximising cost savings is optimising overall system efficiency. Finally, manufacturers tend to add compressors as their needs grow, rather than re-thinking their overall compressed air requirements and how best to meet it. Linked together, two or more compressors form a 'cascade', and offer a welcome element of redundancy in the event of a breakdown. But the impact on efficiency can be decidedly unwelcome. This was the case at Smurfit Kappa, for example, points out Graham Coats, MD of compressor control manufacturer EnergAir. A second 90kW compressor was often running when just a small amount of additional capacity was needed – meaning that 84% of energy consumed by that compressor was wasted. Poor overall management of a compressed air installation adds to the problem. As the British Compressed Air Society's Chris Dee notes, air leaks and loss through inappropriate use remain big sources of loss, just as they have for years – and that there's not much the compressed air industry can do about it, except warn, educate, and point to the cost in terms of waste and carbon emissions. And as Carbon Control's Andrew Fletcher observes, almost every manufacturer knows about the importance of leak control, but few tackle it seriously enough. Yet just one 3mm leaking hole costs roughly 3kW, equivalent to around £2,000 per annum. "We go into so many businesses where although they know they should be controlling leaks, they don't seem to be doing much about it," he says. "Very few undertake regular leak testing, never mind remediation programmes. You can ask the maintenance manager: 'Do you test for leaks?', and get an answer in the affirmative – but when you ask to see the logs, or the date of the last test, it's simply not been done." But according to John Forman, communications manager at Atlas Copco Compressors, some quite simple measures can help – cutting costs and easing environmental concerns at the same time. "The waste associated with compressed air often results from the fact that many compressed air systems have no one with overall responsibility for their performance," he argues. "Making someone responsible for the compressed air system is essential for its cost-effective performance, and can make a big difference." Like most compressor manufacturers, Atlas Copco will readily advise on energy-saving initiatives that manufacturing customers can undertake – and, stresses Forman, most of these only require either low or medium levels of investment. A leak reporting and repair programme might result in savings equivalent to 20-40% of compressed air usage – or higher. A chemical company in South Wales, for instance, achieved savings of 50% implementing a six-monthly leak detection and repair programme, with the first survey alone detecting 412 leaks, equivalent to an annual loss of £66,500. Other low-cost initiatives include proper system maintenance, user education and awareness programmes, and avoiding leaving the system pressurised during unproductive periods. Together, such measures cost almost nothing, but can result in savings of as much as 32%. Reducing the system pressure is another easy option: as a rough rule of thumb, every 0.5 bar reduction in pressure saves 3% of electricity running costs. Even medium-level investments – such as installing compressor drive controls – cost under £10,000, and can save up to 15% of electricity consumption. More significant investments can make even bigger impacts on energy saving. Traditional piston compressors have long been supplanted by modern screw compressors, which are much more efficient. But that efficiency comes at a price: they are best run at a fixed speed, and at low output levels are far less economic. With today's newer generation of variable speed drive compressors, the savings in electricity can be up to 35%, as the air supply matches the demand for air – no matter how cyclical. It was just such a replacement compressor that was installed at Smurfit, for instance, and the payback time is generally comparatively short. Better still, modern installations can actively recover the waste heat that compressors produce. "We're getting more and more enquiries about heat recovery," says Philip McArragher, technical manager at HPC Compressed Air Systems. "Heat recovery is still fairly unusual – even though nearly all manufacturers pay for water and space heating that the recovered waste heat could be used for." While the savings attributable to heat recovery vary with hours of running and compressor size, HPC estimates that a 55kw compressor fitted with heat recovery capabilities might save around £4,500 per year. What is often a surprise to manufacturers, says McArragher, is the level of heat recovery that is practically attainable – 94% is recoverable through a combination of water and space heating. Yet often, compressor installations simply dissipate it to the atmosphere, via oil coolers and vents. "The simplest and most direct method of recovering the heat generated in a compressor is by direct utilisation of the cooling air that carries away the heat from it," he says. "This heated air can be ducted away for use as space heating in warehouses and workshops, or it can be used for other applications such as drying, heat curtains or pre-heating combustion air. Even with a relatively small 18.5 kW compressor, you can easily recover enough energy to heat a small unit or office area." And even the Carbon Trust would applaud that.