10% of UK industrial electricity is used to produce compressed air, at an annual cost to UK PLC of around £750 million (and rising). Compressed air is an expensive resource, and shouldn’t be wasted. It is important to get an independent view of compressed air use within a production environment. Consultant Engineering firm Air Technology, of Loughborough, has conducted over 1900 energy saving investigations in many different industries across the world, so have great experiences of the main ways in which energy is wasted and what can be done about it. By taking a holistic view of air use throughout a plant, the savings achieved have averaged 28%. Savings can be significant both on the demand (machinery) and supply (compressor) sides of the equation.
Compressed air is a very convenient way of transferring energy around a plant but, being so convenient, is often wasted. Distribution networks can lose pressure, and bottlenecks can be formed that increase compressor load. There may be simple remedies such as changing fittings or pipework, but detailed studies need to be undertaken to clearly identify where issues lie. On the demand side, optimisation of processes lead to excellent returns. For example, Air Technology showed a paper recycling plant annual savings of £12,000 simply by isolating production machines when not in use. In air-intensive operations such as aeration plants in wastewater treatment, savings of over 60% have been achieved, amounting to more than a million kWhr/yr.
From the standpoint of an independent consultant, the key first step is to understand the process. Energy losses can exist at a number of points within the system; an inappropriately specified compressor, or poor maintenance is often significant, especially when you consider that around 75% of the lifetime costs of an air compressor are accounted for by the costs of energy. The first part of any project is to look at the energy used and the flow of air through the plant; by cross referencing against a large number of results obtained from similar sites it is possible to determine just how far from the ideal the plant is operating. Once the benchmarking exercise is complete, a project can of course be put in place to remedy the situation.
Supply side savings are really down to improving compressor efficiency, individual and group compressor control with variable speed motors and reducing air generation pressure. There are enormous savings to be made for example on Air Technology’s recommendation, a packaging manufacturer replaced a centrifugal compressor with oil free screw machines and saved over £73,000.
If we look at the air generation and distribution system, there are six key areas where any air system can lose efficiency and where improvements can be made:
Generation: Companies waste substantial amounts of money through poor selection of equipment and wasteful operation. Over the life of a compressor, approximately 75% of the cost will be electricity. New equipment is inevitably more efficient, but it is also vital to choose the right size, type and configuration, to cover the full range of demands seen on site. As the compressors get older the percentage of poorly performing and inefficient compressors can increase. This can easily go unnoticed by site personnel as the deterioration is gradual and the fact that more compressors are running can be put down to other factors such as increased loads. Maintenance issues will also contribute to poor performance; inadequate maintenance schedules for compressors or instrumentation will significantly impact efficiency.
Behandeling: The Compressor not only compresses the air, of course, but also airborne moisture and particulates. Atmospheric air contains about 1.5 million particles per m3; in compressed air at 7 bar, this is more like 1.2 billion. It is vital that managers have a clear understanding of the required air standards because different treatment plant options create different burdens on the system. Improvements in dewpoint sensing control and use of waste heat for desiccant regeneration saved a pharmaceutical plant £36,000, for example.
Pressure losses and other inefficiencies through treatment are important. The key is to treat to a minimum standard and then upgrade treatment locally where the individual process makes it necessary. Measurement of parameters such as dewpoint and levels of contaminants including oil, particulates, microbial burden and atmospheric pollutants help to identify over-or under-treatment and point up opportunities for energy reduction.
Distribution and Network: The way the air gets to the process also needs to be considered. There may be pressure drops due to badly routed pipe work, redundant piping, wrongly sized pipework or bottlenecks affecting efficiency which all add up to the pressure set point being well above optimum.
Leakage: The average leak rate in the UK is estimated to be between 20% and 50%. Identifying and repairing air leaks in the system is a vital part of reducing energy costs in a compressed air system. Ultrasonic leak detection can find leaks even in noisy production areas and allow a programme of leak repair to be planned.
Process Optimisation: improving the efficiency of the process and the air network can give major savings. There are many areas where air supply to equipment can be isolated when that machine is out of operation, for example. Also look for applications where air can be replaced e.g. running air driven pumps can cost around 10 times more than standard electric motor driven units.
Waste heat Recovery: don’t forget the waste heat. For a compressor to be efficient it has to reject 90% of the input energy. Recovery of the waste heat for space heating can result in very worthwhile savings. One recent example was an aircraft manufacturing plant that saved £6,800 just by ducting the heat into the factory!
Compressed air accounts for a substantial amount of industrial energy usage, but often does not receive the attention it deserves when considering energy conservation and energy efficiency. An independent consultant, not tied to a machinery supplier, will help you to save energy, improve process efficiency and drive down costs.
Air Technology Limited
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