Energy-saving in compressed air

Topic editor:
Marion Beaver

Marion is a senior energy auditor with Maxaura Ltd

Air is free – until its compressed, then it becomes one of the most expensive resources on site. However, many users pay it scant attention so long as it supplies processes consistently. This is not wholly surprising - the cost of compressed air is as invisible as the utility itself, remaining comfortably hidden in the electricity bill and as such, unaccountable. The old maxim “If you don’t measure it you can’t manage it” holds as true for compressed air as any other resource.

Before implementing any improvements to a compressed air system, you should establish your current costs and consumption by way of an audit. An audit is necessary to establish a baseline against which improvements can be measured.

To justify an audit it is helpful to estimate the cost of running the compressors, users may estimate it thus:

Annual energy cost = motor power (kW) x % utilisation x operating hours x unit cost electricity (£ per kWh)

e.g for a 75kW compressor running 24/7 at 70% utilisation, 8p/kWh the annual energy cost is:

75 x 0.70 x 8,760 x 0.08 = £36,792

This figure whilst only as estimate may surprise some users and prompt further investigation. Note: this is only the energy cost of the compressor. The electrical consumption of dryers will be additional to this figure, as will servicing costs etc. and capital depreciation.

For establishing the baseline consumption, there are a couple of options:

  1. The “black box”: for an indication of the compressor loading profile, current clamps can be attached to the compressor and a data logger used to monitor the power consumption for a week or representative period of production.

    Such surveys are widely available from compressed air equipment distributors and can be useful as an initial overview for highlighting peak periods of demand and off load running time. However, the flows are derived from the power consumption are therefore only an estimate based on the manufacturer’s compressor output data (when new). Also this method only considers the generation (supply) part of the system, not the efficiency of the system as a whole.

  2. Flow monitoring For an accurate assessment of system performance, the scope of an audit needs to cover the whole system: generation, control, treatment, distribution and point-of-use. It should measure all key system parameters: flow, pressure, power consumption at crucial locations, not just the compressor house. With true measured flow rates you can:
    • Highlight the performance of compressors, dryers
    • Identify control system issues
    • Determine system leakage
    • Size compressors, dryers, receivers, controls and pipework correctly
    • Calculate a benchmark cost for compressed air
    • Calculate a specific energy consumption (i.e. number of kW needed to produce each litre per second of air).
Only by using this holistic approach can you cost effectively minimise your system’s carbon footprint and reduce your energy spend. Future tutorials will look more closely at the barriers to saving compressed air in terms of management and accounting practices.

Useful links

BCAS is the trade association representing manufacturers, distributors and users of compressed air and vacuum equipment and services. It acts as a one-stop shop for all compressed air related matters such as technical, legislation, health and safety and energy efficiency. Follow these links for: compressed air training courses and posters on energy saving in compressed air
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