Plant & Works Engineering
Energy in focus…
Published:  18 November, 2016

PWE spoke to Andy Jones, managing director at Mattei to answer questions on compressor energy efficiency.

PWE: What kind of difference can an energy efficient compressor make to utility bills?

AJ: Some businesses have a misconception that compressed air is ‘free’ once they have invested in the equipment, but this couldn’t be further from the truth. Compressors account for around 10 per cent of the total electricity consumed in the industrial sector, so they can be a good place to start making savings.

We have many examples we can cite where manufacturers have achieved substantial savings through updating the compressors in their factories and processing plants. One of our customers, a leading food manufacturer, is expected to save around £150,000 over five years as a result of replacing large reciprocating compressors (which had been in operation for nearly 40 years) with three Maxima 75 high efficiency low speed compressors. For another customer, replacing a 12-year-old 90kW compressor with a Maxima 75 resulted in energy savings approaching £20k per annum.

PWE: What’s the best way to find out if a compressed air system is inefficient?

AJ: In the first instance we would recommend a data logging exercise to evaluate compressed air needs and the efficiency of the system. Air consumption profiles are recorded and measured over a seven-day period, followed by discussions to identify unusual patterns or planned process changes. Investing in a more detailed energy audit, carried out in accordance with the international standard ISO 11011:2013, Compressed air – Energy efficiency – Assessment, can paint an even more realistic picture.

Flow monitoring is also a very useful tool for understanding how much air is used in total and which areas are the greatest users (especially when combined with data logging), and also helps identify leakage more accurately than other methods.

PWE: If readers have recently changed their production processes – are their compressors still suitable?

AJ: To maximise efficiencies, compressors should be closely matched to the end user’s needs. For example, if you have a variable speed compressor when a fixed speed model would be more appropriate, or vice versa, it can waste energy. A data logging exercise can help to identify which option is most suitable, and whether the output is correct. We found that one company running a 75kW compressor could actually fulfil its compressed air requirements with a 45kW machine, with estimated savings being in the region of £10,000 a year.

PWE: How do you know whether one compressor is more efficient than another?

AJ: There is a lot of confusion, and indeed misinformation, regarding the energy efficiency of compressors. It’s important to analyse a compressor’s specific energy efficiency, as in reality, the electricity consumed during operation over a five-year period accounts for around 75% of the total cost of ownership, including the initial capital outlay for the compressor. So, even if a higher efficiency machine costs more, you’ll save money in the long run because of reduced energy costs.

However, no matter how energy efficient a compressor is, the system needs to be appropriately designed, leak-free and properly maintained for savings to be fully realised.

PWE: Are compressors becoming more efficient?

AJ: It is likely that the majority of, if not all, compressor companies supplying the UK market have been looking at improving the energy efficiency of their compressors for some time, particularly in light of Lot 31 of the Ecodesign Directive (widely referred to as ErP) preparatory study for compressors. Though the recommendations of this initial study are yet to be finalised, a large number of compressors will potentially need to become more energy efficient.

PWE: Should I consider heat recovery?

AJ: Heat recovery should always be considered. Compressors generate a lot of waste heat (around 90% of the electrical input used to power a compressor’s motor is turned into heat), and around 80% can be recovered and used for heating or hot water.

However, although this type of system can deliver energy savings, the factory or plant needs to have an ongoing demand for space heating or hot water to make it viable (a compressor generally runs all year, so it is more efficient to have a continuous demand) – which isn’t always the case. Also, if space heating is required on the opposite side of the factory to the compressor it will be a costly exercise, and won’t actually be that efficient.

PWE: Is there anything else that can be done to improve the efficiency of a compressor?

AJ: It’s important to regularly check for leaks. In many companies, 30% of the air generated is wasted through leaks, which can prove costly over time. Check for leaks at initial installation, re-check frequently and carry out an annual leak detection survey.

Making changes to pipework can also improve efficiency. Excessive lengths and bends lower system efficiency, so pipe runs need to be suitably designed and laid out.

In addition, compressed air needs to be used appropriately and as intended; for example, using it to dust off machinery or workbenches wastes energy, as well as causing a safety hazard.

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