Plant & Works Engineering
Recover and save?
Published:  07 March, 2017

Heat recovery should be a serious consideration for any business running a compressed air system. Andy Jones, managing director at Mattei, talks through the options.

Energy efficiency remains on the agenda for most businesses, and understandably manufacturers will be interested in learning more about reducing energy costs and carbon emissions. Compressed air is often considered to be industry’s fourth utility, and compressors account for around 10% of the total electricity consumed in the sector. Therefore compressors can be a good place to start making energy savings – indeed insight from the ESOS assessments carried out among manufacturing businesses suggests that compressed air is often an area where energy efficiency can be improved*.

There are many approaches that can be taken to improve the efficiency of a compressed air system, and one effective measure can be heat recovery.

Compressors generate a lot of waste heat – in fact, over 90% of the electrical input used to power a compressor’s motor is lost as heat**. However, a significant proportion of it – around 80% – can be recovered and used for space or water heating, reducing fuel consumption and therefore lowering energy costs and carbon emissions.

It is possible to recoup the cost of a heat recovery system that recovers warm air in under a year**. If the compressors operate for long periods of time, the savings can be substantial – according to the Carbon Trust, a 55KW/350cfm compressor will have 88,000kWh of heat available per year (if the compressor operates for 2000 hours a year), resulting in potential savings of £2588 if gas heating was replaced (based on a boiler efficiency of 85 per cent and a gas cost of 2.5p/kWh). For a 110kW/700 cfm compressor, the potential savings increase to £5,176.

The payback period for hot water systems is about two years**. The hot water can reach temperatures of up to 90ºC, and can be used for a number of purposes, such as to pre-heat boiler-feed water or for sanitary use.

However, having said all of this, using the recovered heat for local space or water heating is not always practical. When considering heat recovery, it is important to assess how much heat is available and at what temperature, and, importantly, where and when the heat will be used.

For example, if the heat is needed on the opposite side of the factory to the compressor, heat recovery will be a costly exercise, and won’t actually be that efficient. And the benefits can only be realised if a factory or plant has a constant need for space heating or hot water (a compressor generally runs all year, so it is more efficient to have a continuous demand) – which isn’t always the case. Overheating the premises can also be an issue, especially in summer months.

As using waste heat from compressors for space or water heating is not always practical, the compressed air industry is seeing the development of a new process that generates more useful electrical energy instead.

Heat recovery is only one part of the picture however, and there are other ways to improve the efficiency of a compressed air system. Leak detection and repair and equipment updates (if required) are other important avenues to take.

In many companies, 30% of the air generated is wasted through leaks, which can prove costly over time. According to the Carbon Trust, even a small leak (just 3mm) could cost more than £700 a year in wasted energy – and we often see compressed air systems with around 150 to 300 leaks. Our advice is to check for leaks frequently and carry out an annual leak detection survey – the cost of which is less than 10% of the overall leakage costs.

Meanwhile, we have many examples we can cite where manufacturers have achieved substantial savings through updating the compressors in their factories and processing plants. However, before a business invests in new compressors, it’s important to understand how much compressed air is being used, and how much it costs.

The most straightforward way to evaluate compressed air needs and the efficiency of the system is through data logging. This involves recording and measuring air consumption profiles over a seven-day period, and some 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 about compressor efficiencies, as can flow monitoring.

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