Recovering hot air14 February 2022

Shell-and-tube heat exchanger. Image: berkut_34/

By recovering waste heat, facilities can reduce energy costs and CO2 emissions, while simultaneously increasing energy efficiency. But is it as easy as it sounds? By Tom Austin-Morgan

Energy prices are continuing to rise, which means facilities will face ever more stringent targets for CO2 emissions reduction, so it’s vital that energy is used as sparingly and efficiently as possible for the sake of both economy and the environment.

Solely focusing on the production of sustainable energy will not answer future needs. Energy reduction is the best way to a carbon-neutral economy – and it can start in the compressor room. Approximately 10% of all electricity used across industry is accounted for by compressed air systems. Under ideal circumstances, 94-96% of the heat energy generated by compressors, which would otherwise go to waste, can be recovered using heat recovery systems.

Heat recovery systems, such as compressed air and gas company CompAir’s heat recovery unit, use oil-to-water plate heat exchangers (not shown) which transfer the heat created by air compressors into a cooling oil; the heated oil circulates by pressure and flows through a plate heat exchanger. The heat is then transferred to an incoming water supply, heating the water at the same time as cooling the oil. The hot water can then be delivered for any application where heated water is required.

Additional heat that isn’t captured in the cooling oil radiates into the canopy of the heat recovery unit and exits the canopy roof as hot air. This would, again, ordinarily go to waste. But by installing heat recovery ducting, the hot air can be diverted to a different part of the facility. The air ducting can also divert to the outside during the summer, if hot air is not required inside the building. An additional benefit of heat recovery ventilation is that it prevents condensation from forming moisture that can cause mould and mildew.


“The key thing about heat recovery is being able to use the heat in an effective manner,” says David Ewan, technical services manager, Boge Compressed Air Systems. “We’re limited by the maximum temperatures we can get out of the compressor without degrading compressor life.”

The maximum temperature of recovered heat is around 50°C. One obvious use for this would be in central heating systems – but that is not possible for the many systems that normally run at around 60°C.

He says that a key factor to consider is when machines generate waste heat. “If your compressor is always generating heat and your process is always able to use the generated heat, great. But, if your compressors are operating on and off, or at variable speeds, then it’s not generating heat at all times, and maybe not at the same times as the processes that require that heat.”

The ideal recipient of heat recovery is a process that requires constant heating of mains cold water, since whatever heat can be recovered, can be used. “The main application of our compressed air heat recovery customers is to use the waste heat to pre-heat boiler feed water. This leads to electricity savings because they don’t need to heat up the boiler feed water from cold,” says Oliver Kent, pictured left, product marketing manager – industrial air and oil free air, Atlas Copco Compressors UK. “The waste heat can of course also be used for space heating, but that only saves electricity during the colder months, whereas boiler feed water makes savings all year round, as many companies use hot water in all sorts of manufacturing processes.”


Ewan says that a full heat recovery system, incorporating equipment like Boge’s Duotherm, could pay for itself in as little as three years, though this would likely require the compressor to run hot at all times, which increases the chance of reliability and maintenance issues. Five years is more likely to be a representative time frame for a return on investment.

Kent expands: “One food production customer who used heat recovery to pre-heat their boiler feed water saw electricity savings of around £10,000 after the heat recovery installation had only been running for 7,000 hours.”

The biggest pitfall, according to Ewan, is specifying heat recovery on a compressor without properly considering the needs of using waste heat elsewhere, especially when it comes to plumbing. “You’ll come across lots of machines on site that have heat recovery systems with two pipes sticking out the side of the compressor that have never been hooked up because they didn’t consider the wider implication of how to get water through the machine to heat it up,” he says. “It’s not quite as simple as getting a plumber to add some push-fit pipework.”

He suggests that it’s worth consulting a specialist, such as the compressor manufacturer (rather than a consultant), get them to come to your site and advise where recovered heat can be used.

Ewan adds: “The system you’ll get then is more complicated and more expensive, but it’ll be much more effective and it can utilise variable speed pumps, smart control and valves so you’re not running water over heat exchangers when you’re not able to recover heat.

“Energy surveys are great to build up a picture of how energy efficient the system is. The more knowledge you can get about how your compressor runs the better, because then you understand what heat you’re generating and when you’re generating it, and you can make informed judgements about how much energy can be recovered and where it can best be used. You can log this data into a master controller to give greater control of the process.”

Being able to recycle the majority of waste heat from air compressors can have a huge environmental benefit. Kent adds: “If a compressor installation consumes 500kW for 8,000 hours per year, its annual energy consumption equals 4 million kWh or 2,828 MT of CO2. If 94% of the energy is recovered, this is a saving of 3.76 million kWh or 2,658 MT.

“Compressed air heat recovery has the potential to save 1.99% of the total electricity consumption in the UK. This is the equivalent of removing the emissions from 913,000 cars per year, or recovering the energy required to power 1.5 million households per year.”


Alexander Pavlov, general manager, Atlas Copco Compressors UK, says that there are a number of other factors to consider as well as heat recovery to save money on air compressors:

1.Matching equipment to applications

2.Air storage, pipework design and air treatment

3.Air purity requirements

4.Air volume and flow pattern

5.Compressor type

6.Control systems

7.System maintenance

8.Remote monitoring of the system

Tom Austin-Morgan

Related Companies
Atlas Copco Compressors
Boge Compressors Ltd
Compair Demag Ltd

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