Cut down on the carbs

Compressor manufacturers are looking for new ways to help their clients reduce losses and conserve energy, as Mark Venables reports.

Energy saving is one of the big trends driving compressor development in the current economic and political climate. To put energy waste into perspective, the UK spends £60 billion a year on energy, yet some 20 per cent of that total — £12bn — is wasted. That is equal to one tenth of this country’s gross trading profit.

‘[Energy saving] is the way that most compressor manufacturers have to go as a result of Kyoto,’ Geoff Taylor, managing director of Boge UK, said. ‘With the Government’s initiatives to reduce carbon and to invest in new technologies the compressor manufacturers have had to go along those lines.’

The Carbon Trust says 73 per cent of the lifetime costs of an industrial screw compressor will be consumed in energy. The capital cost amounts to about 18 per cent of cost, maintenance about seven per cent and installation costs add another two per cent.

‘A lot of compressors go into existing sites and those sites evolve over a period of time in a factory and there are a number of inefficiencies that can be identified,’ Taylor said. ‘A good example is that many factories buy a compressor and operate it at eight bar, when they only need 7 or 7.5 bar.’ When you consider that every bar of pressure that is generated and not required consumes about seven per cent of the compressor’s energy cost, that is a lot of waste.

Compressor control then comes into the equation. Most manufacturers now use microprocessor systems that decide where these types of efficiencies can be drawn from. The systems also offer controls that can join a number of compressors on a site together with a transducer in the airline to control the pressure within 0.3 bar.

‘Traditionally, with the cascade system, you would have to set nominally a half- or one-bar differential so if you had three or four compressors you would have massive inefficiencies built into the system,’ Taylor said. ‘Pressure control – optimising the system pressure – is a very energy-efficient move.’

However, compressor users can enjoy practical and proven ways to save on their fuel bills and, eventually, on their energy tax liabilities. Another manufacturer, Atlas Copco, provides a monitoring system that ensures a compressed air system is working efficiently.

‘Our unique measuring box can reveal just how much compressed air is being used on site and more importantly, how much energy is being wasted in producing it,’ John Forman of Atlas Copco said.

‘The box is installed free of charge and attached to the customer’s compressor’s control panel for a week. Then the usage patterns are analysed and a detailed report is prepared on how the compressed air system could be improved for maximum efficiency and reduced costs.’

Another way that manufacturers influence the end user is by looking at the system and identifying areas of misuse. Again, the Carbon Trust quotes some healthy figures. Up to 40 per cent of air that is generated can be lost through leakage.

‘When you go round a factory you can quite often hear air leaks,’ Taylor said. ‘It is claimed that a 3mm hole in an airline that is running non-stop would cost £600 a year in energy to sustain. Some sites have massive air losses and reducing those losses obviously saves energy and the compressor has to work less.’

Independent research confirms that motor-speed regulation by variable speed drives (VSDs) produces outstanding energy savings. As much as 35 per cent at partial load can be achieved with the added bonus of extended component life, constant air pressure within 0.1 bar limits to improve process stability and low-current, smooth start-up to avoid peak penalties and simplified electrical installation.

‘VSD-derived energy savings can see equipment investment pay back easily realised within a one to two-year timeframe,’ Forman said.

‘Combined with the added benefit of energy recovery systems, VSDs can play a major role in the mitigation of climate change levy liability.’

This sentiment is echoed by Boge’s Taylor. ‘Another energy-saving tool that is extremely popular is frequency control – that’s where the motor that drives the air is at a variable speed so that you get a variable output,’ he said.

‘If you have a system that has greatly fluctuating demands, the traditional way was to use a fixed-speed compressor to pump the system up. It would reach the pressure, fill the receiver, run off-load for a period of time and when the pressure decayed to a minimum level it would start up again. So you would get all the surges in power and off it goes and builds up the pressure again.

‘With a variable-speed compressor it actually maintains the system pressure at what you want and operates at the required level to generate the exact amount of air that you need. If you had 100bhp compressor with a nominal 4200cfm of air that it was generating, and the system was only called on 2100cfm it would just run at 50 per cent, produce the exact amount of air and maintain an absolutely firm pressure band, which would save energy.’

This approach is not only suitable for new installations. Taylor confirms there are many examples where a frequency-controlled machine can be put into an existing system with a number of fixed-speed machines, saving considerable amounts of energy.

Most of the electrical power used by a screw compressor is transformed into heat. This means 94 per cent of the heat dissipates into the cooling medium (which can be air or water), while four per cent remains in the compressed air and two per cent is radiated into ambient surroundings.

According to Atlas Copco there are substantial savings in energy costs to be made by re-using the heat created when compressing air — up to 90 per cent of energy used by a compressor can be recovered. Air-cooled systems provide large, warm airflow rates at relatively low temperatures for direct space heating. Heat can also be distributed to remote areas of buildings efficiently using a water-cooled system. The heat produced can be recovered as hot water for washroom supply, space heating, water radiators, water-to-air heat exchangers, industrial process water, and boiler feed water.