Plant & Works Engineering
Going oil-free for cleanliness
Published:  04 August, 2016

Oil-free compressors are appropriate for hygienic applications such as food or pharmaceutical production and can be designed for maximum efficiency, says Mark Whitmore, general manager at BOGE.

Oil may be a crucial element in the smooth running of machinery, but can be a hindrance when strict cleanliness is required. It is therefore a balance to ensure that machines such as air compressors run efficiently but do not generate contamination – especially in industries such as food or pharmaceuticals.

Whether compressed air is generated via positive displacement compression or dynamic compression, compressors fall into two broad ranges – oil-injected and oil-free. An oil-injected air compressor works by injecting oil into the compression cavities of the unit, which achieves two things: it helps seal the unit, which reduces pressure drops caused by escaping air; and it provides cooling – because compressing air produces heat that must be dissipated.

Banishing oil

In an oil-free air compressor, air is compressed entirely by the action of rotary screws. It is a ‘dry running unit’ whose compression chamber is not lubricated, and whose screws operate contact free. It means there is no oil to contaminate the compressed air. A high proportion of oil-free compressors are of the rotary screw type, and are commonly used where large volumes of compressed air are needed – such as in commercial or industrial applications.

Air is pulled into the compressor and pressurised in the compression chamber, before being released into the wider compressed air system. The air does not come into contact with oil, so is ideal for use in applications requiring high levels of hygiene such as food and drink processing and pharmaceutical manufacturing.

It should be noted that ‘oil-free’ compressors do actually contain oil – but only for lubrication. High performance seals ensure that oil cannot come into contact with the air, by getting into the compression chamber.

A second type of compressor – which also comes in oil-injected and oil-free versions – is the reciprocating or piston compressor. These use a piston to compress gases to high pressure. In oil-free piston compressors, bearings are filled with grease and encapsulated.

Beyond this, there are very large process compressors – or turbo compressors, which are normally centrifugal. The key component here is the impeller, which contains a set of rotating blades that gradually raise the energy of the air. This type of compressor is always oil-free.

Using oil-free compressors alone, however, does not mean that the compressed air produced will be totally oil free. Any impurities suspended in the ambient air will still be present after compression. To totally remove these impurities, some manufacturers, install a catalytic converter at the compressor outlet. Any residual oil in the air is run through the catalyst directly after leaving the compressor, where long chain hydrocarbons – which consist largely of extended carbon chains bonded to hydrogen atoms – are ‘opened’ until only carbon dioxide and water remain. This results in oil-free compressed air and eliminates the need for expensive downstream condensate treatment. Catalytic converters are not subject to wear, have a service life that is at least as long as the major components of the compressor, and do not require filtration, which reduces maintenance to a minimum.

Off-the-shelf compressed air systems are not always appropriate. In this case, manufacturers can offer customised packages. This might include design and project management, while the bespoke package could be a containerised or skid-mounted system.

Customised compressed air systems tend to be more common in oil-free applications, because oil-free systems are highly engineered and may need to meet specific compliance standards or certification. This customisation can take many forms. A good example is nitrogen generation systems. These require oil-free compressed air, and can be bolted onto compressor packages.

Efficiency boost

Once you have decided on which type of system you need, the next consideration is ensuring maximum energy efficiency. Around 75% of the lifetime cost of a compressor is spent on energy, while the rest is split between capital cost and maintenance. Anything that can boost energy efficiency will help to minimise the cost of running the system. There are three main ways to do this in a compressed air system.

Using smooth bore pipes helps to reduce friction losses in pipes – so there is less of a pressure drop across the system. The lower the pressure drop, the less air needs to be generated at source – and the more energy you save. The effects can be surprising: a 1 bar reduction in pressure generation equates to 7 per cent in energy costs.

Fitting variable speed drives (VSDs) is another well-proven method of energy saving. These can either fitted with a compressor, or retrofitted to a fixed-speed compressor. A VSD automatically slows down or speeds up the compressor to match demand, and can banish inefficient behaviour.

For instance, a fixed-speed compressor can run ‘loaded’ or ‘unloaded’. When running loaded, it draws in air, compresses it and delivers it. When running unloaded, system pressure builds until reaching a set point – after which the compressor begins to unload. Pressure then falls as the compressor vents its internal pressure, before reloading and returning to full capacity and producing compressed air.

When a compressor is running unloaded, it shuts the intake valve – but the motor continues to turn even though there is no load on it. So, although there is a reduction in power consumption, the system continues to use some power – typically, 20-40% of its input power. If, however, a fixed-speed compressor continues to use air, the pressure will fall to its minimum and the system will then ramp up again.

VSD technology eliminates unloaded running because it operates to a single constant target pressure, delivering compressed air according to demand.

Significantly, more than 90% of the electrical energy used by a typical air compressor is lost as heat, according to the British Compressed Air Society. However, using heat recovery allows the heat to be recovered – for space heating, for example, by venting hot air out of the roof of the compressor and ducting it directly to where it is needed. By adding heat exchangers, waste heat can raise the temperature of water – to around 95°C on an oil-free compressor.

Potential savings are impressive: if a 160kW compressor runs at 70% capacity and 90% of the input energy can be recovered in heat, you can recover 100kW of heat energy. This can equate to a significant amount of money: for a compressor that runs for 6,000 hours – and an electricity tariff of 10p per kWh – the savings amount to £60,000.

Final choice

Choosing the right compressed air system depends on a range of factors. Even before the system has been designed the user should consider several factors: what the compressed air is to be used for; what air quality, flow and pressure are needed; and, what expansions to the system might be needed in future?

Users also need to look at the ambient conditions and whether there is enough power on site to run the system. Factors like relative humidity and altitude can have a huge effect on the specification.

Specifying the best system is complex and time-consuming – and requires extensive knowledge – which makes it worthwhile consulting the experts before making a final decision. These experts can be found at supplier companies or at the British Compressed Air Society in the UK, and the Compressed Air and Gas Institute in the USA.