The Importance of Proper Air Quality in Pneumatic Systems

Pneumatics can be used for driving and controlling various operations, from small and precise to substantial industrial applications. However, air quality is a critical factor in using compressed air. Contaminants such as oil, water vapour, liquid water and particulates affect the air quality in pneumatic systems. They will interfere with their proper operation and should be eliminated. If moisture and contaminants aren’t removed from your system, it will experience damage or premature wear.

For optimal performance in a pneumatic system, air leaving the compressor must be reduced in pressure, and contaminants must be filtered out. Some applications also require adding clean oil to the filtered air to lubricate downstream equipment. The most common method is a combined filter/regulator/lubricator unit (FRL). Every pneumatic system is configured differently, but they all need air of the correct pressure, temperature and quality to function optimally. 

Cleaning Compressed Air Of Contaminants

When compressed air exits the compressor, it’s wet, dirty and hot. It will probably also be higher in pressure than you need. This is where the FRL unit comes in, with the filter being the most critical element in removing moisture and contaminants.

Oil

Most oil you’ll find contaminating the compressed air in your system comes from the compressor itself. The compressor needs oil to lubricate it, but when the air has been compressed and leaves the compressor, it no longer retains any lubricating quality. At this point, the oil has changed from a helpful lubricant to an aggressive contaminant. 

Oil can contaminate compressed air systems in three forms: oil vapour, aerosols or oil/water emulsions. Standard airline filters will remove emulsions, but aerosols or vapours require more sophisticated filtration. 

Removing oil vapour in most industrial processes is unnecessary since it’s only present in minute quantities. But it must be eliminated in clean environments like breathing air applications, pharmaceuticals or food and beverage processing. The air is usually passed through a pre-filtration unit, a coalescing filter and an adsorbing bed to extract the oil vapour for these sectors. 

Aerosols are minute oil particles found in suspension in the air. Most are too small for standard airline filters to remove with centrifugal action. This means you’ll need special coalescing filters, which require protection from water and particulate contamination. To remove aerosols effectively, you should mount airline filters immediately upstream of the coalescing filters. 

You can eliminate enough oil with a standard airline filter for most applications. However, there are some situations where the air must be completely oil-free. It’s possible to get oil-free compressors, but these don’t get rid of water and dirt. A more cost-effective solution might be to use a combination of lubricated compressors, after-coolers and ordinary airline filters. Then, you can install speciality filters only where oil-free air is necessary.

Water Vapour

A filter designed to remove water in its liquid state won’t eliminate moisture in the air. About 0.25% of all air comprises water vapour, which may condense further in the filtration process. There are several ways to remove condensation in pneumatic systems, including drain valves, water separation, after-coolers and air dryers. 

If your application demands zero water contamination, you’ll have to consider an air dryer. This will reduce the water vapour content to a very low temperature so that its dew point is lower than any surrounding air. Most applications commonly use drain valves, but after-coolers and dryers are necessary when water content is critical.

Liquid Water

Hot air from the compressor contains water vapour, which will condense as the air cools. How much moisture there is depends on the air pressure and temperature: more liquid water will form at the highest and lowest temperatures. This is the most efficient point to remove it with an after-cooler. Collected water can be removed with automatic drain valves, filters or drip leg drains. 

Solid Particles

You’ll find particulates in all pneumatic systems, such as dirt from ambient air or carbon build-up and corrosion. Particulates vary significantly in size but are generally categorised as acceptable (<40 μm) or coarse >40 μm). Applications like process control instrumentation or high-speed pneumatic tools typically use fine filtration (10-15 μm). Applications like food and beverages, breathing air or paint spraying require ultra-fine filtration with high-efficiency filters. For coarse particles, you can use ordinary airline filters and install them as a pre-filtration system in addition to finer filters.

Selecting The Right Filter

Once you know what type and quantity of contaminants you’re dealing with, you can determine how clean the air needs to be in each section of the industrial process or plant. Contaminant levels are defined in air quality classes, as set out in ISO 8573. You should consult the class requirements to determine the recommended filtration level for your application. You’ll benefit from reduced energy and maintenance costs when you choose the suitable filters and place them in the appropriate locations. 

Crucial Factors In Designing Filtration

Generally, you should always locate standard airline filters for water and large particles upstream of coalescing filters. Choosing the appropriate filter is critical according to how much air you’ll need. 

You’ll find your energy and maintenance costs will increase if you have undersized, inappropriate filters, so always:

• Choose the correct type of filter

• Select the suitable rating for particle removal

• Choose filters that you can drain and maintain easily 

• Make sure liquids are removed efficiently, with no possibility of re-entrainment

• Select filters with easy visual monitoring to ensure proper function and maintenance 

Proper air quality in pneumatic systems can’t be too strongly emphasised. Correct air preparation is critical to achieving optimal performance in all pneumatic systems. When effective contaminant filtration is installed, you can move on to design issues such as lubricants and pressure optimisation.