Liquid water, water vapor, particulates and oil can all interfere with proper compressed air operation. Removing moisture and contaminants ensure that the system does not experience premature wear or damage.
Pneumatics is a versatile, proven technology for powering or controlling the operation of an amazing number of applications, from neo-natal respirators to building-size industrial equipment. The range of pneumatics capabilities is illustrated by the variety of typical systems, their uses and requirements.
• General pneumatic circuits (e.g. directional control valves and cylinders in machine cleaning, air motors and high-speed tools)
• OEM machines
• Breathing air
• Heavy duty lubrication
• Direct injection lubrication, such as required for conveyor chains
• Oil-free applications like paint spraying or film processing
• Critical pressure control and instrumentation
• Motion control for industrial automation or equipment operation
• Continuous processes like those in paper mills or chemical plants
While the configuration of pneumatic components for each of these systems varies, they all require air of the proper quality, temperature and pressure to function most productively. The air leaving a compressor is hot, dirty and wet, and is generally at a higher pressure than desired. Before this air can be used, it needs to have contaminants removed, pressure reduced and, in many cases, oil added to lubricate downstream equipment. This article examines the first requirement: removing moisture and contaminants.
Air exiting the compressor outlet will contain water vapor, but as the air cools, the moisture condenses. The amount of water vapor in any given volume of compressed air is directly proportional to the air temperature and inversely proportional to the pressure, so there is more liquid water when the temperature is lowest and the pressure highest. This is the point where removing it is the most efficient. An aftercooler should be used to cool the air coming out of the compressor to within 8º C of the temperature of the water entering the after cooler for most efficient water removal.
At this point, the outgoing air should be piped to a receiver in the coolest location available, definitely not within the compressor house itself. Further cooling—and condensation—may occur in the distribution mains. These should be laid out with a pitch in the direction of air flow, so gravity and air flow will carry the water to drain legs. Except for these drain legs, all other air take-off points from the distribution mains should be taken from the top of the main to prevent water from entering the take-off lines.
As discussed earlier, water condenses (and is most efficiently removed) at high pressure, so anything that produces a pressure drop in the distribution system should be avoided. Filters should be located upstream of any pressure-reducing valves.
Maintaining consistent pressure also conserves energy, helping to control costs. Make sure to properly size piping and eliminate complex flow paths with undue bends.
Water can be removed using drip leg drains, automatic drain valves or filters. These devices should be located where liquid water is present in amounts large enough to be removed. Because air may cool as it passes through distribution mains and branch lines, it is more effective to install smaller individual filters as near to the actual point of air usage as possible, rather than rely on one large filter at the air receiver.
A system properly designed to remove liquid water will still not remove moisture from the air, and this moisture can condense later in the process. If the application requires complete freedom from water contamination, then the water vapor content must be reduced to the point that the dew point is lower than any temperature to which the air in the system will be exposed.
To remove water vapor from a compressed air system, air dryers must be employed. Dryers are most efficient at the lowest possible temperatures, and performance is diminished when air is contaminated by water, oil or water/oil emulsions, so dryers should always be used in conjunction with filters and coolers.
There are three types of dryers: refrigerant, regenerative adsorbent desiccant and deliquescent absorbent. (See Table 1, Dryer Comparison.) Here are some considerations for making the most cost-effective dryer choices.
Does your process truly require dry air? Air dryers are most commonly needed in general industrial applications where high ambient temperatures exist.
Do not specify extremely low dew points if the process does not warrant them.
Limit the volume of air being dried to that actually needed for the particular process, plus some capacity for expansion. For example, only one area of a process plan may require a dryer.
General recommendations for air drying can be difficult, since this depends on the temperature of the compressed air main adjacent to the operation, the level of pressure reduction and air flow rate. It also depends on the relative humidity and ambient temperatures of the local environment.
Particulates enter every compressed air system, either through ambient air intake, corrosion, or carbon build-up. Dirt particles can range in size from a fraction of a micron to several hundred microns, (See Table 2) but generally fall into two categories: coarse (40 microns and above) or fine.
Most normal airline filters will remove coarse particles. Fine filtration in the region of 10-15 microns is normally required for high-speed pneumatic tools or process control instrumentation. Filtration of 10 microns or finer is essential for air bearings and miniature pneumatic motors. Even finer filtration may be needed for paint spraying, breathing air or food-related applications. These require high efficiency (oil removal/coalescing) filters. Standard airline filters should be used as pre-filters to avoid overburdening the finer filters with coarse particles and triggering premature failure.
Oil as a contaminant
The principle source of oil contamination in a compressed air system is the compressor. Oil lubricates the compressor, but by the time it emerges with the compressed air, it has lost any lubricating capability and in fact is an aggressive contaminant that must be removed.
Normal airline filters will remove enough oil to leave the air suitable for most pneumatic tools and cylinders, but certain applications require completely oil-free air. Oil-free compressors eliminate oil but not water and dirt. So it can be more economical to use lubricated compressors with after coolers and standard airline filters and fit high efficiency oil removal filters only at the points in the system where oil-free air is absolutely required.
Oil in compressed air systems can exist in three forms, oil/water emulsions, aerosols or oil vapor. While emulsions can be removed by standard airline filters, more sophisticated filtration is required for aerosols or vapors.
Aerosols are small oil particles suspended in the air. Approximately 90% of these are between 0.01 and 1 micron—too small to be removed by the centrifugal action of standard airline filters. Special coalescing filters are required, and these should be protected against particulate and water contamination by airline filters mounted immediately upstream.
For most processes, removal of oil vapor is unnecessary, since quantities are minute. Exceptions include food or beverage processing, pharmaceuticals or breathing air applications.
The most common method of removing oil vapor is to pass the air through an adsorbing bed, usually comprised of activated carbon, after it has been through a pre-filter and a coalescing filter. Note: this system will not, as is sometimes thought, remove carbon monoxide or carbon dioxide.
Once all the contaminants have been considered, the degree of cleanliness of air for each part of the industrial plant or process can be determined. Table 3 shows the levels of contaminants allowed in each class of air quality as defined by ISO 8573. Employing the correct filters in the right locations can keep energy and maintenance costs to a minimum.
When designing filtration to clean compressed air, be sure:
• The correct type of filter and element rating is selected for particle removal.
• Liquid removal is efficient and re-entrainment is not possible.
• It is easy to maintain the filters and remove liquid condensate.
• There is easy visual monitoring of condensate or filter elements for proper function or prompt maintenance. This may be a pressure drop device, liquid level indicator or transparent bowl.
With the system in place to remove contaminants from compressed air, designers can move on to the challenges of optimizing pressure and adding lubrication. Correct air preparation is the best way to get maximum performance from any pneumatically controlled system.