Compressed air quality

Contaminants can be introduced into the compressed air stream by the inlet air, compressor, receiver tank, piping, and other installed components. Contaminants include solids such as dirt, dust, pipe scale, and compressor wear particles; liquids/aerosols such as oil and water; and gases/vapors including water, oil (hydrocarbons), carbon monoxide, and chemical pollutants.

10/15/2002


Key Concepts
 
  • Quality levels

  • System components

Sections:
Dryers
Filters
Drain traps
Breathing air
Sidebars:
Monitoring air quality
Startup protection

Contaminants can be introduced into the compressed air stream by the inlet air, compressor, receiver tank, piping, and other installed components. Contaminants include solids such as dirt, dust, pipe scale, and compressor wear particles; liquids/aerosols such as oil and water; and gases/vapors including water, oil (hydrocarbons), carbon monoxide, and chemical pollutants.

Compressed air is treated by cooling, drying, and filtering for particulate matter, oil aerosols, and oil vapor. Because the specific application determines the type of air treatment required, the first step in meeting air treatment needs is to look closely at the application and the air quality it requires. There are six basic levels of compressed air treatment. At each level different components work together to achieve the required purity levels.


Dryers

Atmospheric air entering a compressor always contains water vapor. For example, at 75 F and 75% relative humidity 20 gal of water will enter a typical 25-hp compressor during one day of operation. When air is compressed, this water content is concentrated.

When the air heats up during compression, the water remains vaporized. However, when the compressed air travels downstream and cools, the vapor condenses into liquid droplets. This water can lead to corrosion, air leaks, pressure drops, and scale formation.

Refrigerated air dryers are the most common and economical type of dryer. Warm and saturated air from the air compressor is cooled to between 35 F and 50 F. At these temperatures, the water condenses and is mechanically separated and discharged from the system.

Once the air is free of liquid moisture, it can be reheated and discharged into the compressed air system. This air now has a 35 F to 50 F pressure dew point, which means the air temperature has to drop below this temperature before condensation occurs.

Desiccant air dryers are used in applications that require compressed air at dew points as low as -150 F. Air flows alternately through two identical drying towers, each containing a desiccant bed. While one tower is on-stream the other is being regenerated. Purge air is used to regenerate the desiccant.

Desiccant dryers are either cold regenerative or heat regenerative. In cold regenerative dryers, 15% of dried, unheated purge air is diverted from the air outlet and is used to regenerate the desiccant.

In heat regenerative desiccant dryers, purge air is heated to between 300 F and 400 F and directed through one of the desiccant towers. Depending on the heated dryer type (internally heated, externally heated, blower purge, etc.), only a small percentage of purge air, 1% to 7%, is diverted from the dried air stream. Valuable purge air is saved, reducing operating costs up to 40% in applications more than 500 cfm.

Membrane dryers pass compressed air through a bundle of tubular membrane fibers. Water vapor permeates the membrane walls. Dried air continues down the tubes and into the downstream air system. The amount of "sweep air" is lost through the membrane walls along with the water vapor is an important evaluation for membrane dryers.

Filters

Airline filters should remove liquid aerosols, rust, scale, dirt, and other solid particles 1-micron and larger. They operate with two stages: a first stage of coarse media collects larger particles and a second stage of finer media separates dirt, water, and oil-aerosols.

In oil-lubricated systems, a good filter can also be used as an oil removal filter with more than 70% efficiency, or it can be used in combination with other filters to remove particulates. In nonlubricated systems, filters are used upstream and downstream of desiccant dryers.

The most basic filtration is provided by a filtered centrifugal separator , which uses centrifugal force and impaction. A well-designed filter's first stage is 99% efficient in removing particles 10-microns and larger. The second stage is a replaceable filter sleeve that removes solids and liquids down to 3-microns through coalescence.

Coalescing oil removal filters remove down stream oil aerosols that can contaminate end products and gum up air tools. A good coalescing oil removal filter has a liquid oil removal rate of better than 99.999%. Air is directed through a maze of submicronic glass fibers where the oil aerosols are coalesced into larger droplets and continuously removed.

Oil vapor adsorbers are final stage filters, which adsorb oil vapor by passing the compressed air through two levels of activated carbon. They should be installed after the dryer and oil removal filter, because liquid oil aerosols will prematurely saturate the activated carbon and significantly reduce adsorptive capacity.

High-temperature afterfilters are primarily designed as afterfilters to heated desiccant dryers, although they can be used wherever large amounts of solid particles are present in dry air.

Drain traps

Drain traps are a critical, but often overlooked, component in compressed air systems. A properly operating drain trap reduces plant-operating costs, lowers maintenance, and prevents airlines from flooding.

Demand-operated drain traps automatically discharge moisture and oil-containing condensate from the system. It is important that drain traps discharge only water, not costly compressed air, and that they do not require a maintenance-intensive strainer upstream of the condensate inlet. They can be used on air receivers, inter- and aftercoolers, refrigerated dryers, separators, filters, and header piping.

Timed electric drain traps are not a practical option. Not only do they release costly compressed air in addition to condensate, but the discharge process can also create a stable emulsion, which cannot be easily separated and increases condensate disposal costs.

Breathing air

Air for facemasks, hoods, helmets, and other supplied-air breathing apparatus, requires a breathing air system. These are complete, unitized, purification systems producing "Grade D" air under OSHA standards and designed to remove excessive moisture, solid particulates (dust and dirt), oil and oil vapor, carbon monoxide, and other hydrocarbon vapors commonly found in ordinary compressed air.

Edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, jfoszcz@reedbusiness.com

Complete compressed air treatment system components

Minimum Additional
AftercoolersOil/water separators
DryersOil vapor adsorbers
FiltersBreathing air systems
Drain trapsAir main charging valve
Air quality monitor


Monitoring air quality

With more and more companies operating under ISO certification, plant engineers need a method of ensuring air quality. Air quality monitors use capacitance hygrometers and photometers to accurately and reliably analyze compressed air contamination, pressure dewpoint, temperature, and pressure at a given point in the system.

Startup protection

An air main charging valve is an economical and simple way to limit pressure to appropriate levels and prevent overloading the system with excessive airflow. A ball or butterfly valve, installed after the air treatment system, is controlled by a pressure switch. When the compressor is started — for the first time or after a weekend shutdown — the pressure switch closes the valve as soon as maximum system pressure is reached. The valve prevents a high-velocity flow of air from travelling downstream and depositing residual condensate back into the air system.



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