All About Compressed Air

Purification of Compressed Air from Generation to Application:

Compressed air is widely used through industry as an essential and safe power source in production process. However, the compressed air will contain too much much contaminants, such as water, dirt, wear particles, and degraded lubricating oil, which rapidly wears tools and pneumatic machinery, blocks valves, and corrodes piping systems causing high maintenance and costly air leaks. 
The quality of air required throughout a typical compressed air system can vary. Treatment of compressed air prior to entry into the distribution system, as well as at each usage point or application, is highly recommended. This approach to system design provides the most cost effective solution to system purification.

Sources of Contamination Found in a Compressed Air System:

1. The quality of air being drawn into the compressor (from surrounding atmosphere in a large volume)
2. The type and operation of the air compressor (wear particles, coolants, and lubricants)
3. Compressed air storage devices and distribution systems (air receiver and system piping)

Types of Contamination Found in a Compresses Air System:

1. Atmospheric dirt (typically 140 million dirt particles for every cubic meter of air in an industrial environment)
2. Water vapor, condensed water and water aerosols
3. Rust and pipescale (can be found in air receivers and piping of wet systems)
4. Micro-organisms (such as bacteria and viruses, ambient air can typically contain up to 3850 micro-organisms per cubic meter)
5. Liquid oil and oil aerosols (during operation, lubricating oil is carried over into the compressed air system as liquid oil and aerosols)
6. Oil vapor (oil vapor concentrations typically can vary between 0.05 to 0.5 mg per cubic meter of air)

Compressed Air Quality Standards:

ISO 8573 - is group of International Standards relating to quality of compresses air and consists of 9 separate parts.
Part 1 specifies the quality requirements of compresses air and parts 2-9 specify the methods of testing for the range of contaminants. 

ISO 8573.1 (2001) is primary document used from ISO 8573 series and allows the used to specify the air quality or purity required at key points in a compressed air system.

Specifying Air Purity in Accordance with ISO 8573.1 (2001) - When specifying purity of air required, the standard should always be referenced, followed by the purity class selected for each contaminant if required.

Cost Effective System Design - to achieve the accurate air quality level required for today's modern production facilities, a careful approach to system design, commissioning and operation must be employed. It is highly recommended that the compressed air is treated prior to entry into the distribution system, to a quality level suitable for protecting air receivers and distribution piping.

Point-of-use purification also employed, with specific attention being focused on the application and on the level of air quality required. This approach to system design provides the most cost effective solution to high quality compressed air.

How Water Gets Into the Air System:

compressed air has become an indispensable source of energy in modern industrial processes.
All atmospheric air contains a certain amount of water vapor which is mixed with other gases, such as nitrogen, oxygen, and carbon monoxide. This water vapor is drawn into the air compressor with the incoming air during the compression cycle. The air which is drawn into the compressor has water in gaseous form. The exact amount of water is called the humidity of the air.
a) Relative Humidity - The amount of water vapor that can be held in air is dictated by the temperature of the air (more water vapor in hot air than cold air). The proportion of the maximum vapor holding capacity is referred to as relative humidity.
b) Dewpoint and Condensation - When air with a given relative humidity is cooled, it reaches a teperature at which it is saturated. At saturation, the relative humidity is 100% (the air contains as much water vapor is it can hold). The temperature at which the air is at 100% relative humidity is determined as dewpoint of the air. Cooling air beyond that temperature results in condensation of the water vapor.
c) Cooling and Condensation in Compressed Air - The effect of the air temperature rise as the air is compressed. The increased temperature of the compressed air increases its vapor holding capacity which reduces relative humidity of the air because the actual water vapor content has remained constant. Compressing the air also increases the dewpoint of the air. Using an aftercooler can remove a significant proportion of the water vapor from the air through the principle of condensation. When leaving the aftercooler, the compressed air is saturated. Any further cooling of the air will result in condensation. 
d) Sources of Cooling - There are many ways to cool saturated compressed air:

  • Ambient Conditions:
    • Expose compressed air lines to cooler outdoor temperatures
    • Expose compressed air lines to unheated rooms
  • Pressure Reduction:
    • Pressure regulators, vortex tubes, expansion vessels, and receiving tanks
  • Process Equipment:
    • Aftercoolers, Dryers, etc.
The water vapor becomes a major hazard in compressed air systems, given that it is distributed together with the compressed air itself. as the compressed air is cooled while passing through the plants air piping, this water vapor will condense. It is because the compressed air, at normal ambient temperatures, cannot hold as much water vapor as air at atmospheric pressure. However, the heat generated during the compression cycle increases its ability to hold water vapor. When the compressed air is cooled between the compressor and the point of use, this water vapor will condense and become liquid water, depositing itself in the system piping, air receiver, tools, etc. The quantity of water vapor condensed will be that amount which is in excess of the saturated temperature of the compressed air. This condensed water will corrode system components resulting in increased maintenance and reduced system efficiency.

How Much Water Can Be Found In A Typical Compressed Air System?

The amount of water in a compressed air system is huge. A small 100 cfm  (/min) compressor and refrigeration dryer combination, operating for 2000 hours in a typical climatic conditions can produce approximately 5000 litters or 1100 gallons of liquid condensate per year.
The resulting condensate can falsely resemble oil in an oil lubricated compressor. Although the amount of lubricated oil is very small (less then 0.1 % the overall volume). If a compressed air system was operated in a warmer, more humid climate,or with larger compressors installed, the volume of condensate would increase significantly.

How is Water Removed from the Air System:

Getting the Water Out: Usually, compressed air contains water in both the liquid and vapor phases. Drying can range from trapping the condensed water, to preventing additional condensation of water vapor, to removing virtually all the water present. The more water removed, the higher the cost of drying is.
However, if too much water is permitted to remain in the compressed air supply, the price is paid in maintenance costs, corrosion, and product losses. These costs support the importance of specifying the proper drying technology for a given application.

Available Drying Methods:

  • Aftercooler - Reduces the tempreature and water content of the compressed air.
  • Bulk Liquid Separators - Remove bulk liquid condensed in the distribution system.
  • Particulate Filters - Remove solid particle contaminants down to 5 micron and the separation of bulk liquids.
  • Coalescing Filters - Remove aerosol water and other liquids, which bypass the water traps.
  • Pressure Reduction - Drying through expansion.
  • Refrigeration Dryers - Drying to dewpoints of approximately 37 F .
  • Desiccant Dryers - Drying to dewpoints of approximately -40F to -100F.
  • Membrane Dryers - Variable drying capabilities to approximately -40F dewpoint.
At the Compressor:
The standard compressor installation consists of a compressor, an aftercooler, and a receiver. The distance from the receiver to the filter is not important in a system with an efficient afftercooler. It is often convenient to install the filter immediately after the receiver since the filter is usually maintained by a personnel responsible for the compressor.
Some compressor installation do not have an aftercooler. It is not recommended. Air saturated with water vapor leaves the compressor at temperatures between 230F and 392F and cools to approach room temperature in the distribution lines. 


An efficient aftercooler is essential to all compressed air systems and will condense up to 75% of the water vapor. Installation of coalescing filters at various points in the system will remove much of the condensate in the absence of an aftercooler, but if the air temperature at any filter is higher than room temperature, water will condense downstream from the filter as soon as the air cools a few more degrees. 

Bulk Liquid Separators:

These are high efficiency water separators used to remove bulk condensed liquids after the aftercooler, receiver, or anywhere within the distribution system. These separators also help protect filters in systems where excessive cooling takes place. They remove more than 98% of bulk liquid contamination through centrifugal separation techniques.

Coalescing (Oil Removal) Filters:

Coalescing filters are essential to remove compressor lubricant, water droplets, and particles from the compressed air supply. They are designed to remove only liquids and particulate from a compressed gas stream down to 0.01 micron in size.

Particulate Filters:

Particulate filters are used fro the removal of solid particle contaminants down to 5 micron and the separation of bulk liquids.  This type of filter is generally used in industrial applications, and should be used as a pre-filter for the coalescing  filter.

Refrigeration Dryers:

The refrigerated dryers work by cooling the air to low temperatures; thus condensing much of the water vapor. It is not possible to achieve dewpoint below freezing with this type of dryer. Refrigeration dryers remove the heat from the inlet air and use it to reheat the air at the outlet. Dried air is returned to the air line at reasonable temperature. Refrigeration dryers are not suitable for installations where piping is installed in ambient temperatures below the dryer dewpoint.

Pressure Reduction:

In air distribution systems not subject to freezing temperatures, the function of the filter is to prevent condensed water from entering the air-operated equipment. This application requires care in selecting the filter and in positioning it correctly on the air line. Virtually all air supplies are regulated from a higher line pressure to a lower line pressure at the use point. As such, it is possible to take advantage of the drying effect of pressure reduction. Air at lower pressure holds more water vapor than air at higher pressure at the same temperature. Therefore, less water vapor will condense out of the air at the reduced pressure.

Adsorption Dryers:

Adsorption dryers are used in those applications where very dry air is required; they are generally either installed downstream of the after-cooler and/or the refrigeration dryer.

Membrane Air Dryers:

Membrane materials selectively permeable to water vapor are an excellent medium for producing dry air from standard compressed aire. The water vapor in the compressed air is removed by the principle of selective permeation through a membrane. The membrane module consists of bundles of hollow membrane fibers, each permeable to water vapor. As the compressed air passes through the center of these fibers, water vapor permeates through the walls of the fiber. Membrane dryers can be located near the point-of-use and can supply clean dry compressed air with dewpoints as low as -40F and 35F.

Important Note Regarding Compressed Air Dryers:

As refrigeration, adsorption, and membrane dryers are designed to remove only water vapor and not water in a liquid form, they require the use of coalescing filters and possibly a bulk liquid separator to work efficiently.

Specifying the Right Dryer:

In specifying the right dryer for a compressed air installation, the following information to be kept in mind:
  • Do not over-specify - Drying the entire compressed air supply in a factory to dewpoints less then -40F is wasteful. It is more sensible to subdivide the compressed air supply by application, treating each end use point as needed to provide appropriately dry air for the downstream application served.
  • Do not under-specify - Damage caused by wet air costs money in maintenance time and supplies, downtime, and lost product. A drying system should be designed in a way to meet specific needs.
  • A drying system which only contains an after-cooler and a coalescing filter could create problems with condensation downstream from the after-cooler. The air is still saturated with water vapor which is likely to condense if the ambient temperature is lower than the compressed air temperature. 
  • Utilize the drying effect of pressure reduction - For application which use air at lower temperatures than the main compressed air line and will tolerate some water vapor, install filters or filter-regulators at the point-of-use to maximize the drying effect of pressure reduction.
  • Specify membrane dryers for those parts of the system which require dewpoint of 35F to 52F and flow rates up to 1200 SCFM.
  • Specify membrane dryers for instrument quality air, air exposed to freezing temperatures, and water sensitive applications requiring flow rates up to 100 SCFM.

Source: Republished from an undisclosed sources with slight modification.