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Purification System

The need for total purification can be readily appreciated when considering that ambient contaminants combine with contaminants generated by the compressor such as liquid oil and water, oil mist, water vapour, traces of carbon and particulates.

The purification process begins with the location of the intake of the compressor. As noted earlier, the compressor should be located in a well-ventilated area that is free of contaminants such as harmful fumes, engine exhaust and other dangerous solvents. The intake filter should remove particulates down to 10 microns in size. As the air is compressed, oil mist and water vapour saturate the air stream. Inter-stage coolers and condensate traps ensure the removal of gross contamination. Liquid oil and water can be manually or automatically drained to a reservoir for proper disposal in compliance with local environmental regulations.

As compressed air leaves the final stage of compression, discharge temperatures often exceed 300 degrees Fahrenheit. For efficient purification, a properly designed after-cooler (air or water heat exchanger) is required to reduce air temperature to approximately 100 degrees Fahrenheit. After the compressed air stream is cooled, the remaining gases and contaminants will be removed as the air is routed through the purification system.

The first stage of the purification process is the mechanical separator. The mechanical separator is responsible for 90% of the entire purification process. Air entering the mechanical separator has been cooled by the compressor's after-cooler, causing oil and water vapour to form droplets. These droplets, as well as particulate contaminants, are removed by impingement and typically, a coalescing filter. The droplets are collected in a moisture sump where liquids are periodically drained. The remaining gaseous and particulate contaminates (water vapour, oil mist, hydrocarbons, carbon dioxide, carbon monoxide, etc.) are removed by passing the air through a series of chemical beds. Chemicals can be packaged within disposable purification cartridges or supplied as bulk chemical beds. The first chemical bed is an adsorbent desiccant specifically designed to remove all entrained water vapour. Exiting the drying desiccant, the air stream is directed through a bed of activated carbon where foul tastes and odours, typically associated with compressed air, are eliminated. Activated carbon is an excellent adsorbent for removing organic vapours and hydrocarbons. The final chemical bed is a catalyst designed to convert toxic carbon monoxide to carbon dioxide. Exiting the final purification bed, an absolute particulate filter should be incorporated to remove all particulates down to 1 micron in size.

The effectiveness of a purification system depends a great deal on its design and level of maintenance, as well as the condition of the purification chambers and purifier cartridges. Purification systems that are improperly maintained and/or improperly designed, are hazardous to equipment and the health of the user.

For instance, breathing apparatus airflow can be blocked when valves and regulators become clogged with solid particles or freezing moisture. Also, purifiers which are not designed properly may not remove all contaminants. Even properly designed purifiers will allow impurities and noxious gases to pass through if they are not serviced properly. The end result of a poorly designed and/or maintained purifier can be a failed breathing apparatus, leading to illness or death of the user.

Purification System Design
A purification system should be designed to ensure that delivered air meets or exceeds the Compressed Gas Association's Commodity G-7.1 (1989), Grade E Specification.

As a minimum, the purifier should include the following:

• The mechanical separator should include an automatic condensate drain valve.
• To prevent processed air from escaping from the condensate drain system, a check valve should be positioned between the mechanical separator and purification chambers.
• A pressure-maintaining valve should be provided on the outlet of the purifier to prevent the purifier from operating below a pre-set minimum pressure, typically 1800 psi or higher. Increasing the operating pressure of the purifier increases the efficiency of the purifier.
• To facilitate service, the purifier should be equipped with a pressure gauge, bleed valve and outlet shutoff valve.

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