SPECIFIC AIR QUALITY REQUIREMENTS

The diverse uses of air are accompanied by specific air quality requirements. These vary from high purity requirements through the need to introduce materials into a system to be carried along with the air. This section will discuss these specific air quality requirements.

Commercial Air

Commercial compressed air is graded according to its purity. The purest is grade A running alphabetically to grade J, the least pure. The Compressed Gas Association has set guidelines for the grading of commercial compressed gas. The application of commercial compressed air is varied and generally specified for each individual installation by engineers. The full extent of the quality requirements for commercial compressed air applications can be located in the Compressed Air Association publication Commodity Specification for Air, G-7.1 (ANSI 286.1-1973).

Breathing Air

Breathing air must be of high quality for obvious reasons. Federal Specification BB-A-1034 (fig. 11-2) outlines the specific requirements for breathing air.

Figure 11-2.-Breathing air requirements, Federal Specification BB-A-1034A AM 1.

Special attention must be given to eliminating carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons, odor, and water from breathing air. Carbon monoxide has first priority as its effects are cumulative and very small concentrations can cause problems. Whenever possible, carbon monoxide monitoring should be provided at the compressor intake. This monitoring equipment should sound an alarm or shut down the system when CO is detected.

Carbon dioxide is found in combustion flue gases such as boiler stacks. Do not place compressor intakes near or downwind of the stacks.

Systems should be kept free of oil to limit the possible concentration of hydrocarbons or petroleum products. For breathing air, compressors should be oil free rather than using auxiliary petroleum removal equipment. The heat caused by compression may cause thermal breakdown of oil or an explosion danger may exist as a result of drawing hydrocarbons into the air system.

Water content is kept below saturation to prevent condensation at points that cannot be cleaned. It is recommended that refrigerant or dessicant dryers be used to remove moisture from a breathing air system. This will limit the vapor clouding (fogging) of glasses and visors.

Medical Air

Medical air quality must be the same or better than breathing air. Whatever quality is established must be strictly adhered to.

Instrument and Control Air

Air quality requirements for instrument and control air should place emphasis on cleanliness and low moisture content. The Instrument Society of America (ISA) has established the following requirements:

. Dew point, exterior: 18F (-7.8C) below minimum recorded ambient temperature

. Dew point, interior: 18F (-7.8C) below minimum interior temperature but not higher than 35F (1.7C) l Particle size: 3 microns maximum . Oil content: As close to zero as possible but not over 1 ppm

Contaminates: No corrosives or hazardous gases

Water content must be low enough to prevent condensate accumulations. Special attention should be given to ensure that intake air is filtered and oil or water removed. A refrigerant dryer with a dew point at least as low as 30F (-1.1C) is recommended for these services.

Aircraft Starting and Cooling Air

Aircraft starting air requires reasonably clean air to prevent introduction of excessive levels of oil, water, or particulate into engine systems. Normal intake filtering and oil/water separation should be adequate.

Aircraft cooling air is intended for electrical load cooling to prevent malfunction of the equipment. Cooling air should be reasonably clean. This air may also be used for breathing. If it is, then breathing air quality standards should be maintained.

Air for Pneumatic Tools

When compressed air is intended for use with pneumatic tools, it should be filtered for particulate and water should be separated out. Oil is usually required to be ingested into the air for tool lubrication. Mist injection is preferred for tools to ensure dispersion and maximum settlement. Note that pressures in excess of 400 psig may cause compression combustion when oil is present.

High-Pressure Air Systems

Air quality must be carefully analyzed to minimize not only the normal hazards of high pressure, but the internal explosive hazards that exist with high-pressure systems. Of particular danger is the introduction of oil and hydrocarbons during compression and their remaining and accumulating throughout the system. A high-pressure system of 500 psig or higher is subject to rapid local heat buildup whenever there is a rapid filling of a component or vessel. The heat buildup (combined with oil and foreign material) that permits the oil to wick or vaporize can readily cause an explosion or fire. Any explosion in the system may produce several shock waves to travel the system, compounding the damage. Because of this problem, special attention is required to clean the intake air, limit the introduction of lubrication oil, and remove oil after completion of the compression process.

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