- Untreated compressed air isn’t necessarily safe for breathing air purposes unless the air has been dried and atmospheric contaminants have been removed.
- Fixed-cycle dryers typically have a constant purge using significant volumes of compressed air and this incurs high operating costs.
- Dew point or moisture-loading dependent controls can save costs on main air dryers and purifiers.
Many industrial facilities need breathing air to protect workers from hazardous airborne contaminants. Often, general plant compressed air is used as a source. However, the conditions at the air compressor’s intake can’t be guaranteed. Purifiers are needed to clean and condition the air to ensure it meets required breathing air standards. Purifiers using fixed-cycle heatless desiccant dryers might use significant amounts of expensive compressed air for purging. This purge flow usually is higher than required because breathing air systems are subject to inlet flow rates that are less than the purifier rating point. Let’s explore the effects of purge flow in breathing air purifiers and discuss measures that reduce purge flow and save operating costs.
The environment around a paint booth or cleanroom might be clean, dry and free of vapors, but there’s no guarantee these same conditions exist at the intake of the compressor that provides breathing air. “We’ve been using breathing air filters to condition our air for our welding hoods”, says the welding supervisor at a highway coach manufacturer. “But we make vehicles, so, despite warning signs posted around our compressor rooms, very often idling vehicles cause our carbon monoxide alarms to ring regularly.” Contaminants that could affect the health of breathing air users include oil aerosols, oil vapors, solid particles, carbon monoxide and microorganisms.
Compressed air for breathing air applications must be purified properly to ensure that it meets the relevant local breathing air standards. Standards exist in most every country and dictate the protection required for workers exposed to hazardous environments. One example is the use of breathing hoods or masks. The compressed air sent to these devices must meet breathing air quality criteria, as determined by OSHA in the United States and CSA in Canada. These standards define acceptable levels of oxygen, nitrogen, carbon dioxide, carbon monoxide, moisture and other trace gases.
Figure 1. A typical breathing air purifier system contains a filtration system, a desiccant air dryer and a catalyst element. (Source: SPX)
Often desiccant-style breathing air purifiers are used to help condition the air to meet the required standards. These units (Figure 1) typically contain a filtration system, a desiccant air dryer and a catalyst element. The filtration system removes particles, liquid aerosols, oil vapors and odors that might be transmitted from the air compressor intake, through the distribution system to the worker. The catalyst converts carbon monoxide that might be present to more tolerable carbon dioxide. The desiccant dryer removes the water vapor so the catalyst can operate without contamination, as these are sensitive to the presence of water vapor.