- 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.
Energy issues with purifiers
Fixed-cycle heatless desiccant dryers, the most common type of dryer used in breathing air purifiers, need significant volumes of air for internal purge. The dryers are designed to process the full rated compressed air flow at an inlet condition of 100° F compressed air temperature, 100 psig air pressure, saturated with water vapor and an ambient temperature of 100° F. These conditions require an average purge flow of about 15% of the dryer’s rated flow. For example, a dryer rated at 100 scfm would consume about 15 scfm of compressed air, equivalent to 4 hp of air compressor power. For purifiers using fixed-cycle dryers, this purge is constant and consumes significant volumes of expensive compressed air, often leading to unnecessarily high operating costs.
On average, purifiers are subject to much less than design conditions. Often these units are only partly loaded because they’ve been sized for future conditions of maximum production capacity. These factors often make a constant purge flow of 15% of the dryer rating unnecessary.
Breathing air consumption also is highly variable because of the nature of the work. For example, painting operations aren’t continuous. There are many changes in demand as personnel take breaks, set up the work or aren’t painting during evening and weekend shifts. It’s not usually a common or safe working practice to turn off the breathing air manually during work breaks to save purge flow, so, in most cases, the units are left to run 24 hours a day, seven days a week.
A recent study highlighted a large highway coach manufacturer that installed breathing air purifiers to protect its paint booth workers. Flow meters were installed on sample units to measure inlet and outlet flows. This manufacturer was in a production slowdown and operating painting activities only 8 hr/day, but the purifiers ran continuously, including evenings and weekends. Measurements showed that the actual average breathing air loads were less than 5 scfm, or about 3% of the capacity of the installed purifiers. The purge flows averaged 35 scfm per unit, which is more than 7 times the actual breathing air used, and 47 times the purge required by this reduced flow. There were six breathing air purifiers at the site, consuming a total of 315 scfm in purge flow to process an average of about 30 cfm of breathing air. This compressed air amounted to $45,000 per year in electricity costs.
Measures that can be applied
Fortunately, some manufacturers now offer breathing air purifiers with controls that limit the purge flow based on inlet dew point or desiccant loading using sensor-based inputs. The controls sense conditions that allow the reduction of purge flow and automatically reduce it to compensate. This lowers operating costs significantly and, depending on the purifier manufacturer, might reduce the purge to near zero in conditions of very low usage.
Consider the highway coach manufacturer. When a purifier having a purge control system was installed on a new paint booth, the measured purge requirement dropped by 85% during evenings and weekends because of the favorable inlet conditions (Figure 2).
Figure 2 – Purge flow on this purifier is reduced to 85% during off shifts.
Table 1 shows estimates of the savings under a number of scenarios and assumes compressed air is produced at 20 kW per 100 cfm and power cost $0.10 per kWh. Note that some purifiers have watchdog cycles that limit savings to no more than 85%.
|Item||Flow (%)||Annual cost||Saved (%)|
Table 1: Estimated savings from dewpoint control - 305 cfm rated purifier.
Where you shouldn’t save
Breathing air purifiers are critical systems that protect your workers. You need to stress that these systems must be maintained exactly to the manufacturer’s specifications to ensure adequate operation at all times in all conditions. This thinking also applies to the maintenance of breathing air instrumentation that is monitoring carbon monoxide or other contaminants. These devices need to be operating at all times and subject to regular testing and calibration in accordance with the instruction manuals and local codes and standards.
Can these measures be applied to air dryers?
If you don’t have a breathing air purifier, but have a desiccant air dryer on your main compressed air system, these savings might also apply to you, and more, depending on your air dryer type. Dew point or moisture-loading dependent controls can save you just as much on your main air dryers as for purifiers. Retrofits are available to convert existing dryers to moisture-dependent control.
If you have externally heated or heated-blower style dryers, there are further savings to be gained if the inlet air temperature is less than rated conditions. That’s because the heated style dryer, not the heatless type, can reduce its purge requirement in proportion to the amount of moisture it processes, rather than simply the air flow through it.
Example calculation of moisture loading
Let’s look at an example of compressed air moisture content at varying conditions. For a heated dryer at rated inlet conditions of 100 psig and 100° F, the wet, saturated air entering the dryer would contain 0.40 lb of water vapor for every 1,000 standard cubic ft. But, if saturated air at 70° F enters the unit, the water content is 0.15 lb of water vapor per 1,000 standard cubic ft, 38% of the previous moisture content, according to Best Practices for Compressed Air Systems, Appendix 2.B.1, Compressed Air Challenge. If this dryer is passing only 50% of its rated flow at 70° F, the purge requirements would be 38% x 50%, or 19%, thereby saving 81% of the rated purge.
Contact your compressed air equipment supplier for more information. Additional information about compressed air dryers is available at the Compressed Air Challenge website (www.compressedairchallenge.org), where you can download the document titles “Improving Compressed Air System Performance: A Sourcebook for Industry,” or by ordering CAC’s Best Practices for Compressed Air Systems manual.
Ron Marshall is a member of the Project Development Committee at the Compressed Air Challenge. Contact him at firstname.lastname@example.org and (204) 360-3658.