SCBA AIR PURIFICATION
MAINTENANCE
A fire fighter’s protective breathing apparatus will not be of much help if it is filled with air that is not properly purified.
Air is a combination of gases, consisting of 78 percent nitrogen, nearly 21 percent oxygen and traces of nine other gases. Oxygen and nitrogen are critical to the human respiratory system.
If air is contaminated, the oxygen content is reduced. When the oxygen content drops to about 16 percent, a person will experience anoxia, with symptoms of mental confusion, blurred vision and reduced muscular coordination. When the oxygen content is reduced to 11 percent, unconsciousness results; prolonged exposure below 11 percent causes death.
Therefore, to maintain a consistent supply of pure air from a compressor, it is essential that an efficient and troublefree filtering system be utilized. There are two distinct phases of respiratory air purification, the mechanical phase and the chemical phase.
Mechanical phase
Air leaving the final stage of compression from a compressor is often in the 250°F range. This air is saturated with moisture, contains some oil vapor and of course, is unfit for breathing use at this point.
The mechanical phase makes use of an important relationship between temperature and pressure in the purification process. Saturated air at 1000 psi and 100°F contains four times as much moisture as the same air at 2000 psi and 80°F. Therefore, three important equipment components are incorporated in this phase to take advantage of this temperature/pressure relationship.
Aftercooler – This is installed after the final stage of compression. This aftercooler cools down the hot discharge air to within 10°F of the ambient air. In doing so, the aftercooler condenses much of the moisture in the airstream. For every 4°F decrease in temperature, there is a 10 percent reduction in air moisture content.
This means that when the air reaches the filters, there is less moisture to be removed and the filter life is increased by 10 percent.
- Separator – This coalesces the water and oil vapors into droplets, which are carried to a sump at the bottom of the separator to be drained away either manually or automatically.
- Pressure maintaining valve —This is also called a priority valve as well as a back-pressure regulator. This valve maintains the system pressure at a minimum of 1800 psi, and will prevent air from passing through the filters until this pressure is reached. The reason for this is that for every 100 psi increase in pressure between 1000 psi and 2000 psi, there is a reduction of 14 percent in air moisture content. Again, filter life is increased by 14 percent.
In summary, the mechanical phase initially removes much of the contaminants in the airstream, increasing the life of the chemical filters downstream.
Chemical phase
The most effective method of removing moisture, oil vapors and most gaseous contaminants is by the process of absorption. Several of the most popular and effective desiccant chemical filters for removing moisture and oil vapors are molecular sieves, activated alumina, activated carbon and silica gel. To remove any remaining aerosols or odors, the air is passed through a catalyst, such as hopcalite. This chemical simply converts the deadly carbon monoxide to carbon dioxide.
Filter limitations
Each of these chemical filters has limitations as to the maximum air tem>perature and moisture content they can handle and still be effective. The following table summarizes the filters’ maximum operating temperatures:
Silica gel is mainly used for moisture removal. It is capable of producing -100°F dewpoint air, and has a low tolerance for oil vapor and can become quickly contaminated, reducing its effectiveness.
Molecular-sieve type 13X is excellent for oil and moisture removal. Its affinity for absorbing moisture is greater than that for oil vapor; therefore, if the air is quite moist, it will make this filter inefficient for oil removal.
The catalyst filter must be kept very dry for it to function properly in its role of converting carbon monoxide to carbon dioxide. For this reason, it is always installed downstream from the dehydrating-type filters.
If a nonlubricated or oil-free compressor is being used, the filtration system can be simplified considerably. Since separators on this compressor will remove about 95 percent of the moisture in the airstream, it is possible to breathe the air directly from this compressor without using filters. If rescue work utilizing respiratory air is to be performed in freezing conditions, then a desiccant-type filter should be included in the nonlubricated compressor system. This will ensure that the air will be sufficiently dry so as to prevent freezing of critical air regulator parts.
Whether using a lubricated on nonlubricated compressor, care should be taken to ensure that the air inlet to the compressor is kept free of contaminants as much as possible. An air intake next to a parking lot or near a traffic-congested street can provide a source of air that is alarmingly high in carbon monoxide.
Protective accessories
Different types of protective accessories that can be added to a purification system to increase its safety effectiveness are:
Carbon monoxicJe monitor – This is an electronic instrument that samples the airstream continuously to determine the CO content. A series of lights or a directreading meter indicates from 050 ppm. The compressor will shut down upon reaching a preset contamination level.
Visual moisture/CO indicator — This is a dual airstream indicator that detects the carbon monoxide content and excessive moisture. Activation time is 1-4 minutes.
Noxious gas detector kit – Handheld, this chemically activated device is used to make periodic checks of the output air.
Providing purified air for respiratory use can be a matter of life and death. Therefore, good air management means (1) reducing chances for contaminated air entering the compressor, (2) having continuous air monitoring devices installed on the compressor purification system, (3) using filters with CO removal capabilities and (4) using a high-quality synthetic oil in the c ompressor (see Fire Engineering, October 1981, ‘Air Compressor: Lubricate Properly’).
