Compressor
Under normal conditions, the following information will provide satisfactory
guidelines for selecting the right compressor for your needs. Unusual operating
conditions may require a larger unit than indicated. If you are not certain,
select a larger unit to ensure adequate initial capacity and provide for future
needs.
Power Source
It is very important to determine the correct electrical service available to
your location. Make certain the compressor you have selected will have the
correct power available for proper operating. Incorrect voltage or phase will
delay installation. Contact a professional electrician to determine the
electrical service in your facility. Gasoline or diesel engines are available if
a mobile unit is preferable to a stationary design.
Compressor Intake Requirements
It is important to provide fresh, uncontaminated air to the compressor's intake
since gross contamination of intake air will adversely affect the efficiency of
the purifier. If the compressor is operated near running vehicles or close to a
gas heater, the CO and CO2 levels in the ambient air may be affected resulting
in an adverse impact on the air quality of your system. If the compressor is
located in a room where chemicals are stored, fumes or particles from the
chemicals can be drawn into the system and affect air quality. Your compressor
should never be located where volatile gases can enter the compressor.
To safeguard your system's air quality, a remote intake may need to be installed
in another room, on an outside wall, or on the roof. The induction pipe should
be of sufficient diameter to prevent restriction of air flow to the compressor.
A rule of thumb is to keep the induction pipe three times the diameter of the
compressor's inlet. Ninety degree elbows and the distance of intake piping,
should be kept to a minimum to prevent any restriction of the air flow. A drip
leg with drain should be provided to prevent water from entering the compressor
intake.
Flow Requirements
There are three major factors to consider when determining your compressed air
output requirements:
- The time required to fill the BA cylinders.
- The number of BA cylinders to be filled on a regular basis.
- The capacity of your storage system.
Flow Rate
The Compressor Data Chart (Chart A) indicates the
number of 45 cubic foot SCBA cylinders that may be filled during a one hour
period using compressor capacities that are typical for compressed breathing air
systems. As illustrated, each compressor has two ratings for delivered flow
rate: free air delivery (FAD) and average cylinder charge rate (CR). When the
same compressor is rated using these two methods, the results are dramatically
different - average charge rate will exceed free air delivery.
Free Air Delivery
Free air delivery (FAD) measures the compressor's output at a specific operating
pressure. The result, normally expressed in actual cubic feet per minute, rates
the outlet flow based on inlet conditions. This method for measuring FAD is
specified by the American Society of Mechanical Engineers (ASME PTC-9) and the
International Standards Association (ISO 1217).
A FAD of 15 ACFM means that if the total output of a compressor for one minute
was free to expand to the barometric pressure, temperature and humidity that
existed when it was compressed, the compressor's output would expand to fill 15
cubic feet of space. If the compressor was operating continuously at 5000 psig
during the test, then the FAD would be 15 ACFM at 5000 psig.
Cylinder Charging Rate
Charging rate measures the average output of a compressor as it fills a BA
cylinder. Since there are no recognized standards governing how a charging rate
is determined, the method of determining the rate is left up to the individual
compressor manufacturer. As a result, there are numerous ways to calculate a
charging rate. The rate is usually obtained by timing how long it takes a
compressor to fill a BA cylinder, perhaps an 80 cf SCUBA cylinder, from 0 to
3000 psig. The cylinder volume (80cf) is then divided by the filling time (5
minutes, for example). The result would be a charging rate of 16 cfm.
In actual practice, charging rate is meaningless. To fill BA cylinders, the
compressor must be fitted with a purifier equipped with a minimum pressure
valve. If the minimum pressure valve is set at 2800 psi, then the average
charging rate for a 3000 psi BA cylinder would have to be measured from 2800 to
3000 psi (instead of 0 to 3000 psi) if the test were to be accurate. Since
compressors used for breathing air are equipped with purifiers and minimum
pressure valves, average cylinder charging rates (typically determined without a
minimum pressure valve) are significantly overstated for breathing air
applications. Although charging rates are still widely used in today's market,
the inaccuracy of this method often conceals inefficiencies that occur with
normal operation at higher pressures. Also, this method can mask problems that
would normally be revealed by a free air delivery rating.
For the purposes of this document, both FAD and CR ratings are referenced.
However, all cylinder refill system calculations utilize FAD.
Pressure Requirements
Choose a compressor that is rated at a higher operating pressure than is needed
to complete your cylinder refill requirements. Higher operating pressures aid in
the purification process as well as making it possible to store a greater volume
of air in the same physical space.
Compressor Design
Multiple stage reciprocating compressors are utilized to produce high pressure
air. High pressure compressors are available in a number of configurations
employing either a lubricated or a non-lubricated design. The basic difference
between the two designs is the presence of lubricant within the compression
process. Non-lubricated compressors are designed with closer tolerances within
the compressor's valve assemblies and cylinder walls. A non-lubricated
compressor will have a significantly higher initial cost as well as a higher
cost of maintenance, when compared to a lubricated compressor. The
non-lubricated compressor will also require more frequent service intervals.
Although lubricating oil (synthetic or mineral based) must be viewed as a
contaminant, it also serves many critical functions. Lubricating oil reduces the
operating temperature of the compressor, reduces friction (thus increasing
compressor operating efficiency and output), minimizes air leakage around piston
rings, and it reduces the possibility of internal corrosion. Lubricating oil
will also assist in the removal of dirt, wear particles and liquid water within
the compressor.
Inner-stage and final-stage coolers (air or water cooled) with internal
condensate traps, provide a medium for the removal of gross oil contamination.
Further filtration of oil vapour within the purification process ensures the
elimination of condensed oil and other hydrocarbons.
Compressor Components and Controls
It is highly recommended that your compressor be equipped with automatic
controls to prevent damage due to human error or mechanical failure. The
following controls and components are recommended for a high pressure breathing
air compressor:
- Magnetic starter (with overload protection)
- Belt guard (OSHA requirement)
- Intake filter
- Inter-stage coolers
- After coolers
- Pressure safety relief valve for each stage of compression
- Elapsed time recorder
- Condensate drain traps
- High temperature alarm and/or carbon monoxide alarm when using an
oil-lubricated compressor (OSHA requirement).
Over recent years, breathing air compressors have become more automated with
more elaborate controls and sophisticated components. Examples include:
- Automatic condensate drain with muffler/reservoir assembly
- Low oil level alarm and/or shutdown
- Inter and final stage pressure gauges
- Emergency stop and restart controls
With the advent of automated compressors, programmable controls have begun to
replace electro-mechanical designs of the past. A programmable electronic
control system will provide an operator with numerous diagnostic features
monitoring all normal and abnormal operating conditions. | 