By Don Kirkham
Imagine this scenario: A fire chief ventures out to purchase a new air compressor for the station at a local discount store. It has a compressor capable of delivering 90 psi and has a large tank. The motor operates on 110 volts. The unit arrives and is installed successfully. The next week, a firefighter notices the tire air pressure on the engine is low. He begins to fill the tire to the recommended pressure, 110 psi; however, the air compressor is designed to deliver 90 psi. When quizzed, the fire chief says 90 psi is okay. Some time later, the engine is involved in a traffic accident, and the cause is determined to be low tire pressure. Now there are liability issues and safety concerns galore.
The air compressor installed in your fire station often goes unseen and unappreciated until something goes awry. Purchasing and installing an air compressor is a straightforward process; however, there are some terms and sizing considerations.
Air receiver: The tank into which compressed air is delivered and stored. The tank should be sized for psi, capacity as dictated by the manufacturer.
Compressor efficiency: The ratio of the theoretical to the actual work required to compress air.
Design pressure: The maximum continuous operating pressure as designed by the manufacturer.
Pressure: The force per unit of area.
Pounds per square inch gauge (psig): Gauge pressure is the pressure above the atmospheric pressure.
Pounds per square inch absolute (psia): Equal to gauge pressure plus atmospheric pressure.
Following are typical uses of compressed air and their required cubic feet per minute (cfm).
SIZING AIR COMPRESSOR If your compressed air requirements are between 0 and 80 psi, a single-stage air compressor may satisfy your needs. If the compressed air requirements are between 80 and 250 psi (many stations are), a two-stage air compressor is required.
When sizing an air compressor, list all air-powered equipment and their consumption—both continuous and intermittent. To determine the compressor’s required horsepower, first determine the total cfm necessary. It is a good idea to add approximately 20 percent for variables. To this sum add 40 percent of that total for future applications. If the resulting figure above is less than 100 cfm, divide by 4 to find the horsepower necessary. If the total is more than 100 cfm, divide by 5.
Tank size. The general rule is, bigger is better. Size the tank with at least one minute of volume of your compressor. In this case, a receiver larger than 60 standard cubic feet per minute (scfm) is recommended.
Type control. Use a stop-start (pressure) switch for small systems up to 15 horsepower. The motor stops when the compressor unloads and starts again when the pressure in the receiver drops below a given set point.
Electrical power supply. Before purchasing an air compressor, a quick review of your station’s electrical supply is required. Ensuring proper voltage, amperage, and circuitry is critical at this time.
Piping. Again, more is usually better. It is not unusual to find uses for compressed air once this resource is available. The cost of installing air plumbing at the time of construction or remodeling is miniscule compared with installation at a later date. A rule of thumb would be a drop or two at the shop area, wash bay, each column between the overhead doors, and each column in the apparatus bay. Each area should have a compressed air outlet within 50 feet of any area in the apparatus bay. If compressed air is used to supply air to the engine’s air system, more air outlets are necessary. An important consideration of the air piping system is that it should be looped and tied together so there are no dead ends, just like a well-designed water system.
Condensation. ALL compressors create condensation in the process of compressing air. Capturing this condensate and properly disposing of it are mandatory. Several states require permits to be obtained prior to installing air compressors. Condensation will contain between 300 and 500 parts per million of oil and other contaminates that should not be flushed down the drain.
Maintenance. This is a critical issue for continued long-term compressor life. This valuable piece of equipment requires periodic maintenance according to the manufacturers’ recommendations.
Don Kirkham is a retired firefighter/medic from the Delaware City (OH) Fire Department. He ha a bachelor of science degree in fire science and engineering, a master’s degree in public administration, and a Ph.D. in business administration. Kirkham is facility manager for Velocys, a research and development company, and has been the construction project manager for Ohio University’s newest satellite campus in Pickerington, Ohio.