Compressed air is the fourth utility. It is just as vital as water, gas, and electricity in today's industrial and aircraft maintenance operations. It runs tools and machinery, applies paint and coatings, and if properly designed can even ensure clean, breathable air in contaminated environments.
When purchasing a compressor or compressed air system, there are several factors to consider including the total cost of producing compressed air, the specific compressor features, maintenance requirements, and product warranty. Remember that the most cost-effective unit is not usually the lowest priced. Reliability and efficiency are key. An unreliable compressed air system can be disastrous to the bottom line.
Consider the following factors when evaluating your compressor options:
Electrical expense: As much as 70 percent of compressed air cost is electricity to operate the compressor. Look for a compressor that offers the most air per electrical consumption. Look at the package efficiency and check with the manufacturer for engineering data. Members of the Compressed Air and Gas Institute (CAGI) will publish their data on official CAGI data sheets so that compressor efficiencies can be easily and accurately compared.
Installation: Consider noise and vibration when choosing a compressor. Compressors that do not require special rooms or custom foundations will save on installation costs. Many units available on the market today can be purchased as package units with integral dryers and filtration, or tank mounted to minimize the system's space requirements.
Maintenance and repair costs: The easier the system is to service, the more you save in the long run. If either in-house personnel or contracted professionals can easily and properly maintain the air system, overall operating costs and downtime may be reduced. Proper maintenance increases an air compressor's reliability and efficiency. A compressor that is difficult to service may not receive adequate preventive maintenance, possibly resulting in costly repair and downtime. Some air compressors and air compressor components are specially designed for easy maintenance and repair.
Components to evaluate in a compressor include:
Cooling systems: Coolers and fans must be sized to provide low discharge temperatures in high ambient temperatures. In air-cooled units, low noise radial fans generally provide better cooling while using less electricity than axial fans. Also consider the ability to easily duct air in and out of the compressor package. Topside exhaust often simplifies ducting and reduces footprint.
Drive: Drive efficiency and simplicity are important. Belt drives require only simple maintenance and offer advantages such as flexibility in pressure selection. Automatic belt-tensioning devices are a must to ensure transmission efficiency and protect bearings from excess stress. Larger facilities with large air requirements may see additional advantages with recent direct drive technology. True direct drive units -- ones without gear connections -- offer the best efficiency with no loss in transmission efficiency and no maintenance requirements.
Operator interface: The control panel must be reliable, readable, and run the compressor efficiently. It should indicate operational status as well as offer maintenance interval reminders, diagnostic information, and external communications capability for remote monitoring and control.
Interconnecting piping: Look for rigid piping with flexible connections and high quality fittings to eliminate leaks.
Vibration isolation: Vibrations can loosen fluid and air fittings as well as electrical connections. Some compressors mount the motor and airend on vibration isolators to eliminate this source of stress. Additional isolators under the compressor package offer another layer of vibration protection, and for most rotary screw compressors, these isolators eliminate the need for special foundations.
Motor: Motor efficiency affects electrical consumption. Ensure your compressor motor meets or exceeds Energy Policy Act (EPAct) standards. TEFC motors offer much better protection from airborne dirt and dust than ODP motors.
Sound enclosures: An insulated enclosure can reduce the compressor noise emissions well below safety limits, eliminating the need for a separate compressor room. This can save you thousands of dollars in site preparation costs.
A warranty is the manufacturer's pledge to the customer. Make sure you have a comprehensive warranty on your equipment that is backed by solid manufacturer and distributor support.
Look for experience and expertise from the compressor supplier and service provider. A collaborative and consultative selling process is more likely to produce an efficient and reliable system. Consider whether the salesperson works to understand your facility's specific operational requirements and can clearly explain product features and relate their benefits to your needs.
Specific considerations for the aircraft maintenance industry
Without a consistent supply of compressed air, standard air tools and other pieces of vital shop equipment will not run. If the tools are not running, neither is the service shop. Productivity losses due to compressor reliability problems can mean real losses in hard dollars.
Determine pressure and flow needs
When designing a compressed air system, pressure and flow are the essential considerations. When asked about compressor size, many end users say they need 145 or 175 psig compressors. Compressor size is not determined by pressure requirements. It is determined by the compressor's output capacity in cubic feet per minute (cfm).
Consult the tool manufacturer's manual to determine pressure and flow requirements. Very few tools actually require pressures above 100 psig and over pressurizing will only cause premature wear. Most tool and compressor manufacturers publish charts with flow guidelines. Adding all of these together yields the total flow requirements. However, it does not take into account the percentage of time each tool is used. This requires some study of how the different parts of the facility shop operate throughout the day. To determine actual flow requirements, data logging devices are ideal for recording compressor usage over time.
Many users do not understand the inverse relationship between pressure and the volume of air delivered in a compressed air system. End users often complain about "not enough air" and will increase the pressure setting on a compressor to compensate. This actually reduces the volume of air delivered and increases the air consumption. In most cases, the problem is inadequate flow due to an undersized compressor, poor compressor performance, inadequate pipe size, and/or leaks. Increasing system pressure will increase the amount of air lost through leaks ? and everyone has leaks. Countless compressed air system studies have confirmed that as much as 35 percent of all compressed air produced is lost through leaks.
What air quality do you need?
Refrigerated dryers and particulate/oil removal filters provide the clean, dry air today's expensive tools and equipment require. Water or moisture in various forms causes excessive wear in tools or rust in iron pipes. It also accumulates in tanks, reducing the volume available for storage and causing the compressor (piston type only) to run beyond its recommended duty cycle.
Particulates also build up in piping causing pressure drop and excessive tool wear. Oil, usually in the form of vapors or mists, combines with particulates to clog tools. It may also build up in piping and cause significant pressure drop.
Do not forget to include high quality, automatic drain traps in the system. If the filtered and separated contaminants are not removed, they will find their way into the system again.
Piston vs. rotary compressor
Piston compressors are still commonly found in hangars. A piston compressor may provide adequate flow for a short period, but its allowable duty cycle must be considered. The duty cycle is the percentage of time a compressor may operate without the risk of overheating and causing excessive wear to the compressor. Most small piston compressors have an allowable duty cycle of 60 to 70 percent. For this reason, piston compressors are usually oversized to allow the compressor to periodically shut down and cool off because of the relatively high operating temperatures.
Rotary vane and screw compressors have a 100 percent allowable duty cycle with lower operating temperatures of only 170-200 F, compared to 300-400 F in a piston.
Reliability and maintenance over time
Compressor reliability, maintenance requirements, and productivity are essential considerations. Consider how a compressor performs as time passes. Pistons, cylinders, rings, and valves wear over time, causing the compressor to deliver less air, and send more lubricating oil past the rings into the compressed air system and right to the point-of-use. Preventive maintenance will slow this process, and rebuilding the compressor will reverse the gradual loss of flow and reduce the oil carry-over. However, it can be expensive.
Rotary screw compressors do not wear because the rotors do not touch each other or the rotor housing. Some screw compressors can operate for more than 50,000 hours before requiring an overhaul -- that's more than 10 times as long as a typical piston compressor and they are much quieter too!
A thorough system analysis goes a long way in building a reliable, cost-effective system. Carefully consider each system component and its impact on the application. Remember, value is more than initial price and purchasing quality equipment now will save time and money for years to come.
