W&WP July 2004
The air compressor is a frequently overlooked component in the spray finishing equation.
By Michael Camber
In recent years, many woodworking companies have discovered the significant impact a properly designed compressed air system can have on product quality and shop profitability.
The compressed air system is critical for two main reasons. First, compressed air is a utility; it is an energy source for most shop equipment. Second, compressed air mixes with the product in finishing and spraying operations. Both air quality and supply directly impact the quality and the appearance of a product's finish. "Re-dos" increase labor and material costs, and interrupt workflow. Costly rework due to poor quality finishing and workflow interruptions due to inadequate air supply can increase cycle times, causing real losses in hard dollars.
A little basic information on the source of compressed air and the appropriate air treatment provides valuable insight for selecting the right system or finding cost-effective ways to improve an existing one.
Increasing system pressure will increase the amount of air lost through leaks, wasting both air and electricity. Everyone has leaks. Industry studies confirm that as much as 35 percent of all compressed air produced is lost to leaks!
Flow requirements: Compressor size is not determined by pressure requirements; it is determined by the compressor's output capacity in cubic feet per minute (cfm). To properly size a compressor, find out how much air is needed in terms of volume, not pressure. Most equipment manufacturers provide clear information on air consumption rates. However, the percentage of time each tool is used is also critical. Electronic data logging devices are a convenient way to measure and record compressor usage.
Compressor types: The piston (or "recip") compressor is still commonly found in small to medium cabinet and woodworking shops. A piston compressor may provide adequate flow for a short period, but consider its allowable duty cycle. The duty cycle is the percentage of time a compressor may operate without the risk of overheating. Most shop piston compressors are air-cooled with an allowable duty cycle of 60-70 percent. They are often intentionally oversized and operate over a wide pressure band to allow the compressor to frequently shut down and cool off because of the relatively high operating temperatures (often 300-400F).
Rotary vane and screw compressors have closed circuit, thermostatically controlled cooling systems that provide a 100 percent allowable duty cycle with operating temperatures of only 170-200F. This is an important consideration for finish spraying and other moisture sensitive applications since moisture vapor content decreases with temperature. An important rule of thumb is that every 20F decrease in temperature cuts moisture vapor content in half, making it easier to remove moisture from the system.
Compressor location: Compressors are often installed where the noise, vibration and heat will least bother people rather than where they will best perform and be easily serviced.
Rotary compressors offer much lower operating temperatures, up to 200F cooler than the typical piston and generate much lower noise levels, giving the shop owners more flexibility on compressor location.
Maintenance and long-term performance: Maintenance requirements and long-term compressor performance are essential factors to consider. Piston compressors require relatively little preventive maintenance other than periodic oil changes, replacing the air inlet filter and maintaining proper belt tension. Rotary compressors require these as well as oil filter and air/oil separator changes.
However, the pistons, cylinders, rings and valves in reciprocating units wear over time, causing the compressor to deliver less air and send more lubricating oil past the rings into the compressed air system. Without proper filtration and more frequent filter maintenance, this will cause finish problems. Preventive maintenance will slow this process, but rebuilding a piston compressor periodically is necessary to reduce the oil carry over and reverse the gradual loss of performance. Because the rotors in rotary screw compressors do not touch each other or the rotor housing, performance does not change over time.
* Water/moisture in vapor, mist and liquid form may cause defects in finishes and excessive wear in tools. For example: On a 75F day with 75 percent relative humidity, a 10-hp compressor can introduce 7 gallons of water into a compressed air system. Can you imagine how much water is produced on a 90F summer day with 90 percent humidity?
* Ambient particulates such as dirt and dust build up in piping causing defects in paint and finishes, pressure drop, and excessive tool wear. Oil vapors and mists cause finish defects, and combine with particulates to clog tools and spray guns.
Air compressor systems have several built-in features to counter these contaminants. They include:
* Receiver tanks provide the first stage of moisture separation and store air. Tanks stabilize system pressure and provide an "air buffer" to compensate for fluctuations in air demand. Regulators placed at the point of use will further regulate the air pressure for the specific tool. For example, spray guns equipped with regulators supply a steady stream of air resulting in a more consistent spray pattern.
* Dryers are used to remove moisture in a variety of ways. Refrigerated dryers are the most common. A refrigeration system lowers the compressed air temperature well below the ambient temperature, condensing much of the moisture vapor into liquid that can be drained from the system. Refrigerated dryers are designed to produce dew points between 35F and 50F at rated conditions and are ideally suited for use with rotary screw compressors.
High temperature refrigerated dryers include an after-cooler and are primarily used with piston compressors as the air must be pre-cooled prior to entering the dryer. These are usually designed to achieve 50F dew points at rated conditions.
* There are many filters available through specialty and industrial suppliers. Most fall into a few basic categories based on the contaminants they capture. Moisture separators mechanically separate liquid water and oil from the air stream. Particle filters capture dirt and dust, but may remove some water and oil mists. Coalescing oil filters remove oil aerosols/mist and capture fine particles.
* Condensate drains/traps are a critical but often overlooked component of the compressed air system. If the separated contaminants are not drained from tanks, dryers and filters, they build up and find their way back into the air system. Liquid accumulation in tanks gradually eliminates air storage capacity, causing periods of inadequate air flow/pressure.
Price and True Cost
Carefully consider each system component and its impact on the application. Remember - value is more than initial price. Installing quality equipment now will save time and money for years to come and improve the quality of your finish in the process.
Michael Camber is marketing services manager of Kaeser Compressors Inc. based in Fredericksburg, VA. He can be reached at (540) 898-5500 or via E-mail firstname.lastname@example.org.
Bill Mehall, regional manager of Kaeser Compressors, will present "Tuning Up Your Air System" at the Wood Finishing 2004 Conference Aug. 25 in Atlanta.
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