For some time, MRO experts have been analyzing shop practices and procedures to increase shop productivity rates while minimizing resource costs. However, the significant impact of compressed air systems on workflow and product quality is often overlooked.
The compressed air system is critical for two main reasons. First, compressed air is a utility; it is the energy source for most shop equipment. Second, clean air systems are often required. Compressed air mixes with the product in paint spraying operations. Moisture or oil can also contaminate critical structures like composites during the repair process. Both air quality and supply directly impact repair facilities. Rework increases labor and material costs, and interrupts workflow. It can increase turnaround times, causing real losses in hard dollars.
There are three major areas to consider:
• Air source
• Air treatment
• Piping and distribution
Avoid a common pitfall — don’t over-pressurize. If asked, many users say they need 145- or 175-psig compressors. In fact, very few tools require pressures above 100 psig. Refer to the tool manufacturer’s manual to determine your shop pressure and flow requirements.
Don’t Confuse Pressure and Air Volume
Many users do not understand the inverse relationship between the air pressure and the air volume 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. 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, wasting both air and electricity. Numerous compressed air industry studies confirm that as much as 25 percent of all compressed air produced is lost through leaks.
Determine 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. Some tool and compressor manufacturers publish charts with air consumption rates for many common tools. Adding these rates together for all tools will yield the total potential flow requirement. 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 shop operate throughout the day. Electronic data logging devices are a convenient way to measure and record compressor usage.
The piston (or “recip”) compressor is still the most common type found in body shops. 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 shop piston compressors are air cooled and have an allowable duty cycle of 60 to 70 percent. They are often 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 F to 400 F).
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 F to 200 F. This is an important consideration for paint spray booths and other moisture sensitive applications since moisture vapor content decreases with temperature. An important rule of thumb is that every 20 F decrease in temperature cuts moisture vapor content in half, making it easier to remove moisture from your system.
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