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.
Compressors are often installed where their noise, vibration, and heat will least bother staff and customers rather than where they will best perform and be easily serviced.
Rotary compressors offer much lower operating temperatures, up to 150 F cooler than the typical piston — and much lower noise levels, allowing for more flexibility on compressor location.
Maintenance and Long-Term Performance
Maintenance requirements and long-term compressor performance are essential factors to consider. Piston compressors and rotary compressors have different maintenance and service requirements.
Piston compressors have proven to be very reliable and require relatively little preventive maintenance other than periodic oil changes, replacing the air inlet filter, and maintaining proper belt tension. Rotary compressors also 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 contamination issues. Preventive maintenance will slow this process, but rebuilding a piston compressor periodically may be necessary to reduce the oil carry-over and reverse the gradual loss of performance. However, rotary screw compressors are designed so that the rotors do not touch each other or the rotor housing, therefore performance does not change over time.
Many sources recommend improving paint finish quality by eliminating contamination in the spray booth, but compressed air quality plays an equally important role as well. Here are three basic types of air system contaminants and their effects: Water/moisture in the form of vapor, mist, and liquid may:
• Cause defects in finishes (“fish eyes”)
• Contaminate the repair surface
• Cause excessive wear in tools
• Cause rust in iron pipe
• Accumulate in storage tanks, reducing the volume available for storage and causing the compressor (piston) to run beyond its recommended duty cycle
For example: Assuming 24-hour operation, on a 75 F day with 75 percent relative humidity, a 10-hp compressor can introduce 7 gallons of water into a compressed air system. On a 90 F day with 90 percent humidity, the same compressor can introduce 15 gallons.
Particulates (dirt, dust, rust, etc.) build up in piping causing:
• Defects in paint finishes
• Surface contamination
• Pressure drop
• Excessive tool wear
Oil, usually in the form of vapors or mists, causes defects in finishes and can combine with particulate to clog tools and spray guns. It may also build up in piping and cause significant pressure drop.
In summary, these contaminants reduce system pressure, increase maintenance costs, and cause product defects (both cosmetic and safety critical). They increase labor and material costs for each aircraft worked on.
Air storage tanks (receiver tanks) are a critical air system component. Almost every facility has one, and it is important to understand some of its functions and benefits:
• Provides the first stage of moisture separation • Stores air for short periods of high air demand • Allows the compressor to shut off to save energy and prevent piston units from overheating (if properly sized)
Here are a few guidelines for tanks:
• Allow at least 3 to 4 gallons per cfm • Pressure rating must exceed highest possible system pressure • Must have safety relief valve, pressure gauge, and drain to remove liquids • Must meet ASME or other required code (check with local authorities)
Choose the right dryer for your needs. Dryer performance is stated in terms of specific conditions (ambient temperature, compressed air inlet pressure, and compressed air temperature).
Refrigerated dryers are the most common. They employ a refrigeration system to lower the compressed air temperature well below the ambient temperature. This condenses the moisture vapor into liquid that can be drained out of the system. This lowers the “pressure dew point” of the compressed air, and as long as the compressed air does not cool below this new dew point, any remaining moisture will remain in vapor form. Refrigerated dryers are designed to produce dew points between 35 F and 50 F at rated conditions.
High temperature refrigerated dryers are similar to standard refrigerated dryers but include an aftercooler 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 50 F dew points at rated conditions.
Desiccant dryers operate very differently from refrigerated dryers. They work by directing the compressed airflow across a bed of desiccant material that adsorbs moisture vapor out of the air. Desiccant dryers are used to produce dew points as low as –100 F and are recommended when air quality requirements are extremely high.
Filters are categorized based on the contaminants they are designed to capture. They may be designed to capture more than one type of contaminant.
Moisture separators are designed to mechanically separate liquid water and oil from the air stream. Particle filters are designed to capture dirt, dust, etc. but may remove some water and oil mists.
Coalescing oil filters are finer filters designed to remove oil aerosols/mists and fine particles. These are usually placed after a refrigerated dryer.
Vapor adsorbers are designed for eliminating oil vapors only and should be placed after all other filters and dryers. Staging filters in the system provide more effective filtration, lower pressure drop at each filter, and longer filter life. Some systems have differential pressure gauges, liquid level indicators, and/or built-in drains.
Proper filter maintenance will minimize pressure drop and ensure good air quality. Failure to change filters will guarantee higher pressure drop and greatly reduce filter performance.
The drain trap is a critical but often overlooked component of the compressed air system. Drains remove liquid contaminants from the system. If the filtered and separated contaminants are not removed from tanks, refrigerated dryers, and filters, they build up and find their way back into the air system. Liquid accumulation in tanks will gradually eliminate the air storage capacity in the tank. This will cause periods of inadequate airflow/pressure, and could cause a reciprocating compressor to exceed its duty cycle and overheat.
There are several types of drains:
• Manual drain
• Timed drain
• Automatic demand drain — a mechanical or electric device that activates when the liquid level reaches a certain point inside the drain.
Piping and Distribution
Pressure drop. Restrictions in airflow create air turbulence that results in a reduced system pressure. This occurs in many components, including the dryer, filters, valves, and piping. The degree depends on the choice of material and pipe size. Pressure drop can be greatly reduced with proper system design and maintenance, but there will always be some. Be sure to account for the total pressure drop when selecting the compressor’s operating pressure.
Pipe size has a major impact on system performance. Pressure drop changes exponentially with pipe diameter. Bigger is better. Look ahead when planning a system and allow for business growth. It is time-consuming and expensive to install a larger distribution system later. There are standard charts published that provide guidelines. Check with a compressed air specialist or call the Compressed Air and Gas Institute (www.cagi.org).
For example, the pressure drop of 40 cfm (from a 10-hp compressor) through 500 feet of straight 3/4-inch smooth pipe would be about 8 psi. If the shop added another 10-hp compressor the total flow would be about 80 cfm and the pressure drop would increase to 32 psi! An increase to 1-inch diameter pipe would change these numbers to 2 psi and 9 psi, respectively. Pressure drop increases with the number of turns in the system and with rough interior surfaces.
There are several piping materials available and the choice has a large impact on air quality and flow. Common materials include:
• Black iron
• PVC or other plastics
• Modular aluminum systems
Each of these materials has its advantages and disadvantages. However, experience shows that copper has the best combination of performance, material cost, and installation cost. The modular aluminum systems show promise, but are still relatively new.
Here are a few piping recommendations
• Plan for future growth and install the largest pipe diameter feasible.
• Minimize the use of pipe “T”s and right angles.
• Install a flexible hose between the compressor/or tank and the piping to eliminate stress on pipe connections caused by compressor vibration.
• Provide adequate bracing/support when hanging pipe from ceilings or walls.
• Use only full flow ball valves to minimize pressure drop.
•Loop distribution to balance pressure and flows at all points of use.
• Connect point of use pipe drops to the top of the header to reduce moisture carry-over.
• Install drip legs at each point of use to capture residual moisture.
An important rule of thumb is that every 2-psi increase in pressure increases energy consumption 1 percent. In addition, the higher the system pressure the greater the volume lost through leaks. A 1/16-inch leak loses 7-8 cfm at 120 psig. At 150 psig, it loses 9-10 cfm. A 1/8-inch leak loses 30 cfm at 120 psig and nearly 38 cfm at 150 psig! 38 cfm is more than many 10-hp compressors can produce.
Price and True Cost
A simple yet 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. Purchasing quality equipment now will save time and money for years to come.