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Air Compressors: What to Consider When Buying

Rotary type compressors are now taking a larger bite out of piston compressors in the collision repair market, with their manufacturers claiming this technology is more energy efficient and provides a reliable supply of clean, dry air.

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kaeser as 25 t aircenter rotary screw.atlas copco gx2ff rotary screw.champion hydrovane rotary vane.curtis toledo se 25 rotary screw.ingersoll rand up-series rotary screw.mattei ac series rotary vane.quincy qgs-25 rotary screw.sullair shoptek st 1500 rotary screw.bendpak tri-max piston.Compressor technology is changing, and many shop owners are finding out what large facilities and industrial manufacturers have known for years: There are significant benefits to using rotary type technology, not just the traditional piston technology. Rotary type compressors are more energy efficient and provide a reliable supply of clean, dry compressed air.

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In one of his famous science lectures, Dr. William H. Cosby asked the pertinent question: “Why is there air?” Being the astute man of knowledge he is, he answered his own question: “To blow up basketballs.”

Of course, Bill Cosby’s rationale for why air exists is different than any of yours. After all, he was a phys ed major and comedian. Air – like any number of other things – was, for him, a laughing matter. For you, air (or more precisely the compression, quality and storage of air) is a very serious business.

Do Your Research

Today’s collision repair facility relies on a steady supply of compressed air to run tools and equipment – but determining how you get that air can be a challenge. Most assuredly, it will require research to make sure you’re meeting your shop’s needs in the most efficient manner possible.

Surprisingly, the biggest investment you’ll make when it comes to compressed air is not the compressor. For every dollar you spend on air, 70 cents will be used for electricity and 15 cents will be used for maintenance. Obviously, it’s imperative that you maintain your air system properly to ensure the lowest possible operating cost.

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While piston compressors are still the most common type found in the collision industry, technology is changing. Rotary screw compressors and rotary vane compressors are now becoming more popular. They’re more energy efficient, quieter and provide a reliable supply of clean, dry compressed air. When evaluating a facility’s compressed air system, consider duty cycle and performance, energy efficiency and maintenance requirements.

Duty Cycle

Duty cycle is the percentage of time a compressor may operate without the risk of overheating and causing excessive wear.

A piston compressor may provide adequate flow for a short period, but its allowable duty cycle must be considered. 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. Even with adequate air receivers and storage, this can cause capacity problems during peak hours or busy periods. Further, if the shop expands or business picks up, capacity can become even more of an issue.

Proponents of rotary type compressors say these machines have a 100 percent allowable duty cycle and operate continuously if the need arises. While both piston and rotary type compressors are positive displacement machines, rotary type are fluid cooled. The fluid removes the heat generated by compression and also removes impurities from the air.

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Air Compressor Spec Chart


Rotary type compressors also have lower operating temperatures of only 170° to 200° F, compared to 250° to 400° F in a piston. Lower operating temperatures are an important consideration for body shops with expensive tools, spraybooths and other moisture-sensitive applications; hot air carries more moisture and requires additional components to dry and clean. Modern rotary type compressors now come with built-in aftercoolers designed with ample surface area and a powerful fan to lower the compressed air’s temperature as it exits the compressor. Even with specially designed high-temperature dryers, the air exiting a piston compressor is very hot and hard to dry. Even with an aftercooler and dryer, it’s difficult to reach the same dewpoint as a rotary screw compressor.

Advocates for rotary vane compressors, however, cite issues rotary screw compressors have with volume and efficiency due to the “blow hole” and air leakage (back to the intake). They say claim that to make up for these inefficiencies and keep up air volume, the designers of rotary screw compressors incorporate higher speed, which they say is detrimental to service life.

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Rotary vane fans claim the volumetric efficiency of these compressors is 90 percent, which they say leads to a reliable continuous volume of air while providing a substantial savings in energy, wear-life and maintenance costs.

Maintenance Requirements

Consider how a compressor performs as time passes. Ironically, while piston compressors don’t use oil in the compression chamber, oil carryover is a serious problem in most piston units. Pistons, cylinders, rings and valves wear, 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. Rebuilding a piston compressor will reverse the gradual loss of flow and reduce the oil carryover, but is expensive and time ­intensive.

Fans of rotary screw say these compressors maintain their ability to deliver air. There’s no drop in performance or “wear” because the rotors don’t touch each other or the rotor housing. Some screw compressors can operate for more than 10 times as long as a typical piston compressor before requiring an overhaul.

As far as oil carryover, rotary vane manufacturers claim they offer the lowest in the industry – less than 1 to 3 ppm. They use a multilevel internal separation system before the vapor is introduced into
the separators.

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Regarding maintenance and life expectancy, rotary vane manufacturers say these compressors don’t degrade over time either because during rotation, the vanes slide on an oil film, preventing direct contact with the internal surface of the stator – meaning there’s zero wear on the vanes. Also, they contain no bearings, only bushings, which they claim make them last two to four times longer than rotary screw.

Noise Levels and Vibration

The standard shop compressor has a reputation for being loud and is often placed in a separate room, in a forgotten corner or worse – outside exposed to the elements. The physical, mechanical pounding of piston compressors is certainly effective as a compressive process, but the resulting noise – approximately 85 dB(A) or significantly more – and vibrations affect the entire shop and its workers, especially in a small enclosed space.

Rotary screw fans say these compressors run smoothly and are available with sound-attenuating enclosures featuring anti-vibration mounts. Typical noise levels start at 65 dB(A) – a significant drop and low enough to have a normal conversation while standing in the immediate vicinity.

Those who believe rotary vane compressors are superior say these machines are inherently quiet, too, due to a direct-coupled, 1,800 rpm design that ensures perfect alignment of the compressor and motor. They say this eliminates the noise associated with sheaves, belts or gears, citing noise levels as low as 65 dB(A) with or without an enclosure.

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The Real Cost

The main reason cited for selecting piston or reciprocating compressors is often lower purchase price. But the actual cost comparison should extend beyond the initial transaction. Because rotary type compressors don’t need to be oversized for duty cycle, proponents say, and because they’re up to 20 percent more efficient than piston models, some shop owners are surprised to find that their 15-hp piston compressor can be replaced by a 10-hp rotary type.

A 10-hp unit will use less electricity and reduce a shop’s overall operating costs. Air that doesn’t contain significant amounts of oil or moisture won’t clog tools and will extend air tool and equipment life. Consider also the cost associated with unscheduled downtime – reliable air compressors keep employees working and productive.

Those on the rotary vane compressor side counter by saying their machines operate at a lower rotational speed (1,800 rpm vs. 3,600-8,500 rpm), which translates to longer bearing and airend life. Also, the airends don’t use ball or roller bearings. Those rotary screw compressors that do, they say, create airend damage in the event of bearing failure. Bearing failure, they claim, allows the male and female rotor to make contact, resulting in damage to the rotors, shafts and stator housing.

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Consider all the facts when setting up a new shop or retrofitting an existing facility. Look at capacity and the potential for growth and make your selections based on an honest evaluation of available technology. There are many factory-qualified representatives who can advise you on the right system for your applications and your day-to-day needs.

Particulate Buildup

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. Don’t forget to include high quality, automatic drain traps in your system. If the filtered and separated contaminants aren’t removed, they’ll find their way into the system again.

As compressed air piping is responsible for actually delivering the compressed air to the point of use, its material, age and condition also impact both system reliability and air quality. Iron piping will rust and corrode, creating buildup on the interior and reducing the functional diameter. This buildup will create pressure drop in the system and contribute to poor air delivery.

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One type of pipe that isn’t recommended is plastic. After continued interaction with compressor lubricant, the plastic can degrade, leading to weak spots in the pipe wall that could rupture under certain conditions.

Appropriately sized copper or aluminum piping offers the best performance over time, say experts. Easily installable, modular piping products are now available and are ideally suited for shop upgrades as well as new facilities.

Avoiding Pressure Drop

Restrictions in airflow create air turbulence that results in 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. 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. There are standard charts published that provide guidelines. Check with a trained compressed air professional or call the Compressed Air and Gas Institute (www.cagi.org).

Fix and Identify Leaks

Numerous industry studies confirm that as much as 35 percent of all compressed air produced is lost through leaks…and every facility
has leaks.

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Discharging valuable compressed air to the air in your shop is costly, creates artificial demand and can impact tool/equipment performance. Many shops compensate by increasing the compressor’s pressure setting. Unfortunately, this just increases the amount of air lost to leaks – wasting both air and electricity.

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 to 8 cfm at 120 psi. At 150 psi, it loses 9 to 10 cfm. A 1/8-inch leak loses 30 cfm at 120 psi and nearly 38 cfm at 150 psi! That’s more than many 10-hp compressors can produce.

Whether or not you’re in the market for new air system equipment, identify leaks and fix them…sooner rather than later to maximize efficiency.

Go With the Flow

In order to determine your requirements, it’s a good idea to look at your usage. Most tool and compressor manufacturers publish charts with flow guidelines. Adding all of these together yields the total flow requirements. However, they don’t take into account the percentage of time each tool is used. This requires some study of how the different parts of your shop operate throughout
the day.

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To determine your actual flow requirements, data logging devices are ideal for recording compressor usage over time.

Look for experience and expertise from a factory-trained compressor supplier and service provider. Consider whether they understand your facility’s specific operational requirements and can clearly explain compressor features and relate their benefits to your needs. A collaborative and consultative selling process is more likely to produce an efficient and reliable system.

Value for the Money

Remember – it’s about getting the right system for your application. Carefully consider each system component and its impact on the application. No one wants to spend more on compressed air equipment. However, building the right system for your facility and applications now will more than pay for itself in the long term, especially when you consider reliability, efficiency and improved product quality.


Air Compressor Spec Chart


Doug Kaufman is editor of Engine Builder magazine. He can be reached at (330) 670-1234 ext. 262 or [email protected].

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