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 Engine Builder’s readers. 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 creation, movement and storage of air) is very serious business.
Today’s automotive repair shop and certainly the modern engine building 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 the electricity and 15 cents will be used for the maintenance of your compressor. Obviously, it’s imperative that you maintain your air system properly to ensure the lowest possible operating costs.
While piston compressors are still the most common type found in the automotive service industry, 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 screw technology. These compressors are more energy efficient, and provide an extremely 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 is the percentage of time a compressor may operate without the risk of overheating and causing excessive wear to the compressor. 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 60-70 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.
Rotary screw compressors have a 100 percent allowable duty cycle and operate continuously if the need arises. While both are positive displacement machines, rotary screw compressors are fluid cooled. The fluid removes the heat generated by compression and also removes impurities from the air. They also have lower operating temperatures of only 170-200° F, compared to 250-400° F in a piston. Lower operating temperatures are a very important consideration for automotive facilities with expensive tools, paint spray booths and other moisture sensitive applications; hot air carries more moisture and requires additional components to dry and clean.
Modern rotary screw 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 special-designed high-temperature dryers, the air exiting a piston compressor is very hot and hard to dry. Even with an aftercooler and dryer it is difficult to reach the same dewpoint as a rotary screw compressor.
Maintenance Requirements and Performance
Consider how a compressor performs as time passes. Ironically, while piston compressors do not 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 between 2,000 and 5,000 hours will reverse the gradual loss of flow and reduce the oil carry over but is expensive and time intensive.
Rotary screw compressors maintain their ability to deliver air. There is no drop in performance or “wear” because the rotors do not touch each other or the rotor housing. Some screw compressors can operate for more than ten times as long as a typical piston compressor before requiring an overhaul.
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 effect the entire shop and its workers especially in a small enclosed space. Rotary screw compressors run smoothly and are available with sound-attenuating enclosures featuring anti-vibration mounts. Typical noise levels start at only 65 dB(A) a significant drop and low enough to have a normal conversation while standing in the immediate vicinity.
The Real Cost
The main reason cited for selecting piston or reciprocating compressors is often lower purchase price. But the actual cost comparison really extends beyond the initial transaction. Because rotary screw compressors do not need to be oversized for duty cycle, and because they are up to twenty 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 screw.
Obviously a 10 hp unit will use less electricity and reduce a shop’s overall operating costs. Air that does not contain significant amounts of oil or moisture will not clog tools and will extend air tool and equipment life. And consider also, the cost associated with unscheduled down time reliable air compressors keep employees working and productive, not waiting for the compressor to come back on-line.
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.
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 your system. If the filtered and separated contaminants are not removed, they will find their way into the system again.
Plan-out Piping and Distribution
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 build-up on the interior and reducing the functional diameter. This build up will create pressure drop in the system and contribute to poor air delivery.
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, which 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.
Tips on Avoiding 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. 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. 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.
|When you’re looking to install or upgrade piping for your shop’s compressed air system, make sure you do it right:
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 wall;
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; and
Install drip legs at each point of use to capture residual moisture.
Storage and control – Receiver tanks provide a first stage of
moisture separation, and store air for later use.
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.
Guidelines for tanks:
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)
For more information on equipping your shop, visit http://us.kaeser.com.
An important rule of thumb is that every 2 psi increase in pressure increases energy consumption 11 percent. In addition, the higher the system pressure the greater the volume lost through leaks. A 1/16? leak loses 7-8 cfm at 120 psi. At 150 psi, it loses 9-10 cfm. A 1/8? leak loses 30 cfm at 120 psi and nearly 38 cfm at 150 psi! 38 cfm is 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, it does not 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. 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.
With special thanks to Angela Kelly at Kaeser Compressors and Dave Monyhan at Goodson for their assistance with this article. For information visit http://us.kaeser.com.