MRO Magazine

What Does Air Cost You? Understand compressed-air problems and what to do about them


Industry

October 24, 2011
By PEM Magazine

Compressed air systems are rarely installed with instrumentation that can provide a baseline to monitor system energy performance and associated cost. Operators usually run these systems blind to the real operating costs and perhaps unaware of the consequences of the problems plaguing their systems and associated compressed air powered machinery. Studies have show that a few hours of basic awareness training can go a long way in increasing system efficiency. This “efficiency” training often proves to have significant side benefits of a more productive and reliable system due to more stable pressure, improved compressed air system reliability and increased plant productivity.

So, how about the question? How much is your compressed air system costing your company?

Odds are that your management information system can track and control your pencil and paper clip purchases down to the penny, but to put an exact number on the cost of your compressed air, well that’s another story.

If you go to your compressor room and look around, you may understand the reason for the lack of information. The room will often be a noisy, hot and dirty place. The equipment within will have various instrumentation options to observe your system temperatures, pressures and operating hours, but missing will be any indication of flow and power consumption.

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Track down your operators and ask about their operating strategy and compressor control coordination plan. More times than not the question will receive a blank stare, the reality of the situation will be that the plan and coordination strategy will be left to the default settings of the compressor manufacturer, or well meaning, but untrained operator.

But Isn’t Air Free?
Many end users think so — and don’t know what it costs to operate their air compressors, nor can they assess the cost of using 10, 50, or 100 cubic feet per minute. Many are surprised to discover that the operating costs throughout the life of compressed air equipment greatly exceed the initial purchase price. In fact, in most cases, the annual energy cost alone will exceed the purchase price in the first year of operation. It’s essential to determine the current annual costs in dollars and communicate this to all involved. In this way, better decisions can be made on new equipment selection and mode of operation of existing equipment.

Compressed Air Versus Other Energy Sources
Compressed air is a necessary part of most plant operations, but is not the most efficient source of energy in a plant. To operate a one-horsepower (HP) air motor, you need seven to eight HP of electrical power into the compressor. At higher than typical pressures, even more power is needed. The overall efficiency of a typical compressed air system can be as low as 10 to 15 percent.

Annual energy costs for a one-HP air motor versus a one-HP electric motor five days per week for a two-shift operation at $0.05/kWh is $1,164 (compressed air) vs. $194 (electric). This means the compressed air powered motor can cost six times as much for the same power output — more if plant pressures are higher than the estimated level of 90 psi.

Cost of Poor Performance
Systems with leaks waste money. The diagram shows the cost of leaks through a perfect orifice in a compressed air system.

Costs calculated using electricity rate of only $0.05 per kWh, assuming constant operation, 100 psig and a typical compressor.

Leaks make an air system less efficient and increase the cost per unit produced.

Why Use Compressed Air At All?
Compressed air systems are used in almost every sector of the economy, and there are thousands of different uses for compressed air. Uses in the manufacturing sector include powering pneumatic tools, packaging, automation equipment, conveyors, controls systems and others. Although they have many advantages, pneumatic tools are generally much less energy-efficient than electric tools. Many manufacturing industries also use compressed air and gas for combustion and process operations such as oxidation, fractionation, cryogenics, refrigeration, filtration, dehydration and aeration.

There are many applications where compressed air is the best solution. However, if there are other, more cost-effective sources of power, compressed air may be used inappropriately. Air may be free, but compressed air is expensive: when it is needed to make a product, it should be used wisely.

The Systems Approach
Compressed air systems consist of a supply side, which includes compressors and air treatment, and a demand side, which includes distribution and storage systems and end-use equipment. A properly managed supply side will result in clean, dry, stable air being delivered at the appropriate pressure in a dependable, cost-effective manner. A properly managed demand side minimizes wasted air and uses compressed air for appropriate applications. Improving and maintaining peak compressed air system performance requires addressing both the supply and demand sides of the system and how the two interact.

Hundreds of manufacturers produce the various pieces of equipment that are used in a compressed air system, from the compressor packages to the end use tools. There are generally many different options for accomplishing a given task with compressed air, and it is important to apply the equipment properly. Often, if a system is performing poorly, it is not because the equipment is faulty, but because it has been applied improperly or poorly maintained. Almost every compressed air system has room for performance improvement, from a modern system in a two-year old plant to one that has been modified and updated over the last forty years.

Improving and maintaining peak compressed air system performance requires not only addressing individual components but also analyzing both the supply and demand sides of the system and how they interact. This practice is often referred to as taking a “systems approach” because the focus is shifted away from components to total system performance. Applying the systems approach usually involves the following:

• Developing a basic block diagram of your system.
• Measuring your baseline (kW, pressures, and leak load) and determining costs, with available tools.
• Working with your compressed air system specialist to implement an appropriate compressor control strategy.
• Once controls are adjusted, re-measuring to get more accurate readings of kW and pressures, and to determine leak load.
• Walking through to check for obvious preventive maintenance items and other opportunities to reduce costs and improve performance.
• Identifying and fixing leaks and correcting inappropriate uses — knowing costs, re-measuring, and adjusting controls as above.

Understanding Your System
System operators need be able to understand their system problems and what to do about them. One of the ways to solve this need is efficiency-awareness training. The Compressed Air Challenge has developed two levels of training to aid in awareness: “Fundamentals of Compressed Air Systems” and “Advanced Management of Compressed Air Systems.”

A very high portion of end-users reported using materials directly from the training to make efficiency improvements to their compressed air systems. As a point of reference, compressed air system efficiency experts find that, for the typical compressed air system, 30 percent of system energy usage can be saved through cost-effective measures.

As well, studies also showed that end-users who implemented compressed air system efficiency measures also experienced significant non-energy benefits. End-users reported experiencing benefits such as: reduced downtime, improved system reliability, reduced moisture and contamination in the system air, more consistent system pressure, and restored delivery of adequate pressure to all system components.


Ron Marshall is an industrial systems officer with Manitoba Hydro and Bill Scales is CEO of Scales Industrial Technologies Inc. For more information about the Compressed Air Challenge, visit www.compressedairchallenge.org.