Pulling Money Out of Thin Air

Feb. 17, 2005
Raising the efficiency of compressed-air systems could save thousands of dollars yearly.

Steven Coll
Parker Hannifin Corp.
Automation Group
Cleveland, Ohio

Save air with a regulator added to a valve bank.

Placing FRL elements on scheduled maintenance intervals reduces pressure drop and lets air flow.

Compressed air is considered the "fourth utility" in many industrial plants after electricity, gas, and water. A study by the U.S. Dept. of Energy found air compression accounted for approximately 8.6% of overall industrial energy consumption. Generating compressed air for a pneumatic system requires a huge amount of energy, but many companies do not consider it as a cost of production.

Compressed-air production is a variable expense. There can be significant savings in operating costs when it is monitored and controlled. These savings can be divided into three categories: energy, maintenance cost, and better operational efficiency. Following are techniques to manage both the supply side (compressors and air-treatment equipment) and demand side (distribution, storage systems, and end-use equipment) of a compressedair system. They can save dollars while work continues at an equivalent or improved level.

The most obvious source of "found money" from a compressed-air system is in fixing air leaks. Much has been written about air leaks. However, there are new audit and detection methods worth revisiting. It is not unusual for a well-maintained plant to have a leak rate of around 20% of total compressed air production. A single 1/16-in. leak can cost $750/yr while a 1/4-in. leak can run close to $12,000 using an energy cost of $0.07/kW-hr.

The majority of leaks will not be found at the primary header and piping, but at the valves, connections, fittings, tools, and at the point of use. It is often possible to repair the leak by simply tightening threaded connections, or removing the fitting and properly applying thread sealant. Push-to-connect fittings on plastic tubing can often be restored to leaktight condition simply by extracting the tubing, cutting an inch or so off, and reinserting into the fitting. The goal is to restore the tubing ends to a smooth, round sealing surface as when new.

Some fittings, such as live swivels, are notorious leak points, and should be replaced if possible. Equipment no longer used should be isolated with a shutoff valve so as to not add unnecessary leakage. A quality leak-prevention program will identify, tag, repair, and track leak points. A leak audit is a good idea performed by an outside professional air-management firm or by in-house personnel.

A proactive leak program can use the old soapybubble liquid method for detecting leaks, but modern ultrasonic leak-detection equipment is the serious way to attack this problem. These detectors use microphones, audio filters, amplifiers, and headphones to let an operator locate the high-frequency hissing of air leaks in a plant. In most plants, a payback analysis will reveal that these units typically pay for themselves quickly.

The other source of air loss is the leakage designed into the system. This refers to components from which air exits during normal operation such as blow guns, air nozzles, and condensate traps. There is now a whole new generation of compressedair components dedicated to operation efficiency. For example, electronic-sensor condensation drains let compressed air escape only if there is water present. Dew-point controllers automatically measure outlet dew point and adjust an adsorption dryer's cycle time accordingly. Plants should eliminate wasteful inappropriate uses of compressed air for low priority tasks such as blowoff, dusting, and sparging. Dedicated blowerbased air knives and efficiency nozzles for air drying and blowoff can save substantial volumes of more costly compressed air. Some air-tool manufacturers now design with an eye on efficiency. Certain models carry features such as multiple speeds to reduce wasted air. There's even a version of the simple blowgun that uses a multiposition trigger mechanism to meter the flow appropriate for that use. These incremental efficiencies add up to save air and money.

Both contamination (particulates as well as compressor oils), and moisture will make systems inefficient. Contaminants can lead to component-blocked orifices, sticking valves, and leaks from seal damage. Overall, actuators or tools can act sluggishly and can likely damage components as moisture washes lubrication away.

The cost of properly preparing compressed air with filters, coalescers, drip legs, and dryers is much less than that of damaged components or living with poor operational efficiency and low output. The same can be said for putting air-filter-element replacement on regular maintenance intervals. It is a challenge to improve air quality because typical systems have no way to measure it. But there are methods to monitor whether filters, dryers, and other devices are doing their job. Newer devices such as filters have built-in differential pressure indicators, and use of a portable dewpoint meter can be helpful.

It's possible to consume less energy with a change as simple as locating compressor-filtered air intakes outside to supply colder air. This happens every time the outside air is cooler than inside the plant. Lower temperature intake air retains less water vapor, which consumes less energy for moisture removal. Cooler air is also more dense, which reduces compressor operating time. Dense cooler air has a higher molecular weight with lower relative humidity, resulting in a higher mass flow without the compressor working any harder. Assuming 80% relative humidity in intake air, the dry air will drop by about 4%. Reducing the humidity level to 25% only reduces dry air by 1.38%. From a temperature perspective, each inlet temperature drop of 10°F will save about 1.9% in energy. Of course, the trade-off is the cost of potentially dirtier air taken in.

System pressure has a tremendous influence on both costs and efficiencies. Many plants have found their compressors work more efficiently at lower pressures, so they use less air and compressors don't work as hard. An additional, little acknowledged benefit is that lower operating pressures reduce leakage rates throughout the entire plant. A rule of thumb is that every 2 psi decrease in operating pressure yields a 1% savings in operating energy costs.

In some plants, high pressures throughout the entire system are justified by only one or two pieces of equipment. From a cost perspective, pressures in a main system would easily pay for adding a point-of-use compressor, or other device called a pressure booster or amplifier, for these few machines.

Let's look at this pressure-versus-cost variable at the component level. Take a cylinder that functions with enough force and speed at 85 psi, but instead runs at 110 psi. Twenty-five percent more compressed air is required to fill that cylinder at 110 psi. This unnecessary demand forces the compressor to run longer with no appreciable increase in cylinder function.

Another potential savings comes from using a regulator to lower pressure on the return of larger bore cylinders that require little or no force. A cylinder using 100 psi on the power stroke, reduced to 50 psi on the return, will use 30% less air during each cycle. These individual savings can multiply throughout the plant. For example, three 250-hp compressors operating year round at just 10 psi too high costs an additional $17,000/yr in energy based on a rate of $0.07/kW-hr.

Pressure drop throughout the system costs a surprising amount. Pressure drop is often the cause of wideopen regulators and high pressures in the plant-distribution system. But many sources of pressure drop can be reduced or eliminated. For example, reduce multiple combinations of fittings and shorten tubing and pipe runs to the minimum. Use straight connectors instead of elbows wherever possible, and replace multiple T fittings with a manifold.

Piping and tubing should not be sized to equal the valve or connecting device. It is better to use larger lines for lower pressure drop and have end components "bush up" to the piping. Air-line filters and other air prep components are often sized smaller to get cost down without considering the pressure drop. An FRL or dryer may cost less initially, but will add expense daily with higher pressure drop and lost efficiency.

Advanced compressor-control configurations can save both energy and improve reliability. Proven money savers include networking or sequencing compressors together, remote monitoring, automatic start and stops, as well as turning compressors off that can serve as back-ups. Primary receiver volume needs to be evaluated as well. A compressor-service provider with training in advanced management of compressedair systems can take a full system approach to analyze a system and recommend efficiency improvements.

Improving the efficiency of a compressedair system saves air, boosts equipment life, reliability, and output. The benefits far outweigh the time and resources required to implement many of these savings scenarios.

Compressed-air management software

There are several air-management software programs available for in-house evaluations. These tools assess the present performance of a compressed-air system as the first step. They can then model existing and future system upgrades to help evaluate savings and effectiveness of energy efficiency measures. Managers will have a more precise picture of how much can be saved by adding extra reservoir capacity, or lowering overall system pressure several psi. For example, stand-alone software such as the Airmaster+ may model "what if" scenarios by plant engineering personnel or DOE qualified specialist air-system service providers. Additional information on Airmaster+ is at www.compressedairchallenge.org. Other software such as Atlas Copco's Measurement Box or EnergAir Solution's Erecon Compressed Air Management System work with other components and are incorporated as part of the overall air-control system.

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