Compressed air is the typical manufacturer's most important utility. Pneumatic tools and controls, air cylinders for machine actuation, product cleansing and blow-offs each use compressed air. Without a consistent supply of quality compressed air, a manufacturing process can stop functioning. Compressed air systems provide direct indications that a problem exists, but finding the right solution isn't always obvious.
A new, super-fast packaging machine is supposed to process 1,000 widgets per hour. However, the air pressure delivered to the machine fluctuates by 20 psi during the day. To avoid shutdowns, the output of the packaging machine has been reduced. It seems to run properly at 300 widgets per hour and the pressure fluctuations don't result in shutdowns. However, the costly result is lower productivity.
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A CNC milling machine, which can mill a custom component in two hours, is critical to meeting a contract from a major customer. The machine needs to produce three components per shift. Momentary pressure dips during the day are tripping the low-pressure safety switch, causing the machine to shut down. Scrap rate is two pieces per shift. The end result is less productivity and overtime.
Many compressed air systems behave erratically. Approximately 90% of companies use compressed air in some aspect of their operations. Approximately two thirds of these users have improper downstream equipment, inappropriate controls, unsound practices or simply the wrong compressor for the application. Reduced equipment life and higher operating costs are symptoms of a larger problemthe general lack of understanding of compressed air systems and their operation.
Room for improvement
There is no such thing as a fool-proof compressed air system. Every system, new or old, has room for performance improvement. Improving the performance of a compressed air system will reduce energy costs and:
Reduce scrap rate.
Improve product quality.
Extend equipment life.
Efficient compressed air systems affect cost, productivity and reliability significantly, which are, in turn, affected by air supply and demand. A properly managed supply side results in clean, dry, stable air delivered at the appropriate pressure in a dependable, cost-effective manner. A properly managed demand side minimizes air wasted on inappropriate applications.
Improving and maintaining peak compressed air system performance requires addressing both the supply and demand sides of the system and the way they interact. This systems approach shifts the focus away from individual components in favor of total system performance. The systems approach usually involves:
Developing a system block diagram.
Measuring system baselines (kW, pressure, leak load) to determine operating costs.
Working with a compressed air system specialist to implement an appropriate control strategy.
Re-measuring after controls are adjusted to get more accurate kW, pressure and leak load readings.
Checking for obvious preventive maintenance issues and other opportunities to reduce costs and improve performance.
Identifying and repairing leaks, correcting inappropriate uses; knowing costs, re-measuring and adjusting controls.
Implementing continuous improvement and awareness programs.
System dynamics, or changes in demand over time, are especially important. Using controls, air storage and demand management to design a system that meets peak requirements and operates efficiently at part load are the keys to a high performance compressed air system.
Production interruptions usually originate on the demand side. Common areas where energy savings exist are many.
Repair leaks, which can represent as much as 30% of the total compressed air demand. Check point-of-use connections for the slightest hissing sound. An ultrasonic leak detector can identify leaks, even in a noisy industrial plant.
Avoid the improper, yet common, practice of leaving manual condensate drains partially open in an effort to ensure moisture-free performance at a particular point-of-use. Even a timed electrical drain operating for 10 seconds every 30 minutes can cost hundreds of dollars in compressed air per year. Investigate zero-air-loss drains.
Use a quality regulator to limit point-of-use operations to the lowest practical pressure. Poor quality regulators drift and track, which can increase operating cost.
Modify and, if possible, eliminate blow-offs. Many blow-off stations use compressed air simply because it is available. A blower or fan may accomplish the same objective. Engineered nozzles are a substitute for open pipes or hoses.
Shut off the air supply to idle production equipment.
Install a dedicated compressor for a user requiring air pressure higher than others on the system. Don't set the system pressure for a single use or point-of-use application. Consider using a separate compressor, amplifier or booster sized for the function.
Piping of the proper diameter ensures air gets where it needs to go, when it needs to get there, close to the originating pressure, and in the quality and quantity required. Minimizing pressure drop requires a systems approach in system design and maintenance. Select air treatment components, such as aftercoolers, moisture separators, dryers and filters, with the lowest possible pressure drop at specified maximum operating conditions of flow and temperature. When installed, follow and document the recommended maintenance procedures. The pressure drop through the system also increases as the square of airflow rate (velocity). High volume, intermittent demands create peak airflow rates, causing significant pressure fluctuations.
In a multiple compressor system, every compressor should be base loaded, except the one trimming.
A modulating compressor operating at 40% output could still be consuming 80% of its power. Other compressor controls may be better suited for trimming.
Reduce the output pressure: For every two psi change from rated pressure, the brake horsepower required will change 1%. Increase the pressure by 10 psi and the bhp rises by five percent. Decrease the pressure by 20 psi and the bhp drops 10%.
The electrical energy the compressor uses is converted to heat. A properly designed heat recovery system can capture 50% to 90% of this thermal energy. The recovered heat can warm space, heat processes and water, treat makeup air and preheat boiler water.
Use pressure and flow controllers because the higher the pressure delivered to the plant, the higher the artificial demand and the leakage. Pressure and flow controllers produce stored air volume to handle peak requirements and reduce artificial demand and leaks.
Frank Moskowitz is with DRAW Professional Services, Cave Creek, Az. He can be reached at 480-563-0107, email@example.com. For more information on Compressed Air Challenge training and events, visit http://www.compressedairchallenge.org