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Do you know how to maximize the productivity of your compressed air system?

by Frank Moskowitz

Compressed air systems can give you fairly direct indications that a problem exists, but finding the right solution and fixing the problems aren’t always obvious. Some examples:

• Your new super-fast packaging machine was supposed to package 1,000 widgets an hour. However, the compressed air pressure delivered to the machine can’t be sustained. It fluctuates 20 pounds per square inch (PSI) during different times of the day. In order to avoid shutdowns, the machine’s output is reduced to 300 widgets per hour. The machine seems to run properly at this level and the pressure fluctuations don’t result in shutdowns, but the costly end result is lower productivity.

• The computer numeric control (CNC) milling machine, which can mill an aluminum component in two hours, is critical for a contract your company received from an aerospace manufacturer. You need to make three components per shift. The low-pressure safety switch on the milling machine is set at 95 pounds per square inch gauge (psig). Any pressure below that will shut it down. Momentary pressure dips during the day indeed cause the machine to occasionally shut down. The scrap rate is at two pieces per shift. The end result is less productivity, overtime to make up lost parts and high-dollar scrap.

There are many more examples of how compressed air systems behave erratically. Perhaps you have your own. You’re not alone.

Widespread problems
In industry today, approximately 90 percent of companies use compressed air in some aspect of their operations. Of this percentage, approximately two-thirds have some problem with their systems (either obvious or not). Some problems occur from installing incorrect types of compressors, improper cleanup equipment, inappropriate control methods or unsound installation practices. The bottom line is that the problems are costly and lead to reduced equipment life and noticeable operating costs. These are all symptoms of a much larger problem: the general lack of understanding of compressed air systems.

Knowing how your system functions and what forces (outside or inside) influence it can have a significant effect on costs as well as increase productivity and reliability.

For a compressed air system to work efficiently and reliably, you must manage both the supply side (the compressors, air treatment equipment and primary storage) and the demand side (the distribution, secondary storage systems and the end-use equipment). A properly managed supply side results in delivering clean, dry, stable air at the appropriate pressure in a dependable, cost-effective manner. A properly managed demand side minimizes wasted air and uses air for appropriate applications.

Systems approach
Addressing both the supply and demand sides of the system and how they interact is referred to as taking a “systems approach” because the focus is shifted away from the individual components to the total system performance.

Applying this approach involves the following:
1) Develop a basic block diagram of your system.
2) Measure your system’s baseline (kilowatts, pressure and leak load) to determine the costs to operate.
3) Work with your compressed air system specialist to implement a proper compressor control strategy.
4) Once controls are adjusted, remeasure to get more accurate readings of kilowatts, pressure and leak load.
5) Walk through to check for obvious preventive maintenance items and opportunities to reduce costs and improve performance.
6) Identify and fix leaks, and correct inappropriate uses; then, knowing the costs, remeasure and adjust the controls.
7) Evaluate the results and implement an awareness and continuous improvement program.

With compressed air, system dynamics (changes in demand over time) are especially important. Key to achieving a high-performance system is the use of controls, storage and demand management to effectively design a system that meets peak requirements, but also operates efficiently at part load.

Demand-side issues
Production interruptions are usually caused by the demand side. Let’s identify several common areas where energy savings are available:

• Leaks constantly occur in an operating system, and often consume up to 30 percent of a plant’s total demand. Check all of your point-of-use connections for the slightest hissing sound. An ultrasonic leak detector can identify leaks, even in a noisy 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 each year. Look into “zero air loss” type drains.

• Regulate all point-of-use operations at the lowest practical pressure using a good quality regulator (poor quality models tend to drift and track). If the regulator tracks or drifts up 5 PSI, the application will use more air.

• Modify and, if possible, eliminate blowoffs. Since many blowoff applications use compressed air simply because it is there, check to see if a blower or fan could accomplish the same objective. Engineered nozzles are an excellent substitute for open pipes or hoses.

• Shut off the air supply to idle production equipment.

• If one point of use requires air pressure at a much higher level than the rest of the system, consider putting it on a dedicated system. Don’t run the entire system’s pressure for a single use or point- of-use application. Consider using a separate compressor, amplifier or booster sized for the function.

Piping issues
The piping should be the proper diameter to ensure that air gets to its destination, at the proper time and at the required pressure quality and quantity.

Other opportunities:
• Minimizing pressure drop requires a systems approach in 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 recommended maintenance practices.

• The pressure drop through the system also increases at the square of airflow rate (velocity). High- volume intermittent demands can create peak airflow rates, causing significant pressure fluctuations.

Supply-side issues
In a multiple compressor system, all compressors should be base-loaded except for one, which should be trimming.

Other opportunities:
• Evaluate your need for modulating compressors. Such a compressor operating at 40 percent output could still consume 80 percent of its power. Other compressor controls may be better suited for trimming.

• Lower the output pressure. For every 2 PSI change from rated pressure, the brake horsepower (BHP) required will change 1 percent from the rated BHP. Increase the pressure 10 PSI and the BHP will increase 5 percent. Decrease the pressure 20 PSI and the BHP will drop 10 percent.

• The electrical energy used by an industrial air compressor is converted to heat. A properly designed heat recovery system can recover 50 to 90 percent of this thermal energy. Use recovered heat for supplemental space heating, industrial process heating, water heating, makeup air heating and boiler makeup water preheating.

• Utilize pressure/flow controllers. The higher the pressure delivered to the plant, the higher the artificial demand and the leakage. Pressure/flow controllers are high-performance pressure regulators installed on the supply side of the compressed air system. They have two simple effects on systems: they create stored air volume to handle peak requirements and lower system pressure to reduce artificial demand and leaks.

Frank Moskowitz is a consultant with Draw Professional Services, a firm specializing in compressed air system education. To learn more, call , e-mail or visit www.aircare-seminars.com.

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