Blow away waste in compressed air systems
Compressed air is one of the most commonly used industrial utilities, but also one of the most wasteful. A U.S. Department of Energy study noted that poorly designed and operated air systems in American factories waste from $1 billion to as much as $3.2 billion annually. And those statistics were compiled before energy costs began to skyrocket.
Although almost all industrial plants consume pressurized air in their manufacturing processes, many maintenance executives fail to realize that 20% to 50% of this air is dissipated in typical industrial settings through misapplication of air, leaks, improper compressor control and poor system design and maintenance. According to Scott Stroup, president and founder of Airometrix Mfg., Inc., a consulting firm specializing in performance testing of compressors, education and evaluation of compressed air systems, less than 15% of compressor input energy is actually converted into usable work at the end of the pipe.
Opportunities to improve
While improvements can be achieved in many areas, Stroup claimed that some of the most significant cost savings and efficiency gains can be made in the areas of leak management, compressor controls, proper maintenance and reduction of wasteful air applications. He said that a typical industrial plant can have leak rates ranging between 20% and 40% of total delivered air volume, with some facilities exceeding 60%.
“While zero air leakage sounds like a realistic goal, attaining it is generally not cost-effective because of a diminishing return to achieve those last few savings percentages,” Stroup said. “Depending on the complexity of the system and industry, goals between a 5% and 15% leak rate are more realistic and acceptable.”
Damaged hoses and fittings, valves, piping, tubing and connectors are causes of compressed air, ‘literally disappearing into thin air,’ Stroup added. He said the challenge for plant personnel is to cost-effectively find and mitigate the leaks in the plant.
“There are several techniques for finding the leaks, ranging from simply walking the air lines and listening for the leaks to using specialized equipment, such as ultrasonic leak detectors. Many large leaks can be found by ear when the plant systems are down and the areas are quiet, but this becomes more difficult when machinery is operating.
“Finding leaks with soap bubbles or by feel %%MDASSML%% the ‘calibrated hand method’ %%MDASSML%% works, but is very time consuming. Ultrasonic is the preferred method of finding leaks around operating equipment and accelerating the detection process,” Stroup said.
Ultrasound detectors easily pick up high-frequency sounds generated by escaping air that are beyond the range of human hearing. The high-frequency sounds are electronically translated down into the audible range where they are simultaneously heard in headphones and noted, usually as decibels on a display panel. These ultrasounds are produced not only by compressed air leaks, but also by other escaping gases such as nitrogen, oxygen and steam. Ultrasonics offers additional applications for finding leaking steam traps, bearing analysis and identification of electrical emissions, such as arcing, to name a few.
Using an ultrasonic detection instrument, a maintenance technician wearing headphones hears and is able to track the escape of compressed air at its source. Sound levels can be recorded and used for cost analysis with specialized software or simple spreadsheet tools. Data and sound samples of mechanical wear in bearings can also be recorded, diagnosed and trended, allowing personnel to prepare an action plan toward preventing equipment outages.
Ensure suitable applications
In addition to leakage, misapplications of air are sources of waste and unnecessary load on compressors, Stroup indicated.
“Compressed air used for blowing, drying, chip removal or parts cleaning is generally very inefficient,” he said. “In most instances, there are other alternatives that work just as well or better and cost far less to operate. Most applications require a high volume of air, not high pressure. Blowers, amplifiers and other techniques can supply the required volume of air at a lower pressure, resulting in significant energy savings.”
He also suggested the installation of zero-loss moisture drains in place of timer drains, which blow air at regular intervals, or cracked drain legs, which remain open continuously. Other solutions can include reducing the air pressure to the actual PSI required for the job.
“Many companies are using 110 PSI when 90 PSI or less is more than enough,” Stroup said. “As a rule of thumb, for positive displacement compressors, every 2 PSI rise in the set point requires 1% more energy consumption. A system that could operate at 80 PSI requires an additional 10% more energy usage if the compressors are set at 100 PSI. That could prove to be quite costly.”
Stroup suggested that plant personnel regulate pressure to meet application requirements.
“Air pressure requirements from plant-to-plant can be vastly different, even within an industry or between sister plants. Excessive pressure is typically an indication of other problems in a plant, such as high leak rates, improper receiver sizing or placement, improper piping design or inadequate control of the compressors,” he said.
Other areas of energy reduction include heat recovery from the compressors. Stroup noted that about 75% of the input energy to an air compressor is converted directly into heat.
“Most of this heat is recoverable, usable for space heating, air drying and heat for various manufacturing processes such as heating boiler feedwater,” he said. “This is ‘zero-cost energy’ that can be readily recovered and put to use in a plant. Equipment required for this purpose is relatively inexpensive and readily available.”
One of the key elements to fighting waste in an air systems is education. “You can’t manage what you don’t measure,” Stroup said. Most plant personnel %%MDASSML%% operators, maintenance technicians and management %%MDASSML%% have no idea just how much an air system costs to operate.
“They must be informed that compressed air is not free!” he said. “It must be compressed, filtered, dried and distributed. All of these steps require costly energy. In general, a compressor running year-round will consume its initial cost in energy each year. That means if you buy a $40,000 air compressor, expect to pay about $40,000 a year in energy costs to run it. On top of that, you have maintenance and upkeep costs. As compressors age, or if maintenance processes are not done regularly and correctly, the overall delivery efficiency of the compressor will slip and costs will start to rise.”
Education and awareness programs should include shop-floor training for maintenance personnel to better understand how their decisions regarding maintenance, filtration and setpoints affect overall costs. Operators need to understand the consequences of using air for menial tasks such as cooling, cleaning and drying applications, and everyone from operators to management to maintenance staff should be an active part of a leak detection program. If maintenance personnel are expected to use ultrasonic detectors, the company should be investing in proper training for these personnel to take full advantage of their investment in the technology. Plants can likely save a minimum of 20% of their current compressor energy with a properly conducted energy audit and implementation of those recommendations, Stroup asserted.
Results summarized from several thousand compressors tested showed that the majority of them were not producing their rated flow at rated pressure. The primary reason was not worn out compressors but insufficient or improper maintenance of the compressors and controls. Fixes are generally inexpensive and involve recalibration or replacement of sensors and gauges, cleaning control lines and cleaning or replacing control valves.
Proper compressor sequence of operations is also important for minimizing air system costs. This includes making sure the individual compressor controls are operating properly as well as sequencing multiple compressors to meet loads efficiently. For example, if multiple compressors are being run %%MDASSML%% all featuring inlet modulation controls %%MDASSML%% only the ‘trim’ compressor should be throttling. The rest should be fully loaded.
“Running multiple compressors where all are throttled is a big waste of energy,” said Stroup, who advised that energy audits be conducted by independent third parties instead of compressor vendors. “In our experience, the majority of plants could turn compressors off instead of buying new ones.”
|Alan Bandes is vice-president of marketing for UE Systems, Inc. in Elmsford, NY. Since 1973, Bandes has served as a panelist at ultrasound conferences, presented and published articles in trade journals and helped develop several training programs to advance the understanding and use of airborne and structure-borne ultrasound.|
The Bottom Line…
While compressed air is an essential component of most manufacturing processes, it’s wasteful in and of itself, with 20% to 50% of it being dissipated in typical industrial settings.
Leak management is key to eliminating waste in compressor applications. Use of ultrasonics is beneficial in identifying leaks.
Knowledge is power. Keeping plant personnel, including operators, maintenance and management, up to speed on operation and maintenance of air compressors is the surest way to eliminate waste in compressor systems.