Plug an electrical device into an outlet. Does it work? Great! For some people that’s all that matters. When it comes to compressed air, many manufacturing plants operate the same way. As long as there is enough air, that’s all that matters.
But what if cost control also matters to your company? Smart compressed air users may already know how much air they’re producing, but they also want to know how much air they’re using—and whether they’re using it productively. To find out, they’re taking accurate, real-time measurements using flow meters.
Flow Meters Provide a Direct Measurement
CDI 5200 flow meter serving an individual piece of equipment in a ceramics manufacturing
Instead of attempting to calculate air flow based on assumed relationships between system pressure and a compressor’s electrical consumption, or by extrapolating air flow from the time that a compressor is loaded or unloaded, you can measure the air flow at various points in your facility.
“To begin fixing nonproductive demand, you first need to measure it,” according to Jeff Wright, President of Compressor Energy Services (CES), LLC, a compressed air consulting firm based in Merrimack, NH. “CES uses flow meters on every project because they provide a direct measurement of what the compressor is supplying. The Department of Energy states that 50 percent of compressed air goes to nonproductive uses. But, by actually measuring air flow, CES has found the figure is closer to two-thirds of compressed air produced is nonproductive.”
Wright explains how flow meters can help you locate where air is being used nonproductively. “Data from air flow meters lets you see the signature of demand events as they happen,” he explains. “You can see how compressors respond, how they sequence. Flow meters open up a world of understanding in a language the layman speaks. Anyone can look at the LED readout on the flow meter and know what’s happening. We get a lot of buy-in from plant workers because they don’t have to trust a calculation. They just have to look at the display. There’s no black box, no smoke and mirrors trying to relate amps to pressure. Nothing is calculated, only measured.”
Flow Metering is Critical to Air Compressor Sizing
Wright, who describes CES as vendor-neutral and energy-biased, points out that flow metering is critical for properly sizing a compressor. “No matter what compressor brand you’re looking at, you need to look at your actual flow characteristics and match the manufacturer’s curve to get the right fit. Too many people oversize compressors, especially variable speed drive compressors, but buying too big a compressor without addressing nonproductive uses just means you waste air more efficiently than before. If a compressor is too big for demand and has to turn down, it frequently runs on the low end of the performance curve. You can easily find yourself using more power with a larger variable speed drive unit than with a smaller fixed-speed compressor operating in its sweet spot.”
Wright recalls a company that conducted an air use audit based on amps and pressure. The results suggested they needed another compressor and they brought in CES to help them size the new unit. “Their impression was they needed more machines to meet demand. We put flow meters on their equipment and demonstrated that they could actually run fewer compressors at full load, where they operate most efficiently, and turn off the ones running at part load.”
Using flow data, CES created a blueprint and implemented improvements, which included:
- Fixing leaks
- Programming a solenoid valve so air was only used on equipment that’s running
- Installing more efficient nozzles to reduce peak demand
- Modifying piping to resolve pressure problems
- Installing booster pumps where higher pressure was required
- Installing a small, point-of-use compressor on a sand blaster used occasionally
- Improving compressor operating efficiency by utilizing storage
Wright emphasizes that air losses are not always leaks, per se, but waste designed into the equipment by the manufacturer. “Vacuum devices that grab and release work pieces typically have two-position valves,” he explains. “If they remain open when not in the vacuum position, they blow off compressed air continuously. In a noisy environment, you may never hear it.”
When Compressor Energy Services presents to plant managers, they begin by asking how many use flow meters. “Typically it’s about 10 percent,” Wright says. “When we ask how many are actively looking at their metered data, there are only a few left. Facilities already measure consumption of water, gas and electricity, so why not air? It’s the most expensive plant utility—measure it!”
Where to Install Flow Meters
Flow meters range from simple displays of air flow to smart devices that log air flow data, send out alerts if flow exceeds set limits, and send data to networked or remote computers for analysis by engineering staff. By sharing the results of analysis regularly with equipment operators and maintenance technicians, it’s possible to directly relate production output to compressed air consumption. This is valuable for companies that want to assign the cost of compressed air to specific machines, production lines or plant operations with a high degree of accuracy.
“It is worthwhile to learn where expensive compressed air is going, and it’s not all that hard to do,” according to Roger Dennison, President of CDI Meters, Inc., in Woburn, MA. “Measuring compressed air flow throughout your facility allows you to identify leak loads and ineffective uses, which are problem areas you can quantify and correct.”
Thermal mass flow meters employ a simple, direct method of measuring flow, without imposing a significant pressure loss. They work by sensing the temperature of air in the pipe and the amount of heat required to maintain a heated sensor at a fixed temperature difference above the measured air temperature in the pipe. “The faster air is flowing, the more heat is required,” Dennison explains. “The heat requirement depends on the mass velocity of the air, which means that the meter is measuring the mass of air going by as opposed to the volume. As long as the compressed air has been filtered to remove lubricating oil and dried to remove water droplets, thermal mass dispersion flow meters provide accurate flow data independent of pressure and temperature.”
Dennison designed CDI’s meters as a low-cost method of measuring and saving compressed air. Meters clamp directly to a pipe. No welding is required. Two 3/16-inch holes are drilled in the pipe using a drill jig to precisely locate the holes. The flow element and reference element are in separate sensors, one in each hole. The meters operate on 24V DC power. A digital display reads real-time flow in scfm (standard cubic feet per minute), cubic meters per minute or cubic meters per hour. CDI offers an optional Modbus RS485 serial communication output so data can be fed into a local Ethernet network or directly into a host computer. CDI also provides software free for download that will collect and plot the data.
“All CDI flow meters are CE certified and RoHS compliant, have built-in LED displays, and require no calibration,” Dennison says. “They are available for steel, copper or aluminum piping, and their cost is typically low enough to make them appropriate for installation throughout an air distribution system. Just place them strategically, measure air flows and data log the information.”
Dennison recommends placing a flow meter ahead of each production machine to record the real-time flow for each machine. “Note the flow to each machine when it is idle, either between cycles, at the end of a shift or during a shutdown. Any flow at such times is suspicious, as is any increase in this flow or any difference in flow among similar machines doing similar work. Also consider whether the flow is reasonable for the benefit provided, and whether the cost is sufficient that alternatives to compressed air should be considered.”
Dennison also points out that maintenance personnel will be more motivated to repair leaks when they can see the reduction in air usage.
CDI 5400 flow meter installed on a major air distribution line in a lamp manufacturing facility
Biogen Idec Inc. Uses Flow Meters in Many Areas of Their Facility
Consider how a biopharmaceutical company is using flow meters: “We have an intense energy consumption profile,” says John Kelliher, Senior Plant Engineer with Biogen Idec, Inc., in Cambridge, MA. “That’s why we’re always looking at systems that improve energy consumption and capacity utilization. With compressed air, the approach we used before was simply a timer that recorded how many minutes a compressor was loaded or unloaded. It was pretty crude, actually. Now we use flow meters in many areas of our facility, predominantly labs and research buildings, so we know how much air we are really using. We’re also beta testing flow meters with nitrogen applications.”
Kelliher says that Biogen Idec is constantly upgrading facilities and equipment, so when it comes time to replace a compressor or a dryer, flow meters provide data he can use in selecting a machine this is right sized. “As we upgrade our buildings, we’re often able to downsize our air compressors and dryers,” he explains. “By measuring flow we can determine the right size based on the true connected load rather than on what the system was designed for when it was installed years ago. Currently about two-thirds of our flow meters are dumb, where we go around and take readings, but as we upgrade our building management system we will integrate the whole compressed air system including smart meters so we’ll be able to spot trends in real-time. We’re starting to tap resources for better measurement and management of compressed air usage, and flow meters get us in the game without substantial cost.”
A Tool to Manage Energy
In his work managing energy with manufacturers, Richard Feustel, CEM, Vice President of the Wisconsin Chapter of the Association of Energy Engineers (WAEE), says he used to attempt flow calculations from the electrical side. He initially used this approach with a manufacturer of marine engines that had a large washing machine with nozzles for blowing off parts.
“This equipment took large gulps of air for periods of two to 30 minutes,” Feustel explains, “and it triggered alarms in other places in the network. I thought another compressor was needed. We used pressure gauges and electrical consumption and tried to relate those figures to calculate flow, but trying to determine flow from the electrical side was challenging, expensive and intrusive. It didn’t give us a lot of confidence. Measuring air flow was the solution because it gave us hard data to work with.”
Measuring, logging and analyzing air flow data led Feustel to a solution involving a large storage tank to support the entire compressed air system. The tank addressed peak demand issues, allowed the existing compressor to operate more efficiently then go on standby, and was less expensive than a new compressor.
Cost of Goods
For increasingly lean, efficient and accountable manufacturing operations, it makes sense to know the cost of compressed air in order to calculate an accurate cost of goods. The compressed air component of that cost is rarely known for particular production lines or production runs, and rarely built into the cost of goods. Typically, the cost of air is estimated or simply rolled up and spread out as overhead across all products.
Using data gathered from flow meters, the cost component of compressed air directly related to a manufacturing process can be determined with a high degree of accuracy. (You might mention this to your accounting staff when you’re requesting funding for flow meters!)
For an average investment of \$800 (range of \$500 to \$2,000 per meter, depending on size, application and features) and just a few minutes for installation, flow meters give you real-time data that can help you make meaningful financial decisions. With electricity costs approaching at \$200 per cfm per year in many parts of the country, it’s easy to see the advantage. Go with the flow.
For more information on the organizations discussed in this article, visit Compressor Energy Services (www.compressorenergy.com), CDI Meters, Inc. (www.cdimeters.com), Biogen Idec (www.biogenidec.com), and the Association of Energy Engineers (www.aeecenter.org).
To read similar Measurement Technology articles, visit www.airbestpractices.com/technology/instrumentation.