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The ABCs of air compressors

WHETHER HE IS checking a small shop or a large manufacturing facility, Jim Dawson invariably finds that air compressors are being improperly used and the company is wasting money.

Dawson, senior systems engineer for Ingersoll-Rand, says the average shop often uses multiple compressors that run in competition with each other, causing the shop to use considerably more electricity that does not necessarily provide the “on demand” need of compressed air in the volume required at various stations or tools throughout the shop.

“Everybody buys an air compressor, and they almost always oversize it, thinking they need all that air,” he says. “When you're in a body shop or garage, the usage is all intermittent — they're all operator-dependent. A guy picks up a tool, uses a sandblaster — it's on-off usage. If they turned every tool on at once, could they consume twice as much air? Yes, they could. But that in reality rarely happens. A guy picks up a tool, uses it, set its down.”

Dawson recommends that a company do two things:

  • Perform a systems analysis.

  • Integrate a variable-speed-drive, rotary-screw compressor.

Ingersoll-Rand performs an IntellSurvey Supply Side Audit in which it records the power, flow, and pressure of a shop's air system 24 hours a day for seven days.

“By doing it, we get a profile of how they run throughout an average week,” he says. “We can say, ‘Hey look, guys, you have a 25-hp air compressor, but on average you're using 10 hp worth of air.’ Compressors are designed to run at full load, and when they come down off full load, they're very inefficient — unless it's a variable speed drive unit. Fixed-speed units draw 90-95% of full-load power to make anything off their design point. The inlet valve will throttle, making the air needed, but they consume 90-95% of full-load power doing that. Even when they unload when the inlet valve is closed, they still draw 50% of full-load electric hp (kW) that you pay for. If you're not running at full load and start to throttle back, you're running less than efficient. And then at no load, brochures might say, ‘Well, you use 20% of unloaded horsepower.’ That's mechanical horsepower. That's not what you pay for. You pay for kilowatts, electric horsepower.

“By doing IntelliSurvey audits, you're able to determine, ‘Wow, I can run off a much smaller air compressor if I added storage capacitants to my system — meaning air receiver tanks.’ And then you need to store that gas at a higher pressure and then regulate or flow-control it out at the bare minimum pressure that the plant will operate at. The larger differential in pressure that I have between the tank and what is being sent out to the floor, the more pounds of stored gas I have. So store the gas at 125 psi but flow-control it out at 100 psi because all pneumatic tools operate at 90-100 psi, so that a 25-pound differential in pressure from 125 in a big receiver tank coming out of a flow controller at 100 gives me a lot of pounds of stored gas.

“So if a bunch of people pick up tools at same time, the chunk of air volume required or demanded by the system comes out of the tank and the compressor never sees it. Then they set the tool down, the tank has recovery time, then they pick it up. So you can handle these simultaneous spikes with storage capacitants.”

Dawson says a variable speed drive rotary screw compressor also can add to energy savings.

“You still have a tank, but as you ramp up and down, the rotary screw has a linear power curve with the variable speed drive, meaning if I have 25 hp but I need to make only 10 hp worth of air, that thing will slow down the drive to a low RPM so it's only sucking in 10 hp worth of air,” he says. “So you're not getting killed on electric usage. But if you need 25 hp, it'll ramp right up to 25.

“The other advantage is that when it starts up, it doesn't spike your system. It's a soft start, so you don't get the surge of inlet-rushed current. Power company electric meters all read every 15 minutes and take the average every 15 minutes. If you have a spike in there, it skews your average and kicks on your ‘on demand’ charges for the entire month.”

He says audits show a shop how much a system is lacking in storage capacitants.

“You need flow control,” he says. “You need to have a differential in pressure to properly use storage. A lot of people have the air pressure oversized. They're compensating for ‘artificial demand.’ They run more kW in the compressor room to compensate for a lack of storage, lack of flow control, and lack of response from the compressor to get air from the compressor room out to where guys are using it. So they crank the pressure up higher, put a larger unit in. Sure, if you put enough power behind it, you'll get the air there, but you're running so inefficiently. All your air leaks are leaking at this elevated pressure, so therefore you have ‘artificial demand.’”

Example of big savings

Dawson gives the example of Allstate Tool and Die, which operates a 60,000-square-foot facility in Rochester, New York, specializing in the manufacture of precision-machined, close-tolerance, complex components and assemblies for various precision industries.

“They had 25 hp and 30 hp air compressors fighting each other,” Dawson says. “They thought, ‘We need to buy a bigger compressor. We want to buy a 75 hp cause we're running 55 hp.’”

The survey showed that Allstate was running average compressed air usage of 76.7 cfm.

Operating the system in that fashion utilized 186,391.2 kW in total energy during the year. The compressed air system operated 8760 hours per year, yielding 21.3 kW employed for each hour of compressed-air operation.

IntelliSurvey analyzed the environment in which Allstate's compressed-air system operated: atmospheric pressure, 14.4 psig; average temperature, 85 degrees F; average humidity, 60%; and altitude above sea level, 500 feet.

Dawson says compressed-air trains are compressor systems that couple air-treatment equipment with each compressor in the system, producing multiple, independent-operator compressor systems that cause equipment to cycle unnecessarily and air quality to decline. At Allstate, there was one compressed air train operating. By moving the system control-pressure signal location from each individual air compressor to a common downstream dry-side receiver location, the negative consequences of compressed air trains were eliminated.

Dawson says that in order to obtain system reliability, it is critical to ensure enough supply-side capacity to obtain compressed air-system integrity. Allstate had no existing dry-side storage. In order to achieve system integrity so that the largest system event — 120.6 cfm — would not interrupt production, 660 gallons of dry-side storage needed to be added.

He also says that large changes in pressure at the point of use results in lost production, increased scrap, and excessive down time. And to compound those effects on the production floor, pressure variability on the supply side results in lost energy and a significant decrease in system reliability. Pressure variability is caused by the increase in duty cycle on the compressors in the system and is inherent to any operation. At Allstate, by decoupling the supply side from the point of use, the production system reduced pressure variability to 0.75 psig, which also resulted in an increase in system reliability.

Variable-speed compressor

Allstate also switched to a variable-speed compressor that yielded 9kW in savings, or $4826 in energy cost reduction of the compressed air system.

“A conventional, fixed-speed air compressor is controlled by an inlet-control valve that modulates between open and closed positions,” Dawson says. “But using the inlet valve to meet air-system air demands results in extreme pressure fluctuations and wasted energy, greatly reducing efficiency whenever the compressor operates outside its optimum performance range. Using a frequency inverter, an air compressor can deliver air at a constant pressure, regardless of demand, at maximum efficiency. Efficiency and pressure are maintained by varying the compressor inlet speed versus modulating the inlet valve.”

With all of those changes, Allstate's new system energy cost was $6400 a year — a savings of $7359.

“In reality, their air demand on average was 69 cfm, which is a 20 hp air compressor,” he says. “When I got done with them, I also picked up their air leaks. I found out they were leading 41 cfm, so their air demand was even considerably below that. They were running two air compressors tripping each other. They really should have 15 hp on it. They were running four times what they should have. So their opportunity for energy savings was almost $7500 to their bottom line.”

As another example, Dawson cited Hawk Frame and Axle, which operates a 60,000-square-foot shop in Fairport, New York.

“If everybody turned on the tools and a guy was sandblasting, they'd use over 100 hp of air,” Dawson says. “But they averaged 51 cfm, which is under 15 hp. You'd look at that shop and think, ‘I need 50 or 60 hp.’ His sandblast nozzle does use 125 cfm — that's free air flowing into atmosphere. He needs 30 hp when that is running full-out. But how often does he use that? Not very often. So he should have a large storage tank at that point of use so he doesn't drain the rest of the system down when that guy first starts blasting. That tank will allow a permissive start of a second air compressor to come on and help out the first when he's sandblasting. But yet the sandblaster doesn't suck all of the volume out of air piping with his big nozzle when he first starts up.”

He says the IntelliSurvey costs $975, but provides a wealth of information to explain how the system is operating.

“Of the four utilities — gas, water, electric and air — compressed air is by far the most expensive,” he says. “There's no mechanic or body shop that can run without them, but compressed air is typically four to 10 times the electric rate. Eastman Kodak in Rochester generates their own power at 4 cents a kilowatt hour, but their cost to compress air is 43 cents per mcf. So it costs them more than 10 times as much to generate compressed air than to buy or make electricity.

“One, it's an energy hog for you. Two, you're running a bigger machine. It's no good to run a 30, 40, 50 hp air compressor, and you're using 10 hp. You'll have more oil leaks and the machine will rapid-cycle, beating up the bearings.

“The key thing in a system is system analysis — that will determine what you should put in. If you don't have data, you are what I call a ‘catalog salesman.’ You're guessing. You're coming in with a brochure and saying, ‘Well, you've got 25 and 30 hp. You want a 75 hp? Here it is, sir.’ If I wouldn't have done that audit, the guy would have bought a 75 hp and spent all that money for big air, and his electric bill would have gone through the roof. It's not the machine's fault. You misapplied the machine to an application. Why? Because you had no data.

“A variable speed drive costs more money. So some won't opt for those. But irregardless, they should do the system analysis to find out what their system is doing. How bad is it? Once they know that, they can determine, ‘Do I want to change anything? Spend some dollars? Upgrade?’”

This story was based on a presentation given at the Truck Frame & Axle Repair Association (TARA) meeting in Rochester, New York, October 25-28.

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