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Compressed air efficiency, part 2: managing supply

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Compressed air is a hugely expensive resource. Efficiently control your compressors and manage your compressed air supply to help ensure you don't pay more than you have to.

In July, compressed air specialist Roger Dean, recently retired from a long career with Kaeser Compressors, offered tips about managing compressed air demand. This month, the focus is on supply.

And watch for Part 3 of this series later this fall, which will walk through the compressed air audit process and give insight into how these studies can help you save.

Part 2: Managing compressed air supply

Roger Dean suggests paying attention to the following elements of your compressed air supply:

Controls

There are generally four types of controls used on compressors to regulate how hard they work: inlet modulation, variable displacement, on-load/off-load, and variable speed drive.

Of the first, Dean says, "Inlet modulation is an energy pig. It's the most inefficient compressor control. Inlet modulation increases the compression ratio by choking back the inlet air, so while you get nice smooth air flow out, it's done at the expense of energy consumption.

"For example, at approximately 60-65% of machine capacity, you're still consuming over 90% of full load horsepower. It's convenient, because the system holds its pressure, but it's very inefficient."

Dean says inlet modulation is like holding one foot on the brake at the same time as pressing the gas pedal in your car. The car is ready to accelerate at a moment's notice, but you're burning extra energy in the meantime.

Dean recommends never using an inlet modulation control system.

With an on-load/off-load system, the compressor fills storage tanks up to a desired pressure, then unloads.

"The compressor still runs, it's just not compressing any air," says Dean. "When the pressure falls 10-15 psi, or whatever amount you've set, it loads up and starts compressing air again."

The key with this system is to have adequate storage capacity, so that the compressor is loading and unloading efficiently.

"To have an efficient system, you need to have at least three gallons of storage per cubic foot per minute (cfm) capacity. So for a 100 cfm compressor, you need a 300 gallon storage tank," says Dean.

He cautions that this refers to storage capacity located at the compressor, serving the entire system – additional air accumulators in the system used to serve individual machines don't count as system storage. Since the cost of a 400-gallon tank, including installation, is approximately $3,000, Dean says this is a worthwhile investment in efficiency.

"There are so many systems out there that could benefit simply from getting bigger tanks," he says.

However, the most efficient control system is a variable speed drive (VSD) – assuming your system has fluctuating requirements.

"A VSD speeds up and slows down in response to system needs. Unlike inlet modulation, it does this efficiently; it's without a doubt the best choice of controls.

"For example, at 60% of full load flow, you're drawing about 63% of full load horsepower. The only time you wouldn't want a VSD is if you do not have a varying flow. If you have a continuous process making 20,000 widgets all day, everyday, a VFD might not make sense."

For this reason, Dean recommends doing an air study before determining if a VSD is right for your system.

Appropriately sized compressors

"Let's say you have a requirement on Wednesdays for 300 cfm of air, but the rest of the time, you only need 100 cfm," says Dean. "Don't get a larger machine just to meet the requirement for Wednesday; get several smaller machines, and turn one off when you don't need that extra capacity.

"You need to look at the consumption profile unique to your plant to come up with the best solution, the best efficiency at all levels of consumption."

Another approach is to isolate high-pressure machines. "I've seen pressures maintained at 130 psi because one small operation, such as a hoist, needs the higher pressure. So we have a 500 cfm plant producing 130 psi air for this one single requirement.

"Take it off the loop and put a little piston compressor there just for that one high-pressure operation, and then you can reduce the overall plant pressure ... but don't over-pressurize your whole system just for that one need."

Sequencing and automation

Using several compressors together efficiently requires effective sequencing.

"Theoretically, one machine starts and runs, and the second should not start until the first machine is fully loaded, and the third shouldn't start until the second is fully loaded," says Dean. "But what we often see is machines fighting each other, or three machines all running only partly loaded. That's inefficient. Sawmills are big targets for this; they have multiple machines and it seems they're rarely controlled properly."

Traditionally, cascade controls were used to connect a series of compressors – simple pressure sensors that launch the next compressor when system pressure falls to a set point. However, to accommodate the second compressor's startup time, the sensitivity setting must be well above actual system needs, wasting energy.

"We've gone beyond that type of system now," says Dean. "Today's intelligent automation stores a lot of information and basically remembers what happened previously, so it can anticipate when additional air is going to be required and bring a compressor on in time.

"If you're a multi-compressor user, you really should look at automation. It's probably good for a 25% energy reduction right off the bat."

Storage and accumulators

Air tanks for storing compressed air are an important part of an efficient system. Tanks near the compressor – air receivers – help maintain a steady system pressure. Others, which Dean calls accumulators, are located strategically where individual machines require them.

"Let's say you have a machine that just gulps air. A valve opens and a big amount of air rushes into a cylinder; it's momentary use," he explains. "What you don't need is a huge air system, with the compressor trying to satisfy that instantaneous need.

"You situate an accumulator next to the machine that it draws from. It doesn't make a difference to operations, but it helps with energy use because you don't need so much system-wide pressure."

Again, Dean cautions that such accumulators should not be counted in calculations of overall system storage.

Filters

"Over-filtration is a chronic problem in our industry, because people make money selling filters and replacements," says Dean.

Improperly designed filtration causes excessive pressure drop, and thus requires the system pressure to be higher than it needs to be in order to compensate. A rule of thumb is: for every 2 PSI increase in discharge pressure, energy consumption will increase by 1 percent.

"I've often seen two or three filters in a system, and they can be anywhere from 3-6 psi across each filter. Let's say across three filters you have 14 psi – that means 7% of the energy you're paying for is being used just to drive air across those filters."

Dean recommends checking the manufacturer's requirements for cleanliness of compressed air within your system, then ensuring you have the correct filters but nothing more.

"You can filter the whole system as a first stage, and if there is a critical use that needs even more filtration, provide that filtration at the specific point of use so you're not suffering that pressure drop for your whole system."

Air dryers

As air exits a compressor, it is naturally saturated with water, which must be removed by running it through an air dryer to protect equipment down the line. There are two types of air dryers: refrigerated and desiccant.

Dean says a common inefficiency is the use of a desiccant dryer – which uses more energy – where it is not required.

Desiccant dryers can dry the air to a dewpoint temperature well below freezing – making them necessary in outdoor operations where winter temperatures are low. However, there are options for limiting the use of desiccant dryers.

First, Dean says they should never be used indoors where spaces are heated. Where outdoor distances are short, the option of insulating and warming air lines should be considered rather than switching the full system to a desiccant dryer.

Alternatively, a desiccant dryer can be used only to dry the air going outdoors, rather than over-dry the air for the entire plant. Finally, operations can save money by installing two dryers, and only using the desiccant dryer during the coldest months of the year.

Drain valves

Drain valves are also critical in helping remove moisture as it accumulates in the compressed air system. But some drain types bleed air as they expel water, wasting energy.

"The ones most often in use currently are either the float type, which always fail, or the solenoid type which are electric, and they typically aren't reliable and dirt and debris is a big problem for them. Plus, both of those types blow away compressed air when they operate.

"By comparison, zero-loss drains sense the presence of water electronically," says Dean. "There are no floats to stick or anything like that, and they close before the last of the water is expelled, so they shut before any compressed air is lost."

Dean says zero-loss drains cost about $50 more than solenoid models. "It's not necessarily worth going and replacing all your drains at once. But certainly all new installations should use zero-loss drain valves, and anytime you need to replace a valve, it's worth putting in a zero-loss."

Piping

As with plumbing, it's important to size the piping across your compressed air system to ensure the pressure drop across your system is not too high.

Says Dean: "You shouldn't have a pressure drop of any more than 3 psi – because you have to crank up the system pressure to overcome that drop and so you consume more energy."

The choice of piping material also affects efficiency. "For example, old galvanized pipe has a higher pressure drop because it's very rough on the inside. Today we have modular aluminum piping, which is like a mirror on the inside, and so there's less pressure drop and it takes less energy to drive that air through the pipe."

Dean says old iron pipe systems with threaded connections tend to spring leaks within a few years, but newer modular systems don't leak. "The modular systems go in very quickly, so they save labour. And from an energy standpoint, they're significantly better than what you could get 15 years ago."

There are many components to ensuring effective management of your compressed air supply. Knowing what technologies to choose and where to start on upgrades and retrofits is best determined by performing an air audit.

Part 3 of this compressed air series will take a closer look at how air audits are done, and what information they provide. Stay tuned!

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