Compressed Air Efficiency: Part 1, managing demand
After spending most of his working life assessing, designing and installing compressed air systems, compressed air specialist Roger Dean has an unflattering comment about this mainstay of industrial operations.
"At the best of times," he says, "compressed air is a disgustingly expensive utility."
That perspective gave Dean a keen eye for efficiency throughout his career with Kaeser Compressors. Dean has retired from full-time work. But over the next few issues of Current, he shares tips about compressed air (CA) efficiency, gleaned from long experience in varied industrial settings.
This month, Dean focuses on the demand side of CA systems. In September's issue, Part 2 will explore supply side issues. Part 3, later in the fall, will walk through an air audit – the study you use to identify which demand and supply side options will provide you the best efficiency.
Part 1: Managing compressed air demand
"People tend to focus first on compressed air supply when they're looking for efficiencies – compressors, filters, tanks and drains," says Dean. "The demand side is ignored. But the demand side is where you get your lowest cost savings."
Dean suggests a few key demand-side focus points:
- artificial demand
- excess pressure
- inappropriate use of compressed air
Leaks: 'Biggest bang for your buck'
There's nothing good to say about leaks in a compressed air system: you pay money to produce the air, and leaks let it slip away before you make use of it. And they cost dearly.
"A single leak in your entire system of just 1/4 inch at 100 psi is the equivalent of losing the power of a 20 hp compressor," says Dean. "In a 24x7 operation, that's about 122,000 kWh of electricity, or about $6,600 a year that you're just blowing away.
"And I've seen systems where – forget a single 1/4 inch leak – they're getting 40% leakage in some cases. Fixing leaks is your biggest bang for the buck."
Dean suggests using an ultrasonic leak detector to zero in on leaks. "And the 80/20 rule applies," he says. "About 80% of your leakage will come from about 20% of your leaks, so just tracking down the major ones will make a big difference."
Artificial demand: system design flaws
Artificial demand refers to any additional compressed air your system consumes beyond what is required to do a specific piece of work.
"For example, perhaps you have a pneumatic cylinder in a piece of equipment. If the pressure is too high for what is required by the cylinder, it takes more compressed air than it needs to perform the work, resulting in additional waste," says Dean. "Artificial demand is related to pressure. As you lower pressure, you reduce artificial demand."
Poorly designed systems may "demand" more air than they should in other ways too. For example, piping that's the wrong size can allow pressure to drop across your system, creating a perceived need for more pressure from the compressor. (More about piping in September's supply-side story.)
Excess pressure: too much for the job
The flip side of artificial demand, Dean says systems that are producing more pressure than required by operations are common.
"If you don't need it, don't generate it," he says. "Every time you can lower your pressure by 2 psi, you save 1% on your energy cost."
He says pressure is often increased because of poor compressor controls, or inadequate air storage, which can cause header pressure to fluctuate.
"People tend to raise the pressure in a fluctuating system, so that at its lowest it's not too low. If the pressure is going up and down 20 psi, you can't start at 100 psi because when it drops 20 psi, things will shut down. So everyone just jacks the pressure up to avoid that."
Operators sometimes confuse capacity and pressure. When a non-continuous consumer of air is started, and the compressor station cannot meet that demand, pressure will fall. Some operators will try to compensate by raising the system pressure, which will have little or no effect, but will consume more energy.
The key to determining how much pressure you need (how much your system demands) lies in doing an air audit, or compressed air optimization study. If you're guessing at your demand needs or increasing pressure because of large fluctuations, it's time for a study.
Inappropriate use: Use a blower or fan instead
"Inappropriate use of compressed air is drying things with compressed air where a blower will work," says Dean. "It's sweeping floors with compressed air – get a broom or a vacuum cleaner. It's man-cooling, or blowing dust off people, or blowing chips away from production equipment."
Compressed air is easy, and convenient – but it's a high-cost resource. Using it inappropriately not only wastes it at the point where it's used; it also sends false signals about overall system demand, which may cause you to increase pressure and capacity when you don't need it.
"Anywhere you can use a blower or fan to do the job, or anything other than compressed air, that's a better use of your money," says Dean.
Dean says B.C. industries tend to be wasteful in their use of compressed air because of relatively inexpensive energy, and because most industrial facilities produce it for themselves.
"In other parts of the world, there are often compressed air suppliers. There, industries buy compressed air based on a cost per cubic foot; the supplier meters them," he says. "Here, compressed air is often taken for granted. It's a utility that we supply for ourselves, so as long as we have some flowing, we don't worry about it."