Compress efficiently

From inlet filters to oil separators, airends to controls, compressor and compressed-air equipment manufacturers are driving down the amount of energy it takes to generate a given flow — your kWh per 100 cfm. Here’s how they say they’re doing it, and what you need to know to take advantage of their labors.

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By Paul Studebaker, CMRP, editor in chief

PlantServices.com

Experts are quick to point out that the biggest gains to be made in compressed air system energy efficiency are on the demand side. Most plants are running higher system pressures, full of leaks and using compressed air for purposes where less energy-intensive options exist. And Plant Services has published thousands of words about where to find and how to fix those demand-side excesses.

But the fact that the demand side is by far the larger part of the problem hasn’t fazed the engineers who toil daily to increase the efficiency of the compressors themselves. And like good engineers always do, they’ve made quiet but excellent progress during the past 20 years.

From inlet filters to oil separators, airends to controls, compressor and compressed-air equipment manufacturers are driving down the amount of energy it takes to generate a given flow — your kWh per 100 cfm. Here’s how they say they’re doing it, and what you need to know to take advantage of their labors.

Principles start with p

A few percent here, a few percent there — it doesn’t sound like much, but the purchase price of a compressor typically pales in the face of its life cycle cost. “What’s the cost of a compressor?” says Mike Bakalyar, manager, enhanced services, Gardner Denver (www.gardnerdenver.com). “Over its life, a 150-hp machine at $0.10/kWh will cost $1,200,000.”

Some of those percentage improvements are obtained by limiting pressures. Minimizing the maximum compression pressure has a direct and well-known relationship with efficiency — the guideline is that every 2 psi reduction increases efficiency 1%. Annual savings from reducing a pressure drop between the airend and the point of use is shown in Table 1.

“It’s all about specific performance,” says Harold Wagner, national sales manager, Kaeser (www.kaeser.com). “You must compare the total amount of energy put into a compressor to produce the compressed air at the specified pressure at the compressor discharge. That includes everything from cooling fans to drive motors and transmission losses.”

Begin at the end

Specific performance starts with the right airend — the compressor itself — for the job. Highly developed over many years, each commonly-available industrial airend technology has settled into a range of efficiency, shown in Table 1 for units under typical conditions at full load. Size, manufacturer, efficiency of the driver (typically a motor with or without a variable-speed drive) and the operating conditions (pressure, turndown, etc.) affect this basic efficiency range.

There are reasons other than efficiency to choose one technology over another, but Table 2 is an interesting place to calibrate your expectations. The advances we discuss apply to compressors based on many types of airends, but screw compressors have become the most common type for industrial applications, so the examples tend to reference screw impellers. As Table 2 shows, multiple stages can offer a significant efficiency advantage at common industrial system pressures.

Another fundamental is the drive system. “Direct drive is most efficient,” says Hannu Heinonen, global rotary line manager, Fu Sheng Industrial Co. Ltd. (www.fusheng.com). “Belts can handle lower horsepower, but start at 1% loss and can slip to 6%. Gears require less maintenance and last longer, but add 2% to 4% loss.”

Table 2 also shows a difference between lubricant-injected and lubricant-free screws, due to the sealing effect of injecting oil or water into the screws. But what goes in must come out. “Oil-flooded screw compressors are more efficient than oil-free, but oil must be separated,” says Steve Centers, manager, electrical engineering, Quincy Compressor (www.quincycompressor.com). “New separation element materials take the oil content down to 1 ppm instead of 3 ppm to4 ppm, but separators can build up back pressure.”

The choice of lubricant for oil-injected compressors can materially affect energy efficiency by reducing friction and dimensional changes caused by varnishing. “Using water as a lubricant offers about the same efficiency as oil lubrication, about 15% better than dry,” says Heinonen, “Water-injected units  also offer almost adiabatic compression because of good cooling, but must be run at higher speed due to the lower viscosity. There’s no oil to change, but service life is shorter than typical oil-lubricated compressors. You have to plan for 24,000- to 30,000-hour rebuild intervals.”

That good-guts feeling

Manufacturers are making incremental gains in airend efficiencies by matching sizes and designs more closely to requirements, by using simulation software to refine designs, and by producing components with higher accuracy and precision. “You need the right size rotors with the right profile,” says Harish Shah, associate fellow, Sullair (www.sullair.com). “Do you make specific machines or go mid-path to cover ranges of speed, horsepower and capacity?” Manufacturers with more choices of housings and rotors are more likely to offer the highest efficiency at a given pressure and flow.

“Several years ago, we began an initiative not only to optimize the design of our airend, but to specifically match the airend size to the horsepower range,” says Wagner. “We have more than 30 airends to select from when determining which one is going to be the most efficient.”