Compress efficiently

“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.”

Of his company’s most recent line of 75-plus horsepower machines, “It’s not just different rotors — it took us six new airends to cover the range,” says Bryan Fasano, marketing manager, rotary screw compressors, Gardner Denver.

Sizing and design are more critical for direct-drive applications, where speed choices are limited by available motors. “Direct drive has to be optimized in the motor speed or by altering the airend,” says Heinonen. “Most compressors in the U.S. are optimized for 125 psi; it’s 110-115 psi elsewhere. And most equipment needs only 75-80 psi.”

There’s more at stake than efficiency. “When matching the airend to the horsepower, more options are better, to avoid overkill and marginality,” says Bill Kennedy, rotary screw product manager, FS-Curtis (www.curtistoledo.com). “A marginal system raised from 125 psi to 150 psi or 175 psi will require a higher-performance-factor motor.”

As they approach more closely the optimum size and RPM for each application, engineers are gaining incremental improvements by improving rotor profiles and housing designs using computational fluid dynamics.

“A lot of work has been done using computer simulation on the inlet valves, etc. to reduce pressure drops throughout the machine,” says Fasano. “It’s ideal to have a first-stage compressor with a rotor wrap optimized for the interstage pressure — 40 psi to 60 psi — then take that up to 100 psi or 125 psi or even 150 psi.”

On the design of screw impellers, Centers says, “We’re continually working on improving profiles. The geometry is pretty complex, and also the machining.”

Impellers traditionally were made in uniquely timed, matched sets. Moving from machining to hobbing and grinding has allowed tighter tolerances that increase efficiency while allowing drop-in replacement of individual parts. “Improvements in manufacturing have lead to tighter tolerances and better volumetric efficiencies,” says Kennedy. “For example, we no linger need a seal strip at the end of a rotor.”

Several experts commented on the trend to smaller airends. North American designs have typically been larger and have operated at lower rpm than European models, but that’s no longer the case. Smaller, higher-speed airends can hold tighter tolerance, making them at least as efficient and durable as larger models with lower initial cost.

Engineers have worked on package design, eliminating pressure drops through tubing, fittings, coolers, etc. A differential of 3.5 in. WC across an intake air filter equals a 1% loss in capacity. “Optimizing all this can pick up 5% to 8% versus 10 years ago,” says Shah. “New filter designs start with a lower pressure drop that increases half as fast over time. They cost a little more but they pay it back in three days.”

When comparing airends, don’t rely solely on the manufacturers’ spec sheets. See if the performance data has been verified by the Compressed Air and Gas Institute (CAGI, www.cagi.org). “The CAGI Third-Party Performance Verification Program is an important tool for evaluating and verifying specific package performance,” says Wayne Perry, technical director, Kaeser.

Improvements at partial loads

While the mechanical engineers have been extracting higher efficiencies from airends, the electrical contingent has been working on motors and variable-speed drives (VSDs). “The 1990s brought the Motor Challenge,” Shah says. “A premium-efficiency motor offers a 1% to 3% improvement.

“Then came the variable-speed drive, which is now on one of every four machines sold,” says Shah. Screw compressors are efficient at full load and unload. “Part-load efficiency can be all over the map,” he says. In the 1970s, partial loads were handled by throttle suction modulating, in the 1980s came variable displacement, where the unneeded portion of the compressed air is returned to the suction side. On/off, modulation and VSD power consumptions are compared in
Figure 1.

“The shape of the on/off-controlled power consumption curve depends on the air storage capacity,” says Heinonen. With smaller volume, the power consumption curve approaches the modulation control curve. In very large volume systems (relative to compressor capacity), it approaches the VSD curve. “The savings of VSD controlled compressors are illustrated at 60% of full load as this is often thought to be the average screw compressor load in air systems,” he says.

Along with often improving efficiency at part-load, VSD adds a soft start that reduces peak electrical loads compared to wye-delta or across-the-line motor starters. It can reduce pressure swings to 1 psi to 2 psi, from up to 10 psi, which reduces wasteful peak pressures and improves consistency of quality in the plant. “The motor lasts longer — there’s no limit to the number of starts — and it has a high power factor so you don’t need to add capacitors to correct it,” Shah says. “The costs are coming down — a drive that cost $18,000 in 1975 now can be had for $2,000, and there are rebates and tax incentives. You can add one to an existing machine.”