Variable speed drive slashes compressed 
air costs, helps ensure consistent product quality

Manufacturing and other users of compressed air can improve energy efficiency, start/stop motors repeatedly, plus reduce maintenance costs and noise levels with rotary screw compressors incorporating variable frequency drives.

by Ed Sullivan

To most industrial business operations, compressed air is a vital utility. Compressed air runs essential tools and machinery, provides power to material handling systems and clean air to processes. Yet choosing the right compressor for many applications can be difficult, especially given new technological developments, added concerns about even and steady pressure, maintenance and, that traditional nemesis, electrical power.

The cost and dependability of compressed air can have a tremendous impact on production processes and costs. Surprisingly, compressed air costs are most often considered in terms of equipment. Yet energy consumption can represent up to 70 percent of the total cost of producing compressed air, and with energy costs escalating nationwide, selecting an energy-efficient air system has become critical. Plus, there are other significant factors to consider, such as reliability, productivity, systems support, automated features and noise.

Making a proper evaluation about air compressor selection will ultimately be based on a study of the available technologies that may be appropriate for your applications, as well as evaluating existing equipment. In some instances, such as when there is a continuous demand at full load, a fixed speed rotary screw compressor may be the best solution. In cases where the base load varies with an additional load, it might be best to consider supporting the base load with fixed flow compressors and adding a unit with a variable speed drive (VSD) as a “trim” device to carry the variation in the load. 

Many industrial compressed air users are improving air system energy efficiency, reducing maintenance costs and lowering noise levels with rotary screw compressors incorporating VSDs. It's a solution that might be right for you. Since many applications do not have a consistent demand (90 percent to 100 percent) for air, a VSD system can meet the changing demand “on the fly,” plus offer many other cost and quality benefits, depending on the system design and use.

Rotary screw compressors with fixed-speed drives are limited in the number of times they can be stopped and started in a given time frame.  In applications with a variable compressed air demand, drives may run in idle for long periods to avoid overheating caused by frequent restarts.  Although not producing compressed air while in idle mode, a fixed-speed compressor running with modulation control still consumes about 70 percent of full load electrical power, which translates into substantial electrical costs with no benefit.  A fixed-speed compressor operating under dual control (stop/start or on-line/off-line mode) will use only 25 percent of full load electrical power and offer some energy savings. However, compressors equipped with VSDs are much better able to match their variable demand requirements, virtually eliminating the need for the compressor motor to “rest” in the energy-consuming idle mode. 

Electric motors equipped with VSDs have been around for some time. Traditional VSD applications include fluid pumps, HVAC, conveyor systems and positive-displacement rotary-lobe blowers. Only recently have they been applied to rotary screw air compressors. 

Kaeser Compressors Inc. of Fredricksburg, Va., has taken the VSD technology to new levels of efficiency, flexibility and noise control with a proprietary VSD known as Sigma Frequency Control (SFC). SFC rotary screw compressors can typically save users 20 percent to 35 percent on electricity in situations where they have variable loads.

The principal of variable frequency control is accurately measuring the actual air main pressure with a pressure transducer so that the volume of compressed air generated varies, thus achieving a preset final pressure. Highly accurate sensors provide operational data to Kaeser’s proprietary Sigma Control system. Combined with the responsive SFC Drive System, pressure is controlled precisely to +/- 1.5 psig.

Some manufacturers are retrofitting VSDs to their existing compressors, rather than purchasing a system that combines both features. This is an approach that Kaeser technical director Wayne Perry thinks inadequate. 

“In rotary screw compressors, the efficiency is based in part on airend speed," he said. "The efficiency range can be plotted in a bell curve. In an efficient range there is a flat top to the bell curve, but when you get out on the edge it falls off very rapidly. In other words, as you go too slow or too fast, you use more electricity and produce less air. If you just add a VSD onto an existing compressor design, you don’t know whether that compressor design is in the middle of the bell curve or on the edge of the bell curve. I’ve worked with some companies that have compressors that are right out there on the edge of the bell curve, and if you slowed them down at all, they become very inefficient. Plus, in a retrofit scenario, the existing motor may not be designed to handle the conversion from fixed speed to variable speed.”

Perry said it’s necessary to design the compressor airend to operate in the flat part (top) of the bell curve, and keep that whole speed range in the top of the bell curve so they can maintain maximum efficiency in terms of kilowatts in and compressed air out. Compressors with large airends, such as Kaeser offers, produce a much flatter curve.

Kaeser’s design philosophy also calls for a low-speed operating range, from 1,800 rpm down to 450 rpm. 

“This is where we produce the optimum specific performance. To go slower would really not make sense, because we lose efficiency. To go faster makes no sense because then you would have to have a larger drive,” Perry said.

The torque required by the airend determines the drive size. When you have a small airend with higher speeds, you can use a smaller, less expensive drive. But if you need the efficiency and operating range of a big airend with the higher torque, you also have to have a bigger drive.

An inherent advantage of a VSD-equipped compressor is the ability to start and stop the compressor as often as desired. Unlike fixed-drive systems, VSD systems are “soft starting,” and incur the lowest inrush current requirement. This enables unlimited starts and stops of the motor. With a 100 horsepower fixed-drive system, for example, you’d be limited to two or three starts and stops per hour because the inrush current required to start it would heat up the motor windings. That motor has to run for 20 minutes in order to cool the windings down before you can turn it off and then turn it back on. Plus, the user may be penalized by the power company for even one spike on the demand chart from high inrush motor starts.

SFC drive systems also help stabilize plant air pressure, thereby enhancing quality in the plant. 

“We have a customer who manufactures a precision automotive driveline system who was assembling finished products using pneumatic torque wrenches attached to a fixed drive air system," said Perry. "Due to changing available air pressures, the torque values of the wrenches were changing so much that it likely affected proper sealing. Until they changed over to a SFC system, they were forced to reroute their finished products back through a separate line to re-torque them, just to be sure they didn’t ship anything that was not fastened with the correct torque values.”

Perry explained that with fixed-drive rotary screw compressors there is a 10-15 psig. swing built into the controls. “When you put in a VSD compressor, you have only a 1-2 psig. swing, which alone can make a substantial difference in product quality,” he says.

One of the issues with retrofitting a compressor with a VSD is the danger of harmonics backing into the plant electrical system, which could disrupt or even destroy some of the other equipment in your plant. However, with a completely integrated system, such as Kaeser offers, all feedback should be isolated or eliminated so that no harmonic distortion goes into the electrical grid of the plant.

Perry said some VSD-equipped compressors fail to keep the power factor very near unity. 

“Normally, when you start to unload an electric motor, the power factor gets worse and worse," he said. "The power company may penalize you for that, based on how far off unity (1.0) the power factor is – 0.8 or 0.7 – they get a power factor correction penalty every month on their electric bill. With our VSD we can maintain a power factor close to 1.0 throughout the entire speed range. So, even as you unload it, you don’t have this power factor going down – and you don’t get penalized by the power company.”

A factor often overlooked when evaluating plant air systems is noise.

 “Noise is partially due to compressor speed, but also the packaging of the equipment,” Perry said. “If you just take a standard compressor and put a variable frequency drive VSD on it, you get a lot of electrical noise that you can hear. It makes a high-pitched chirping sound in the motor. With our large motors, large airends, low speeds, radial fan and enclosed package, that sound is practically eliminated.” 

Perry adds that some systems blow the air through the oil cooler and out, resulting in a high noise levels. 

“When you divert the air upward as we do, you direct it away from people, and that also reduces the apparent noise considerably,” he said.

In addition to its other proprietary mechanics and features, the new Kaeser Sigma Frequency Control (SFC) compressor line includes the Sigma Control. Based on an industrial PC with Intel microprocessor, the Sigma Control is a compressor controller designed to optimize energy efficiency while significantly increasing operational reliability. Fitted to all Kaeser SFC models as standard, the system can constantly log and process performance data, checking that the compressor precisely fulfills working requirements. It is easily adapted to changing operation conditions, even after installation, with programmed compressor control modes. 

The provision of open system architecture and communications interfaces means that Sigma Control can just as easily communicate with modern data systems, as with older compressors that do not possess these controllers. These interfaces also provide remote data access to the compressor via a modem and telephone network with huge potential benefits to the user.

Kaeser does not manufacture its own drives, but outsourcers Master Drive series drives from Siemens. 

“This is important for machines that are being exported out of the U.S.,” Perry said. “Siemens provides worldwide support, so in most countries you’ll find a Siemens representative who is familiar with the Master Drive series.

Standard Kaeser SFC compressor packages are available from 100-215 hp and flows from 130-1001 cfm providing pressures from 80-145 psig. For its Sigma Frequency Control (SFC) Series of compressors, Kaeser added a number of features to the Siemens Master Drive series drives, including an electromagnetic interference filter, total harmonic line reactors and galvanic separation to separate incoming electrical power from the drive for added safety.

Kaeser products are found in virtually all areas of manufacturing and processing, including the metal, automotive, chemical, plastic, printing and textile industries. Commercial applications include auto-body shops, car repair operations and machine shops. Non-industrial facilities such as hospitals, laboratories and public municipalities also rely on Kaeser to supply their air system needs.

For more information, contact Kaeser Compressors Inc., (800) 777-7873; e-mail kaeser2@kaeser.com; or visit the Web site www.kaesercompressors.com.

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