For many finishers, compressed air is a vital utility. Compressed air runs essential tools and equipment, provides power to material handling systems and supplies clean air to processes. Choosing the right compressor for these applications can be difficult 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% of the total cost of producing compressed air. 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 is ultimately based on a study of the available technologies that may be appropriate for your application 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 compressed air users are improving air system energy efficiency, reducing maintenance costs and lowering noise levels with rotary screw compressors incorporating VSDs. Since many applications do not have a consistent demand for air, a VSD system can meet the changing demand on the fly.
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% 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 online/offline) will use only 25% of full load electrical power and offer some energy savings. However, compressors equipped with VSDs are much better able to match their demand requirements, virtually eliminating the need for the compressor motor to rest in the energy-consuming idle mode. Rotary screw compressors with VSD technology save users 20-35% on electricity in situations where there are variable loads.
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.
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, achieving a preset final pressure. Highly accurate sensors provide operational data. Combined with a responsive drive system, pressure can be controlled to +/- 1.5 psig.
Some manufacturers are retrofitting VSDs to their existing compressors rather than purchasing a system that combines both features, but this may not always be the best approach. In rotary screw compressors, the efficiency is based in part on airend speed. 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 the efficiency falls off very rapidly. In other words, as you go too slow or too fast, you use more electricity and produce less air. By adding a VSD only onto an existing compressor design, you don't know whether the compressor design is in the middle of the bell curve or on the edge of the bell curve. When a compressor is on the edge of the bell curve, if you slow it down at all, it becomes very inefficient. Plus, the existing motor may not be designed to handle the conversion from fixed speed to variable speed.
Therefore, it's necessary to design the compressor airend to operate in the flat part (top) of the bell curve and keep the whole speed range in the top of the bell curve so the user can maintain maximum efficiency in terms of kilowatts in and compressed air out. Compressors with large airends produce a much flatter curve.
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," incurring the lowest inrush current requirement. This enables unlimited starts and stops of the motor. With a 100-hp fixed-drive system, for example, the user would 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. The motor has to run for 20 min 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. Also, newer drive systems help stabilize plant air pressure, enhancing quality in the plant.
With fixed-drive rotary screw compressors there is a 10-15 psig swing built into the controls. VSD compressors have only a 1-2 psig swing, which is a significant advantage and can make a substantial difference in product quality.
One of the issues with retrofitting a compressor with a VSD is the danger of harmonics backing into the plant's electrical system, which could disrupt or even destroy some of the other equipment in the plant. However, with a completely integrated system, all feedback should be isolated or eliminated so that no harmonic distortion goes into the electrical grid of the plant.
Some VSD-equipped compressors fail to keep the power factor near unity. When unloading the electric motor, the power factor gets worse and worse. The power company may penalize the user based on how far off unity (1.0) the power factor is. The user gets a power factor correction penalty every month on its electric bill. However, some VSDs can maintain a power factor close to 1.0 throughout the entire speed range. So, even as the motor is unloaded, the power factor does not go down, eliminating any penalty from the power company.
A factor often overlooked when evaluating plant air systems is noise. Noise is partially due to compressor speed and the packaging of the equipment. If a VSD is added to a standard compressor, a lot of audible electrical noise will be generated. It makes a high-pitched "chirping" sound in the motor. However, the noise can be reduced with large motors, large airends, low speeds, a radial fan and an enclosed package.
Choosing the right air compressor can be quite complicated. But, the right choice can save you lots of money and help you produce better parts.
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