Compressed Air Challenge: VSD Compressors — Turn On Cruise Control

Accurate control of pressure in compressed air systems is always of primary concern, but there are many ways to achieve it. Some are more efficient than others.  One of the biggest innovations in the field of compressed air efficiency is the invention of VSD-controlled compressors. VSD compressor control can put your air system pressure on “cruise control.” Let’s turn to an automobile analogy in comparing compressor control strategies.

One could use modulation control mode, which is similar to driving a car with the pedal to the metal and using the brakes to provide constant speed. Modulation control chokes off the inlet flow to the compressor to control the output pressure. This mode of operation is the least efficient way to provide constant pressure, with the compressor consuming 85% power even at only 50% output flow.

Another control mode involves loading and unloading a compressor between two set pressure points, with the average of the two readings providing the desired pressure.  This approach is similar to driving down the highway and controlling the speed by throwing the vehicle’s transmission alternately into drive and neutral. Air compressors in this mode of operation use less power than modulation—but can still consume between 70 and 85% power at a 50% loading level, depending on the frequency
of cycles.

A third mode is akin to a driver on a busy highway who repeatedly starts and stops his engine (slowing down or going faster) to reach a desired average speed. This method would be equivalent to a start/stop compressor operating mode: an efficient way to run small compressors, but hard on the motor.

In the three modes described above, average pressure could be adequately achieved, but it would come with either higher-than-desired energy consumption or wider pressure fluctuation. In a compressed air system, the desired result is a constant steady pressure—one set high enough to provide sufficient power to compressed air consumers, yet low enough to limit the energy consumption of the compressed air system.

Leveraging VSD control
VSD-controlled air compressors have accurate controllers on board that sense the actual pressure and speed up or slow down the compressor so as to keep a constant discharge pressure. The benefit is that the pressure can be set at a lower, more efficient level. Moreover, as the motor slows, the power consumption is almost linear to the speed reduction, saving even more. These units are more expensive and more complex than standard fixed-speed compressors but often, especially when an air compressor needs to be replaced anyway, the new VSD compressor will pay back the extra cost very quickly.

While these types of units are most appropriate for smaller single- and two- compressor systems, they can save significant energy in larger multi-compressor systems—if applied and controlled appropriately. To determine if VSD compressor control is appropriate for your plant, have an energy analysis of your system performed by a qualified compressed air energy-service company.

More information on this topic and many others can be found on the CAC Website (www.compressedairchallenge.org), in our online Library and our Best Practices for Compressed Air Systems Manual. MT