Control options in rotary screw compressors

They've evolved considerably and now contribute to energy savings and system reliability.

By Harold Wagner

The purpose of any compressor control system is to match compressed air supply to the compressed air demand as efficiently as possible. Although old style electro-pneumatic controls have been supplanted by microprocessor-based controls and software updates that optimize system and control parameters, any control type can be put into one of two categories — fixed speed controls and variable speed controls.

Fixed speed compressor controls

As the name implies, the idea is to hold compressor speed constant. The exact speed is a function of the speed of the drive motor and the ratio of the gearing or V-belt system in use, if any. The capacity output control is based on either modulation or a poppet valve.

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Modulation control

This form of control modulates the compressor's inlet valve according to the compressor's outlet pressure. When the controller detects rising outlet pressure, it causes the inlet valve to start closing. While this control is effective, it's the least efficient. The reason is that compressor efficiency is inversely proportional to the compression ratio, which is the ratio of inlet pressure to outlet pressure. The closing inlet valve produces a vacuum at the compressor's inlet side while the outlet pressure remains relatively constant. This has the same effect as raising the compression ratio.

A compromise is to restrict the modulation control range to something like 40% and automatically convert the scheme to a load/no load control any time output drops below 60%. Unfortunately, this form of control doesn't lend itself to use with multiple machines.

Variable displacement control

Controlling the effective rotor length varies the compressor's output. This is accomplished with internal bypasses machined into the airend housing and controlled by either poppet valves or a turn (or spiral) valve. While the efficiency is better than with modulation control, poppet-style controls are effective only above 50% to 60% of capacity. Also, they can be complex and difficult to troubleshoot.

Online - idle/offline (dual-control)

This simple, effective control uses a pressure switch at the compressor discharge to close the inlet valve completely at a high limit (cut-out) pressure and open it completely at a low limit (cut-in) pressure. The difference between this approach and the modulation control is that, in this case, the compressor is internally unloaded. The reduced compression ratio reduces idle power consumption. This type of control can connect multiple compressor installations easily using a sequencer (master) controller. On the other hand, the air system will need a properly sized air receiver tank.

Start/stop control

This is the most efficient control scheme. The compressor runs either fully loaded or off, depending on the signal from the pressure switch. Unfortunately, motors larger than five to 10 hp can't be started and stopped as often as this control may require without overheating. This control is used on small piston compressors, which typically are mounted on a storage receiver. They use a relatively wide pressure differential of 15 psig to 25 psig.

Variable speed control

Advanced and affordable technology brought variable frequency drives into compressor applications. The principle seems simple--adjust the compressor/motor speed and output to match system demand exactly. Designed correctly, variable frequency control is the most advanced and energy-efficient trim compressor control. However, the following issues must be considered:

  • The airend must be designed to be efficient over the complete speed range. Airend efficiency is a function of rotor tip speed and may be greatly reduced at lower or higher rpm .
  • The variable speed drive controller needs to be efficient, as it represents an additional link between the power supply and the drive motor. The drive system and motor cables need to be free of power distortions and electromagnetic emissions to prevent electromagnetic interference to computers or other sensitive electronic equipment.
  • The drive motor must be able to handle the higher speeds as well as the lower speeds, where bearing design and cooling issues may be problematic.
  • An intelligent controller makes efficient and reliable connections among air pressure, the drive and the compressor inlet valve. A well-engineered device can control compressor outlet pressure to within one psi, despite wildly fluctuating air demands.

Variable speed control is suited for trimming in multi-compressor applications. It allows a sequencer/controller to operate multiple compressors efficiently. Needed machines are loaded fully. Unneeded compressors are in stand-by mode. And the variable speed control efficiently compensates for fluctuating air demand.

The most efficient control, however, can't compensate for an improperly designed system. Conduct a comprehensive air system audit and give your control system a fighting chance to deliver the efficiency gains it promises.

Harold Wagner is national sales manager at Kaeser Compressors. He can be reached at Harold.Wagner@Kaeser.com or 540-834-4502.

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