By: Michael Camber
There are several different ways to control an individual compressor to match output to demand. In single compressor installations, some are better at handling part-load conditions than others. Many newer compressors can be switched from one to the other via their on-board controller. Others (e.g. modulation, variable speed, and rotor length adjustment) require different mechanical and electric configurations. Below are simplified descriptions of the types in use today.
Dual Control: Operates fixed speed compressors at “full load” and “idle mode” via a minimum/maximum pressure switch plus a timer. Pressure band and timer values are preset to match system requirements.
Quadro Control: An enhancement over Dual Control that includes an additional timer to fine-tune the “idle” period, while bypassing “idle mode” after periods of low air demand.
Vario Control: Uses a “smart” timer to vary the idle time based on the frequency of motor starts—resulting in greater energy savings.
Dynamic Control: Allows compressors to switch from “load” to “stop” at low motor temperatures, and to “idle” when the motor is hot. (The control may bypass “idle mode” if the motor temperature is low and more compressor starts are allowable.)
Variable Speed Control: Air flow is controlled to maintain a specified discharge pressure by controlling electrical frequency (and therefore speed of the drive motor) in response to a pressure signal at the compressor discharge. This is the most advanced and energy-efficient type of compressor control.
Modulation Control: This regulates air delivery using a proportional inlet valve to regulate flow and provide a constant output pressure in response to a pressure signal from the compressor discharge. Prior to variable speed/frequency drive, it was a popular part load control.
Rotor Length Adjustment: Constant speed compressor with valves along the length of the airend that regulate volume output in response to a pressure signal from the compressor discharge. Like modulation, this was more popular prior to the widespread use of variable speed drives (VSDs).
Note that VSD, modulation, and rotor length adjustment controls can be effective in maintaining steady discharge pressures in single compressor applications, but systems with multiple units of any type (or mix of types) can experience wide system pressure swings unless all units are controlled together.
This is an excerpt taken from the whitepaper, “Using Master Controls to Improve the Performance and Efficiency of Industrial Air Compressors”. To download the full paper, visit www.kaeser.com/whitepapers.