Module 12

Antisurge control systems of series compressors

When two or more compressors are connected in series, the use .0£ control system as shown in Figure 12-1 on each individual machine will provide the best protection against surge.

This of course, requires a recycle control valve for each compressor.

In some designs, two compressors in series are driven by the same prime mover, and consequently, they both always run at the same speed.

The machines are sometimes considered as a single unit. The manufacturer may provide one surge curve for the combination and the installation usually includes only one recycle valve, and the control system shown in Figure 12-1 is used.

A typical example for this configuration is a single compressor casing with two impellers (compressors), which most often found on refrigeration compressors.

Figure 12-1

Also, when compressors are independently driven at variable speed, but have a single recycle valve, surge is controlled by the system shown in Figure 12-1

Each compressor is equipped with its own ratio station and~ controller. However, the two could be connected to an auto-selector system to enable either controller to operate the common recycle valve as required.

If, at a time, any of the two compressors operates alone, the three way valves in the controller output lines should be energized to ensure a positive air supply to the selector relay of the controller in operation.

If both compressors will always operate together, the three way valves may be eliminated altogether.

For this type of application the two used controllers are specified as normal PI controllers, i.e. without internal anti-reset windup. The actual output signal to the valve is fed back to both controllers as external anti-reset windup.