The question is: How a flow control loop employing a VFD as the means of flow control should be represented using ISA symbology.

Should the controller be represented as a "SC" for speed control or "FC" for flow control?

The question gets more interesting when using a VFD to control dryness of a product at the outlet of a screw driven by a VFD. The motor load will provide an inferred signal of the dryness. Therfore current is the measured value. In this case should the controller be reffered to as a "IC"?

 

If you plan to modulate VFD directly by the flow PID loop, then FC is the answer. Otherwise it should be both (use dual controller symbol with FC and SC). As to the moisture (dryness) control - it will be another correcting factor for VFD, and yes, you should have a separate controller symbol for that one.

Good luck,
Alex Benim
Compressor Controls Corporation
[email protected]