Cascade control should also be considered when the primary control variable is slow to react to disturbances. Like any feedback control loop, a cascade control loop has a controlled variable, a setpoint and a controller. However, instead of having a valve as its final control element, a cascade controller sends its output to the setpoint of another controller, adjusting this setpoint to correct an error in the controlled variable. This other controller is called the secondary or slave controller. The cascade controller is called the primary or master controller.
If disturbances in the process can be recognized and quickly corrected, the primary control loop will not be affected. This suggests that the secondary control loop must operate faster than the primary loop. In fact, general guidelines suggest that the secondary loop should respond at least five times faster than the primary loop.
Looking again at the example of the furnace, let us assume that the fuel system provides fuel to several other furnaces as well. Over the course of hours, the pressure in the system might well vary as the fuel demand in all of the furnaces changes. A change in the fuel header pressure changes heat transfer in the furnace.
Because heat transfer is a slow process, the outlet temperature controller cannot be tuned well enough to eliminate the effect of changing fuel flow (see Figure 300-15). (For details on controller tuning, see Section 350.)
On the other hand, if the fuel flow remains steady while the pressure is changing, the furnace temperature will be more constant. Fuel flow changes almost immediately when the control valve is moved. Therefore, the flow controller can be tuned to eliminate most of the disturbances in fuel flow.
Such circumstances lend themselves to the use of cascade control: a fast process (fuel flow), a slow process (furnace heat transfer), and a disturbance (fuel pressure) that affects the fast loop. Figure 300-16 shows the cascade control system for the furnace.
Compare Figure 300-17 to 300-15. With cascade control the outlet temperature is much more steady. The fuel gas controller (secondary controller) has eliminated almost all fuel pressure disturbance from the furnace.