Tutorial 9 Cascade Control
Description: The tutorial gives a series of exercises on the proper design of cascade controls, which can dramatically improve the performance of feedback control systems, when it is designed and implemented correctly. The first exercise is about determining whether the cascade control was possible in the design of furnace coil outlet temperature control. If it not properly designed, appropriate changes must be made. The same thing is done with the second exercise but this time; bottoms composition analyzer control for distillation was designed. The last two exercises are about choosing the correct characteristics of primary and secondary variable sensors. Generalization: Cascade is possible only if it meets the cascade design criteria. For the first exercise, which is the furnace coil outlet temperature control, in fuel supply pressure, cascade is better. The flow controller will compensate for the disturbance. Whether the secondary corrects for the complete disturbance depends on the flow sensor. In an orifice meter, the density changes with pressure. Therefore, maintaining the flow measurement (ΔP) constant does not maintain the actual flow constant. The mass flow rate can be measured by a mass flow meter, such as a coriolos meter. The total heat release depends on the mass flow rate for light gas hydrocarbon fuels without hydrogen. Therefore, maintaining mass flow rate constant will completely compensate for pressure changes. Cascade control with mass flow control would perform better than with an orifice meter. However, the mass flow meter will be more costly. The density of the liquid does not depend on the pressure. Therefore, the orifice meter provides a good measurement. In terms of fuel density composition, cascade is better. In terms of feed temperature, cascade is neither better nor worse. For a cascade control design, the sensor for the secondary variable should provide good reproducibility. A sensor with good reproducibility is often less expensive than a highly accurate sensor. For a cascade control design, the sensor for the primary variable should provide good accuracy. Nothing can correct errors in the primary sensor. Therefore, the primary sensor must achieve the accuracy needed for the process application. Rating: 1. 2. 3. 4.