Furnace, cascade control

Schemes to control the outlet temperature of a process furnace by adjusting the fuel gas flow are shown in Figure 13. In the scheme without cascade control (Fig. 13a), if a disturbance has occurred in the fuel gas supply pressure, a disturbance occurs in the fuel gas flow rate, hence, in the energy transferred to the process fluid and eventually to the process fluid furnace outlet temperature. At that point, the outlet temperature controller senses the deviation from setpoint and adjusts the valve in the fuel gas line. In the meantime, other disturbances may have occurred in the fuel gas pressure, etc. In the cascade control strategy (Fig. 13b), when the fuel gas pressure is disturbed, it causes the fuel gas flow rate to be disturbed. The secondary controller, ie, the fuel gas flow controller, immediately senses the deviation and adjusts the valve in the fuel gas line to maintain the set fuel gas rate. If the fuel gas flow controller is well tuned, the furnace outlet temperature experiences only a small disturbance owing to a fuel gas supply pressure disturbance.

There are many advanced strategies in classical control systems. Only a limited selection of examples is presented in this chapter. We start with cascade control, which is a simple introduction to a multiloop, but essentially SISO, system. We continue with feedforward and ratio control. The idea behind ratio control is simple, and it applies quite well to the furnace problem that we use as an illustration. Finally, we address a multiple-input multiple-output system using a simple blending problem as illustration, and use the problem to look into issues of interaction and decoupling. These techniques build on what we have learned in classical control theories. 

Figure 10.1. Cascade control of the temperature of a furnace, which is taken to be the same as that of the outlet process stream. The temperature controller does not actuate the regulating valve directly it sends its signal to a secondary flow rate control loop which in turn ensures that the desired fuel gas...

Figure 10.2a. Block diagram of a simple cascade control system with reference to the furnace problem.

Example 10.2 Consider the temperature control of a gas furnace used in heating a process stream. The probable disturbances are in the process stream temperature and flow rate, and the fuel gas flow rate. Draw the schematic diagram of the furnace temperature control system, and show how feedforward, feedback and cascade controls can all be implemented together to handle load changes. 

Figure 20.3 Examples of cascade control for (a) heat exchanger (b), (c) distillation column (d) process furnace.

In one application, the TRC/TRC cascade control was used with four thermocouples installed, in parallel, in the convection section of the heater which was installed in thermowells. The thermowells were inserted in the heater wall between the tubes. Because the thermocouples receive the same heat as the convection section tubes, an instantaneous detection of furnace temperature was made. 

As an example of where cascade control may be advantageous, consider the natural draft furnace temperature control problem shown in Fig. 16.1. The conventional feedback control system in Fig. 16.1 may keep the hot oil temperature close to the set point despite disturbances in oil flow rate or cold oil temperature. However, if a disturbance occurs in the fuel gas supply pressure, the fuel gas flow will change, which upsets the furnace operation and changes the hot oil temperature. Only then will the temperature controller (TC) begin to take corrective action by adjusting the fuel gas flow. Thus, we anticipate that conventional feedback control may result in very sluggish responses to changes in fuel gas supply pressure. This disturbance is clearly associated with the manipulated variable. 

Figure 16.2 shows a cascade control configuration for the furnace, which consists of a primary control loop (utilizing TT and TC) and a secondary control... 

Figure 16.2 A furnace temperature control scheme using cascade control.

The block diagram for a general cascade control system is shown in Fig. 16.4. Subscript 1 refers to the primary control loop, whereas subscript 2 refers to the secondary control loop. Thus, for the furnace temperature control example,... 

The following manipulated variables are available toluene feed (Fi), hydrogen feed (F2), gas recycle (Fr), purge (Fp) and furnace duty (q h). Furnace duty may be used to control the reactor inlet temperature. The setpoint of this control loop may be coupled, in a cascade manner, with other variable from the previous list. 

Fig. 6.9. Direct-charged aluminum melting furnace with cascaded temperature control and regenerative burners. On the next 20-sec cycle, two air valves, two exhaust valves, and two fuel shutoff valves will reverse positions. Ma = milliamps. Se = suction exhaust. SP = setpoint. T/s = temperature sensor. Courtesy of North American Mfg. Co. 

For processing of polymeric materials and composites, a number of industrial microwave equipment manufacturers offer equipment for the production of continuous cast-resin components, in which the microwave unit (3.6 or 7.2kW) processes high-viscosity resin systems with flow rates up to 5.0 kg min" The control system provides easy integration into other and/or existing systems. Several furnaces can be switched in cascades to achieve an increase in temperature difference between feeding flow and drain flow and/or an increase in the flow quantity of the medium to be treated. The microwave flow heater is available, which can also be applied in other fields, for example, food, plastic, and chemical... 

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