Furnace feedback control

Under slower heating rates in heat-flux DSC, the deviation of sample temperature from the setpoint during a self-feeding reaction may be maintained adequately small so as to be neglected. If the furnace feedback control is set to act based on the temperature of the sample (that is the sample temperature thermocouple is the control thermocouple), then the control system may be able to allow the transforming sample to heat itself at a constant rate, and the heat input from the furnace will retreat as needed. 

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. 

A block diagram for a feedback control furnace system, used in thermal analysis instrumentation, is shown in Figure 2.10. The SCR receives a control instruction, and in turn permits a... 

We next try a more aggressive heat recovery alternative as shown in Fig. 5.24. The heat input to the furnace is quite small and most of the heat is provided by the large feed-effluent exchanger. With, our choice of measurement lags (two 1-minute lags in series) and the lag in the furnace., this system cannot be stabilized by feedback control around the furnace if the quench controller is in manual. However, it is possible to stabilize the system with just the quench controller in automatic and the furnace controller in manual. Subsequent tuning of the furnace controller is then easy since the new system is open-loop stable. 

Substrate heating is required in CVD reactors. Since the films are grown under isothermal conditions, the substrate must be held at a constant growth temperature for an extended period of time. The achievement of this requirement is facilitated in two ways, and reactors are classified into two groups depending on how the substrate is heated. In a hot-wall design, the entire reactor is placed in a tube furnace and the substrate, the region of forced gas convention, and the walls of the reactor are maintained at the same temperature. Of course the ends of the reactor are cooler than the middle because of heat loss from the ends and the introduction of cold gas at the entrance of the reactor. To remedy this imbalance, a three-zone furnace, with independent feedback control for each zone, is usually employed. Substrates are loaded only in the portion of the reactor where the temperature can be accurately maintained. 

The fine adjustments are made using a feedback control system. An important element of that system is an optical sensor which is used to control the hydrocarbon emissions. As more volatiles are released into the furnace, the sensor detects the... 

The catalysts were contained between quartz wool plugs inside a horizontal quartz tube reactor (1-cm-i.d. by 35-cm-long). The reactor was heated with a split tube furnace, and the temperatures were controlled by a feedback controller. The reactants were preheated to 175°C before entering the catalyst zone. The temperature of the product stream was maintained at 150°C to prevent condensation in the sample chamber or capillary inlet to the mass spectrometer. 

Fig. 9.23. Muscular-cybernetic analogy. Feedback control of furnace temperature...

Prior to extrusion the fused silica preform is usually treated with dilute hydrofluoric acid to remove any imperfections and deformations present on the inner and outer surfaces and then rinsed with distilled water and followed by annealing (55). In a cleanroom atmosphere, the preform is vertically drawn through a furnace maintained at approximately 2000°C. Guidance and careful control of the drawing process is achieved by focusing an infrared laser beam down the middle of the capillary in conjunction with feedback control electronic circuitry in order to maintain nniformity in the specifications of the inner and outer diameters in the final prodnct. 

Feed-forward control takes quick action on any disturbance while feedback ensures that any persisting or increasing offset in the level is taken care of. Feed-forward control is often used in case of slow changing processes where the state variable reacts slowly on a distiubance. This happens, for example, in distillation columns with many trays, which are subject to large disturbances (for example feed changes from furnaces). Most process units, however, can be well controlled by using feedback control only. 

Figure 16.1 A furnace temperature control scheme that uses conventional feedback control.

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. 

Since we do not have the precise model function Gp embedded in the feedforward controller function in Eq. (10-8), we cannot expect perfect rejection of disturbances. In fact, feedforward control is never used by itself it is implemented in conjunction with a feedback loop to provide the so-called feedback trim (Fig. 10.4a). The feedback loop handles (1) measurement errors, (2) errors in the feedforward function, (3) changes in unmeasured load variables, such as the inlet process stream temperature in the furnace that one single feedforward loop cannot handle, and of course, (4) set point changes. 

Handling of disturbance in the inlet process stream temperature is passive. Any changes in this load variable will affect the furnace temperature. The change in furnace temperature is measured by the outlet temperature transducer (TT) and sent to the feedback temperature controller (TC). The primary controller then acts accordingly to reduce the deviation in the furnace temperature. 

Moveable throat armor is used for additional control. Hanging plates or horizontally adjustable guides are used to adjust the throat diameter for each charge, thus modifying how the layers are formed in the furnace. Bell-less tops (Fig. 6b) provide even greater flexibility, as both the angle and the rotation speed of the rotating chute may be adjusted. Many furnaces use probes, radar, or laser devices to provide feedback on burden distribution. 

Just as we approached reactor control in Chap. 4, we will start by exploring the open-loop effects of thermal feedback. Consider Fig. 5.19, which shows an adiabatic plug-flow reactor with an FEHE system. We have also included two manipulated variables that wall later turn out to be useful to control the reactor. One of these manipulated variables is the heat load to the furnace and the other is the bypass around the preheater. It is clear that the reactor feed temperature is affected by the bypass valve position and the furnace heat load but also by the reactor exit temperature through the heat exchanger. This creates the possibility for multiple steady states. We can visualize the different... 

The cracking catalyst was selected as a bed material due to its ideal fluidizing properties and not for its catalytic properties. The nitrogen gas was preheated to furnace temperature and was fed to the reactor through an inlet on the bottom segment. The reactor was heated by a tubular Lindberg furnace and the temperature was controlled by a thermocouple feedback mechanism connected to the oven control unit. 

A heater, as for example furnace, is required for start-up. Because positive feedback due to heat integration may lead to state multiplicity, the furnace duty can be manipulated in a temperature control loop to ensure stable operation. 

The human factors literature is rich in task analysis techniques for situations and jobs requiring rule-based behavior (e.g., Kirwan and Ainsworth 1992). Some of these techniques can also be used for the analysis of cognitive tasks where weU-practiced work methods must be adapted to task variations and new circumstances. This can be achieved provided that task analysis goes beyond the recommended work methods and explores task variations that can cause failures of human performance. Hierarchical task analysis (Shepherd 1989), for instance, can be used to describe how operators set goals and plan their activities in terms of work methods, antecedent conditions, and expected feedback. When the analysis is expanded to cover not only normal situations but also task variations or changes in circumstances, it would be possible to record possible ways in which humans may fail and how they could recover from errors. Table 2 shows an analysis of a process control task where operators start up an oil refinery furnace. This is a safety-critical task because many safety systems are on manual mode, radio communications between control room and on-site personnel are intensive, side effects are not visible (e.g., accumulation of fuel in the fire box), and errors can lead to furnace explosions. 

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