Heat feedback process

Fig. 3.10 Thermal structure of a combustion wave and heat feedback processes therein.

Fig.s.n Schematic representation of heat feedback processes in a combustion wave. 

Figure 3-10. Thermal structure and heat feedback process of combustion wave.

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 10.2h gives a sketch of the feedback control system and a block diagram for the two-heated-tank process with a controller. Let us use an analog electronic system with 4 to 20 mA control signals. The temperature sensor has a range of 100°F, so the Gj transfer function (neglecting any dynamics in the temperature measurement) is... 

Example 10.1. The closedloop transfer functions for the two-heated-lank process can be calculated from the openloop process transfer functions and the feedback controller transfer function. We will choose a proportional controller, so = K,. Note that the dimensions of the gain of the controller are mA/mA, that is, the gain is dimensionless. The controller looks at a milliampere signal (PM) and puts out a milliampere signal (CO). 

When processes are subject only to slow and small perturbations, conventional feedback PID controllers usually are adequate with set points and instrument characteristics fine-tuned in the field. As an example, two modes of control of a heat exchange process are shown in Figure 3.8 where the objective is to maintain constant outlet temperature by exchanging process heat with a heat transfer medium. Part (a) has a feedback controller which goes into action when a deviation from the preset temperature occurs and attempts to restore the set point. Inevitably some oscillation of the outlet temperature will be generated that will persist for some time and may never die down if perturbations of the inlet condition occur often enough. In the operation of the feedforward control of part (b), the flow rate and temperature of the process input are continually signalled to a computer which then finds the flow rate of heat transfer medium required to maintain constant process outlet temperature and adjusts the flow control valve appropriately. Temperature oscillation amplitude and duration will be much less in this mode. 

A recent study of the deflagration of RDX (Ref 107) presents the following model for the deflagration process (1) partial decompn in the liq phase (2) vaporization and gas phase decompn (3) oxidation of products (particularly HCHO) by N02. As system press increases, (1) and (3) become progressively more prominent. Although the reacting liq layer at high pressures is thin, its heat feedback into the still unreacted material increases... 

Using the simulation program given in Appendix A for the three-heated-tank process and a relay feedback test, determine the ultimate gain and ultimate frequency for the loop in which the temperature in the third tank T3 is controlled by manipulating the heat input to the first tank Qi. Compare these results with the theoretical values obtained in Example 8.8. 

An example of a feedback control loop is shown in Figure 8.1 for heating the process water with live steam injection. The temperature of the water leaving the process is measured by a temperature sensor, and a transmitter sends the signal to the controller. The desired temperature setpoint is adjusted in the controller, and the difference between the water temperature out of the process and the setpoint is called the error. In this example, the process water temperature out of the process is the controlled process variable, that is, controlled to a setpoint. The manipulated variable is the steam flow rate, which is manipulated by the automatic valve that is connected to the controller output. 

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