Inlet temperature feedforward

From Equation (6.19) we know that the gain in the bias feedforward is given by 

From Chapter 2, we know that (Kp)m varies inversely with feed rate. If we were to configure the output of the bias algorithm to manipulate the fuel flow directly, rather than the fuel-to-feed ratio, then we would need to include adaptive tuning to automatically adjust K to keep it in proportion to feed rate. 

For simplicity we assume that feed specific heat (cp) and heater efficiency (rj) are constant. If ATis the change in inlet temperature then the required change in fuel flow (AF energy units) is given by the heat balance 

Combining with Equation (10.11) confirms the dependence of Ton the feed flow Ffi j). 

This is not a problem, of course, if feed rate varies little. But if we can retain the feedforward ratio, the MV becomes the ratio SR By dividing Equation (10.13) by Ffgg we get 

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The example simulation THERMFF illustrates this method of using a dynamic process model to develop a feedforward control strategy. At the desired setpoint the process will be at steady-state. Therefore the steady-state form of the model is used to make the feedforward calculations. This example involves a continuous tank reactor with exothermic reaction and jacket cooling. It is assumed here that variations of inlet concentration and inlet temperature will disturb the reactor operation. As shown in the example description, the steady state material balance is used to calculate the required response of flowrate and the steady state energy balance is used to calculate the required variation in jacket temperature. This feedforward strategy results in perfect control of the simulated process, but limitations required on the jacket temperature lead to imperfections in the control. 

The feedback control in loops 1 and 2 is combined with a feed-forward controller in loop 3 which measures the inlet, temperature T calculates the change in cooling water flow rate Few which is required to bring the reactor temperature Fback to its set point Ts and sends this signal to the feedback controller (the feed-forward controller consists of a model of the process and is therefore not of P-, PI- or PID-type). The feedforward control loop will therefore theoretically eliminate any disturbances in inlet temperature Tv The feedback part of the control system, loop 1, will compensate for any inaccuracies in the feed-forward control model as well as eliminate the effect of other, unmeasured disturbances, e.g. in inlet flow rate Fr... 

Feedforward control of a heat exchanger (shown in Figure 21.2a) The objective is to keep the exit temperature of the liquid constant by manipulating the steam pressure. There are two principal disturbances (loads) that are measured for feedforward control liquid flow rate and liquid inlet temperature. 

V.21 Derive the nonlinear, steady-state feedforward control system that will keep the exit temperature of a stirred tank heater at the desired set point despite any changes in the inlet temperature or flow rate, T, and F,. The feedforward control system should be capable of (1) rejecting the effect of disturbance changes, and (2) tracking any set point changes. Identify all relevant transfer functions. 

However, this type of scheme prevents the application of many of the techniques covered in this chapter. It would be difficult to devise a feedforward scheme to deal with disturbances in feed rate, heater inlet temperature or fuel heating value. A better solution is illustrated in Figure 10.11. 

The most easily understood demonstration of feedforward is in the control of a heat exchango. The computation is a heat balance, where the correct supply of heat is calculated to match the measured load. The process is pictured in Fig. 8.3. Steam flow W, is to be manipulated to heat a variable flow of process fluid Wp fixm inlet temperature T to the desired outlet temperature Ti. 

A feedforward-only control system is to be designed for the stirred-tank heating system shown in Fig. E15.il. Exit temperature T will be controlled by adjusting coolant flow rate, qc. The chief disturbance variable is the inlet temperature Ti which can be measured on-line. Design a feedforward-only control system based on a dynamic model of this process and the following assumptions ... 

Because the pressure control loop responds rapidly, the supply pressure disturbance will have little effect on furnace operation and exit oil temperature. Some engineers prefer that flow control, rather than pressure control, be employed in the slave loop to deal with discharge pressure variations. If the performance improvements for disturbances in oil flow rate or inlet temperature are not large enough, then feedforward control could be utilized for those disturbances (see Chapter 15). 

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. 

In this example, the reactor is equipped with a feedforward controller that calculates the flowrate F and the jacket temperature Tj required to maintain the reactor temperature constant for variations in inlet feed concentration CA0 and temperature T0. 

Design a feedforward controller for the two-heated-tank process considered in Example 10.1. The load disturbance is inlet feed temperature 7. ... 

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. 

Since the controlled variable (the outlet oil temperature) in Figure 34A is being measured, this is a feedback control system. Since the disturbance (the inlet oil temperature) in Figure 34B is being measured, this is a feedforward control system and not a feedback control system. Since both the outlet and inlet oil temperatures in Figure 34C are being measured, this is not a feedback control system. Therefore, the correct answer is Figure 34A. 

Problem 1. Control the concentration cA in the presence of changes in the inlet concentration and temperature. The temperature of the coolant, Tc, is the manipulated variable. Since we have two disturbances, we need two distinct feedforward controllers. To develop the design equations for the two controllers, put Ca(s) = 0 in eq. (9.15a). Then we take... 

Returning to the tank heater example, we realize that we can use a different control arrangement to maintain T = Ts when T, changes. Measure the temperature of the inlet stream T, and open or close the steam valve to provide more or less steam. Such a control configuration is called feedforward control and is shown in Figure 1.4. We notice that the feedforward control does not wait until the effect of the disturbances has been felt by the system, but acts appropriately before the external disturbance affects the system, anticipating what its effect will be. The characteristics of the feedback and feedforward control systems will be studied in detail in subsequent chapters. 

Consider the stirred tank heater shown in Figure 1.1. The control objective is to keep the temperature of the liquid in the tank at a desired value (set point) despite any changes in the temperature of the inlet stream. Figure 1.2 shows the conventional feedback loop, which measures the temperature in the tank and after comparing it with the desired value, increases or decreases the steam pressure, thus providing more or less heat into the liquid. A feedforward control system uses a differ-... 

Feedforward control of a CSTR (shown in Figure 21.2d) Inlet concentration and temperature are the two disturbances, and the product withdrawal flow rate and the coolant flow rate are the two manipulations. There are two objectives to maintain constant temperature and composition within the CSTR. 

Microprocessor-based control (Figure 49.11b) in which the outlet-air temperature is controlled by regulating the feed rate, with feedback from the moisture content measurement and feedforward from the atmospheric humidity, feed specific gravity, and inlet air-temperature measurements... 

In this example, the inlet flow of liquid W and the temperature Ti are measured to determine the amount of steam required as per Equation 6.3. The desired outlet temperature Ti is the set point into the feedforward controller. The feedback temperature... 

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