Feedback control constant flow

Figure Bl.27.8. Schematic view of Picker s flow microcalorimeter. A, reference liquid B, liquid under study P, constant flow circulating pump and 2, Zener diodes acting as heaters T and T2, thennistors acting as temperature sensing devices F, feedback control N, null detector R, recorder Q, themiostat. In the above A is the reference liquid and C2is the reference cell. When B circulates in cell C this cell is the working cell. (Reproduced by pemiission from Picker P, Leduc P-A, Philip P R and Desnoyers J E 1971 J. Chem. Thermo. B41.)...

The components of the basic feedback control loop, combining the process and the controller can be best understood using a generalised block diagram (Fig. 2.29). The information on the measured variable, temperature, taken from the system is used to manipulate the flow rate of the cooling water in order to keep the temperature at the desired constant value, or setpoint. This is illustrated by the simulation example TEMPCONT, Sec. 5.7.1. 

We will assume constant holdups in the reflux drum Aij> and in the column base Mg. Proportional-integral feedback controllers at both ends of the column will change the reflux flow rate and the vapor boilup V to control overhead composition and bottoms composition Xg at setpoint values of 0.98 and 0.02 respectively. 

Figure 11.5h shows a combined feedforward-feedback system where the feedback signal is added to the feedforward signal in a summing device. Figure 11.Sc shows another combined system where the feedback signal is used to change the feedforward controller gain in the ratio device. Figure 11.6 shows a combined feedforward-feedback control system for a distiltetion column where feed-rate disturbances are detected and both steam flow and reflux flow arc changed to hold both overhead and bottoms compositions constant. Two feedforward controllers are required.

Now, from its essential notion, we have the feedback interconnection implies that a portion of the information from a given system returns back into the system. In this chapter, two processes are discussed in context of the feedback interconnection. The former is a typical feedback control systems, and consists in a bioreactor for waste water treatment. The bioreactor is controlled by robust asymptotic approach [33], [34]. The first study case in this chapter is focused in the bioreactor temperature. A heat exchanger is interconnected with the bioreactor in order to lead temperature into the digester around a constant value for avoiding stress in bacteria. The latter process is a fluid mechanics one, and has feedforward control structure. The process was constructed to study kinetics and dynamics of the gas-liquid flow in vertical column. In this second system, the interconnection is related to recycling liquid flow. The experiment comprises several superficial gas velocity. Thus, the control acting on the gas-liquid column can be seen as an open-loop system where the control variable is the velocity of the gas entering into the column. There is no measurements of the gas velocity to compute a fluid dynamics... 

The following data illustrates the performance of feedback flow control samplers under test conditions which represent field conditions. Figure 11 shows the flow rate stability versus time for a sampler operated on its battery. The flow control system maintained a constant flow rate even though the battery was discharging(3). 

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. 

Isocratic elution is commonly used for the elution of analytes from the column. In isocratic elution, the mobile phase is kept constant throughout the analysis. The mobile phase can be a single solvent or a solution of two or more miscible solvents. The major requirements of isocratic pumps are accuracy and smoothness of flow. Because the pump delivers only one solvent system, simple, inexpensive pulse dampeners and rudimentary flow or pressure feedback control circuits can be used. The basic setup of an isocratic system is illustrated in Figure 3.10. 

In a heat flow calorimeter, a feedback controller is used to maintain a constant desired reactor temperature by adjusting the jacket temperature. From (1), with a constant calibration probe heat flow, at steady state (dT/dt = 0), the overall heat transfer coefficient can be found from... 

The sensor system is composed of the sensor, the transmitter, and the sampling system that allows it to make measurements. The performance of a sensor can be assessed by determining its repeatability, time constant, and (sometimes) accuracy. Accuracy is important for a composition analyzer on a hnal product to ensure that the product meets specifications. The accuracy of a flow transmitter is usually not important, because the flow rate is adjusted incrementally by a supervisory controller so that its actual flow rate is unimportant. The dynamics of the sensor can affect the feedback control performance if it is too slow, and a large repeatability can increase the variability in the controlled variable. 

Figure 15.52 shows the application of ratio control to the effluent pH for a wastewater neutralization process applied in a mixing tank. This controller can effectively handle wastewater feed flow rate changes when the chemical makeup of the wastewater remains relatively constant. Small changes in the chemical makeup of the wastewater can usually be handled by the feedback controller, which adjusts the reagent-to-wastewater ratio to maintain the specified effluent pH. 

II.ll Figure PII.10 shows a simplified representation of a drum boiler. Feed water enters the boiler with a flow rate F1 (mass/hr) and a temperature T i and it is heated by an amount of heat Q (Btu/hr) which is supplied by burned fuel. The generated steam flows out from the top of the boiler, with a flow rate F2 (mass/hr) and a pressure p (psig). A simple feedback control system has been installed to keep the level of the water in the drum boiler constant by manipulating the flow rate of the feedwater stream. 

To maintain a constant mass flow rate to the flare hp, flow control valves must continually be opened to compensate for the decreasing tank pressure. Historically, this was done manually by skilled operators who watched a pressure gauge connected to the flare tip fuel plenum and continuously opened a valve in the fuel supply line in an attempt to keep the hp pressure constant. This process had severe shortcomings. It was a manual feedback control dependent on the operator s skill. During a short dura-hon test, the operator tended to perform an overshoot-... 

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