Feedback flow control system

Thus it is shown that the feedback flow control system will act to correct for any condition that would cause flow variation. 

Figure 7. Feedback flow control system for low-speed pump...

A simple positive-displacement sampler system is shown in Figure 3. The basic system contains a battery and motor connected to a positive-displacement pump mechanism and provides an efficient means for moving air through the sampler. In order to provide feedback flow control the system must be expanded to include the means to monitor air flow through the pump to compensate for flow variations. 

Figure 4 shows a simple version of a feedback flow control sampler system. An orifice and pressure switch are used to monitor air flow. 

Figure 4. Feedback flow control sampler system...

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). 

R Feed Forward Combustion Control. Largo consumers of fuel gas are discovering that the gas flow control systems they have employed for years car no longer deliver a controlled heat flow to their burners. The typical industrial combustion process consists of pressure or flow control loop and a feedback loop— see Figure 2— to reset the pressure or flow of gas to the burners based on load. 

Operation of HPLC pumps for delivering micro- to nanoliters per minute flow rates is based on the capability to make precise flow measurements and the feedback control through an electronically driven purge valve situated before the flow sensing area. Such an electronic flow control system may be placed after formation of the required mobile-phase composition (from two or more high-pressure pumps) or may be used for each specific pump. In the first case, the mobile-phase flow exceeding the threshold is purged as a waste, while in the second case the solvent may be recirculated back to the reservoir. The two setups are depicted in Fig. lOA and B. 

The analysis results of the reactor coolant flow control system failure (transient) are shown in Fig. 7.100. Since the reactivity feedback from the coolant density is smaller than that in the Super LWR, the increase in the power is smaller and hence the reactor is not tripped. 

One such approach is called cascade control, which is routinely used in most modern computer control systems. Consider a chemical reactor, where reac tor temperature is to be controlled by coolant flow to the jacket of the reac tor (Fig. 8-34). The reac tor temperature can be influenced by changes in disturbance variables such as feed rate or feed temperature a feedback controller could be employed to compensate for such disturbances by adjusting a valve on me coolant flow to the reac tor jacket. However, suppose an increase occurs in the... 

A cocurrent evaporator train with its controls is illustrated in Fig. 8-54. The control system applies equally well to countercurrent or mixed-feed evaporators, the princip difference being the tuning of the dynamic compensator/(t), which must be done in the field to minimize the short-term effects of changes in feed flow on product quality. Solid concentration in the product is usually measured as density feedback trim is applied by the AC adjusting slope m of the density function, which is the only term related to x. This recahbrates the system whenever x must move to a new set point. 

Fig. 4.1 Block diagram of a closed-loop control system. R s) = Laplace transform of reference input r(t) C(s) = Laplace transform of controlled output c(t) B s) = Primary feedback signal, of value H(s)C(s) E s) = Actuating or error signal, of value R s) - B s), G s) = Product of all transfer functions along the forward path H s) = Product of all transfer functions along the feedback path G s)H s) = Open-loop transfer function = summing point symbol, used to denote algebraic summation = Signal take-off point Direction of information flow.

Continuous in-line measurements and control of the mass material balance in the process, with automatic feedback to the reactants dosing devices (performed either by computerized system or by traditional flow control loops). 

Operating near the washout point maximizes the production rate of cells. A feedback control system is needed to ensure that the limit is not exceeded. The easiest approach is to measure cell mass—e.g., by measuring turbidity— and to use the signal to control the flow rate. Figure 12.5 shows how cell mass varies as a function of t for the system of Examples 12.7 and 12.8. The minimum value for t is 2.05 h. Cell production is maximized at F=2.37h. 

The simple feedback control system below consists of a continuous-flow stirred tank, a temperature measurement device, a controller and a heater. 

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 8 shows basic elements of a feedback control system as represented by a block diagram. The functional relationships between these elements are easily seen. An important factor to remember is that the block diagram represents flowpaths of control signals, but does not represent flow of energy through the system or process. 

However, we can describe the basic structure of several feedforward control systems. Figure 8.7 shows a blending system with one stream which acts as a disturbance both its flow rate and its composition can change. In Fig. 8.7a the conventional feedback controller senses the controlled composition of the total blended stream and changes the flow rate of a manipulated flow. In Fig. %.lb the manipulated flow is simply ratjoed to the wild flow. This provides feedforward control for flow rate changes. Note that the disturbance must be measured to implement feedforward control. 

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... 

As pump load is increased, this error effect causes the flow rate to increase more than necessary. This problem is eliminated by using an accumulator to filter the flow through the orifice. Figure 7 shows the completed feedback control system which incorporates an accumulator between the pump and orifice. 

The use of high or low limits for process variables is another type of selective control, called an override. The feature of antireset windup in feedback controllers is a type of override. Another example is a distillation column with lower and upper limits on the heat input to the column reboiler. The minimum level ensures that liquid will remain on the trays, while the upper limit is determined by the onset of flooding. Overrides are also used in forced-draft combustion control systems to prevent an imbalance between airflow and fuel flow, which could result in unsafe operating conditions. 

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