Control flow

Flow control is usually associated with inventory control in a storage tank or other equipment. There must be a reservoir to take up the changes in flow-rate. 

Reboiler control systems are selected as part of the general control system for the column and are discussed in Section 5.8.7. 

Flow control systems are critical components of most of the energy systems involving fluid flow and heat transfer. These systems are essential for performance optimization of both macroscale and microscale devices. Micropumps, microvalves, microshear stress sensors, and microflow sensors are integral components of flow control systems. Capillary micropump, MHD micropump, thermocapillary micropump, and electrokinetic micropump have been presented in earlier chapters. The present chapter reports various microactuators and shear stress sensors for flow control systems. More details on microvalves and microflow sensors can be found in other references (Nguyen and Wereley, 2006). 

Pressure, velocity, and temperature information are used to characterize a flow field. A flow field is broadly classified as laminar or turbulent. In turbulent flows, these flow variables are random functions of space and time. The coherent and random flow structures/vortices and their mutual interactions influence the overall behavior of a turbulent flow field. The location of the wall surface, Reynolds number, Mach number, buoyancy force due to temperature, concentration gradients, and so on influence the turbulent flow field characteristics. The nature of flow field also depends on the convective or absolute instability mechanism leading to turbulent flow. 

The flow control is a process by which the flow field is manipulated to obtain the required behavior in comparison to the natural uncontrolled case. The flow control is broadly classified as (1) passive control, where no auxiliary power source is required, and (2) active control, where there is expenditure of energy. In passive control, parameters like geometry, compliance, temperature, porosity, and so on are varied. Boundary oscillation, acoustic waves, blowing, suction, and so on are used for active control. The active control schemes use actuators for manipulating the flow behavior. The size of these actuators depends on the nature of the flow field. When the Reynolds number is increased, the required size of the actuator is reduced. 

Transport Phenomena in Microfluidic Systems, First Edition. Pradipta Kumar Panigrahi. 

The availability of MEMS fabrication technique has contributed toward small-scale actuator development and enhanced the capability of flow control systems. 

MATLAB has several flow control structures that allow the program to make decisions or control its execution sequence. These structures are for, if, while, and switch which we describe briefly below  

You can also define an else clause, which is executed if the condition in the i/statement is not true  

Two useful commands in programming are break and pause. You can use the break command to jump out of a loop before it is completed. The pause command will cause the program to wait for a key to be pressed before continuing  

Flow is the manipulated variable as well as the controlled variable, so it seems as though the process is unity. But this is not the case. Opening a valve does admit flow, but the response is not quite instantaneous. If the fluid is gaseous, it is subject to expansion upon a change in pressure therefore the contents of a pipe vary somewhat with pressure drop, hence with flow. In a liquid stream, inertia is significant-flow cannot be started or stopped without accelerating or decelerating. To demonstrate the dynamic character of inertia, the time constant of a column of liquid in a pipe will be derived. 

In the steady state, the velocity of flow in a pipe varies with pressure 

Therefore the pressure drop due to flow in the steady state is 

To find the time constant, the differential equation must be reduced to its standard form  

Flow coefficient can be replaced by its steady-state equivalent  

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Mol ding of parts for a wide variety of plumbing and irriga tion appHcations consumes as much acetal resin as the industrial appHcations. Rod and slab stock can be machined into components for precision flow control devices. 

The pressure-volume curve of a proposed centrifugal fan has a different shape. This fan curve must be drawn for the anticipated fan inlet density expected at its location in the system. The point of intersection of these two curves locates the flow rate and pressure rise at which the fan and system operate. This intersection represents a desirable operating combination for fan and system. The system curve intersects the fan curve in the middle of its maximum efficiency range and also at a point where the fan pressure produced varies smoothly but distinctly in a constant trend with flow rate which is desirable for flow control. 

Fig. 4. Selection of fan size where the soHd line represents a typical setting and the dashed lines the operating extremes, (a) Desirable sizing. The system resistance curve intersects the fan curve near its maximum efficiency. Changes in system resistance from a flow-control element also intersect the fan curve at desirable points for good flow control. The dashed curves also intersect system resistance curves at desirable locations, (b) A fan essentially too large for the system. The intersection of the system curve near the peak of the fan curve results in poor system flow control and perhaps surging.

For air-flow control, the system may contain a control valve or damper that automatically or manually modulates system pressure drop. The dotted curves in Figure 4a on each side of the system resistance curve might represent operating extremes of the system resistance as the control valve is varied from maximum to minimum opening. These curves also intersect the fan curve at desirable operating portions of its range both for efficiency and flow control. 

The wisest fan choice is frequently not the cheapest fan. A small fan operates well on its curve but may not have adequate capacity for maximum flow control, future needs, or process upset conditions. It may be so lightly constmcted that it is operating near its peak speed with no provision for speed increases in the future, if needed. As fan size is increased, efficiency generally improves and wheel speed is lower. These factors decrease operating cost and provide reserve capacity for the future. However, it is also possible to oversize a fan and impair its performance. 

Device Air flow control Initial Cost Operating Noise... 

Two-speed motors are typically used on noncondensing services where the process is not sensitive to temperature but mostly seasonal or variable throughput of fluids in the air cooler requires some degree of air flow control. This is a simple, rather inexpensive means to control air flow when volume air flow is not critical. Typical motor ratings are 1800/900 rpm, although 1800/1200 rpm types are available. 

Air control louvers or dampers, popular in the past for air flow control, are used primarily for only very low scale air flow control. Louvers are used in many winterized heat exchangers in extremely low ambient temperature locations to retain and recirculate warm air in completely enclosed heat exchangers, sometimes in very compHcated control schemes. The use of louvers on the discharge side of a fan to control air flow is inefficient and creates mechanical problems in the louvers because of the turbulence. A fan is a constant volume device, thus use of louvers to control flow is equivalent to... 

Fig. 11. Flow control system (11). FIC = flow indicator controller.

Table 4. Risk Assessment of Flow Control System ...

Characterization and influence of electrohydro dynamic secondary flows on convective flows of polar gases is lacking for most simple as well as complex flow geometries. Such investigations should lead to an understanding of flow control, manipulation of separating, and accurate computation of local heat-transfer coefficients in confined, complex geometries. The typical Reynolds number of the bulk flow does not exceed 5000. 

Fig. 23. Two types of hollow-fiber modules used for gas separation, reverse osmosis, and ultrafiltration applications, (a) Shell-side feed modules are generally used for high pressure appHcations up to - 7 MPa (1000 psig). Fouling on the feed side of the membrane can be a problem with this design, and pretreatment of the feed stream to remove particulates is required, (b) Bore-side feed modules are generally used for medium pressure feed streams up to - 1 MPa (150 psig), where good flow control to minimise fouling and concentration polarization on the feed side of the membrane is desired.

The ease with which the separated products leave the bowl determines the richness of the fat. Fluid whole milk enters the separator under pressure from a positive displacement pump or centrifugal pump with flow control (Fig. 1). The fat (cream) is separated and moves toward the center of the bowl, while the skimmed milk passes to the outer space. There are two spouts or oudets, one for cream and one for skimmed milk. Cream leaves the center of the bowl with the percentage of fat ( 30 40%) controlled by the adjustment of a valve, called a cream or skim milk screw, that controls the flow of the product leaving the field of centrifugal force and thus affects the separation. 

An air valve, sometimes called the air-activated valve, is widely used for automated food handling operations. Although electronic or electric control boxes may be a part of the system, the valve itself generally is air-activated, and is more reflable than other types. Air-operated valves are used for in-place cleaning systems, and for the transfer and flow control of various products. 

Schemes to control the outlet temperature of a process furnace by adjusting the fuel gas flow are shown in Figure 13. In the scheme without cascade control (Fig. 13a), if a disturbance has occurred in the fuel gas supply pressure, a disturbance occurs in the fuel gas flow rate, hence, in the energy transferred to the process fluid and eventually to the process fluid furnace outlet temperature. At that point, the outlet temperature controller senses the deviation from setpoint and adjusts the valve in the fuel gas line. In the meantime, other disturbances may have occurred in the fuel gas pressure, etc. In the cascade control strategy (Fig. 13b), when the fuel gas pressure is disturbed, it causes the fuel gas flow rate to be disturbed. The secondary controller, ie, the fuel gas flow controller, immediately senses the deviation and adjusts the valve in the fuel gas line to maintain the set fuel gas rate. If the fuel gas flow controller is well tuned, the furnace outlet temperature experiences only a small disturbance owing to a fuel gas supply pressure disturbance.

Fig. 13. Cascade control schemes, where TC = temperature controller FC = fuel gas flow controller and LC = liquid level controller, (a) Simple circuit having no cascade control (b) the same circuit employing cascade control and (c) and (d) Hquid level control circuits with and without cascade control,...

Many misconceptions exist about cascade control loops and their purpose. For example, many engineers specify a level-flow cascade for every level control situation. However, if the level controller is tightly tuned, the out-flow bounces around as does the level, regardless of whether the level controller output goes direcdy to a valve or to the setpoint of a flow controller. The secondary controller does not, in itself, smooth the outflow. In fact, the flow controller may actually cause control difficulties because it adds another time constant to the primary control loop, makes the proper functioning of the primary control loop dependent on two process variables rather than one, and requites two properly tuned controllers rather than one to function properly. However, as pointed out previously, the flow controller compensates for the effect of the upstream and downstream pressure variations and, in that respect, improves the performance of the primary control loop. Therefore, such a level-flow cascade may often be justified, but not for the smoothing of out-flow. 

The results of several studies were interpreted by the Poole-Erenkel mechanism of field-assisted release of electrons from traps in the bulk of the oxide. In other studies, the Schottky mechanism of electron flow controlled by a thermionic emission over a field-lowered barrier at the counter electrode oxide interface was used to explain the conduction process. Some results suggested a space charge-limited conduction mechanism operates. The general lack of agreement between the results of various studies has been summari2ed (57). 

Most of the resin systems used in commodity composites are slight modifications of the standard commercial mol ding grade material. Usually certain selected properties, such as purity or molecular weight range or distribution, are enhanced or carehiUy selected. In addition, special additives, such as flow controllers, thermal stabilizers, or antioxidants, are often added by the resin manufacturer prior to shipment. Many of the conventional or commodity-type resins used in thermoplastic composites are Hsted in Table 1 and the preparation of each of these is described. AH resins and blends described in the hterature are not Hsted, and the synthesis described is not the only procedure available, but is usually the most common commercial process. 

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