Materials flow

Controlling the rate of material flow to various pieces of equipment such as grinders or kilns... 

Belt-conveyor scales determine the amount of material being conveyed on a belt. A section of belt is weighed by placing the belt support rollers on a scale the belt speed is also measured. Weight and speed data are suppHed to a controller which integrates them to arrive at a material flow rate, often stated in tons per hour. The controller may display a flow rate, shut the conveyor down when a predeterrnined amount of material has passed, or it may be used to maintain a specified flow rate. Accuracy is limited because of the number of detrimental influences involved, eg, variable belt tension. 

Ratio and Multiplicative Feedforward Control. In many physical and chemical processes and portions thereof, it is important to maintain a desired ratio between certain input (independent) variables in order to control certain output (dependent) variables (1,3,6). For example, it is important to maintain the ratio of reactants in certain chemical reactors to control conversion and selectivity the ratio of energy input to material input in a distillation column to control separation the ratio of energy input to material flow in a process heater to control the outlet temperature the fuel—air ratio to ensure proper combustion in a furnace and the ratio of blending components in a blending process. Indeed, the value of maintaining the ratio of independent variables in order more easily to control an output variable occurs in virtually every class of unit operation. 

Reinforced Thermoplastic Sheet. This process uses precombined sheets of thermoplastic resin and glass fiber reinforcement, cut into blanks to fit the weight and size requirements of the part to be molded. The blanks, preheated to a specified temperature, are loaded into the metal mold and the material flows under mol ding pressure to fiU the mold. The mold is kept closed under pressure until the temperature of the part has been reduced, the resin solidified, and demolding is possible. Cycle time, as with thermosetting resins, depends on the thickness of the part and the heat distortion temperature of the resin. Mol ding pressures are similar to SMC, 10—21 MPa (1500—3000 psi), depending on the size and complexity of the part. 

Feeding devices, stirring, and baffles all produce material flow that contributes to convection. Material flow between surfaces moving at different velocities... 

By far the most used detector is the thermal conductivity detector (TCD). Detectors like the TCD are called bulk-property detectors, in that the response is to a property of the overall material flowing through the detector, in this case the thermal conductivity of the stream, which includes the carrier gas (mobile phase) and any material that may be traveling with it. The principle behind a TCD is that a hot body loses heat at a rate that depends on the... 

An aerosol container can be considered a special appHcation of airless atomization (see Aerosols). The pressure is usually suppHed by a Hquefied gas in the container at its equiUbrium pressure. The material being sprayed has a very low viscosity to provide easy material flow through the feed tube and to permit fine atomization. 

Preferential Removal of Crystals. Crystal size distributions produced ia a perfectiy mixed continuous crystallizer are highly constraiaed the form of the CSD ia such systems is determined entirely by the residence time distribution of a perfectly mixed crystallizer. Greater flexibiUty can be obtained through iatroduction of selective removal devices that alter the residence time distribution of materials flowing from the crystallizer. The... 

Transfer mol ding minimises preforming, and is usually used for the production of very small parts however, this technique may generate excessive amounts of scrap material. Flow requirements can be quite high, but fluorocarbon elastomers are available that are effective iu this appHcation. 

Composite Samples Obtained by Multiple Sample Extractions Material flow streams are sampled in practice by combining extractions taken at successive time intei vals into a composite sample. Multiple increment collection to obtain representative composite sampfes for specified bulk-material flows is performed according to a... 

Selection of appropriate time intei vals for increment extractions relates to property variation (inhomogeneity) within material flow streams. Ten minute extraction intei vals are generally adequate to obtain suitably representative samples from material flows under practical circumstances. Precise determination of extraction intei vals consistent with individual apphcations can be calculatedthrough autocorrelation of historical sampling data, a statistical method described in references (Gy, Pitard). 

Example 5 Solids Sampling by Linear Traversing Trajectory Cutter lucremeut weight S hy a linear traversing cutter from hulk material flow of fine powder B expressed in unit weight per unit time is calculated hy... 

Spaced-Bucket Centrifugal-Discharge Elevators These elevators (Fig. 21-5<7) are the most common. They are usually equipped with the style 1 or 2 buckets shown in Fig. 21-5/j. Mounted on a Belt or a chain, the buckets are spaced to prevent interference in loading or discharging. This type oi elevator will handle almost any free-flowing fine or small-lump material such as erain, coal, or dry chemicals. Buckets are loaded partly by material flowing directly into them... 

Material-Flow Cbaracteristics Two important definitions of the flow characteristics of a storage vessel are mass flow, which means that all the material in the vessel moves whenever any is withdrawn (Fig. 21-17), and funnel flow, which occurs when only a portion of the material flows (usually in a channel or rathole in the center of the system) when any material is withdrawn (Fig. 21-18). Some typical mass-flow designs are shown in Fig. 21-19. 

The principle of operation of batch-type scales is based on the concept that a flowing stream of material has constant density. If this is true, then if at some point in advance of the desired batch weight the stream is cut off, the amount of material flowing will remain con-... 

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