Gravimetric feeders

A gravimetric feeder rehes on weighing the material to achieve the required rate. Feed accuracies of 1/4% are obtainable using a properly designed gravimetric feeder. A disadvantage of such a feeder is that it is usually mote expensive than a volumetric device. 

Loss-in- Weight Feeders. The loss-in-weight (LIW) gravimetric feeder is used when feed accuracy is essential. It measures the loss in weight of material discharged from the system. As such, it can be used both in continuous and batching systems, and may be used for Hquids by replacing the... 

Standard instrument control and pacing signals are generally acceptable for common feeder system operation. Volumetric and gravimetric feeders are usually adaptable to operation from any standard instrument signals. 

When solution must be pumped, consideration should be given to use of holding tanks between the dry feed system and feed pumps, and the solution water supply should be controlled to prevent excessive dilution. The dry feeders may be started and stopped by tank level probes. Variable-control metering pumps can then transfer the alum stock solution to the point of application without further dilution. Means should be provided for calibration of the chemical feeders. Volumetric feeders may be mounted on platform scales. Belt feeders should include a sample chute and box to catch samples for checking actual delivery with set delivery. Gravimetric feeders are usually furnished with totalizers only. Remote instrumentation is frequently used with gravimetric equipment, but seldom used with volumetric equipment. 

Volumetric or gravimetric feeders may be used, but volumetric feeders are usually selected only for installations where comparatively low feed rates are required. Dilution does not appear to be important, therefore, control of the amount of water used in the feeding operation is not considered necessary. Inexpensive hydraulic jet agitation may be furnished in the wetting chamber of the feeder as an alternative to mechanical agitation. The jets should be sized for the available water supply pressure to obtain proper mixing. 

Fig. 33. Constructional features of loss in weight gravimetric feeder.

FEEDER (Gravimetric). Frequently in the form of belt scales, the function of a gravimetric feeder is lo continuously weigh material as... 

Solids flow measurement is more important in the control and optimization of coal-fired power plants than in alternative energy processes. The mass flow of solids can be detected by impact flowmeters, which are relatively low-accuracy devices (1-2% FS). Better accuracy and rangeability are provided by belt-type gravimetric feeders (0.5% AF over a 10 1 range), which measure both the speed and loading on the moving belt, as shown in Figure 3.89. 

Belt-type gravimetric feeder with digital controls. 

PAC can be fed in the dry state using volumetric or gravimetric feeders or can be fed in slurry form. There are at least three major PAC producers, over 50 manufacturers of volumetric and gravimetric feeders, and over 50 manufacturers of slurry feeders (24-26). There are also many manufacturers of sequencing batch reactors (SBR) (22), dissolved air flotation (DAF) clarifiers (30), and centrifuges. 

Extra equipment could include a volumetric or gravimetric feeder, an alarm system for detecting and reporting low solution levels, a softener for removing hardness constituents from the solution water, a small meter for measuring the amount of water used in solution preparation. As the size and complexity of the fluoridation system grows, the number and complexity of these pieces of auxihary equipment increases, and they become necessary components. 

Conveyors transport solid material streams. With appropriate geometry and speed control, they regulate the volumetric flow of the material and are called volumetric feeders. With the further mass sensing and control modules, they regulate the mass flow of material and are called gravimetric feeders. 

Among the various feeder types (e.g., vibratory, screw, belt), there are two broad classes of feeders volumetric and gravimetric. Volumetric feeders rely on known unit volume (as well as motor speed) to meter materials whereas gravimetric feeders rely on weighing and speed-sensing devices to control flow. If the process follows three simple rules, a volumetric feeder can be the most effective and economical choice. The rules are [2] ... 

The motor drive should have tachometer feedback provisions to eliminate velocity fluctuations from screws, rotors, tables, belts, and so forth. Table 52.4 summarizes the major features of both volumetric and gravimetric feeders. 

Robust can handle a wide range of feed rates, especially if used as a gravimetric feeder... 

At least three separate gravimetric feeders are required one for ground, crumb, or pellet rubber, one for materials in the pellet form, and one for the materials in the powder form. The feeders for pellets and powders may require different types of feeding screws. For feeding of additional solid materials at other barrel locations additional gravimetric feeders are required. 

Gravimetric feeder gra-v3- me-trik- (weigh feeder). A device for feeding extruders that continuously meters, by weighing, the rate at which feedstock enters the feed port of the extruder. 

Hopper ha-par (13c) n. In extrusion injection molding, the bin mounted over the feed opening that holds a supply of molding material. The hopper may be intermittently filled or continuously fed (See hopper loader). Feeding from the hopper is ordinarily by gravity, but it may be aided by vibrators, stirrers, or screw feeders. In some setups the hopper feeds into a metering device such as a gravimetric feeder or vibratory... 

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