Belt weigher

Except for continuous weighing, control of the flow of solids is less precise than that of fluids. Several devices used for control of feed rates are shown schematically in Figure 3.7. They all employ variable speed drives and are individually calibrated to relate speed and flow rate. Ordinarily these devices are in effect manually set, but if the solid material is being fed to a reactor, some property of the mixture could be used for feed back control. The continuous belt weigher is capable ordinarily of 1% accuracy and even 0.1% when necessary. For processes such as neutralizations with lime, addition of the solid to process in slurry form is acceptable. The slurry is prepared as a batch of definite concentration and charged with a pump under flow control, often with a diaphragm pump whose stroke can be put under feedback control. For some applications it is adequate or necessary to feed weighed amounts of solids to a process on a timed basis. 

For many applications, volumetric measurement of feedrate is accurate enough and various proprietary feeders are available. Alternatively, some of the types of transfer equipment described in section 17.3.6.2 may give the required accuracy. Proprietary belt weighers are also available. For the highest degree of accuracy, however, batch weighing is recommended. 

Bin apron feeder for controlled conveyor loading linked to belt weigher for accurate control... 

Sand and soda are drawn from hoppers, dosed via belt weighers and homogenized in screw conveyer mixers (see Figure 22.2). A normal shrinkage in the weight of the charge (ca. 10%) is due... 

It is advisable to perform the sampling as a weight-dependent rather than as a time-dependent operation. In the former case the sampler is controlled direct by a belt weigher. The cumulative sample is homogenized in a special mixer after a certain quantity (tonnage) of material has been collected. With both methods suitable belt conveyors and automatic counting equipment are required. The samplers can be adjusted to take any desired quantity. 

Fig. 40 Combination of a belt weigher with a weigh belt feeder...

A belt weigher incorporated in the belt conveyer bringing the raw material from the quarry records the quantity handled. Under adjustable electronic control, the contents of the mixer are intensively mixed after every 5001 of material passing the weigher. On completion of the mixing operation a quantity of about 200 g is removed from the mixer by a pneumatically powered extraction device and is fed to an automatically functioning pneumatic despatch station which sends the samples to the laboratory. In the laboratory each sample is further dried and prepared for analysis by pulverization in a vibratory mill. 

The feeder consists basically of a belt weigher with electric speed control. The value of such mach ines for producing mixtures correctly proportioned by weight in various industrial processes is obvious (Fig. 40). 

The performance of belt weighers is such that they may be used in applications which are legal for trade . For systems approved in commercial applications the maximum permissible errors in ODvIL class I are at initial verification = 0.5% of the totalised load for any flow between 20 - 100 % of maximum and in service the figure is permitted to rise to 1%. The maximum permissible errors for a class 0.5 system are half those of class 1, whereas those for a class 2 system are double those for class 1 [6]. A summary of the approximate typical performance of belt weighers is presented in Table 2 [7],... 

It is of interest to note that the sole method of verifying legal for trade belt weighers recognised by the OIML [6] is to compare the totalised mass flowrate signal with an independent (and traceable) weight evaluation of the collected product. As part of the procedures, the minimum amount of material to be collected is specified. This method does lend itself to the evaluation of the meter at different flowrates. 

OIML International Recommendation for Continuous totalizing automatic weighing instruments (belt weighers). Organisation Internationale de Metrologie Legale, R50, 1994. 

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