Scraper blades

The scraper or knife discharge consists of a blade that removes the cake from the dmm by direct contact with the filter cake. It is normally used for granular materials with cake thickness greater than about 6 mm. In order not to damage the filter cloth, a safety distance of 1 to 3 mm between the blade and the cloth must be observed. If the residual layer is made not of filter aid but of the product, there is danger of its blocking by fine particles and by successive consoHdation by the scraper blade. 

In conventional disk filters, cake discharge is usually performed by a scraper blade, for cakes thicker than 10 mm, or sometimes by a tapered roU air blowback is often used to assist the discharge. High pressure sprays also have been used for cake discharge. 

FIG. 11-34 Scraper blade of scraped-siirface exchanger. (Henty Vogt Machine Co., Inc.)... 

Scraper-Discharge Filter The filter medium is usually caulked into grooves in the drum grid, with cake removal facihtated by a scraper blade just prior to the resubmergence of the drum (Fig. 18-122). The scraper selves mainly as a deflector to direcl the cake, dislodged by an... 

Circular basin clarifiers are most commonly fed through a centrally located feed well. The overflow is led into a trough around the periphery of the basin. The bottom gendy slopes to the center and the setded solids are pushed down the slope by a number of motor-driven scraper blades that revolve slowly around a vertical center shaft. This design closely resembles a conventional thickener. Like thickeners, circular clarifiers can be stacked in multitray arrangements to save space. Some juice clarifiers are also arranged in this way. 

Cleaning by hand tools is the oldest process. Today this is employed only when power tools are not available and where the surface is inaccessible to power tools and too small. The generally used hand tools are wire brush, scraper blades and chipping hammer and emery or sand paper. Hand held electric or pneumatic power tools for cleaning include sanders, pneumatic scalers, needle gun and rotary steel brushes. 

A second method involves the installation of cooling cylinders and wall scrapers (Figure 9). On large reactors this is complex and expensive. Great problems will ensue if the scraped-off material remains stuck to the scraper blades. Cleaning and dismanteling are extremely difficult. 

Figure 9. Reactor with cooling cylinder and scraper blades...

Scraper blades set to rotate at 35 rpm are used for a pilot plant addition of liquid ingredients into a body-wash product. What should the speed of the blades be in a full-scale plant, if the pilot and the full-scale plants are geometrically similar in design Assume scale-up is based on constant tip speed, diameter of the pilot plant scraper blades is 0.6 m, and diameter of the full-scale plant scraper blades is 8 ft. 

N2 = scraper speed of full-scale plant DA1 = diameter of pilot plant scraper blades DA2 = diameter of full-scale plant scraper blades... 

Intimate mixing is accomplished in change-can mixers in two ways. One method is to have the mixing-unit assembly revolve in a planetary motion so that the rotating blades sweep the entire circumference of the can (Fig. 18-39). The other is to mount the can on a rotating turntable so that all parts of the can wall pass fixed scraper blades or the agitator blades at a point of minimum clearance. 

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