Conveyor pitch

Generally manufacturers will have a standard pitch for each bowl diameter (about 5° pitch angle) plus at least one other pitch generally smaller, a little more than half the standard. Very approximately these would be 30% and 20% of the inside bowl diameter, respectively. Some manufacturers have experimented with wider pitches. 

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Increasing (he conveyor speed will force die solids out of the machine more quickly, thus leaving behind some of die wetter solids and increasing the solids concentration in the cake while decreasing solids recovery. Similarly, (he conveyor pitch will influence solids recovery and cake dryness. If die scroll pitch is increased, the solids will be moved out faster but only (he larger, heavier solids will he pushed out leaving the wetter solids in (he centrate. Increasing (he number of leads in (he conveyor will likewise increase cake dryness at (he expense of solids recovery. 

Conveyor Pitch. The pitch that is required cannot be easily determined because it is dependent on a combination of many factors such as ... 

The rake or cant of the flight is varied for some applications, as is the conveyor pitch or pitch angle (see Sections 2.4.8 and 2.4.10). 

Many manufacturers have experimented with variable conveyor pitches. These have been used to enhance dryness in effluent dewatering. The pitch starts wide at the centrate discharge end. narrowing progressively along the... 

When operating properly in DS mode, a decanter will be virtually full of cake, from end to end, and from bowl wall to pond surface. The conveyor will be pushing the cake towards the discharge ports against a restriction, a baffle or a narrowing of the conveyor pitch, or against the nip between conveyor hub and the beach. 

If the pond level is not set sufficiently deep then the scrolling efficiency is impaired, as is demonstrated by the graphs in Figures 6.26 and 6.27. Once the pond is sufficiently deep, no extra scrolling capacity is obtained, as seen in Figure 6.27. Scrolling capacity is also dependent, as would be expected, on conveyor pitch. This is seen in Figure 6.28, where capacity is seen to increase a little more than the ratio of the pitches. 

Scaling up between two decanter sizes is generally best done when there is geometrical similarity between them. This means the same beach angle, the same conveyor pitch angle, and the same conveyor and feed zone designs. When there are differences then the scale-up may not be reliable. 

Differential is the next parameter value to be calculated. In the absence of any other information, initially at least, the differential for the larger decanter would be fixed to have the same cake thickness, in the bowl, as the test machine. Cake thickness will be proportional to feed rate, and inversely proportional to conveyor pitch and differential. Thus, for the larger machine, conveyor differential ... 

There are three basic ways to achieve mechanical expression, ie, with an indatable diaphragm, with a compression belt, or in a screw conveyor of reducing pitch or diameter. 

Screw presses consist of a single or double screw conveyor that has perforated walls. The soHds being conveyed are squeezed due to a gradual reduction in the pitch or diameter of the screw. Screw presses are used for dewatering of rough organic materials. 

Other designs squee2e the cake between two permeable belts or between a screw conveyor of diminishing diameter, or pitch, and its permeable enclosure. The available filters which use mechanical compression can be classified into four principal categories, ie, membrane plate presses, tube presses, belt presses, and screw presses. 

Several plants employ cooled-belt flakers. These consist of flexible steel belts, ca 1-m wide and up to 50-m long, that have short mbber skirting at the edges. Molten pitch flows from a thermostatically controlled tank over a weir to give a flat thin sheet on the belt, which is cooled from below by water sprays. At the end of the belt, the solid pitch is broken up by rotating tines. The pitch flakes are drained and transported to a covered storage silo by belt conveyor, during which time the surface moisture evaporates. 

Screw Conveyor Ca.pa.city, The volumetric capacity of a horizoatal screw coaveyor is calculated oa the assumptioa that all material coataiaed within oae screw pitch moves oae pitch distance ia oae screw revolutioa. Volumetric coaveyiag capacity is calculated as... 

Because of the steep pitch required, slides are limited in application. They are most commonly used to bridge the gap between roller-conveyor systems on two floors, because the roller conveyor can take the container off the slide rapidly and ehminate or reduce the chance for collisions. Slides may also be used when containers can be chuted from an upper floor to a manually loaded carrier. The use of several rollers at the feed point is recommended for easy dehveiy to the sloping section. If the drop is short and containers light, a roller cleanout will prevent backup of containers on the slide. The slope of gravity slides is a function of container weight, size, and friction characteristics and should be selected with care to be sure that containers do not move either too swiftly or not at all. Slides usually use flat steel sheet. 

In the string discharge system, the conveyor consists of a number of endless strings which are spaced at a pitch of approximately 12 mm over the width of the filter drum. The string spacing may, however, be in the range 6-25 mm depending on the mechanical properties of the solids. 

Figure 5.7. A screw conveyor assembly and some of the many kinds of screws in use. (a) Screw conveyor assembly with feed hopper and discharge chute, (b) Standard shape with pitch equal to the diameter, the paddles retard the forward movement and promote mixing, (c) Short pitch suited to transfer of material up inclines of as much as 20°. (d) Cut flight screws combine a moderate mixing action with forward movement, used for light, fine, granular or flaky materials, (e) Ribbon flights are suited to sticky, gummy or viscous substances.

Various types of screws are used for modern conveyors. These include short-pitch, variable-pitch, cut flights, ribbon, and paddle screws. The Procedure given above also applies to these screws. 

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