Conveyor performance

Table 21-5 indicates screw-conveyor performance on the basis of material classifications as listed in Table 21-4 and defined in Table 21-3. Table 21-6 gives a wide range of capacities and power requirements for various sizes of screws handling 801 kg/m (50 lb/ fU) of material of average conveyabihty. Within reasonable limits, values from Tables 21-5 and 21-6 can be interpolated for preliminary estimates and designs. 

Mainwaring, N. J. and Reed, A. R. 1987. Permeability and air retention characteristics of bulk solid materials in relation to modes of dense-phase pneumatic conveyor performance. Bulk Solids Handling 7 415-425. 

Other standards cover standard terms and definitions (CEMA 102) and dimensional standards (e.g., CEMA 300 for screw conveyors). Other publications describe installation standards and conveyor performance guidelines. 

Pneumatic conveyor performance is determined by the following factors (1) primary-driver output (2) internal surface of the piping or ductwork and (3) condition of the transported material. Specific factors affecting performance include motive power, friction loss, and flow restrictions. 

Screw-conveyor performance is also determined by the volumetric efficiency of the system. This efficiency is determined by the amount of slip or bypass generated by the conveyor. The amount of slip in a screw... 

Improper clearance is the source of many volumetric-efficiency problems. It is important to maintain proper clearance between the outer ring, or diameter, of the screw and the conveyor s barrel, or housing, throughout the operating life of the conveyor. Periodic adjustments to compensate for wear, variations in product, and changes in temperature are essential. While the recommended clearance varies with specific conveyor design and the product to be conveyed, excessive clearance severely impacts conveyor performance as well. 

For property changes directly related to product temperature, preheaters or coolers can be used in the incoming-feed hopper, and heating/cooling traces can be used on the conveyor s barrel. These systems provide a means of achieving optimum conveyor performance despite variations in incoming product. 

This chapter describes the more common basic tj s of chain conveyors to help the reader understand how each type of chain conveyor functions and to recognize the chain attachments required to support, move, and discharge the material from the conveyor. The chain conveyors discussed are arranged according to the type of service the conveyor performs and the types of attachments added to engineering steel chains that help them perform their required function. 

Process industries frequently need to weigh and control the flow rate of bulk material for optimum performance of such devices as grinders or pulverizers, or for controlling additives, eg, to water suppHes. A scale can be installed in a belt conveyor, or a short belt feeder can be mounted on a platform scale. Either can be equipped with controls to maintain the feed rate within limits by controlling the operation of the device feeding the material to the conveyor. Direct mass measurement with a nuclear scale can also be used to measure and control such a continuous stream of material. 

These coaveyors are made up of standard mechanical components that can be configured ia differeat ways to perform a particular task. The types most used ia the chemical iadustry iaclude aproa, flight, and drag chain conveyors. 

Conveyor-Belt Devices The metal-belt type (Fig. 11. 55) is the only device in this classification of material-haudhug equipment that has had serious effort expended on it to adapt it to indirecl heat-transfer seiwice with divided solids. It features a lightweight construction of a large area with a thin metal wall. ludirect-coohiig applications have been made with poor thermal performance, as could be expected with a static layer. Auxihaiy plowlike mixing devices, which are considered an absolute necessity to secure any worthwhile results for this seiwice, restrict applications. 

Heat-transfer aspects and performance were studied and reported on by Depew and Farbar (ASME Pap. 62-HT-14, September 1962). Heat-transfer coefficient characteristics are similar to those shown in Sec. 11 for the indirectly heated fluid bed. Another frequent application on plastics is a sm, rather incidental but necessary amount of drying required for plastic pellets and powders on receipt when shipped in bulk to the users. Pneumatic conveyors modifiea for heat transfer can handle this readily. 

The alternative method of sample extrac tion is termed the ctoss-sti eam samphng method, or ci o.s.s-belt when used in conjunc tiou with a belt conveyor. Sample extraction typically take place with a belt conveyor in motion. However, with a rotary table-feeder conveyor, extractions are made with the table stopped. A cutter can perform extractions by this means from a machined flat surface with neghgible... 

Cross-stream samphng from flat surfaces with material handled on a hnear conveyor or rotary table is best carried out with the conveyor stopped. Sample extraction is then performed by linear traverse. 

Extractions performed with the conveyor stopped allow more assured accuracy by the certainty of including fines in the sample increment. Sampler design to extract increments from a flat belt or rotaiw table sampler while the conveyor is stopped minimizes potential for residual fine particles remaining on the conveyor surface in carrying out extractions. See Fig. 19-6 for rotary table sampler extraction diagram. 

Capacity Definitions In any analysis, the capacity per unit time of dynamic equipment (such as conveyors and bagging machines), as well as the rates at which they ac tuaUy perform, must be defined more precisely and realistically than by a mere statement of kilograms or pounds per hour. Some useful definitions employed by the equipment industi y are the following ... 

Selec tion of the correct conveyor for a specific bulk material in a specific situation is complicated by the large number of interrelated factors that must be considered. First, the alternatives among basic types must be weighed, and then the correc t model and size must be chosen. Workabihty is the first criterion, but the degree of performance perfection that can be afforded must be estabhshed. 

Because standardized equipment designs and complete engineering data are available for many common types of conveyors, their performance can be accurately predicted when they are used with materials having well-known conveying charac teristics. However, even the best conveyors can perform disappointingly if material characteristics are unfavorable. It is often true that conveyor engineering is more of an art than a science problems involving unusual materials or equipment should be approached with caution. 

Many preengineered conveyor components can be purchased off the shelf they are economical and easy to assemble, and they perform well on conventional applications (for which they are designed). However, it is advisable to check with the manufac turer to be sure that the apphcation is proper. 

Continuous production ol charcoal is typically performed in multiple hearth furnaces, as illustrated in the Herreshoff patent shown in Figure 2. Raw material is carried by a screw conveyor to the uppermost of a series of hearths, /kir is supplied counter-currently and burns some of the wood to supply process heat. As the layers of wood carbonize, they are transported to the lower (hotter) hearths by rakes. The hot charcoal product is discharged onto a conveyor belt and cooled with a water spray. 

Most baghouse systems are provided as complete assemblies by the vendor. While the unit may require some field assembly, the vendor generally provides the structural supports, which in most cases are adequate. The only controllable installation factors that may affect performance are the foundation and connections to pneumatic conveyors and other supply systems. 

The key performance properties of conveyor belts, particularly belt cover compounds, are flex resistance, abrasion resistance, and low heat buildup. The effect of l,3-bis(citraconimidomethyl) benzene on these key properties, in addition to cure characteristics and tensile properties, has been evaluated in a typical, NR-based belt cover formulation listed in Table 14.49. 

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