Modes of conveying

1 Dilute phase If the material is conveyed in suspension with the carrier gas through the pipeline it is referred to as dilute phase conveying. Provided that a material can be fed reliably into a pipeline almost any material can be conveyed in dilute phase, regardless of the particle size, shape or density. 

With moving bed flows, solids loading ratios of well over 100 can be achieved if materials are conveyed with pressure gradients of about 20 mbar/m of horizontal pipeline. For plug-type flows the use of solids loading ratio is not as appropriate, for as the materials have to be very permeable, maximum values are only of the order of about 30. Despite the low value of solids loading ratio, materials can be reliably conveyed at velocities of 3 m/s and below in plug-type flow. 

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Application to Long-Distance Pneumatic Conveying. From the above three classifications, there is sufficient evidence to suggest that powder classification (i.e., to select ultimately the most suitable mode of conveying for a given product and its behavioral properties) depends on the following properties ... 

The selection criteria for these various modes of conveying are completely different. The capacity, power, and interfacing needs occupy distinctively separate considerations. For example, the nature of the material handled and the need to minimize wear may compromise the speed of operation of conveyors. The size of an elevator may be determined more by the casing span than the handling capacity, and the design of a feeder is affected by both the arching potential of a powder and the extraction pattern that it is necessary to generate. 

Where the qualified volunteer negligently operates a motor vehicle, eiircraft, boat or other powered mode of conveyance. If the actionable conduct of the quedified volunteer is covered by a policy of liability insurance, whose liability for ordinary negligence shall be limited to the amount of the coverage provided... 

The value required depends very much upon the materials being conveyed and the mode of conveying. Typical values for Cmin were presented in Figure 4.41 with a recommendation that Cl should be about 20% greater than this (Equation (4.12)). For fine powders capable of being conveyed in dense phase the value of the solids loading ratio also needs to be... 

Vendors have used certain types of dense phase conveying operations. In the trade, these vendors and their unique modes of transport have been classified in Fig. 3. 

Figure 5 is a plot of permeability factor Kp versus (Ap/Ah)mf and contains the boundary Vmf = 50 mm S 1. Figure 6 is a plot of rcd/ps versus (Ap/Ah)mf-md contains the boundary X= 0.001 nf s kg-1. By using both Figs. 5 and 6, it is possible to propose the following three criteria relating to the potential modes of pneumatic conveying in conventional pipelines. 

However, this mode of flow usually is limited to conveying distances of L < 500 m. The Group D materials that possess wide size distributions (especially a considerable amount of fines) and/or unusual particle shape (e.g., crushed coal, petroleum coke, crushed bath, rice hulls) usually are not suited to low-velocity conveying. In these cases, the options are dilute-phase or single-slug dense-phase (i.e., these materials usually are too coarse for bypass conveying). However, when long distances are involved, dilute-phase may be the only practical option. 

For the general purpose of minimizing air flow, transport velocity, wear and power, the fluidized dense-phase mode of flow is preferred for long-distance applications. Efficient blow tank feeders, rotary-screw compressors, refrigerated dryers and stepped-diameter pipelines also are recommended. For products that are not suited to fluidized dense-phase, the possible modes of flow include dilute-phase (suspension flow) or bypass conveying (Wypych, 1995a). 

Special care should be taken when selecting the mode of solids-gas flow. For example, flow separation and roping could occur even in very dilute-phase conveying systems (e.g., m < 1 for coal-fired boilers). Fluidized dense-phase also is possible for some systems and can offer many... 

Mainwaring, N. J., and Reed, A. R., Permeability and Air Retention Characteristics of Bulk Solid Materials in relation to Modes of Dense-Phase Pneumatic Conveying, Bulk Solids Handling, 7(3) 415-425 (1987)... 

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