Solids elbow

Nodule Papilla Psoriasis Small node that is solid to the touch Small, nipple-shaped elevation A skin disease characterized by the formation of scaly red patches, particularly on the extensor surfaces of the body (elbows, knees)... 

A slurry pump operating at 1 atm must be selected to transport a coal slurry from an open storage tank to a rotary drum filter, at a rate of 250 gpm. The slurry is 40% solids by volume and has an SG of 1.2. The level in the filter is 10 ft above that in the tank, and the line contains 400 ft of 3 in. sch 40 pipe, two gate valves, and six 90° elbows. A lab test shows that the slurry can be described as a Bingham plastic with v = 50 cP and t0 = 80dyn/cm2. 

For dust extraction systems, the concentration of solids usually is quite low. For this reason, the methods employed to calculate pressure loss are based on air-only conditions. Comprehensive information is available (ASHRAE, 1985 ACGIH, 1992) to assist the designer in estimating the pressure loss caused by pipe branches, ducts, elbows, etc. 

Frictional losses are evaluated separately for the air and the solid. To each of these, contributions are made by the line itself, the elbows and other fittings, and the receiving equipment. It is conservative to assume that the linear velocities of the air and solid are the same. Since the air flow normally is at a high Reynolds number, the friction factor may be taken constant at f, = 0.015. Accordingly the frictional power loss of the air is given by... 

A pneumatic transfer line has 300 ft of straight pipe, two long radius elbows, and a lift of 50 ft. A two-stage cyclone is at the receiving end. Solid with a density of 125 lb/cuft is at the rate of 10 tons/hr and the free air is at 5000ft/min. Inlet condition is 27psia and 100°F. Investigate the relation btween line diameter and power requirement. 

Assume air and solid velocities equal. Elbow radius = 12D. 

The mechanical erosion of a solid surface such as a pipe wall in a gas—solid flow is characterized by the loss of solid material from the solid surface due to particle impacts. The collisions of the particles either with other particles or with a solid wall may lead to particle breakup, known as particle attrition. Pipe erosion and particle attrition are major concerns in the design of a gas-solid system and during the operation of such a system. The wear of turbine blades or pipe elbows due to the directional impact of dust or granular materials, the wear of mechanical sieves by the random impact of solids, and the wear of immersed pipes in a fluidized bed by both directional and random impacts are examples of the erosion phenomenon in industrial systems. The surface wear associated with the erosion phenomenon of a gas-solid flow has been exploited to provide beneficial industrial applications such as abrasive guns, as well. 

The solids flow rate can be controlled by nonmechanical valves such as the L-valve, as noted in Chapter 8. The L-valve has a long horizontal leg. Thus, it is convenient to characterize the pressure drop across an L-valve by two terms. One term is the pressure drop through the elbow (A/ V). This term can be described by the equations developed for the mechanical valve because the solids flow patterns between the two are similar... 

The work being done on the solid at the rate of Ib/sec is made up of the kinetic gain at the entrance ((Vj), the lift ( 3) through an elevation Az, friction in the fine w ), and friction in the elbow (ws). Accordingly,... 

This type of flow affects sampling in two ways First, because of the helical motion, it is very difficult to align the probe with the fluid velocity vector. Consequently, and because of the inertial effect, sample concentration will be always less than the upstream concentration (49), Second, the inertial effects on the elbow plane and the centrifugal force on a plane perpendicular to that of the elbow will produce a nonuniform solids distribution downstream of the elbow. 

A few studies considered the solids distribution downstream of elbows. Ayukawa (50) and Toda et al. (5i) observed an accumulation of coarse particles at the outer wall of vertical bends. Toda et al. (52) noted some changes in the solids distribution downstream of 90 bends. However, no concentration measurements were taken. 

Figure 18 shows the solids concentration profile 22 pipe diameters downstream of a short-radius elbow. The concentration profile is symmetrical, and a minimum solids concentration appears at the center of the pipe. Also, the solids concentration gradually increases toward the pipe wall. This variation in concentration across the pipe is evidently a consequence of the centrifuging action of the secondary flow that is generated by the bend upstream. Figure 18 also shows that the concentration profiles are concentration dependent, and as the solids concentration is increased, the profiles become flatter. Other results (55) showed that these profiles are also functions of the particle size and the radius of curvature of the elbow. 

An attempt was made to measure the particle local velocity at 1.5 pipe diameters downstream of the elbow by using a particle velocity probe (56). However, the technique failed, presumably because the strong secondary fiow prevented the velocity probe from being aligned with the velocity vector. For this reason, velocities obtained at 22 pipe diameters downstream of the elbow had to be used to estimate the concentrations at this level (1.5 pipe diameters). Figure 20 shows the estimated solids concentration normalized by the discharge concentration CJC ) for fine and medium sand particles 1.5 pipe diameters downstream of the elbow. Most of the relative concentrations are lower than unity, and consequently the mean concentration based on these measurements would be lower than the true value. Similar findings were obtained by Sansone (57) in gas-solid systems downstream of a 90 elbow. This phenomenon occurs because the velocity vector and the probe axis are not colinear, so that the concentration results are only of qualitative value. 

Figure 20. Effect of particle size on solids concentration profile downstream of a short-radius elbow. (Reproduced with permission from reference 55. Copyright 1987.)...

To establish a uniform concentration profile downstream of a 90 elbow, straightening vanes 10 cm long were inserted just downstream of the short-radius elbow. Figure 21 shows the effect of these vanes on the concentration profile. Although the concentration becomes flatter, a distinct minimum at the center of the pipe still exists. These results imply that the solids are already distributed at the exit of the elbow, and the vanes merely increase the rate of diffusion of the particles. 

Flow properties of the dry solids are important in developing a milling operation for scale-up. Flowability of the unmilled solids primarily affects delivery of solids to the mill, but if the milled or partially milled solids are very cohesive or prone to accumulating electrostatic charge, overall process productivity and yield may be reduced. Milled material that tends to build up in low flow areas of the milling system (cyclones, pipe elbows, and bends) can be removed periodically but at the cost... 

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