Page 240 - Fluid mechanics, heat transfer, and mass transfer

218 TWO-PHASE FLOW SYSTEMS

also common for use in conventional slurry systems & Fine particles, 10–100 mm, are usually fully sus-
because of their high-pressure capability. Positive pended, but gravity causes concentration gradients.
displacement pumps are limited by the maximum & Medium size particles, 100–1000 mm, may be fully
particle size, approximately 2.4 mm, which will pass suspended at high velocity, but often form a moving
through the valves. deposit at the bottom of the pipe.
& Line sizes of 45 cm, handling 50–60 wt% coal par- & Coarse particles, 1000–10,000 m, are rarely fully
ticles with particle sizes up to 14 mesh, using veloc- suspended and are usually conveyed as a moving
ities of about 1.5 m/s, have been used successfully in deposit.
the past. & Ultracoarse particles larger than 10,000 m are not
. Give reasons why particle settling velocities in slurries suspended at normal velocities unless they are un-
are lower than free settling velocities of single particles, usually light.
other factors remaining the same for both cases. & Turbulence in the line helps to keep particles in
& Hydrodynamic interaction between particles and
suspension. It is essential, however, to avoid dead
upward motion of displaced liquid. spaces in which solids could accumulate and also to
& Increased viscosity of suspension over the liquid make provisions for periodic cleaning of the line.
without particles. . What is deposition velocity?
. What are the two categories of slurries handled in slurry & The velocity below which particles tend to settle out
flow? What are the issues involved in piping design for and form a deposit in the pipe is called the deposition
slurry transport for these two categories? velocity. The pipe diameter should be selected such
& Nonsettling slurries are made up of very fine, highly that the velocity in the pipeline is maintained above
concentrated, or neutrally buoyant particles. These the deposition velocity over the operating range of
slurries are normally treated as pseudohomogeneous flow rates.
fluids. They may be quite viscous and are frequently . What are the flow conditions in slurry flow in horizontal
non-Newtonian. pipes?
& Slurries of particles that tend to settle out rapidly are & Flow conditions can be classified into four categories:
called settling slurries or fast settling slurries. ➢ Homogeneous Flow: Homogeneous flow implies
& For fast settling slurries, ensuring conveyance is that the solid particles are uniformly distributed
usually the key design issue while pressure drop is across the pipeline cross section. Examples in-
somewhat less important. clude sewage sludge, drilling muds, paper pulp,
& For nonsettling slurries conveyance is not an and many finely ground materials.
issue, because the particles do not separate from ➢ Heterogeneous Flow: Slurries at low concentra-
the liquid. Here, viscous and rheological behaviors, tion with rapidly settling (coarse particles) solids
which control pressure drop, take on critical generally exhibit heterogeneous flow. Typical ex-
importance. amples are sand and gravel slurries and coarse coal
& Fine particles, often at high concentration, form slurries.
nonsettling slurries for which useful design equa- ➢ Intermediate Regime: This type of flow occurs
tions can be developed by treating them as homo- when some of the particles are homogeneously
geneous fluids. These fluids are usually very viscous distributed while others are heterogeneously dis-
and often non-Newtonian. Shear thinning and Bing- tributed. Examples are tailings slurry from mineral
ham plastic behaviors are common. Dilatancy is processing plants and transportation of coal–water
sometimes observed. Rheology of such fluids must slurries.
in general be empirically determined, although ➢ Saltation Regime: The fluid turbulence may not be
theoretical results are available for some very lim- sufficient to keep fast settling particles in suspen-
ited circumstances. Sewage flow is an example of sion. The particles travel by discontinuous jumps
such slurries. or roll along a sliding or stationary bed at the
. Describe settling characteristics of particulates in bottom of the pipe. This type of flow will occur
slurry flow in horizontal piping in relation to particle with coarse sand and gravel slurries.
size ranges. ➢ The regimes are illustrated in Figures 7.17
& Ultrafine particles,10 mm or smaller, are generally and 7.18.
fully suspended and the particle distributions are not . What are the common problems in slurry handling
influenced by gravity. systems?