Page 239 - Fluid mechanics, heat transfer, and mass transfer

TWO-PHASE FLOW 217

➢ Easy to retrofit. TABLE 7.4 Minimum Recommended Conveying Velocities
for Good Ventilation
➢ Low particle attrition (no chipping or surface
abrasion). Nature of Material Industrial Examples V fmin (m/s)
Disadvantages
Vapors, gases, smoke All vapors, gases, and smoke 5–10
➢ High pressure drop. The conveyed solids lose most Fumes Welding 10–12
of their momentum during the impact and must be Very light fine dust Cotton lint, wood flour 12–15
reaccelerated downstream of the bend. As a result, Dry dusts and Fine rubber dust, bakelite 15–20
pressure drop across a blind tee can be as much as powders molding powder dust,
three times that of a long radius bend. cottondust,lightshavings,
soap dust, leather shavings
➢ Not suitable for moist, cohesive, or sticky
Average Grinding dust, dry buffing 18–20
materials.
industrial dust lint, coffee beans, granite
➢ May result in cross-contamination if the pocket dust, silica flour, general
does not self-clean. materials handling, brick
& Blind Bend: cutting, clay dust, foundry,
Advantage limestone dust, asbestos
dust
➢ Better erosion resistance.
Heavy dusts Heavy/wet saw dust, metal 20–23
Disadvantages
turnings, foundry tum-
➢ Same as those for blind tee. bling barrels and shake-
➢ Secondary impact wear zone on the inner radius. out, sand blast dust, wood
blocks,brassturnings, cast
. “In pneumatic conveying of abrasive solids, pipewear is
iron boring dust, lead dust
a problem reducing its life, especially severe wear Heavy or moist Lead dust with small chips, >23
effects at points of change in direction.” How is this moist cement dust, asbes-
problem solved? toschunks from machines,
sticky buffing lint
& Use of elbows with a wear-resistant ceramic liner or a
replaceable impact plate insert. Examples include Source: ACGIH. Industrial Ventilation: A Manual of Recommended
high-density alumina ceramics, zirconium corun- Practice, 21st edition, American Conference of Governmental Industrial
dum, hardened cast iron, silicon carbide, and tung- Hygienists, Inc., Cincinnati, OH, 1992.
sten carbide.
& Use of a blind tee elbow.
& Whenever possible, piping should be designed to be
& Use of a spherical chamber elbow. self-draining.
& Use of proprietary elbow designs. & Manual draining should be installed to drain sections
. What are the minimum conveying velocities for various of the piping when self-draining is not possible.
types of materials for good ventilation? & Blowout or rod-out connections should be provided to
& Table 7.4 gives minimum conveying velocities for clear lines in places where plugging is likely to occur.
different types of materials for good ventilation & Access flanges should be provided at T-connections.
requirements. & Manifolds should have flanged rather than capped
connections to allow for easy access.
7.1.2.4 Solid–Liquid (Slurry) Flow & Clean-out connections should be provided on both
sides of main line valves so that flushing can take
place in either direction.
. Give important examples of slurries that are transported
in pipelines. & Break flanges should be provided every 6 m of hor-
izontal pipe or after every two changes in direction.
& A coal–oil slurry used as fuel and a lime slurry used
for acid waste neutralization are two examples of . What type of pump(s) is suitable for slurry handling?
slurries involved in process applications. & In short process lines, slurries are readily handled by
& Long-distance movement of coal, limestone, ores, centrifugal pumps with open impellers and large
and others. A few such installations have been made clearances.
with lengths ranging from several kilometers to & Positive displacement pumps, notably diaphragm
several hundred kilometers. pumps, are another type of pumps that are widely
. What are the general considerations involved in the used in slurry transport in long-distance lines, in
design of a slurry pipeline? pumping stages. Plunger or piston-type pumps are