Page 237 - Fluid mechanics, heat transfer, and mass transfer

TWO-PHASE FLOW 215

➢ Capacity can then be increased by the same & Another way is to use abrasion-resistant materials for
amount as the reduction in line restriction. the line.
➢ Line length and capacity are inversely ➢ The higher the material is on the Mohs scale, the
proportional. better resistive it is for abrasion with lower Mohs
➢ Reducing line length by 10% will increase capac- materials.
ity by 10%. & The fourth method is to use bends that have a
& If the system is a dilute phase and the conveying different configuration than radius bends.
velocity is higher than that required, reducing air ➢ Any bend that forms a pocket to allow material
volume will reduce required pressure making in- accumulate in it will have a greatly increased life
crease in capacity possible. as the conveyed material now impacts itself in-
➢ There are systems in which slowing down the stead of the pipe wall.
blower reduces pressure and increases conveying Venting of Feeder/Air Locks
capacity. & Improper or lack of venting is the primary cause for
& Increasing air flow by speeding up the blower: it is most feeding problems of conveying systems.
observed that the capacity, instead of increasing, ➢ If material is to flow into a vessel, chamber, feeder,
reduced. conveying line, and so on, the air that is filling the
➢ A given material requires a minimum air flow to be volume must be vented.
conveyed in dilute phase, but increasing the ve- ➢ When feeding into a pressure conveying system,
locity above that point only raises the required some air tries to escape from the pressure line, both
pressure and has little effect on capacity. through clearances in the feeder and through the
Line Wear and Maintenance displacement volume of the rotor. If the feeder is
under a silo or storage bin, the air must be vented
& Line wear is caused by the impact of an abrasive
out for smooth flow.
material on the pipe and is most noticeable at changes
in direction. ➢ With coarse materials that are not fluidizable,
➢ Abrasive wear depends on particle characteristics venting is simple.
such as hardness, particle size and shape, and ➢ A channelinstalled inside thebin allows the leakage
particle density. air to pass upward a small distance, where an
➢ Hardness is measured on the Mohs scale with opening allows it to escape through a filter bag or
diamond having a value of 10 and talc having a continue upward to the top of the bin.
value of 1. & If the above method is used for fine fluidizable
- For example, a No. 6 material will scratch a No. material, leakage air fluidizes the material and then
5 material, but a No. 1 material will have little flows like water, seeking its own level, up into the
effect on a No. 6 material. venting area, and quickly plugs the vent line.
& Particle size affects the rate of wear, since wear is a & The approaches that are used fairly successfully are
function of the energy of impact between the material venting the feeder body, separating the feeder from
and the pipe wall. the air lock, and venting the air by venturi.
➢ Larger particles produce greater attrition on the ➢ Venting of the feeder body is done through ports
pipe wall than smaller particles. For example, either on the side of the feeder or on the end plates.
pneumatic conveying of broken glass pieces will There are problems with this arrangement:
have a much greater impact on the line than finely - There must be an adequate number of blades on
pulverized glass. the rotor to assure separation from the feed bin to
the vent and from the vent to the conveying line,
& Shape of the particle influences the abrasive wear.
regardless of the position of the rotor.
➢ If the particle is spherical, it will impact the wall
- This is often not considered and, after a short
surface over a larger area than if it is jagged and
period, vent line plugs and feed problems resume.
contacts it at a small point.
- If the material does not drop out of the rotor at
& Velocity of conveying determines the energy of
the bottom, maybe due to too high speed of
impact that influences wear rate.
2 operation, the material is carried along with the
➢ Impact is a function of (velocity) .
rotor past the dropout point and is then carried
➢ To improve life of the line, velocity must be kept to
into the vent line. This also plugs the vent over
the minimum.
time. Injecting air into the rotor in the discharge