Page 234 - Fluid mechanics, heat transfer, and mass transfer

212 TWO-PHASE FLOW SYSTEMS

. For upflow of gas–solid systems in vertical pipes, what ➢ Positive system is most widely used.
is the recommended minimum conveying velocity for & Pressure Negative (Push–Pull) Combination Sys-
the solids? tem: Normally used when conveying from several
& The minimum recommended conveying velocity for pickup points to several discharge points.
low loadings may be estimated as twice the terminal ➢ The pressure negative system is ideal for unload-
settling velocity of the largest particles. ing railcars and also for conveying light dusty
& Choking occurs as the velocity drops below the materials because the suction at the inlet helps
minimum conveying velocity and the solids are no the product enter the conveying line.
longer transported, collapsing into solid plugs. ➢ Positive systems are poorer than negative systems
. How does conveying distance affect capacity and con- for handling such materials because feeding into
veying pressure? the pressure line can be difficult and can present a
& Conveying capacity is inversely proportional to con- dust collection problem at the pickup location
veying distance. because of blowback or leakage air through the
rotary valve.
& Conveying pressure is directly proportional to con-
veying distance. ➢ Also the negative system works better if there are
lumps at the pickup end.
. What are the general types of pneumatic conveying
systems? State briefly their uses. ➢ The positive pressure system tends to pack at the
lump, while the negative system will often keep
& Negative (Vacuum) System: Normally used when
solids moving around the lump and gradually wear
conveying from several pickup points to one dis-
it away.
charge point.
➢ The negative system usually sucks on a cyclone ➢ For the pressure negative system, a single blower
and/or filter/receiver mounted above the receiving can be used for both the negative and positive sides
storage hopper or bin. Solids are usually sent down of the system.
to the hopper with a rotary air lock feeder. Air is ➢ Lack of flexibility with a single blower usually
sucked into the transfer pipe or duct at the pickup dictates the need for separate blowers for the
end of the system. negative and positive sides.
➢ Avariety of feeders can be used to introduce solids & Venturi, product systems, or blow tanks.
into the flowing air stream such as rotary air lock . When is a negative (vacuum) system recommended for
feeders, pan-type manifolds under railcar hoppers,
conveying solids? Illustrate.
paddle-type railcar unloaders, screw conveyors,
& When conveying solids from several pickup points as
and so on.
illustrated in Figure 7.14.
& Positive Pressure System: Normally used when con-
& Vacuum systems may require feeders with a good
veying from one pickup point to several discharge
seal to minimize leakage of air/gas into the pipeline.
points.
& An acceleration zone is required after the feed point
➢ In the positive pressure system, air is blown into
to obtain steady velocity for transport.
the pickup duct often up to a cyclone with atmo-
. When is a positive system (pressure) recommended for
spheric vent.
conveying solids?
➢ Usually solids are introduced to the conveying air
& When conveying solids from one pickup point to
stream with a rotary air lock feeder.
several discharge points (Figure 7.15).
➢ For sending the solids down to the storage bin from
& Positive pressure systems require devices that can
the cyclone, a simple spout connection can be used
feed material from atmospheric pressure into a pres-
instead of the rotary air lock feeder required in the
surized pipeline.
negative pressure system. The positive system
does not need a vacuum vessel at each receiving & Positive systems are most common. Diverter valves
location. deliver to several receiving vessels.
➢ Also in the positive pressure system conveying to a . What is a closed loop system? Where is it used?
number of hoppers, a simple bag-type cloth can & Closed systems are used whenever hazardous solids
serve as the filter. are pneumatically conveyed. Reason is environmen-
➢ So for conveying from one pickup location to tal regulations and safety prohibiting atmospheric
several receiving locations, the pressure system discharge of entrained solids. Examples include toxic
is often cheaper than the negative system. and combustible solids.