POWER August 2012 - 54

PLANT DESIGN
the harder lumps are less prone to attrition
and crushing. By testing and reporting the
results of the finer fraction, however, the
reported angle of wall friction is conservative,
as intended. The result is that the
coefficient of friction for a representative,
uncrushed sample is often different than
laboratory test results.
Angular changes in direction occur when
the trajectory impacts a chute surface, two
inclined plates at different angles meet,
or as the coal follows a curved surface. In
Figure 9, the downstream particle's speed
(V2
speed (V1
) is slower than the upstream particle's
) and can be calculated as: V2
=
V1 (cos  - µ sine ). Using this equation,
we can compare making a 30-degree angular
change in one and two steps. In this
example, a 0.50 coefficient of friction is
used. So, as seen in the computation, it is
better to design a chute with gradual angle
changes in the chute when the objective
is to avoid slowing the coal stream speed.
On the other hand, if the speed is too high
at the loading point for the receiving conveyor,
a larger directional change can be
advantageous.
Particles near the edge of the stream will
rebound at odd and unpredictable angles
due to the irregular, angular shape of each
particle. That is quite evident in Figure 4,
where coal is spilling through an opened
inspection door. Coal particles are not
nice little spheres, as considered in some
computer models. Bounce a tennis ball
and catch it-that's child's play. Bounce a
wooden block and catch it-playtime has
become much more challenging.
There is another physical issue that affects
how quickly plugging conditions
can accelerate. Once the flowing, aerated
stream of coal within the chute starts to
fill the available chute volume, additional
forces come into play. The chute casing
exerts a compressive force that moves particles
closer together. A good example is
a bend in the chute with some coal buildup
on the surface. The expanded flowing
stream slows significantly due to a directional
change and starts to fill the chute.
The air between the particles seeks a quick
escape path, which is often countercurrent
to the direction of flow. The escaping
air, therefore, becomes a drag force and a
brake on the coal stream, which can hasten
the formation of a plugged chute.
The dimensions of a chute are also important
to control to prevent plugging.
Historically, chute designers would use
a factor based upon the load area of the
inbound conveyor. Design standards are
company specific. One company's rule-ofthumb
was to size the sectional area of the
August 2012 | POWER
AcidGasExperts.com
chute as four times the Conveyors Equipment
Manufacturers Association (CEMA)
load area for the inbound conveyor. However,
this guide does not consider the speed
of the conveyor belt. Higher-speed conveyors
deliver a higher volume (volume = load
area x belt speed). Air currents entrain dust
within transfer chutes, so large chutes that
disregard air currents can have a dust issue
to handle.
Flow control chute suppliers have opted
for different approaches to minimize the effect
of air currents within a chute. Instead of
considering 400% of the CEMA belt area,
one supplier's standard " fills " chute sections
to 60% of the area. Effectively, this is
a 167% (100%/60%) factor. An advantage
for the supplier is that smaller chutes are less
expensive to fabricate and easier to retrofit in
the available space. The disadvantage for all
is that smaller chutes are easier to plug.
Coal-fired plants naturally refuel at the
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POWER August 2012

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