Transcript 5-9 Particulate Control Equipment

5-9 Particulate Control Equipment
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A number of factors must be determined before a
proper choice of collection equipment can be made:
The physical and chemical properties of the particles;
The range of the volumetric flow rate of the gas stream;
The range of expected particulate concentrations (dust
loadings);
The temperature and pressure of the flow stream;
The humidity;
The nature of the gas phase (such as corrosive and
solubility characteristics);
The required condition of the treated effluent.  this
may be the most important piece of information for
particulate control. Why?
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The condition 7 indicates the collection efficiency that must
be met, either by a single piece of equipment or several
operating in series.
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The five basic classes of particulate collection equipments:
Gravity settling chambers
Cyclone (centrifugal) separators
Wet collectors
Fabric filters
Electrostatic precipitators
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5-9-A Gravity Settling Chambers
• Gravitational force may be employed to remove particles in settling
chambers when settling velocity is greater than about __________.
• In general, this applies to particles larger than _____ if particle density is
low, down to _______ if the material is reasonably dense.
• What would be needed if we want to use this kind of device to remove
smaller particles?
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One possible configuration of a gravity settling chamber
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• For a gravity chamber to be effective in preventing reentering
of the settled particles, the gas velocity must be uniform and
relatively _________, certainly less than _______ and
preferably less than _________.
• Theoretically, the minimum particle size that can be removed
with 100 percent efficiency can be derived in the following
manner.
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The fractional collection efficiency for a given particle size dp
• For particle sizes below that given by
Vt = (VH/L) = Q/(LW) = f(dp)
Or dp,min = (18uHV/gLtho_p)^1/2,
The fractional collection efficiency for a given particle size dp
can be estimated for uniform duct flow by the relation
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• For settling chamber with n horizontal baffles (n flow
channels), the settling velocity Vt,
* The collecting efficiency for this kind of settling chamber with
n flow channels:
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Remarks on the use of above equation
• Turbulent within any device,
including the settling chamber, causes deviations in the
particle’s movement under an applied external force,
resulting in deviations in the particle’s movement toward the
collection surface.
 In the case of the settling chamber, it causes deviations in the
particle’s
(1) speed and
(2) direction of flow
when compared to the uniform duct flow model with no
macroscopic mixing.
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Limitation of the Uniform Duct Flow Model
• Therefore, it is important to assess whether the flow through a
pipe or box (settling chamber) is ___________ or __________
(well mixed).
• In general, if the Reynold’s number of the flow through the
device is less than approximately ________, the flow is
considered in a laminar flow regime.
• As Re increases from _______ to ________, the flow
undergoes a transition from laminar to turbulent.
• The flow is considered to be fully turbulent for Re ________.
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For a fully turbulent flow
• The density and viscosity of the fluid in this case are that of air.
• The diameter to be used in the Re calculation is the
characteristic or hydraulic diameter of the pipe or box.
• The hydraulic diameter Dh is defined as
Dh =
• For the case of a box such as a settling chamber,
Dh =
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Example 5-6
• A settling chamber is operating at ambient conditions and has
a height and width of 3 m, and a horizontal gas velocity of 1
m/s. Determine whether the air flow through the device is
laminar or turbulent.
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Development of a more realistic model:
To model flow as “well-mixed” and a “laminar layer”
next to the collection tray
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Example 5-7
• Determine the length of a simple gravity collector required to
obtain an efficiency of 90 percent when collecting particles 50
um in diameter and having a density of 2.0 g/cm3. The bulk
gas velocity is 0.5 m/s and the chamber is 3 m height.
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Basic characteristics of gravity settling chambers
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(1) very low energy cost
(2) low maintenance cost
(3) low installed cost
(4) excellent reliability
(5) very large physical size
(6) low to very low collection efficiency
• The last item is particularly true when the dust loading
contains substantial amounts of fine to medium-size particles.
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