59 Particulate Control Equipment

1
5-9 Particulate Control Equipment

• 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?

2

• The condition 7 indicates the collection
  efficiency that must be met, either by a single
  piece of equipment or several operating in
  series.
• The five basic classes of particulate collection
  equipments
• Gravity settling chambers
• Cyclone (centrifugal) separators
• Wet collectors
• Fabric filters
• Electrostatic precipitators

3
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?
• ?

4
One possible configuration of a gravity settling
chamber
5

• 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.

6
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

7

• 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

8
Remarks on the use of above equation

• Turbulent within any device,
• including the settling chamber, causes
  deviations in the particles movement under an
  applied external force,
• resulting in deviations in the particles
  movement toward the collection surface.
• ? In the case of the settling chamber, it
  causes deviations in the particles
• (1) speed and
• (2) direction of flow
• when compared to the uniform duct flow model with
  no macroscopic mixing.

9
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 Reynolds 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 ________.

10
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

11
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.

12
Development of a more realistic modelTo model
flow as well-mixed and a laminar layer next
to the collection tray
13
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.

14
Basic characteristics of gravity settling chambers

• (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.