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# Types of pneumatic transport
# Conveying phases
# Dilute Phase transport
# Dense Phase transport
# Air mover
# Roots Blower
# Roots Blower performance curve
# After Cooler
# Airlock Rotary Valve
# Product inlet / Injector
# Choking velocity
# Conveying speed / velocity
# Air volumetric and mass flowrate
# Pipe Equivalent Length
# Solids particle velocity in pipe
# Solids Breakage
#Pipe Diameter or Bore
# Selecting dilute or dense phase
In a pneumatic conveying system, the air conveying velocity below which the solids being conveyed starts to settle at the bottom of vertical pipe, leading either to a blockage either to dense phase conveying, is the choking velocity. In order to be able to convey in dilute phase a bulk solids, the minimum air conveying velocity in vertical part of the line must be HIGHER than the choking velocity.
The choking velocity is better defined through trials in a pilot plant. However, in case one tries to make a 1st design assessment or to try troubleshooting an existing line, correlations are available.
The one below is due to Punwani and al :
Equation 1 : calculation of the choking velocity
Gs = Mass flux of Solids (kg/m2/s)
ϵCH = the voidage of the pipe at the choking velocity UCH = (1 - volumetric concentration of solids)
ρs = Particle density (kg/m3)
ρg = Gas density (kg/m3)
UCH = Choking velocity (m/s)
D = Pipe diameter (m)
All in S.I. units
Both equations must be solved simulateously by trial and error to calculation ϵCH and UCH
Note : The choking velocity, which happens in vertical lines, should not be confused with the saltation velocity which happens in horizontal lines. The saltation velocity is higher than the choking velocity, thus designing the system to run above the saltation velocity will allow to avoid crossing the choking velocity in vertical pipes.Source
Principles of Powder Technology, M.J. Rhodes, 1990, page 148
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