Efficient and well-designed process duct system handle the air properly through
out the system without any fluctuation in the process. The correct design of duct
requires the following considerations.
- Gas velocity
- Pressure handled and drop
- Dust load in ducting
- Temperature of gas handled
- Duct supporting structure/ Type of support and span between two supports
The sizing of the duct mainly depends on maintaining the proper velocity across duct.
The other parameters like pressure, dust load and temperature of gases will decide
the construction features of the duct such as wall thickness, insulation and supports
etc. Supports could be at a longer distance or at shorter distances depending
upon the weight of the duct and wall thickness.
Gas Velocity
Since the velocity is prime parameter for any duct sizing, this has to be
critically analyzed. The correct velocity prevents any settlement or falling of dust
particles inside the duct and reduces the pressure drop in the system. Higher
velocity will cause high wear of the duct and high pressure drop across the duct.
Lower velocity will have settling effect of dust particles leading to clogging of
duct and increase the pressure drop.
The table below shows the various design velocities considered in various sections
of a cement plants like Pyro processing and Cement/Raw mill/Coal mill.
9. Process Duct Sizing Criteria
Ducts Velocity M/Sec
Pyro Processing
Down comer duct 08-10
Duct between cyclones 10-14
Calciner 8-10
Cyclone inlet 10-14
Cyclone outlet 10-12
Kiln inlet riser duct 20-25
Kiln hood 4-6
Tertiary air duct 25-30
Cooler vent from Cooler exit 16-18
VRM Raw Mill Circuit
Hot Air duct to Mill Inlet 16-18
Mill Outlet to Cyclone 16-18
Cyclone to fan 16-18