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INDUSTRIAL VENTILATION DUST COLLECTION SYSTEM DESIGN

HOOD DESIGN

 

Proper design of exhaust hoods is a must If you want effectively control athmospheric contanimation at its source with minumum air flow and power consumption.The theory of capture velocity depends on the creation of air flow past the source of contaminant sufficent to remove highly contaminated air around the source and to draw air into an exhaust hood.

Dust particles in micron sizes ,even if throwed high velocities travel very short distance and after a few inches follow the iar currents.The same for mist ,fumes ,vapors and gases.

Larger dust particles relased at high velocities have a trajectory in air.Unless they are directed to hood ,they cannot be captured..

 
Capture Velocity V fpm Air velocity at any point in front of the hood or at the hood opening ,neccesary to capture the contaminated air at that poinby causing it to flow into the hood.
       
Face Velocity Vf fpm Air velocity at the hood opening
       
Slot Velocity Vs fpm Air velocity through the openings in a slot type hood
       
Plenum Velocity Vp fpm Air velocity in the plenum
       
Duct Velocity Vd fpm Air velocity through the duct after hood.
       
Transport Velocity Vt fpm Minumum air velocity through the duct required to move particules in air stream.
       
Dust Distance X ft Distance outward along center axes of hood.
       
Air Flow Q cfm Required minumum air flow to capture dust particles
       
Area A ft2 Area of hood opening
       
Diameter D ft Diameter of round hoods

 

Hood Design Procedure
 
Elimination of air motion ,important soruce of air motion :
1
Thermal air currents ,especially from hot processes
2
Motion of machinery ,as grinding wheel ,belt conveyor
3
Materila motion ,as dumping or container filling
4
Movements of operator
5
Room air currents ,usually 50 fpm ,0,25 m/sn or higher
6
Spot cooling ,heating equipment
 
 
Other considerations
7
The shape of hood
8
The size of hood
9
Hood must enclose the operation as much as possible
10
Location of hood; must be as close as possible to source
11
Rate of air flow
12
Hood must be flanged whenever possible to eliminate exhauts air from ineffecitve areas

 

Range of Capture Velocities in Hood Design

 

Dust Explosions
A dust explosion is very similar to a gas or vapour cloud explosion, i.e. when a volume of a flammable mixture is ignited, resulting in a rapid pressure increase and fire moving through the cloud. A dust explosion occurs when a combustible material is dispersed in the air forming a flammable cloud and a flame propagates through it. This of course also depends on the supply of oxygen to the fire, and the concentration of the fuel, if either of these are in too high or low then the explosion will not occur.

 

 

 
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