Enclosed Hoods
For enclosed hoods, the exhaust volumetric flow rate can be calculated by the equation:
Q = V x A
Where
• Q = Volumetric flow rate (CFM)
• V = Average flow velocity (FPM)
• A = Flow cross-sectional area (Sq-ft)
The inflow velocity is usually around 100 FPM.
Non-enclosed Hoods
For non enclosed hoods, the capture velocity and the air velocity at the point of contaminant release must be equal and be directed so that the contaminant enters the hood. This results in different volumetric flow rate equations for different type of hoods. For un-flanged round and rectangular openings, the required flow rate equation is:
Q = V x [(10X x 10X) + A]
Where
• Q = Flow rate (CFM)
• V = Capture velocity (FPM)
• X = Centre line distance from the hood face to the point of contaminant generation (ft)
• A = Hood face area (Sq-ft)
L = Long dimension of the slot (ft)
For slot hoods,
the required flow rte is predicted by an equation for openings between 0.5 to 2” in
width:
Q = 3.7 x L x V x X
Where
• Q = Flow rate (CFM)
• V = Capture velocity (FPM)
• X = Centre line distance from the hood face to the point of contaminant generation (ft)
• L = Long dimension of the slot (ft)
If a flange is installed around the hood opening, the required flow rate for plain openings is reduced to 75 percent of that for the corresponding un-flanged opening. The flange size should be approx imately equal to four times the area divided by the perimeter of the face hood. For flanged slots with aspect rations less than 0.15 and flanges greater than three times the slot width, the equation is
Q = 2.6 x L x V x X
A baffle is sold barrier that prevents airflow from unwanted areas in front of the hood. For hoods that Include baffles, the DallaValle half hood equation is used to approximate the required flow rate:
Q = v x [(5X x 5X) + A]
HOOD SYSTEM PRESSURE
The hood converts duct static pressure to velocity pressure. The hood's ability to convert static pressure to velocity pressure is given by the coefficient of entry (Ce), as follows:
Ce = (VP/SPh )0.5 = [1/ (1+K)] 0.5
Where
• K = Loss factor
• VP = Velocity pressure in duct
• SPh = Absolute static pressure about 5 duct diameters down the duct from the hood
