Geoscience Reference
In-Depth Information
four to six diameters downstream and two to three diameters upstream from obstructions
or directional changes, then the average velocity can be estimated at 90% of the centerline
velocity. The average velocity pressure is about 81% of centerline velocity pressure.
3. A more accurate method is the traverse method, which involves taking six to ten measure-
ments on each of two or three passes across the duct, 90° or 60° opposed. Measurements
are made in the center of concentric circles of equal area.
4. Density corrections (e.g., temperature) for instrument use should be made in accordance
with the manufacturer's instrument instruction manual and correction formulas.
Air cleaner and fan condition
measurements can be made with a Pitot tube and manometer.
15.6.4 g
ood
p
raCtiCes
Hood placement must be close to the emission source to be effective. Maximum distance from the
emission source should not exceed 1.5 duct diameters.
1. Keep in mind the relationship between capture velocity (
V
c
) and duct velocity (
V
d
) for
simple plain or narrow flanged hoods. For example, if an emission source is one duct diam-
eter in front of the hood and the duct velocity (
V
d
) = 3000 fpm, then the expected capture
velocity (
V
c
) is 300 fpm. At two duct diameters from the hood opening, the capture velocity
decreases by a factor of 10, to 30 fpm.
2. A rule of thumb that can be used with simple capture hoods is that if the duct diameter
(
D
) is 6 in., then the maximum distance of the emission source from the hood should not
exceed 9 in. Similarly, the minimum capture velocity should not be less than 50 fpm.
Simply, for simple capture hoods, maximum capture distance should not be more than 1.5
times the duct diameter.
System effect loss
, which occurs at the fan, can be avoided if the necessary ductwork is in place.
1. Use of the
six-and-three rule
ensures better design by providing for a minimum loss at six
diameters of straight duct at the fan inlet and a minimum loss at three diameters of straight
duct at the fan outlet.
2. System effect loss is significant if any elbows are connected to the fan at inlet or outlet. For
each 2.5 diameters of straight duct between the fan inlet and any elbow, CFM loss will be
20%.
Stack height
should be 10 ft higher than any roof line or air intake located within 50 ft of the stack.
For example, a stack placed 30 ft away from an air intake should be at least 10 ft higher than the
center of the intake.
Ventilation system drawings and specifications
usually follow standard forms and symbols, such
as those described in the Uniform Construction Index (UCI).
1. Plan sections include electrical, plumbing, structural, or mechanical drawings. The draw-
ings come in several views: plan (top), elevation (side and front), isometric, or section.
2. Elevations (side and front views) give the most detail. An isometric drawing is one that
illustrates the system in three dimensions. A sectional drawing provides duct or component
detail by showing a cross-section of the component.
3. Drawings are usually drawn to scale. (Check dimensions and lengths with a ruler or a scale
to be sure that this is the case. For example, 1/8 in. on the sheet may represent 1 ft on the
ground.)
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