Geoscience Reference
In-Depth Information
Solution:
A
= (10 in. × 10 in.)/144 in./ft
2
= 0.6944 ft
2
VP
=
TP
-
SP
= -0.85 in. - (-1.15 in.) = 0.30 in. wg
V
= 4005 × (
VP
/
d
)
1/2
= 4005 × (0.30)
1/2
= 2194
Q
=
V
×
A
= 2194 × 0.6944 = 1524 scfm
15.6.5.4 Pressure Losses and Velocity Pressure
As the air moves through a duct, losses are created (i.e., static pressure is converted to heat, vibra-
tion, noise.) The loss is usually directly related to velocity pressure:
SP
Loss
=
K
×
VP
×
d
where
SP
Loss
= Loss of static pressure (in. wg)
K
= Loss factor (unitless).
VP
= Average velocity pressure in duct.
d
= Density correction factor.
15.6.5.5 Types of Pressure Loss
Types of pressure loss include hood entry, friction, elbow, branch entry, system effect, air cleaner,
and others. The hood captures, contains, or receives contaminants generated at an emission source.
The hood converts duct static pressure to velocity pressure and hood entry losses (i.e., slot and duct
entr y losses):
H
e
=
K
×
VP
×
d
= |
SP
h
| -
VP
where
H
e
= Hood entry loss.
K
= Loss factor (unitless).
VP
= Velocity pressure in duct (in. wg).
d
= Density correction factor
|
SP
h
| = Absolute static pressure about 5 duct diameters down the duct from the hood (in. wg).
A hood's ability to convert static pressure to velocity pressure is given by the coefficient of entry (
C
e
):
Q
Q
VP
SP
1
actual
ideal
C
=
=
=
e
(
+
K
)
h
h
■
EXAMPLE 15.10
Problem:
What is the hood static pressure (
SP
h
) when the duct velocity pressure is 0.33 in. wg, and
the hood entry loss is
H
e
= 0.44 in. wg? What is
C
e
?
Solution:
SP
=+=+=
VP
H
033044
.
.
0.77 in.wg
h
e
05
.
005
.
Q
Q
=
VP
SP
033
077
.
.
=
actual
i
C
=
=
06
.5
e
deal
h
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