Geoscience Reference
In-Depth Information
15.6.5 F
aCts
, C
onCepts
,
and
e
xample
m
ath
p
roblems
Ventilation is an important form of emission/exposure control. It also provides for health, comfort, and
well-being. All human occupancies require ventilation. Ventilation facts and concepts to know follow:
• Properties of air
• Molecular weight = 29
• Weight density = 0.075 lb/ft
3
at STP
• Standard temperature and pressure for ventilation
• Temperature = 70°F
• Barometric pressure = 29.92 in. Hg, dry air
• Weight density = 0.075 lb/ft
3
at STP
• Density correction factor (
d
) is a factor derived from the ideal gas equations (where BP is
absolute barometric pressure):
=
T
T
(absolute)
(absolute)
×
B
P
BP
TSP
actual
actual
STP
d
• Degrees Rankine (°R) = °F + 460 (absolute temperature)
• Degrees Kelvin (K) = °C + 273 (absolute temperature)
■
EXAMPLE 15.5
Problem:
The temperature is 90°F and the barometric pressure is 27.50 in. Hg. What is density cor-
rection factor
d
?
Solution:
460
+
+
70
27 50
29 92
.
.
530
550
27 5
. 0
29 92
×
d
=
=
×
=×=
096092
.
.
088
.
460
90
.
• Local exhaust ventilation systems are made up of ive components: hood, ductwork, air
cleaner, fan, and stack
15.6.5.1 Pressure
Air moves under the influence of pressure differentials. A fan is commonly used to create the pres-
sure difference. At sea level, the standard static (barometric) pressure (SP) is 14.7 psia = 29.92 in.
Hg = 407 in. wg. If a fan is capable of creating 1 in. of negative static pressure (e.g., 1 in. wg, or
“one inch water gauge”), then the absolute static pressure in the duct will be reduced to 406 in. wg.
• Manometers are used to measure pressure differences (attach both legs to the manometer
to measure velocity pressure).
• The Pitot tube measures both TP and SP.
• S = side = SP.
• TP = tip = top = TP.
Pressures are related as follows:
TP
=
SP
+
VP
TP
SP
VP
Upstream
-
-
+
Downstream
+
+
+
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