Page 42 - Air Pollution Control Engineering

01_chap_wang.qxd 05/05/2004 11:45 am Page 22
22 Lawrence K. Wang et al.

connecting only the static connection to the pressure gage. Using a standard or previously
calibrated Pitot tube one can obtain a test Pitot tube correction factor:
.
C = C (∆ P ∆ P ) 0 5
test std std test (9)
Pitot tubes cannot be used for gas velocities below about 120 m/min (400 ft/min). One
can use Pitot tubes to measure the gas velocity by applying one of the following equations:
For compressible fluids,
c { D [ (γ − ) γ ]} 05
.
(
1
v = [ 2 g γγ − )]( P ρ D) ( P P ) − 1 (10)
C
1
D
where C is the Pitot tube correction factor, v is the velocity, g is a dimensional constant,
c
2
2
(32.174 lb ft / lb s , or 1 kg m / N s ), P is the absolute pressure in the duct or stack,
m f D
static plus barometric pressures, P is the impact tube pressure plus the barometric pres-
sure, γ is the ratio of gas-specific heats (at constant pressure and constant volume), and ρ
D
is the density of gas in the duct or stack.
The generalized Pitot equation is
.
C g (∆ ρ
v = [ 2 P )] 05 (11)
c 1
where ∆P = P − P .
t s
For air, Eq. (10) simplifies to (in metric units)
∆
.
v =147 4 C T P) 05 . (12a)
1 ( 1 1
where v is the air velocity (m/min), T is the absolute temperature (K), and ∆P is the
1 1 1
pressure drop (centimeters of water), or (in English units)
.
v =174 6 C(T ∆ P ) 05 (12b)
.
2
2
2
where v is the air velocity (ft/min), T is the absolute temperature (ºR), and ∆P is the
2 2 2
pressure drop (inches of H O).
2
The average velocities can be obtained by averaging the velocities obtained for each
∆P as calculated by Eqs. (10)–(12b) or, as implied by the equations, the (∆P) 0.5 can be
averaged and a single average velocity calculated.
Multiplying the duct inside cross-sectional area by this average velocity results in an
actual volumetric flow rate (e.g., actual cubic feet per minutes). To correct this to SC of
1 atm and 20ºC, the ideal gas law as shown in Eq. (6) can be applied to obtain normal
3
3
cubic meters per second [(normal)m /s] or standard cubic feet per minute (std ft /min)
as follows:
a( T )( ) 3 3 (13)
Q 293 D P 76 ) = (normal m s or std ft min or scfm
D
where Q is the actual volumetric flow rate, T is the absolute temperature in the duct
a D
(K), and P is the absolute pressure in the duct (cm Hg).
D
6.5. Relative Humidity
The amount of water vapor in a gas stream can be determined by several methods.
One standard method for gases is to use wet–dry-bulb temperature measurements.
This method consists mainly of air or nitrogen and does not have high temperature or