Page 44 - Air Pollution Control Engineering

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24 Lawrence K. Wang et al.

pressure, and Q is the metered gas volume. The value of Q in cubic centimeters is
m c
found by converting the grams of water condensate to vapor:

3
Q = ( )(22 400 cm 18 g T )( m 273 )(76 P ) (16)
M
,
c
m
where M is the mass of condensate (g), T is the meter absolute temperature (K), and P
m m
is the absolute pressure at meter (cm Hg). Finally, the percentage of water content is
W = 100( Q Q ) (17)
w T
where Q is the total volume of metered gas at meter conditions.
T
Use of the Carrier equation is another method for determining water content in flue
gases when the wet-bulb temperature is >180ºF at nonstandard pressure. This equation
includes factors for transfer to heat by conduction and radiation and accounts for diffu-
sion and vaporization. It is accurate at temperatures up to about 400ºF and can be used
with flue gases containing up to about 15% CO . The actual water vapor pressure (P )
2 a
is given in inches of Hg:

P = P − ( [ P − )( t − ) (2 830 − 1 44 t )] (18)
P
.
t
,
a w D w D w w
where P is the saturation vapor pressure of water at duct wet-bulb temperature from
w
steam tables (inches of Hg), P is the absolute pressure in duct (inches of Hg), t is the
D D
dry-bulb temperature in the duct (ºF), and t is the wet-bulb temperature in the duct (ºF).
w
The percentage of water content becomes
a (
W =100 P P ) (19)
D
6.6. Sample Train
Sampling methods and systems have been discussed thoroughly in various publica-
tions. However, the two major variations are the US EPA Method 5 test train (6) and the
American Society of Mechanical Engineers (ASME) Performance Test Code (PTC) 21
method as described in ref. 16 and as supplemental by PTC 27 in 1957 and PTC 28 in
1965. These systems are shown in Figs. 10 and 11, respectively. The basic differences
are as follows. In the US EPA Method 5, an ice bath is used in the train, which can result
in the condensation of insoluble material after the filtering stage. These condensables
are considered as particulates. In the US EPA Method 5, the meter is placed after an
airtight pump, whereas in the ASME method the meter is run under vacuum by placing
it before the pump. In the ASME method, the meter is considered to be airtight (or at
least as leak-free as an airtight pump). It is important that all trains be inspected for
leaks and operated leak-free.
Gas samples are withdrawn proportionally, which means that as the gas flow in the
duct changes, the sample rate is changed proportionally to provide properly weighted
results. Isokinetic sampling is used in collecting particulates and consists of drawing
the sample into the sample probe at a velocity equal to the velocity in the duct where the
sample’s tip is located. This means that duct velocities must be taken simultaneously
with the samples and at the same locations. It is important to be able to relate sample