Page 50 - Air Pollution Control Engineering

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30 Lawrence K. Wang et al.

where Q is the flow rate of the combined gas stream at actual conditions (acfm).
e,a
7.3. Moisture Content, Dew Point Content, and Sulfur Trioxide Calculations
When examining the potential gas stream, the moisture content of the combined stream
must be calculated as a volume percent. The procedures for calculating the volume percent
of moisture for the combined gas stream require that the volume moisture of each stream
be converted to the lb-mol basis, added together, and then divided by the total combined
gas stream volumetric flow rate (Q ). Steps for calculating moisture content of gas stream
e
are provided below as on both a volume percent and a mass percent basis. To determine the
dew point, the mass percent of moisture in the gas stream must be calculated.
The procedures for converting the volume percent of moisture to lb-mol is
)(
M = Q M ( 100% lb-mol 392 scf) (26)
e1,l-m e1
where M is the percent of moisture content of gas stream 1 (% volume) and M is
e1 e1,l-m
the moisture content of gas stream 1 (lb-mol/min). For each gas stream in the emission
stream, this calculation is repeated and the lb-mol/min of each gas stream is summed to
determine the combined gas stream, as shown in Eq. (27). The volume percent of the
combined stream is determined using Eq. (28).
M e,l m = M e ,l m + M e ,l m +K + M en,l m (27)
2
-
-
-
-
1
M = (M /Q )(392 scf/lb-mol)(100%) (28)
e e,l-m e
where M is the moisture content of the combined gas stream (lb-mol/min) and M
e,l-m e
is the moisture content of the combined gas stream (% volume).
To determine the dew point, the moisture content of the combined gas stream must
be presented on mass basis (M ), which is calculated as
e,m
M = (M )(18 lb/lb-mol) (29)
e,m e,l-m
where M is the moisture content of the combined gas stream (lb/min). Equation (30) is
e,m
used to calculate the amount of dry air in the combined gas stream:
DA = Q (lb-mol/392 scf)(29 lb/lb-mol) (30)
e e
where DA is the dry air content of combined gas stream (lb/min). The psychrometric
e
ratio is calculated as
(
Psychrometric ratio lb of water 1b dry air) = M e ,m (DA e − M e ,m) (31)
Using Table 5 with the psychrometric ratio and the gas stream temperature, the dew
point temperature of the combined gas stream is determined.
When the gas stream contains sulfur trioxide (SO ), the dew point temperature of the
3
gas stream increases. If the concentration of trioxide is not considered when determin-
ing the dew point temperature of the gas stream, condensation may occur, resulting in
a gas stream containing liquid droplets. This, in turn, will combine with SO to form
3
sulfuric acid and cause severe corrosion of metal and deterioration of fabric components
in air pollution control equipment (bag house), fans, and filter. Therefore, to protect air