Page 296 - Materials Chemistry, Second Edition

VOC-Laden Air Treatment 279

further shown as actual cfm (acfm) or standard cfm (scfm). The unit of acfm
refers to the volumetric flow rate under the actual temperature, while scfm
is the flow rate at the standard conditions. The standard conditions are the
basis for comparison. Unfortunately, the definition of the standard condi-
tions is not universal. For USEPA, the standard conditions are T = 77°F (25°C)
and P = 1 atm. However, they are 60°F (15.56°C) and 1 atm for the South
Coast Air Quality Management Districts (SCAQMD) in southern California.
In addition, 32°F (0°C) or 68°F (20°C) are also commonly used in technical
articles as the temperature for the standard conditions. One should follow
the regulatory requirements and use the appropriate reference temperature
for a specific project. A standard temperature of 77°F is used in this chapter,
unless otherwise specified.
Conversions between acfm and scfm for a given air stream can be easily
made using the following formula, which assumes that the Ideal Gas Law is
valid:

Q actual @temperature ,inacfm 460 + T (inF)°
T
= (7.11)
Q standard,inscfm 460 +77
where T is the actual temperature in °F, and the addition of 460 is to con-
vert the temperature from degree Fahrenheit to degree Rankine. It should
be noted that if the temperatures are expressed in degree Celsius, Equation
(7.12) can be used for the conversions between acfm and scfm. (The addition
of 273 is to convert the temperature from degree Celsius to degree Kelvin.):

Q actual @temperature ,inacfm 273 + T (inC)°
T
= (7.12)
Q standard,inscfm 273 + 25

Example 7.6: Conversion between the Actual and
Standard Air Flow Rates
A thermal oxidizer was used to treat the off-gas from a soil-venting pro-
cess. To achieve the required removal efficiency, the oxidizer was operated
at 1,400°F. The actual flow rate at the exit of the oxidizer was 550 ft /min.
3
What would be the exit flow rate expressed in scfm? The temperature of the
effluent air from the final discharge stack was 200°F, and the diameter of the
final stack was 4 in. Determine the air flow velocity from the discharge stack.

Solution:
(a) Use Equation (7.11) to convert acfm to scfm as:
Q actual @temperature ,inacfmT = 460 +T = 460 + 1400 = 550
Q 460 + 77 460 + 77 Q
standard,inscfm standard, in scfm
So, Q = 158.8 scfm