Particulate Emission Control - Mathematics and Statistics for the Environment - page 500

■ EX AMPLE 17.4
Problem: A hydrochloric acid mist in air at 25°C is to be collected in a gravity settler. Calculate the
smallest mist droplet (spherical in shape) that will be collected by the settler. Stokes' law applies;
assume the acid concentration to be uniform through the inlet cross-section of the unit (USEPA,
1984b, p. 61).

Given:
Dimensions of gravity settler = 30 ft wide, 20 ft high, 50 ft long
Actual volumetric flow rate of acid gas in air = 50 ft 3 /s
Specific gravity of acid = 1.6
Viscosity of air = 0.0185 cp = 1.243 × 10 -5 lb/ft-s
Density of air = 0.076 lb/ft 3

Solution : Calculate the density of the acid mist using the specific gravity given:

Particle density ( p p ) = Specific gravity of fly ash × Density of water = 1.6 × 62.4 = 99.84 lb/ft 3

Calculate the minimum particle diameter both in feet and microns assuming Stokes' law applies.
For Stokes' law range,

05 .

=

Q
gp BL

18 µ

Minimum d

p

p

( ) ×

2

− 5

18 50

32 299843050

× 1 243 . × 10

=× −

5

= 48210 . ft

. × . × ×

( ) =

− 5 5 5

4 82 × 10 ft × 3 048 . × 10 µ m/ft 14 7 . µ m/ft

■ EX AMPLE 17. 5
Problem: A settling chamber is installed in a small heat plant that uses a traveling grate stoker.
Determine the overall collection efficiency of the settling chamber given the operating conditions,
chamber dimensions, and particle size distribution data (USEPA, 1984b, p. 62).

Given:
Chamber width = 10.8 ft
Chamber height = 2.46 ft
Chamber length = 15.0 ft
Volumetric flow rate of contaminated air stream = 70.6 scfs
Flue gas temperature = 446°F
Flue gas pressure = 1 atm
Particle concentration = 0.23 grains/scf
Particle specific gravity = 2.65
Standard conditions = 32°F, 1 atm

Particle size distribution data of the inlet dust from the traveling grate stoker are shown in Table
17.1. Assume that the actual terminal settling velocity is one-half of the Stokes' law velocity.

Solution: Plot the size efficiency curve for the settling chamber. The size efficiency curve is needed
to calculate the outlet concentration for each particle size (range). These outlet concentrations
are then used to calculate the overall collection efficiency of the settling chamber. The collection