Encyclopedia of Environmental Science and Engineering, Volume I and II

STACK SAMPLING 1101

Velocity, as measured with a pitot tube, is proportional

to the square root of the differential pressure across the two

sides of the pitot tube and the density of the stack gas. Most

sampling trains use a combination inclined–vertical manom-

eter to measure the velocity head, or ∆p. These manometers

usually have 0.01 inch of water subdivisions from 0−1 inch

of water. The vertical section has 0.1 inch divisions from

1−10 inches of water. This type of gauge provides sufficient

accuracy down to 0.05 inches; below that a more sensitive

gauge should be used.

The temperature of the gases is usually measured using

a type K (Chromel-Alumel) thermocouple mounted on the

probe. The absolute pressure is calculated by adding the static

pressure in the stack to the barometric pressure. The molecu-

lar weight of the stack gases is determined using Methods 3

and 4.

Velocity is calculated by the equation below:

V s  K p C p (∆p 0.5 ) avg {T s(avg) /(P s M s )} 0.5

where:

K p = Velocity equation constant

K 34.97

m

sec

(g/g - mole)(mmHg)

( K)(mmH O)

p metric

2

0.5



⎡

⎣

⎢

⎤

⎦

⎥

K 85.49

ft

sec

(lb/lb - mole)(in.Hg)

( R)(in.H O)

p Eng

2

0.5



⎡

⎣

⎢

⎤

⎦

⎥ llish

C p  Pitot tube Coefficient (0.84 for S Type without

interferences)

∆p  pressure difference across the two sides of the

pitot tube (velocity head of the stack gas)

P s  Absolute pressure of the stack, mm Hg or in.

Hg

M s  Molecular weight of the wet stack gases, g/g

mole or lb/lb mole

T s  Absolute stack temperature, ºK (273 + ºC) or ºR

(460 + ºF)

The average dry volumetric stack flow is:

Q sd  3,600(1 − B ws )V s A(T std /T s(avg) )(P s /P std )

where:

Q sd  Average stack gas dry volumetric flow rate

B ws  Water vapor in the gas stream from Method 4

or 5

V s  Average stack gas velocity

A  Cross-sectional area of the stack

T std  Standard absolute temperature 293ºK or 528ºF

P std  Standard absolute pressure 760 mm Hg or

29.92 in. Hg

1

2

3

4

5

6

TRAVERSE

POINT

DISTANCE,

% of diameter

1 2 3 4 5 6

4.4

14.6

29.6

70.4

85.4

95.6

FIGURE 2 Example showing circular stack cross section divided into 12 equal areas, with location of traverse points

indicated.

FIGURE 3 Example showing rectangular stack

cross section divided into 12 equal areas, with a tra-

verse point at centroid of each area.

C019_003_r03.indd 1101C019_003_r03.indd 1101 11/18/2005 11:07:14 AM11/18/2005 11:07:14 AM