Sampling location in stack A stack with an inner diameter of 1.2 m is subject to source emission tests. Determine the sampling points by dividing the stack area into three equal sub-areas.

Question

Sampling location in stack
A stack with an inner diameter of 1.2 m is subject to source emission tests. Determine the sampling points by dividing the stack area into three equal sub-areas.

Accepted Answer

In this problem, n = 3 and D = 1.2 m are given. The sampling points are on the circles with diameters of [latex]D_{si}=D\sqrt{i/2n}[/latex] where i = 1, 3 and 5

[latex]D_{s1}=1.2 × \sqrt{1/6}=0.4899(m)[/latex]
[latex]D_{s2}=1.2 × \sqrt{3/6}=0.8485(m)[/latex]
[latex]D_{s3}=1.2 × \sqrt{5/6}=1.0954(m)[/latex]

These sampling points are depicted in Fig. 15.10, where readers can determine the equal area circle [latex]D_{i}[/latex] by using Eq. (15.15).
[latex]D_{si}=D\sqrt{i/2n}[/latex] where i = 2, 4. . .2n (15.15)

In addition, samples should be taken at each of the 4n sampling points for equal duration, typically being an hour or so. For statistical analysis of the data, at least three replicates are recommended for each sample. This means at least 3 × 4 × n samples are taken for one complete test. Usually averaged results are reported with standard deviation to quantify the variations at the corresponding sampling points. As a result, source tests are time-consuming and a complete stack test may last for many hours to days.
In a real source test, the stack cross section area is often divided into more than two equal areas. The number of points is determined following local standards. In general, more sampling points are needed if particulate matter is of interest. Less sampling points are required for gaseous pollutants because the gases in typical stacks are well mixed by turbulence.

Equal area method is also applicable to rectangular duct, where the determination of the sampling points is much simple. The duct perimeter is first divided into a number of girds and the sampling points are chosen at the center of each grid (Fig. 15.11). The more sampling points the more representative samples can be obtained. However, one has to compromise for time and costs.

Question 15.1: Sampling location in stack A stack with an inner diameter of...