AIR Quality Monitoring and Control

Assignment Task

Correct sampling in flowing gases determines the success of the measurement. There are also special conditions when gases, solids, or liquids in the main gas stream are to be analyzed.

Sampling of Gases

Since gases are completely miscible, there are formally no special requirements for the sampling of a partial gas stream. However the measurement must be made at a point where the concentrations are representative. This is difficult when gases are being mixed or if a chemical reaction takes place. Normally the gas samples from ducts, pipes, or vessels are taken with radial nozzles. Sampling probes may also be used if a concentration distribution has to be determined.

Sampling of Solids

Solids from flowing gases are generally sampled by suction of part of the gas stream. Sampling with a radial nozzle or an inserted pipe leads to incorrect results. Also sampling immediately after a bend or other obstruction is incorrect because of the uneven dispersion of the solids. Depending on the position of the probe, the gas stream entering the sampling device may be loaded with too many or too few particles. Only if the sampling probe is located in a position parallel to the main stream in the undisturbed flow, and sampling is isokinetic will measurements be correct. Differences between the free stream velocity in the duct and the sampling velocity at the probe inlet lead to incorrect results.

Sampling of Liquids

The sampling of liquids in flowing gases is very complicated because usually the state of the liquid is unknown. It may occur as film flow at the pipe wall, as dispersed droplets, or as a fine mist. Combinations of continuous and dispersed flow regimes are possible. Sampling of the gas stream with an opening in the pipe wall will collect mainly the liquid film, and sampling with a probe inserted into the duct will only collect droplets.

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