Volume Flow Measurements in Stacks
A precise volume flow measurement is essential to evaluate emissions from stacks. Many methods exist like pitot tube traverses, averaging pitot tubes, hot-wire anemometers, ultra-sonic flow meters, and critical orifices. The accuracy of these methods is limited when less-than-ideal flow characteristics exist, such as: asymmetric velocity distribution, off-axis flow components due to swirl, turbulence, very low flow, and reversal due to wakes or buoyancy.
These methods also require the measurement of the cross sectional area which can be a significant source of error if the shape and dimensions of the sampling section cannot be determined with sufficient accuracy.
Tracer gas dilution is a volumetric or whole field method for measuring flow. It does not require the measurement of the cross-sectional area of a duct and, with exception of flow reversal, it is insensitive to the non-ideal flow characteristics mentioned previously.
The tracer test is comparable to the flow measurement in a duct. For stacks the standards ASTM E 2029 and the DIN EN ISO 16911-1 are applicable. The detection of the tracer concentration using gaschromatography (GC) with electron capture detection (ECD) is independent of the gas constituents (no cross interference). This differentiates the GC-ECD analysis from e.g. IR-analysis. The latter requires a cross compensation, which of course depends on the concentration of each stack gas constituent. Because concentrations of stack gas constituents vary with stack load, the GC-ECD analysis is more precise and with less effort involved.