On-line Sampling of Solids

Sampling for determining solids concentrations in gas streams and for size analysis needs to be done carefully. Below we highlight some of the intricacies of sampling on-line in the piping both upstream and downstream of a cyclone or swirl tube. 

Sampling on-line involves drawing off a small stream of the solids-laden gas by means of the sampling probe. The small stream of dust-laden gas is led through a filter in which the solids are captured, and from which they can be removed for subsequent analysis. 

We note two extreme cases where isokinetic sampling may be less critical for measuring particle size distribution  

The former of these two cases may well arise in cyclone research. If the maximum and/or minimum particle size and the particle density are not known a priori, however, it is impossible to tell whether one of these conditions ap -ply, and in general it is advisable to sample isokineticaUy whereever possible. Hangal and Willeke (1990) discuss the issue of sampling from a gas stream, and present a model for the capture efficiency of particles as a function of the particle s Stokes number, the ratio of gas to sampler inlet velocities and the angle of the sampler relative to the gas flow. 

The second important point is that the sampling should be representative of the entire cross-sectional area of the pipe. This means that in most cases sampling has to be done at a number of cross-sectional positions. Ensuring representative sampling can be difficult if sampling is done too close to an upstream flow disturbance, such as a bend. The gas velocity profile will then 

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