3 replies to this topic

[sandeep chk]

Can anybody enlighten me on isokinetic flow and its significance in dust concentration measurement.I would also like to know the industrial method/practice of measuring isokinetic flow for flue gas flow of the coal fired boiler.Any help will be much appreciated.
Sandeep

[Andree]

The name "isokinetic" says everything... the point is to maintain constant velocity at all measurement. There are of course relation which allow to recalculate results of dust particle deposition for different velocities to enable comparing them, however the best results are obtained if no recalculation is needed. Velocity affects the deposition and capture of dust particles in impactors, when inertial and interception effects play a role. I do not know if it does effect laser counting (you did not mention what device you refer to) - probably there is a range of velocity at which the device gives accurate results.

[djack77494]

I like to visualize this by thinking about the end of the (sampling) probe. It's sitting in (say) the middle of a duct/pipe facing upstream into the flow. If you want the sample you draw from the stream to have the same solids characteristics as those of the flowing stream, then you must draw your sample isokinetically, or the velocity of the sample gas in the probe must equal the velocity of the flowing gas in the ductwork. If you do things this way, the sampled gas and entrained solids will smoothly flow into the probe. There will be no (minimal) physical effects that would cause the sampled gas (and entrained solids) to differ from the flowing stream.

Now withdraw your sample gas at a higher velocity than the flowing stream. Some of the particulate matter, especially the larger particles, have enough inertia that they do not accompany the gas that they had been entrained in. Instead, the gas "turns" and enters the probe while the particles "don't make the turn" due to inertia. A similar situation develops if the sampling velocity is too low, but here the solids would be over-represented.

[siretb]

Isokinetism is critical for dust measurements.
If the velocity pumped by the nozzle is larger than the one of the gas at the location, you will oversample small particles. If lower, you oversample large ones. djack77494 describes well what happens, they do not take the turn.
5% (or better is required).
There are guidelines and standards for dust sampling (ISO 13284-1)

the main points (from experience, not standard)
5% isokinestism or better
the sampling nozzle should be "aligned with flow lines" deviations of less than 15° are OK
nozzle diameter: no less than 8mm (that will give you the pumping capacity
maintain the sampling lines heated all the way down to the filter
filter temperature: greater than 130°C less than 160°C (at any rate 20°C or more above dew point)
for the filter: prefer quartz filters to glass filters
the sampling probe must be moved across the duct during sampling. Even if you have only very fine dust, it is wrong to assume that sampling in a single location, without further check) is correct. The number of sampling points depends on the size of the duct.
sampling time from 30 min to 2 hours, depending on dust load

sampling location
at least 5D from bend (upstream) and 3D from bend(downstream)
no swirl (beware of fans and bends)
ensure that no obstacle (other probe, .....) interfere with sampling prob