Page 225 - Fundamentals of Air Pollution

188 13. Ambient Air Sampling

13-3 illustrates the flow patterns around a sampling inlet in a uniform flow
field. Figure 13-3(a) shows that when no air is permitted to flow into the
inlet, the streamline flow moves around the edges of the inlet. As the
flow rate through the inlet increases, more and more of the streamlines
are attracted to the inlet. Figure 13-3(b) is called the isokinetic condition, in
which the sampling flow rate is equal to the flow field rate. An exam-
pie is an inlet with its opening into the wind pulling air at the wind
speed. When one is sampling for gases, this is not a serious constraint
because the composition of the gas will be the same under all inlet flow
rates; i.e., there is no fractionation of the air sample by different gaseous
molecules.
Particle-containing air streams present a different situation. Figure 13-
3(b), the isokinetic case, is the ideal case. The ideal sample inlet would
always face into the wind and sample at the same rate as the instantaneous
wind velocity (an impossibility). Under isokinetic sampling conditions,
parallel air streams flow into the sample inlet, carrying with them particles
of all diameters capable of being carried by the stream flow. When the
sampling rate is lower than the flow field (Fig. 13-3c), the streamlines start
to diverge around the edges of the inlet and the larger particles with more
inertia are unable to follow the streamlines and are captured by the sampling
inlet. The opposite happens when the sampling rate is higher than the
flow field. The inlet captures more streamlines, but the larger particles near
the edges of the inlet may be unable to follow the streamline flow and
escape collection by the inlet. The inlet may be designed for particle size
fractionation; e.g., a PM 10 inlet will exclude particles larger than 10 jam
aerodynamic diameter.
These inertial effects become less important for particles with diameters
less than 5 pm and for low wind velocities, but for samplers attempting
to collect particles above 5 /urn, the inlet design and flow rates become
important parameters. In addition, the wind speed has a much greater
impact on sampling errors associated with particles more than 5 jam in
diameter (4).
After the great effort taken to get a representative sample into the sam-
pling manifold inlet, care must be taken to move the particles to the collec-
tion medium in an unaltered form. Potential problems arise from too long
or too twisted manifold systems. Gravitational settling in the manifold will
remove a fraction of the very large particles. Larger particles are also subject
to loss by impaction on walls at bends in a manifold. Particles may also be
subject to electrostatic forces which will cause them to migrate to the walls
of nonconducting manifolds. Other problems include condensation or ag-
glomeration during transit time in the manifold. These constraints require
sampling manifolds for particles to be as short and have as few bends as
possible.