| The SOP's developed by spray
research that govern spray sampling are biased towards ensuring the
measurement of the largest droplets within the spray envelope. Although
these large droplets are typically far from numerous, they can contain a
relatively large percentage of the total liquid volume of the entire
spray.
For example, a single 200µm droplet contains the equivalent volume of
8000 10µm droplets. Obviously, the failure to capture a representative
sample of the larger droplets can profoundly affect the volume-based
characteristic droplet diameters reported by the spray analysis
system.
Most industrial spray processes are sensitive to the mass- and
thermal-transfer characteristics of the spray. These, in turn, are
dependent on the relationship between the total droplet surface area and
volume. In addition, some processes cannot tolerate droplets above a
certain diameter. In gas quenching applications, droplets that do not
evaporate completely within the available residence time may impact on
downstream structures or wet the filters in a baghouse. Incompletely
evaporated slurry droplets may rapidly coat the walls of a spray dryer.
The sampling SOP's share the following procedures:
-
the spray is scanned to identify and locate
any inhomogeneities within the spray envelope
-
weighting factors are derived for
these locations based on flux density, particle concentration, droplet velocity
and characteristic diameters
-
an
appropriate baseline sampling regime is selected and modified by applying the
weighting factors
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