where fl is the number frequency of drop let of size D l .
The Sauter mean diameter corresponds to the particle
diameter with the same volume -to-surface ratio as the
entire spray or powder sample. Sometimes the median
diameter DM is also used in spray drying calculations .
It is that diameter above or below which lies 50% of
the number or volume of droplet s. It is especially
useful when a n excessive amount of very large or
very small particles are present .
Size distribution can be represented by a frequency
or cumulative distributive curve. If occurrence
is given by number, a number distribution results. It
may be obtained by microscopic analysis. If occurrence
is given by area–volume–w eights corresponding
to a given diameter, then an area–volume–w eight
distribution results.
The lognormal an d Rosin–Rammler distribution s
are the most common distributions used in the spray
drying process calculations. The lognormal distribution
ha s two parameters, the geo metric mean size
DGM and the geometric standard deviation S G. The
mathematical form is as follows: