For pressure drop due to solids friction, ZENZ and OTHMER in
their book explained that some of the earlier investigations had
concluded that the term (ƒp·Vp)/(ƒ·Vg) could be ignored but that
the use of this term, in general, will predict pressure drops
greater than those observed and thus can be considered conservative
for design purposes. Further research on this topic
was carried out by ZENZ while he was at PSRI and he concluded
that this term should be replaced by a constant (K) because this
term was dependent upon the physical properties of the solids
being conveyed. His investigation also concluded that because
of many variables it was very difficult to develop an accurate
correlation for calculating the value of K and therefore, the value
of K should be back-calculated from the actual pressure drop
data obtained from lab or plant tests.