found similar agreement for sand 3 with broader size distri-
bution (see Fig. 1 ), transported at a concentration of 15% in
a 0.3 m i.d. pipe. Good agreement was found for concentra-
tions of up to 31% for sand 2. However, predictions made for
sand 1 underestimated friction losses at concentrations of 26-
31%. Predictions made for sand 3 overestimated friction
losses at concentrations of 28-39%. With these two sands,
observed friction losses did not increase linearly with con-
centration, in disagreement with model assumptions. Their
results indicated that the partially-stratified friction loss
model does not predict losses accurately at high concentra-
tions for products with particle size distributions differing
from those of the products for which the model was verified.
The comparatively low friction losses obtained with the broad
distribution sand 3 indicated that particle size distribution and
particles of sizes 0.1-0.5 mm play an important role in reduc-
ing pipe wall friction. Sundqvist et al. [ 14] suggested that
the whole particle size distribution should be included in the
partially-stratified friction loss model, for example in terms
of d85/d15.
So far the partially-stratified friction loss model has not
been validated systematically for solids densities outside the
2650-2900 kg/m 3 range. Furthermore, few results are avail-
able for industrially comminuted products.