APV Dryer Handbook - Umbc

36
way a dryer can be turned down is to reduce the inlet gas temperature. This has
an adverse effect on efficiency.
Figure 15 illustrates the effects on thermal efficiency – of either increasing
or decreasing the evaporator capacity for an application – where it is necessary
to maintain a constant air outlet temperature to maintain acceptable dry product.
Since the gas flow has to be constantly maintained, the only parameter that can
be changed is the inlet gas temperature. The figure refers to total rejection and
demonstrates that an efficiency of 62.4% can be achieved at design rate (this
is reduced to 50% when the dryer is operated at 60% of design capacity).
This illustrates the need to establish realistic production requirements and also
avoid excessive safety factors on the design.
% EFFICIENCY
70
60
50
40
0
140 120 100 80 60 40
% DESIGN EVAPORATION
Figure 15. Variation of efficiency with dryer turndown