APV Dryer Handbook - Umbc

would produce the product at the final required dryness. There are a number of
applications for this technique since second stage dryers are needed for other
functions such as dust removal and agglomeration.
RECUPERATIVE HEATING OF INCOMING AIR
Heat can be saved by using the discharge gas (air)/steam mixture to preheat the
incoming air. This technique is most beneficial when product properties restrict the
gas (air) inlet temperatures, requiring large volumes of gas (air).
UTILIZE DIRECT HEAT
There is less heat loss when the inlet gas (air) is heated directly by the combustion
gases from the gas or oil burner. Clearly, this is not possible for many products.
However, an indirect heating system using a flue gas/air heat exchanger will
typically have an efficiency of 85%. This can be improved to 93 to 95% with air
preheating and flue gas recuperation.
THERMAL INSULATION
Drying equipment tends to be large. Also the equipment operates at quite
high temperatures. As a result, there is a large potential for high heat loss from
both convection and radiation. Insulation of the equipment is vital to ensure
energy efficiency.
CHANGING CAPACITY
One overlooked parameter that can have a significant bearing on efficiency
is changing capacity. With spray dryers and rotary dryers, the mass airflow
can be varied, facilitating modulation of the dryer when operated at lower
throughputs. In the case of pneumatic, fluidized and spin flash dryers, however,
the drying gas medium performs a dual function of providing the heat input for
the drying and also acts as the conveying medium for the transportation of the
product. Since the mass flow of the gas has to remain relatively constant, the only
35