APV Dryer Handbook - Umbc

10
Direct heating is used extensively in industrial drying equipment where much
higher thermal efficiencies are exhibited than with indirectly heated dryers. This
is due to the fact that there are no heat exchanger losses and the maximum heat
release from the fuel is available for the process. However, this method is not
always acceptable, especially where product contamination cannot be tolerated.
In such cases, indirect heating must be used.
With forced convection equipment, indirect heating frequently employs a
condensing vapor such as steam in an extended surface tubular heat exchanger
or in a steam jacket where conduction is the method of heat transfer. Alternatively,
systems which employ proprietary heat transfer fluids can also be used. These
enjoy the advantage of obtaining elevated temperatures without the need for
high pressure operation, as may be required with conventional steam heating.
This may be reflected in the design and manufacturing costs of the dryer.
Furthermore, in addition to the methods listed above, oil- or gas-fired indirect
heat exchangers can also be used.
In general, dryers are either suitable for batch or continuous operation.
A number of the more common types are listed in Table 1, where an application
rating based on practical considerations is given. In the following review, some
of the factors likely to influence selection of the various types are discussed for
particular applications.
BATCH DRYERS
It will be apparent that batch operated equipment is usually related to small
production runs or to operations requiring great flexibility. As a result, the batch
type forced-convection unit certainly finds the widest possible application of any
dryer used today.
The majority of designs employ recirculatory air systems incorporating large
volume, low pressure fans which with the use of properly insulated enclosures,
usually provide thermal efficiencies in the region of 50 to 60%. However,
in special applications of this type of dryer that call for total air rejection, this
figure is somewhat lower and is largely related to the volume and temperature
of the exhaust air. Capital investment and installation cost are relatively low.
The use of fan systems minimizes both power requirements and operating costs.
In contrast, labor costs can be high.