This presentation looks at the differences between direct-fired rotary dryers and indirect-fired rotary dryers, and covers topics such as operation, applications, and materials of construction.

1. Direct Vs. Indirect
Drying
Experts in process design and material
processing for over 60 years.

2. Rotary dryers are a critical component in
many industrial processes, capable of
processing all sorts of materials, from
chemicals and fertilizers, to minerals and
ores, and nearly everything in between.

3. Most dryers are of the direct-fired type,
because they are significantly more efficient
than their indirect-fired counterpart.

4. Most dryers are of the direct-fired type,
because they are significantly more efficient
than their indirect-fired counterpart.
However, in some situations, special material or
process needs require the use of an indirect
dryer over a direct-fired dryer.

5. What follows is an explanation on how these
two types of industrial drying systems differ in
a few key areas.

6. Operation

7. Direct-Fired Dryers
Direct-fired rotary dryers rely on direct
contact between the material and the
drying air to dry the material.

8. Direct-Fired Dryers
Direct-fired rotary dryers rely on direct
contact between the material and the
drying air to dry the material.
Flights, or material lifters, pick up the
material, and drop it through the air
stream in a cascading motion to maximize
heat transfer efficiency.

9. Indirect-Fired Dryers
Conversely, indirect-fired rotary dryers
avoid direct contact between the
material and the drying air.

10. Indirect-Fired Dryers
Conversely, indirect-fired rotary dryers
avoid direct contact between the
material and the drying air.
Instead, the dryer shell is heated from the
outside, and the material is dried through
direct contact with the heated shell.

11. This is beneficial because it allows for the
processing environment inside the drum to
be tightly controlled. This is necessary
when processing materials that cannot
come into contact with the drying air, or
that may be combustible in certain
settings.

12. This is beneficial because it allows for the
processing environment inside the drum to
be tightly controlled. This is necessary
when processing materials that cannot
come into contact with the drying air, or
that may be combustible in certain
settings.
This method also avoids the risk of
entrainment when processing fine
materials.

13. Material of
Construction

14. While direct-fired dryers are typically
constructed of carbon steel, the shell of
an indirect-fired dryer must be able to
withstand much higher temperatures, and
is therefore constructed of a more heat-
resistant alloy.

15. While direct-fired dryers are typically
constructed of carbon steel, the shell of
an indirect-fired dryer must be able to
withstand much higher temperatures, and
is therefore constructed of a more heat-
resistant alloy.
This helps to maintain the integrity of the
drum, despite the constant exposure to
high temperatures.

16. Applications

17. Direct-fired rotary dryers are extremely
robust and reliable. Their ability to accept
variance in feedstock also makes them a
popular industrial drying system.

18. Direct-fired rotary dryers are extremely
robust and reliable. Their ability to accept
variance in feedstock also makes them a
popular industrial drying system.
Direct-fired dryers are incredibly flexible,
and can be used to process a multitude
of materials, including minerals, fertilizers,
chemicals, aggregates, and many more.

19. Indirect dryers are better suited to more
specialty applications. They are ideal
drying solutions for fine and dusty
materials, or materials that cannot be
exposed to drying air during processing.

20. Indirect dryers are better suited to more
specialty applications. They are ideal
drying solutions for fine and dusty
materials, or materials that cannot be
exposed to drying air during processing.
These types of applications are commonly
found with highly combustible materials,
organic materials such as grains and
pulps, and fine materials such as
pigments.

21. Conclusion
Both direct-fired and indirect-fired rotary dryers
are robust, reliable industrial drying solutions.

22. Conclusion
Both direct-fired and indirect-fired rotary dryers
are robust, reliable industrial drying solutions.
Direct-fired dryers are extremely flexible and
used throughout a variety of industries, while
indirect-fired dryers are necessary for more
specialty applications.

23. has been a leader in the thermal processing
industry since the 1950s, providing process &
product development, feasibility testing,
maintenance services, and high-quality,
custom rotary dryers.
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