Rotary kilns are available in both direct-fired and indirect-fired configurations. Each type offers its own unique advantages and disadvantages. This presentation gives an overview of the two types and the advantages and disadvantages each has to offer.

1. Direct vs. Indirect
Rotary Kilns
Experts in process design and material
processing for over 65 years.

2. Rotary kilns are an advanced thermal
processing tool used to cause a chemical
reaction or physical change in a solid
material through the application of
high temperatures.

3. Rotary kilns are available in two configurations:
direct-fired, and indirect-fired.

4. Rotary kilns are available in two configurations:
direct-fired, and indirect-fired.
Each type offers distinct advantages and
disadvantages. And while there can be some
overlap in applications, in general, each type
is better suited for different applications.

5. The following will give an overview on each
type of configuration, as well as their
advantages and disadvantages.

6. Direct-Fired
Kilns

7. DIRECT-FIRED KILN

8. Direct-fired kilns rely on direct contact between
the material and process gas to process the
material.

9. Combustion gas is pushed through the drum in
either a co-current or counter-current fashion,
efficiently processing the material inside
the drum.

10. The main advantage to the direct-fired kiln is
that it is highly efficient.

11. The main advantage to the direct-fired kiln is
that it is highly efficient.
The direct contact between the material and
process gas allows for optimal heat transfer.

12. The main advantage to the direct-fired kiln is
that it is highly efficient.
The direct contact between the material and
process gas allows for optimal heat transfer.
However, a few disadvantages to working
with this type of kiln do exist…

13. More Off-Gases
Because a process gas is used to heat the
material, direct-fired kilns produce more
off-gases that will require treatment than
their indirect-fired counterparts.

14. Risk of Entrainment
Similarly, the direct contact with the
process gas can be a problem when
working with fine materials. The process
gas could potentially pick up and entrain
the material, carrying it out through the
exhaust system.

15. Risk of Entrainment
Similarly, the direct contact with the
process gas can be a problem when
working with fine materials. The process
gas could potentially pick up and entrain
the material, carrying it out through the
exhaust system.
The degree of entrainment depends on a
variety of factors, including gas velocity,
gas density, particle density, and shape.

16. In situations such as this, the design of the
kiln must center around permissible gas
velocities instead of heat transfer
requirements.

17. In situations such as this, the design of the
kiln must center around permissible gas
velocities instead of heat transfer
requirements.
For this reason, when working with fine
materials, an indirect kiln may be a more
suitable option.

18. Applications
Direct-fired kilns are the most common
equipment of choice when it comes to
thermal processing, because of the
efficiency they offer. They are commonly
used to process a variety of materials,
including:

19. Applications
Direct-fired kilns are the most common
equipment of choice when it comes to
thermal processing, because of the
efficiency they offer. They are commonly
used to process a variety of materials,
including:
• Proppants

20. Applications
Direct-fired kilns are the most common
equipment of choice when it comes to
thermal processing, because of the
efficiency they offer. They are commonly
used to process a variety of materials,
including:
• Proppants
• Minerals (Bauxite, Gypsum, Phosphates)

21. Applications
Direct-fired kilns are the most common
equipment of choice when it comes to
thermal processing, because of the
efficiency they offer. They are commonly
used to process a variety of materials,
including:
• Proppants
• Minerals (Bauxite, Gypsum, Phosphates)
• Specialty Ceramics and Clays

22. Applications
Direct-fired kilns are the most common
equipment of choice when it comes to
thermal processing, because of the
efficiency they offer. They are commonly
used to process a variety of materials,
including:
• Proppants
• Minerals (Bauxite, Gypsum, Phosphates)
• Specialty Ceramics and Clays
• Iron Ore

23. Applications
Direct-fired kilns are the most common
equipment of choice when it comes to
thermal processing, because of the
efficiency they offer. They are commonly
used to process a variety of materials,
including:
• Proppants
• Minerals (Bauxite, Gypsum, Phosphates)
• Specialty Ceramics and Clays
• Iron Ore
• And more…

24. Indirect-Fired
Kilns

25. INDIRECT-FIRED KILN

26. Unlike direct-fired kilns, indirect kilns do not
utilize direct contact between the
material and process gas to process
material.

27. Unlike direct-fired kilns, indirect kilns do not
utilize direct contact between the
material and process gas to process
material.
Instead, the drum is enclosed in a heat
shroud, which is externally heated. This
heats the drum itself, and the material is
processed through direct contact with
the shell of the drum.

28. The disadvantage to an indirect kiln is that it is
much less efficient than a direct kiln.

29. The disadvantage to an indirect kiln is that it is
much less efficient than a direct kiln.
However, processing with an indirect kiln offers
many advantages…

30. Inert Processing Environment
Indirect processing is beneficial because it
allows for the processing environment
inside the drum to be tightly controlled.
This is necessary when processing
materials that may form an undesirable
compound with nitrogen or oxygen at
high temperatures.

31. Inert Processing Environment
Indirect processing is beneficial because it
allows for the processing environment
inside the drum to be tightly controlled.
This is necessary when processing
materials that may form an undesirable
compound with nitrogen or oxygen at
high temperatures.
An indirect kiln allows for the processing to
occur in an inert environment.

32. Temperature Control
Indirect kilns also allow for the
temperature along the length of the kiln to
be tightly controlled as well.

33. Temperature Control
Indirect kilns also allow for the
temperature along the length of the kiln to
be tightly controlled as well.
This is beneficial when a material needs to
be brought up to temperature and then
held there for a specific amount of time.

34. No Risk of Entrainment
In addition to these advantages, indirect
kilns are also beneficial because they
avoid the risk of entrainment when
processing fine materials, which can be
an issue when processing in a direct kiln.

35. Applications
Indirect kilns are used throughout a variety
industries to process many different
materials. Commonly processed materials
include:

36. Applications
Indirect kilns are used throughout a variety
industries to process many different
materials. Commonly processed materials
include:
• Carbon Black

37. Applications
Indirect kilns are used throughout a variety
industries to process many different
materials. Commonly processed materials
include:
• Carbon Black
• Chemical Precipitates

38. Applications
Indirect kilns are used throughout a variety
industries to process many different
materials. Commonly processed materials
include:
• Carbon Black
• Chemical Precipitates
• Filter Cakes

39. Applications
Indirect kilns are used throughout a variety
industries to process many different
materials. Commonly processed materials
include:
• Carbon Black
• Chemical Precipitates
• Filter Cakes
• Finely Ground Solids

40. Applications
Indirect kilns are used throughout a variety
industries to process many different
materials. Commonly processed materials
include:
• Carbon Black
• Chemical Precipitates
• Filter Cakes
• Finely Ground Solids
• And more…

41. Conclusion
Both direct and indirect rotary kilns provide a
highly engineered solution to thermal
processing needs, each offering distinct
advantages and disadvantages that make
them better suited to different processes.

42. 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 kilns.

43. Want to learn more?
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