1. The maximum recommended kiln shell temperature varies significantly based on factors like the age of the kiln shell and refractory lining, thickness of the shell, distance between tires, and location of hot spots. 2. Hot spots indicate abnormally high temperatures under 600°C, while red spots that are visible at night always require action as they pose a greater risk of damage. 3. A red spot can generate significant stresses on the kiln shell due to the confined expansion, with a temperature difference of just 200°C potentially exceeding the steel's creep limit and nearing its ultimate strength.

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MAXIMUM KILN SHELL TEMPERATURE
Ricardo Mosci
INTRODUCTION
The maximum recommended kiln shell temperature varies by plant, by
country and by kiln manufacturer, despite the fact that most kiln shells
are made of low alloy carbon steel (v.g. ASTM C27). Kiln control room
alarms are set in a wide range, between 400 C and 550 C.
Three are the most frequent questions on the subject:
1. What is the maximum continuous shell temperature a kiln stands
without permanent damage to the shell?
2. What is the maximum spot temperature on the shell to force a kiln
shutdown?
3. Is it advisable to cool a hot spot with a water mist?
To properly answer to questions 1 and 2, the following additional
information is absolutely necessary:
 Age and condition of the kiln shell.
 Age of the refractory lining.
 Type of refractory lining.
 Distance between tires.
 Proximity of the hot spot to the tires or gear.
 Extension of the hot spot.
 Kiln alignment conditions.
 Whether the hot spot is exposed or under roof.
 If exposed, is it under rain?
 Presence and stability of coating on the lining.
 Shell temperature on the hot spot.
 The presence of shell cracks in the vicinity of the spot.

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In a snapshot, here are the reasons for so many questions.
Old kilns shells have been exposed to creep for a long time and are
more prone to develop fatigue cracks than newer shells.
Old refractory linings are usually infiltrated with salts and less prone
to develop a new coating.
Dolomite products have higher tendency to form a new coating than
magnesia spinel products, and pure magnesia spinel products have
fewer tendencies to form coating than impure magnesia spinel
products. Magnesia chrome products exhibit the same coatability as
dolomite products.
The longer the shell span, the less it will resist high temperatures
without sagging. Therefore, longer spans have more tendency to
develop permanent deformation than shorter spans.
Hot spots near tires and bull gears require immediate action. These
hot spots almost invariably force the kiln down.
The longer the circumferential extension of the hot spot, the greater
the risk of shell permanent deformation or collapse.
Misaligned kilns induce localized stresses along the kiln length. If the
hot spot coincides with an area of stress concentration, the shell
sometimes elongates or twists beyond recovery.
If the kiln shell is directly exposed to the elements and a heavy
rainstorm hits the hot spot, the shell may develop cracks under
sudden quenching. Sometimes the brick results severely crushed in
the hot spot area.
The presence of cracks in the vicinity of the hot spot calls for a
immediate kiln shutdown to avoid shell splitting.

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THE PHYSICS OF KILN SHELLS
Kiln shells are made with structural rolled steel plate, such as
A.S.T.M. A 36. The properties for this type of steel are:
Carbon – 0.25%
Manganese – 0.80% to 1.20%
Phosphorus – 0.04% Max.
Sulphur – 0.05% Max.
Silicon – 0.40%
Copper – 0.20% Min.
The mechanical properties of this type of steel at room temperature,
are:
Tensile Strength – 50,000 to 80,000 p.s.i.
Yield Strength – 36,000 p.s.i. Min.
Elongation – 20% Min.
Linear Thermal Expansion Coefficient – 11.7 x 10 –6 / ºC
Elastic Modulus – 207 GPa
Poisson Ratio – 0.3 in the elastic range, 0.5 in the plastic range.
These properties, as stated before, are measured at room
temperature. What happens to the shell strength as its temperature
is raised? It drops considerably, as shown in Fig. 1.
It is interesting to notice that there is a gain in strength between
room temperature and 200 C, followed by a sharp loss in strength as
the temperature goes up. At 430 C the ultimate strength of the steel
drops from 75,000 p.s.i. to 50,000 p.s.i., a hefty 33% loss. Some
investigators report a 50% strength loss for the same temperature
range.

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80 73
68
70
60
TENSILE STRENGTH
60
50
50
40
30 25
20
10
0
100 200 300 400 550
TEMPERATURE CELSIUS
KILN SHELL DESIGN CHARACTERISTICS
From a purely structural approach, the kiln shell may be compared to a
continuous “O” beam, support in several points along its axis, and
subject to a uniform load comprised of its own weight, the load weight
and the refractory weight. Through finite elemental analysis the
bending momentum and stress on the shell can be calculated at any
point between tires, at any desired temperature. Mathematical modeling
has proven that sagging is not the main source of stress in a rotary kiln.
In modern two-pier kilns, the shell is built purposely flexible to avoid
excessive stress concentration at the rollers and tires. In these kilns
brick crushing in the proximity of the shell became quite common.
It is known to the industry that the kiln shell flattens under load, thus
deviating from its quasi-circular shape. This type of deviation is called
ovality. Even at room temperature, without any load, the cross section
of the kiln is not circular. The greater the ovality, the greater the
pinching stress on the steel and on the refractory lining. In order to
keep the shell format under tires, the steel plate is made progressively
thicker towards the centerline of the tire. The point where the thicker
shell meets the normal shell is a point of great stress concentration as
evidenced by frequent brick shifting at these areas.

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If excessive ovality and stiffening are bad for the kiln shell and the
refractory lining, why not make thicker, more rigid shells? Because the
alignment of the kiln shell is far from perfect. The imaginary axis of the
kiln is not a straight line. During rainstorms, power failures, heat up
and cooling, some parts of the shell develop into a crankshaft. As the
kiln turns, tremendous Hertz pressure develops between rollers and
tires. By resorting to relatively thin and elastic shells, kiln designers are
able to divert the stress away from the tire stations.
Other sources of stress concentration on the shell are misaligned rollers
in the horizontal and vertical directions. The forces thus generated force
the brick lining into diagonal and triangular patterns, followed by
partial or total crushing. Hot spots in these areas are usually
catastrophic for the kiln shell, as the lining collapses instantly.
Table 1 contains some real situations encountered in U.S. kilns.
Shell Thickness Thickness Span Temperature
Diameter Under Tire Elsewhere
(mm) (mm) (mm) (mm) ºC
3,950 50 25 27,700 320
5,182 75 31 34,440 450
3,658 25 20 26,000 480
5,639 100 31 22,631 360
This table indicates that the impact of a hot spot will be different for
each kiln. The reader is encouraged to identify and justify the worse
case scenario on the table.
HOT SPOT OR RED SPOT?
A hot spot is the one that gets the production manager’s attention. A red
spot is the one that gets the corporate office’s attention.
Hot spots are isolated areas on the kiln shell with abnormally high
temperature. Hot spots are quickly detected by a shell scanner or with a
portable infra-red pyrometer. They cannot be seen during the day, and
they can hardly be seen at night. Therefore, based on the visible

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radiation spectrum for hot surfaces, their maximum temperature must
be below 600 C.
Red spots differ from hot spots in that they are visible at night. While a
hot spot is just a warning, a red spot always demands some kind of
action from the kiln operator.
Red spots can be temporary, if caused by sudden coating detachment. If
the brick is thick enough and not deeply densified with low melting
salts, coating may develop again and remedy the situation. Red spots
caused by lining failure are not temporary and require a kiln shutdown.
Only experienced operators, with good knowledge of the residual lining
thickness, can tell the difference. Unfortunately, brick drillings are not
made available to kiln operators, despite the fact that it is a critical
decision tool during emergencies.
Red spots create a relatively small area on the shell that expands faster
than the adjacent areas. Since the shell expansion is confined to a small
region, the hindered expansion develops a tremendous amount of
potential energy. Using the elastic modulus and the thermal expansion
coefficient of carbon steel, the amount of stress developed can be
calculated and compared to the ultimate strength of the steel. A red spot
generates 25 kgf/cm2 for every degree of temperature difference.
Assuming that the steel surrounding the red spot can absorb half of that
stress, the residual stress will be 12.5 kgf/cm2. For a thermal gradient of
just 200 ºC, the creep limit of the steel will be exceeded and its ultimate
strength will be almost reached.
From the previous analysis it becomes evident that not only the value of
the temperature is important, but mostly its distribution along the kiln
length and circumference. If the stress caused by kiln misalignment,
ovality and distortion is added to the temperature stress, it is easy to
understand how bubbles and large cracks develop on the kiln shell. It is
just a matter of time, load and temperature before permanent damage
occurs.

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QUESTIONS AND ANSWERS
Q. What is the maximum continuous temperature a kiln shell stands
without permanent damage to the shell?
A. 450 ºC or 870 ºF for a structural carbon steel shell.
Q. What is the maximum spot temperature on the shell to force a kiln
shutdown?
A. 550 ºC or 1022 ºF if the spot is permanent and persistent. If the red
spot is near or under a tire or bull gear, the shutdown procedure
must start immediately. Any persistent red spot covering more than
10% of the kiln circumference should follow the same previous
procedure.
Q. Is it acceptable practice to cool down a red spot with a water mist?
A. Provided the mist is a mist, and just a mist, yes, it can be tried
without serious consequences to the integrity of the shell. If properly
done, the procedure can avoid a costly permanent deformation to
the shell. If improperly done, the consequences to the shell can be
serious. The goal of this procedure is to cool down the hot air layer
that permanently envelops the kiln shell.

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Kiln shell badly damaged by heat.
Hot spot along the kiln circumference.