1 of 8 MAXIMUM KILN SHELL TEMPERATURE Ricardo MosciINTRODUCTIONThe maximum recommended kiln shell temperature varies by plant, bycountry and by kiln manufacturer, despite the fact that most kiln shellsare made of low alloy carbon steel (v.g. ASTM C27). Kiln control roomalarms 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.
2 of 8In a snapshot, here are the reasons for so many questions.Old kilns shells have been exposed to creep for a long time and aremore prone to develop fatigue cracks than newer shells.Old refractory linings are usually infiltrated with salts and less proneto develop a new coating.Dolomite products have higher tendency to form a new coating thanmagnesia spinel products, and pure magnesia spinel products havefewer tendencies to form coating than impure magnesia spinelproducts. Magnesia chrome products exhibit the same coatability asdolomite products.The longer the shell span, the less it will resist high temperatureswithout sagging. Therefore, longer spans have more tendency todevelop permanent deformation than shorter spans.Hot spots near tires and bull gears require immediate action. Thesehot spots almost invariably force the kiln down.The longer the circumferential extension of the hot spot, the greaterthe risk of shell permanent deformation or collapse.Misaligned kilns induce localized stresses along the kiln length. If thehot spot coincides with an area of stress concentration, the shellsometimes elongates or twists beyond recovery.If the kiln shell is directly exposed to the elements and a heavyrainstorm hits the hot spot, the shell may develop cracks undersudden quenching. Sometimes the brick results severely crushed inthe hot spot area.The presence of cracks in the vicinity of the hot spot calls for aimmediate kiln shutdown to avoid shell splitting.
3 of 8THE PHYSICS OF KILN SHELLSKiln shells are made with structural rolled steel plate, such asA.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 / ºCElastic Modulus – 207 GPaPoisson Ratio – 0.3 in the elastic range, 0.5 in the plastic range.These properties, as stated before, are measured at roomtemperature. What happens to the shell strength as its temperatureis raised? It drops considerably, as shown in Fig. 1.It is interesting to notice that there is a gain in strength betweenroom temperature and 200 C, followed by a sharp loss in strength asthe temperature goes up. At 430 C the ultimate strength of the steeldrops from 75,000 p.s.i. to 50,000 p.s.i., a hefty 33% loss. Someinvestigators report a 50% strength loss for the same temperaturerange.
4 of 8 80 73 68 70 60 TENSILE STRENGTH 60 50 50 40 30 25 20 10 0 100 200 300 400 550 TEMPERATURE CELSIUSKILN SHELL DESIGN CHARACTERISTICSFrom a purely structural approach, the kiln shell may be compared to acontinuous “O” beam, support in several points along its axis, andsubject to a uniform load comprised of its own weight, the load weightand the refractory weight. Through finite elemental analysis thebending momentum and stress on the shell can be calculated at anypoint between tires, at any desired temperature. Mathematical modelinghas 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 avoidexcessive stress concentration at the rollers and tires. In these kilnsbrick crushing in the proximity of the shell became quite common.It is known to the industry that the kiln shell flattens under load, thusdeviating from its quasi-circular shape. This type of deviation is calledovality. Even at room temperature, without any load, the cross sectionof the kiln is not circular. The greater the ovality, the greater thepinching stress on the steel and on the refractory lining. In order tokeep the shell format under tires, the steel plate is made progressivelythicker towards the centerline of the tire. The point where the thickershell meets the normal shell is a point of great stress concentration asevidenced by frequent brick shifting at these areas.
5 of 8If excessive ovality and stiffening are bad for the kiln shell and therefractory lining, why not make thicker, more rigid shells? Because thealignment of the kiln shell is far from perfect. The imaginary axis of thekiln is not a straight line. During rainstorms, power failures, heat upand cooling, some parts of the shell develop into a crankshaft. As thekiln turns, tremendous Hertz pressure develops between rollers andtires. By resorting to relatively thin and elastic shells, kiln designers areable to divert the stress away from the tire stations.Other sources of stress concentration on the shell are misaligned rollersin the horizontal and vertical directions. The forces thus generated forcethe brick lining into diagonal and triangular patterns, followed bypartial or total crushing. Hot spots in these areas are usuallycatastrophic 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 360This table indicates that the impact of a hot spot will be different foreach kiln. The reader is encouraged to identify and justify the worsecase scenario on the table.HOT SPOT OR RED SPOT?A hot spot is the one that gets the production manager’s attention. A redspot is the one that gets the corporate office’s attention.Hot spots are isolated areas on the kiln shell with abnormally hightemperature. Hot spots are quickly detected by a shell scanner or with aportable infra-red pyrometer. They cannot be seen during the day, andthey can hardly be seen at night. Therefore, based on the visible
6 of 8radiation spectrum for hot surfaces, their maximum temperature mustbe below 600 C.Red spots differ from hot spots in that they are visible at night. While ahot spot is just a warning, a red spot always demands some kind ofaction from the kiln operator.Red spots can be temporary, if caused by sudden coating detachment. Ifthe brick is thick enough and not deeply densified with low meltingsalts, coating may develop again and remedy the situation. Red spotscaused by lining failure are not temporary and require a kiln shutdown.Only experienced operators, with good knowledge of the residual liningthickness, can tell the difference. Unfortunately, brick drillings are notmade available to kiln operators, despite the fact that it is a criticaldecision tool during emergencies.Red spots create a relatively small area on the shell that expands fasterthan the adjacent areas. Since the shell expansion is confined to a smallregion, the hindered expansion develops a tremendous amount ofpotential energy. Using the elastic modulus and the thermal expansioncoefficient of carbon steel, the amount of stress developed can becalculated and compared to the ultimate strength of the steel. A red spotgenerates 25 kgf/cm2 for every degree of temperature difference.Assuming that the steel surrounding the red spot can absorb half of thatstress, the residual stress will be 12.5 kgf/cm2. For a thermal gradient ofjust 200 ºC, the creep limit of the steel will be exceeded and its ultimatestrength will be almost reached.From the previous analysis it becomes evident that not only the value ofthe temperature is important, but mostly its distribution along the kilnlength and circumference. If the stress caused by kiln misalignment,ovality and distortion is added to the temperature stress, it is easy tounderstand how bubbles and large cracks develop on the kiln shell. It isjust a matter of time, load and temperature before permanent damageoccurs.
7 of 8QUESTIONS AND ANSWERSQ. 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.
8 of 8 Kiln shell badly damaged by heat.Hot spot along the kiln circumference.