What does kiln thermal analysis examine?

Besides combustion, raw-material reactions and heat exchange between gas and material, it computes heat escaping through refractory and shell. Shell temperature in particular reveals refractory wear or loss, and heat loss ties directly to fuel consumption. In rotary kilns, where material moves while the kiln turns, mixing in the material bed matters too.

What happens to shell temperature as refractory thins?

In the example with a 1400 °C hot face, 220 mm of brick gave a 350 °C shell, but wear to 100 mm raised it to 485 °C and to 60 mm, 578 °C. A red-hot shell leads to deformation and damage, so many kilns scan shell temperature continuously.

A layer of clinker (the burnt product) adhering to the brick in the burning zone, acting as insulation. In the example, 150 mm of coating lowered the shell from 350 to 207 °C and heat loss from 167 to 71 kW per metre. Stable coating also protects the brick from thermal and chemical attack.

Kiln Thermal Analysis

Category: Industrial Heat Treatment | Revised 2026-10-01

CAE visualization for kiln analysis theory - technical simulation diagram

Kiln (furnace) thermal analysis

Theory: heat through the kiln wall

Overview

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On our cement plant's rotary kiln, the shell is hot in just one area. What's happening?

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The kiln interior is protected by refractory brick, and in the burning zone the hot face exceeds 1400 °C. Shell temperature outside is set mostly by brick thickness and the clinker layer stuck to the brick (coating). A local hot spot suggests coating has fallen off there, the brick has thinned, or brick has dropped out. Computing wall heat transfer lets you infer the interior condition from shell temperature — and since heat loss drives fuel use, it matters for kiln energy efficiency too.

Wall heat balance

$$ q = \frac{T_{hot} - T_{shell}}{\sum_i t_i / k_i} = h_c\,(T_{shell} - T_a) + \varepsilon\sigma\left(T_{shell}^4 - T_a^4\right) $$

The sum of layer resistances from hot face to shell (coating, brick, steel) sets the heat flux through the wall; shell temperature settles where this balances convection and radiation from the shell exterior.

$$ q' = q\,\pi D \quad (\text{per metre of kiln length}) $$

Coffee Break Trivia

The invention of the rotating kiln

Until the 19th century, cement raw materials were burned in vertical shaft kilns loaded in batches, limiting output. In the 1880s Ransome in England devised the rotating cylindrical kiln, and once made practical in the US in the early 20th century, it allowed continuous feed and mass production. Kilns over 100 m long were built, but suspension preheaters using exhaust heat to preheat raw meal later shortened kilns and cut fuel use greatly. Today shell-temperature scanners serve as eyes into the kiln.

Worked example: the burning-zone wall

Burning zone of a 4.5 m diameter kiln: hot face 1400 °C, basic brick (2.5 W/mK), 30 mm steel shell. The exterior loses heat to 20 °C surroundings by convection (15 W/m²K) and radiation (emissivity 0.85):

Wall condition	Shell	Heat flux	Loss per metre
Brick 220 mm	350 °C	11.8 kW/m²	167 kW/m
Coating 150 mm + brick 220 mm	207 °C	5.0 kW/m²	71 kW/m
Brick worn to 100 mm	485 °C	22.5 kW/m²	318 kW/m
Brick worn to 60 mm	578 °C	33.3 kW/m²	471 kW/m

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With or without coating, heat loss differs by more than half.

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Coating conducts heat less well than brick, so it acts as insulation. Over a 20 m burning zone, heat loss is 3.35 MW without coating and 1.41 MW with it — about 96,000 GJ versus 41,000 GJ a year. Conversely, brick worn to 100 mm puts the shell at 485 °C, and near 600 °C the shell glows red and risks deforming. At a 300 °C shell, radiation is 54% of exterior loss, so ignoring it overestimates shell temperature. Inferring the interior from scanner readings needs a calculation with radiation and convection done properly.

Analysis workflow

Solve combustion, gas flow and material-bed temperature along the kiln for the hot-face distribution.
From layer resistances (coating, brick, shell) find shell temperature and heat loss.
Include both convection (rotation and wind) and radiation on the shell exterior.
Compare with scanner readings to infer remaining brick and coating condition.
Check thermal deformation of shell and tyres (riding rings) from the temperature field.

Coffee Break Trivia

“The red band on the scanner”

One day a kiln's shell scanner showed a red band of high temperature. Compared with wall heat-transfer calculations, the reading corresponded to brick worn below half or major coating loss. Adjusting combustion to shift the flame slightly rebuilt the coating and the temperature fell — but it rose again weeks later, and at the scheduled shutdown the brick was indeed locally thin. Linking shell-temperature changes to calculations makes shutdown planning easier.

Common mistakes

Mistakes and fixes

Mistake	Effect	Fix
Ignoring shell radiation	Shell temperature overestimated	Include radiation
Ignoring coating	Heat loss overestimated	Calculate per condition
Constant brick conductivity	Profile off	Use temperature dependence
Watching average shell temperature	Local wear missed	Circumferential and axial maps
Ignoring thermal deformation	Brick loosening	Check shell and tyre deformation

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I'd like to try related calculations.

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Try the furnace efficiency calculator, blackbody radiation simulator and radiation network simulator. Related processes include industrial furnace simulation, drying processes and glass forming.

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