Hot air ducting, kiln offtake and clinker cooler connections are high-temperature, almost-no-pressure duties where the whole plant is moving. A rotary kiln grows metres over its length as it comes up to temperature. Nothing rigid survives being connected across that, and the flexible element is there to let the plant expand rather than to carry anything.
What this service does to a hose
Temperature sets the grade and the grade sets the ceiling. SS 304 interlock is good to around 700 °C and SS 321 to around 800 °C as a metal limit. But the packing in the seam, where packing is fitted, has a lower limit than the metal does, and that is the number that actually governs an assembly. Quoting the metal temperature and ignoring the packing is how these get specified beyond what they can do.
Thermal growth is large and it is not subtle. A twenty metre duct in austenitic stainless going from ambient to 600 °C wants to grow roughly two hundred millimetres. That movement is going somewhere. The flexible element is the designed place for it, and it has to be sized for the full excursion including start-up and trip, not for the steady-state condition.
Grit and dust on top of the heat. Kiln gas carries dust, and dust at velocity is abrasive. That is the argument for interlock, whose heavy strip section is a wear allowance in a way that a corrugation wall is not.
Almost no pressure, and interlock is not pressure-tight. These are draught systems running at fractions of a bar, positive or negative. Interlock suits them because pressure containment is not required. Where the duct genuinely holds pressure, interlock is the wrong part.
What we would fit
| Element | What we would supply | Why |
|---|---|---|
| Element | SS 304 interlock to about 700 °C; SS 321 to about 800 °C | The grade sets the metal limit; check the packing limit separately |
| Packing | Fitted where the duct must resist dust or gas escape — and stated with its own limit | The packing usually governs before the metal does |
| Section | Heavy gauge strip where the gas is dust-laden | The strip thickness is the wear allowance |
| Length | Sized on the full thermal excursion including start-up and trip conditions | Steady state is not the design case on a kiln |
| Ends | Flanged, with the assembly designed to be replaced without cutting the duct | It is a maintenance item and should be planned as one |
What we would not fit, and what happens
Interlock on a duct that has to hold pressure. It leaks by design.
An element sized on steady-state growth. Start-up and trip are the extremes and they are what break it.
Annular corrugated hose on dust-laden gas. The convolution faces erode.
An element specified on the metal temperature alone. The packing limit is usually lower and it is the real ceiling.
What we need to size it
| We need | Because |
|---|---|
| Gas temperature, including the excursion at start-up and on trip | The extremes are the design case, not the running condition |
| Duct pressure or draught, positive or negative | It decides whether interlock is possible at all |
| Dust loading and particle character | It decides the strip gauge |
| Movement required and in which directions | Kiln growth is large and axial; the element has to allow it |
| Whether the seam must be gas or dust tight | It decides whether packing is fitted, and therefore the temperature ceiling |
| Duct size and flange arrangement | So it fits and can be changed out |