Pitfalls #3 – Humidity & dew point
Humidity & dew point: “it’s hot ≠ no moisture”
“The gas is at 180°C — there can’t be any condensation.”
This assumption is commonly made. And commonly wrong.
A gas temperature tells you nothing about its moisture content. A hot gas can be perfectly saturated with water vapour. It’s the temperature relative to the dew point that determines whether that vapour stays gaseous — or condenses.
The transient regime trap
The problem generally doesn’t occur under steady-state conditions.
At 180°C, if the dew point of the gas is around 60°C, everything is fine. But at start-up, shutdown, or during a load reduction, the temperature drops. If it falls below the dew point, condensation appears — sometimes localised, on the cold walls of the casing, on the shaft, in the upstream or downstream ductwork.
And if the gas contains SO₂, HCl, or other acidic compounds, that condensation doesn’t produce water — it produces acid. The resulting corrosion can be rapid and severe, well beyond what a simple 304 stainless steel selection can handle.
What we check at AirVision
When a request involves hot gases with a process origin — combustion fumes, drying effluents, industrial process gases — we systematically ask about the dew point, not just the nominal temperature.
This determines:
- the choice of materials (from carbon steel to alloyed refractory steels, depending on the aggressiveness of the condensate),
- whether heat tracing or insulation of cold zones is required,
- the identification of thermal bridges in the circuit — a poorly insulated zone or an untreated metal contact can be enough to locally drop the temperature below the dew point, even when the gas is globally hot.
- and sometimes the positioning of the fan itself within the circuit, to avoid exposure to critical cold-start phases.
Key takeaway
A hot gas can be humid. A humid gas can condense at start-up or shutdown. And if that gas is acidic, transient condensation is enough to permanently damage the equipment.
Nominal operating temperature is a starting point — not a guarantee that condensation is absent.