Hot-start furnace fans: tested cold, corrected by the fan laws, sized for the cold-start spike

A furnace exhaust fan spends its life pulling thin, hot gas — but it gets tested in a cold shop, and it gets switched on for the first time on a cold morning. Those two facts, not the running duty, are what decide whether the fan trips its motor on start-up. Here is how we sized and started one real furnace fan — built for a heat-treatment OEM — so it neither over-current trips nor over-stresses the moment someone hits the button.

The fan, in served-range terms

It is a backward-curved centrifugal fan handling furnace exhaust — an induced-draft (ID) arrangement pulling combustion products off the heat-treatment line, rather than a forced-draft (FD) fan pushing cold combustion air in. The running duty is unremarkable until you notice the temperature:

Parameter	At the hot running duty
Volume flow (actual, at temperature)	~40,000 CMH
Fan static pressure	100 mmWC
Operating temperature	350 °C
Gas density at 350 °C	0.564 kg/m³
Gas density at 20 °C (cold start)	~1.20 kg/m³
Impeller power, hot (350 °C)	13.9 kW
Impeller power, cold open (20 °C)	29.6 kW
Motor	25 HP

At 350 °C the gas is less than half as dense as the air the same fan would move cold. That single number drives everything below.

Tested cold, corrected by the fan laws

We cannot put 350 °C gas through a shop test bay. So the fan is performance-tested cold — at ambient, standard-density air — to the IS 4894 / AMCA 210 method, with the impeller balanced to the ISO 1940 G2.5 grade and vibration recorded against ISO 14694 BV3 tolerances. The catalogue and the witnessed test curve are therefore cold curves.

The hot operating point is derived from those cold curves by the fan laws. Pressure and shaft power both scale linearly with gas density at a fixed speed and flow — this is the density law, the sibling of the speed-cube relationship covered in the cube law. To deliver 100 mmWC of hot static pressure at 350 °C, the cold test curve must show the same duty flow at a proportionally higher pressure; the hot absorbed power falls in the same proportion. Get the density basis wrong and you have quoted a different fan — which is exactly why we state the duty as static pressure at temperature, not a bare "100 mmWC" (see static, velocity & total pressure).

The cold-start power spike

Run the density law the other way and the problem appears. Cold, dense air at 20 °C is about 2.1× denser than the 350 °C process gas. If this fan were allowed to run wide open on a cold furnace, it would absorb the cold power, not the hot power:

• Hot, running at 350 °C → 13.9 kW at the impeller.
• Cold, open damper at 20 °C → 29.6 kW — the same 2.1× the density ratio predicts.

The motor is a 25 HP frame — comfortably above the hot running demand, hopelessly below the cold-open demand. Start this fan cold and open and it draws far past its rating and trips (or, worse, holds on and cooks the windings). The dense cold-start figure — not the hot duty — is the number that governs the start.

Start closed, on a soft starter

You resolve a cold-start spike one of two ways: fit a motor big enough to swallow the full cold-open power, or control the start so the fan never sees it. Fitting a ~50 HP motor to a fan that needs 14 kW hot means it idles half-loaded for its whole service life — poor power factor, poor economics. So this fan is a hot-start design, started under a defined procedure:

• Damper closed. A closed outlet damper throttles the flow at start, and a fan on a closed damper absorbs far less power than one running out on its curve — the cheapest possible protection against the cold spike. The damper is opened only as the system comes up to temperature and the gas thins. High-pressure fans are never left to start wide open.
• Soft starter. A backward-curved impeller of this size carries real inertia and needs high starting torque; a soft starter (or star-delta, mandatory here on a motor above 5 HP) limits inrush current and ramps the machine up without slamming the driveline.
• Never started stone-cold and open. The starting sequence is written into the datasheet, not left to the operator's judgement on the day.

The damper is doing double duty here — start-up protection and running control. That trade-off between a damper, IGVs and a VFD is its own decision, covered in damper vs IGV vs VFD.

What this means for your enquiry

For any kiln or furnace exhaust fan, three lines on the RFQ change the design: the operating temperature, the cold-start temperature, and the intended start procedure. Give us those and we size the motor against the real governing case and specify the start — rather than discovering the cold-start spike at commissioning. If you only remember one thing: a hot-gas fan is sized by the coldest air it will ever move, not the hottest. That is the heart of specifying the duty point.

Talk to us about a furnace or kiln exhaust fan →

Jitamitra Electro Engineering · Fan-engineering notes, written for the engineer.