Learn & Understand

Why Hot Flue Gas Speeds Up: Thermal Expansion in Ducts

The gas velocity calculator divides mass flow by density and area to find how fast gas actually travels through a duct. Buried in that division is density, and in combustion systems density is dominated by one thing: temperature. As gas heats, it thins, and to carry the same mass through the same duct, thinner gas must move faster. This is why hot flue gas rushes through a chimney far quicker than the cool air that fed the fire, and it has real consequences for how combustion equipment is sized.

Same Mass, Less Dense, More Speed

Mass is conserved as gas moves through a duct, the same kilograms per second pass every cross-section. But when the gas is heated, it expands and its density drops sharply. Since velocity is mass flow divided by density and area, a lower density forces a higher velocity to keep the mass flowing. The gas has not gained mass; it has spread out, and spread-out gas must travel faster to carry the same mass past a point each second. Heat, in effect, accelerates the flow.

The Combustion Amplifier

Combustion makes this dramatic, because it raises gas temperatures enormously. Cool air enters a burner, reacts, and leaves as hot combustion products at a temperature many times higher in absolute terms. The gas expands correspondingly, its density plummeting, and the resulting flue gas can move through the exhaust ducting far faster than the incoming air moved through the intake, even though it is essentially the same mass of matter. A duct sized comfortably for the cool intake can be badly overwhelmed by the fast, expanded hot exhaust.

Why This Trips Up Duct Design

The trap is designing exhaust ductwork using cool-gas density figures. Because the hot gas is so much less dense, its actual velocity can blow past the design limits meant to control erosion, noise, and pressure loss, all of which worsen at high velocity. This is why the calculator insists on density "at the actual operating temperature": using a cool-basis density would badly underestimate the true speed of the hot gas and could lead to an undersized, over-fast, noisy, erosion-prone exhaust path.

The Draft It Creates

The same expansion has a helpful side. Hot, low-density flue gas is lighter than the surrounding cool air, so it wants to rise, creating a natural upward draft in a chimney that helps pull combustion products out and fresh air in. The very thinning that speeds the gas also buoys it. So the temperature-driven density change the calculator accounts for is a double-edged effect: it accelerates the flow, demanding careful duct sizing, while also generating the draft that helps the whole system breathe.

For the continuity principle behind mass-conserving flow, see the Gas Flow Calculator; to check the resulting hot-gas velocity against a duct's limits, the Pipe Velocity Calculator.