Why Hot Air Rises: The Ideal Gas Law
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Open the Flue Gas Density Calculator →Hot flue gas rises because it is lighter than the cool air around it — and exactly how much lighter is governed by one of the most useful relationships in all of physics: the ideal gas law. This single equation ties together the pressure, temperature, and density of a gas, and it explains everything from chimney draft to hot-air balloons.
A Gas Spreads to Fill Its Space
Unlike a solid or liquid, a gas has no fixed volume — its molecules fly apart to fill whatever container holds them, and how tightly they pack depends on temperature and pressure. Heat a gas and its molecules move faster and spread out, lowering its density; cool it and they crowd together, raising it. The gas's density is a direct readout of these conditions.
The Ideal Gas Law
The ideal gas law captures this in a compact relationship linking a gas's pressure, temperature, and density (through its molar mass). Its most useful consequence here is that, at fixed pressure, density falls as temperature rises. Double the absolute temperature and the density roughly halves. This is why the same gas grows lighter as it heats, without adding or removing any matter.
| Temperature | Density |
|---|---|
| Cool | Higher — heavier |
| Hot | Lower — lighter |
Lighter Than Air
Hot flue gas, well above the temperature of the surrounding air, is therefore markedly less dense than that air — and less dense means buoyant. Just as a cork rises through water, the light hot gas rises through the heavier cold air. This buoyancy is the origin of chimney draft, of the hot-air balloon, and of the plume climbing from a stack.
Density Drives the System
Flue gas density is not an idle number — it underlies draft, stack velocity, and the pressure a moving gas exerts. Because it changes so strongly with temperature, engineers must calculate it at the actual conditions of interest. The ideal gas law provides it directly from temperature, pressure, and the gas's composition, tying the behavior of the exhaust back to fundamental physics.
Calculating Density
To compute flue gas density, use the Flue Gas Density Calculator. Feed it into draft with the Stack Draft Calculator, and find the mixture's molar mass with the Flue Gas Molecular Weight Calculator.
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