Gas Composition and Moisture: Correcting Blower Pressure and Power for Flue Gas, Biogas and Humid Air

By the NextAir Systems engineering team. Last reviewed October 2026.

Most blower correction advice is about temperature and altitude. Gas composition gets less attention, and it can matter as much. Flue gas, biogas and humid air all have densities different from dry air at the same temperature, and a blower sized on air will deliver the wrong pressure or draw the wrong power if the gas is not air.

We calculate the densities for five common gases from their molecular weights, and show what they mean for pressure and power. The temperature and altitude side is covered in our density correction guide, which this article extends.

The short version

• Gas density is pressure times molar mass divided by gas constant and temperature, so a lighter gas or a hotter gas has lower density.
• At the same temperature, humid air at 40 °C and 90% RH is 91% of the density of dry air at 20 °C, hot flue gas at 200 °C is 63%, and biogas is 89%.
• For the same blower at the same speed and volume flow, pressure and shaft power scale directly with density.
• A fan’s curve is published for a reference density, usually air at 20 °C, and AMCA 201 reminds designers that installed performance depends on the conditions and the system.^[1]

Density from molecular weight

An ideal gas has density ρ = P × M ÷ (R × T), where M is the molar mass, R is 8.314 J/mol·K and T is the absolute temperature. Temperature is the term everyone remembers. Molar mass changes just as much. Water vapor, at 18 g/mol, is lighter than the nitrogen and oxygen it displaces, so humid air is less dense than dry air. Carbon dioxide at 44 g/mol is heavier. Methane at 16 g/mol is much lighter.

The table calculates density at 101.3 kPa. The reference is dry air at 20 °C, 1.204 kg/m³ by this formula. The flue gas composition is an illustration of a gas-fired process, and the biogas one a typical digester mix. Your gas will differ.

Two results stand out. First, humid air at 40 °C is only 91% of the reference density, and the water vapor alone accounts for a drop of about two percentage points compared with dry air at the same temperature. Second, the 200 °C flue gas is about 63% of the reference, and temperature does most of that, with the heavier composition pulling slightly the other way.

What density does to a blower

A centrifugal blower is a volume machine. At a given speed, it moves roughly the same volume of whatever gas enters, and the pressure it develops and the power it absorbs scale with the density of that gas. The fan laws are explained in our fan laws guide. So a blower that develops 4,000 Pa on air at 20 °C develops less on a lighter gas, and absorbs less power.

Worked example. A blower rated for 4,000 Pa and 110 kW on air at 20 °C is used on hot flue gas at 63% of that density, at the same speed and volume flow. It develops about 2,520 Pa and absorbs about 69 kW. If the process needs 4,000 Pa, the fan must run faster, or be a bigger wheel, until the pressure matches, and the cold-start power must be allowed for, because the same fan on cold air at start-up draws the full 110 kW.

Checking your own gas in five minutes

1. Write down the volume fractions of each component and multiply by its molar mass, then add them to get the mixture’s molar mass.
2. Compute density with ρ = P × M ÷ (R × T) at the fan inlet temperature and pressure.
3. Divide by 1.20 kg/m³ to get the ratio to the standard air condition most fan curves use.
4. Multiply the fan’s published pressure and power at your speed and volume flow by that ratio.
5. Compare the result with what the process needs, and look at the cold start as well as the normal run.

If the gas is saturated, remember that cooling will condense water, which changes both composition and density downstream. In a duct that cools, the density at the fan inlet is not the density at the process.

The two traps

• Selecting on air and running on something else. If the process pressure is a fixed number of pascals, a lighter gas needs a faster or larger fan than the air selection suggests.
• Sizing the motor on the hot, light condition. The motor must also handle the cold start, when the gas is denser, and any off-design condition such as a cold day, a wet gas or an air-only commissioning run.
• Ignoring moisture. In humid climates the effect is small, a few percent, but it stacks with temperature and altitude.
• Gases that change. A biogas plant’s methane content varies with feed, so its density varies with it. Size for the range.
• Safety and materials. Flammable, toxic or corrosive gases bring requirements beyond density, which affect the choice of blower construction and are discussed in hazardous atmospheres.

What to put on the datasheet

1. The gas composition, or its molar mass, at the fan inlet.
2. Inlet temperature and pressure, with minimum and maximum.
3. Whether the gas is saturated, and any condensation risk.
4. The duty at each operating condition: start-up, normal, upset.
5. Whether the quoted pressure is at the process density or at standard air, as in reading a blower datasheet.

Where this fits with NextAir

NextAir blowers are selected for the actual gas, with density, temperature and moisture stated on the datasheet, in MS, SS and high-temperature construction. Send the gas composition and conditions to our centrifugal blower manufacturer team. Our centrifugal blower guide covers the range, and power generation is a typical flue gas duty.

Sources

1. AMCA / ANSI Webstore. AMCA 201-23 Fans and Systems.
2. Plant Engineering (US DOE Motor System BestPractices material). Building energy efficiency into fan systems.

Figures and tables on this page are calculated or compiled by NextAir Systems and you are welcome to reuse them with a link back to this article.

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