Understand the physics of air

How does air density affect fan performance?

By Randy Schrecengost April 20, 2022
Courtesy: CFE Media and Technology

Air is a physical mixture of gases (not a chemical compound) consisting primarily of nitrogen (78% by volume), oxygen (21%), other gases (less than 1%) and some amount of water vapor. Air does not have a uniform standard composition in all places and its density (mass of a unit volume) is always different. Density is important in air movement and air handling systems because density affects fan performance.

The density of air or any gas varies per the temperature and pressure conditions, governed roughly by the ideal gas density equation (ρ = PM/RT). Air density is directly proportional to pressure and inversely proportional to temperature. As temperature increases, air density decreases. At higher altitudes as the air pressure drops, there will be a significant decrease in air density.

And in the case of air, moisture content present in the air increases its density and makes it heavier. Because of this variability in density, many applications use a benchmark or a concept of “standard air density,” which is simply air density calculated at standard temperature and pressure conditions. The STP standard used in engineering is the density of 0.0765 pounds per minute/cubic feet for air. This is the density of dry air at an atmospheric pressure of 29.92 in Hg or 14.70 pounds/square inch at sea level.

Because air acts like a fluid, it will move from areas of higher pressures to areas where the pressure is lower. As air temperature increases, the volume increases and pressure decreases and it will also move up and possibly out of a space — similarly to ancient man’s air handling attempts. These passive movements of air as a function of pressure differentials in the environment or with thermal buoyancy using spaces with entry openings at the bottom and exits near the top can be fundamental strategies used in air movement. However, mechanical means using motorized fans or other equipment is most commonly used to induce the flow or movement of air.

Author Bio: Randy Schrecengost is a senior project manager and principal mechanical engineer with Stanley Consultants. He has extensive experience in design and in project and program management at all levels of engineering, energy consulting, facilities engineering and commissioning. He is currently ASHRAE Director Regional Chair for Region VIII and is a member of the Consulting-Specifying Engineer editorial advisory board.