Calculating Air Flow Rate from Pressure and Diameter

March 29, 2021

To calculate flow rate from pressure, several variables must be measured. Mid-West Instrument has a long history of helping companies throughout the Midwest to achieve these calculations via a wide range of state-of-the-art gauges, transmitters, switches, and other instruments used in a variety of industries.

Standard Air Flow Piping System

In any piping system, the airflow rate is relative to pipe diameter. As pipe diameter increases, flow rate decreases, and pressure increases. Regardless of the size of the building or the type of machines that will be producing airflow in the system, the best way to maximize flow rate is by installing the system as a loop–for example, starting at the air compressor and looping around the space, back to the air compressor. This will increase flow rate, minimize pressure drop, and allow smaller pipes to be used, which decreases overall cost.

Bear in mind that the thread size of the port on the air compressor does not determine the ideal pipe size for the system. Instead, pipe size is calculated based on the length of your piping (which is dependent on building size) and the air compressor’s output. It’s important to remember that:

  • Compressed air gets restricted over long distances.
  • The airflow rate decreases the further the air is from the compressor.
  • The farther the system’s pipes must extend, the larger diameter of piping you’ll need.
  • Larger-diameter piping increases airflow rate but also increases system cost.

Air Flow vs. Water Flow

It’s helpful to keep in mind that airflow instruments and measurements differ from water flow measurements. The key reason for the difference is that air is compressible and water is not. The result is that while the calculating water flow rate is straightforward, the volume of a given mass of air may change significantly based on temperature and pressure variations that occur during the measurement process. Thus, calculating the airflow rate in a pipe may require correction for “before and after” pressure and temperature conditions around the flow-sensing element.

Bernoulli’s Principle

The relationship between airflow rate, diameter, and pressure can be explained using Bernoulli’s principle. This mathematical principle states that if the air is flowing through a tube and the tube’s diameter decreases, then the velocity of the air increases, the pressure decreases, and the flow remains constant so long as the air density is constant. This principle also holds true for gases so long as the gas is moving far below the speed of sound and does not vary in temperature (since this would cause an increase in volume).

Bernoulli’s equation is derived from the law of conservation of energy, in that, if the kinetic energy of the air increases due to increased velocity, the corresponding energy associated with pressure (energy per unit volume) must decrease.

Get Quality Measuring Instruments Today

For a terrific selection of high-quality, custom-made measuring instruments, trust Mid-West Instrument. We proudly serve numerous companies across the Midwest–just do an online search for “instrument manufacturer near me” and let us know how we can support your airflow measurement needs.