What Is Volumetric Flow?

Natural Gas

• Consumers using natural gas pay their bills based on thousands of cubic feet. Since the mass or weight, a basic function of the actual number of molecules of natural gas or any gas varies as a function of flowing pressure and temperature, the volumetric expression of thousands of cubic feet is always expressed at standard conditions, which by convention for the North American gas industry is Ps = 14.73 psia and Ts = 60 degrees F. This fixes the exact number of molecules per cubic foot so that cubic feet means the same thing no matter where the gas is purchased, since a higher pressure with a fixed volume would mean more molecules, and a higher temperature with the fixed volume would mean less molecules.

Other Gases

• Flow is generally expressed as the standard volume flowing past a known cross sectional area at a given average velocity. So if the average velocity of air past a 1 square foot section of duct is 60 feet per minute, the volumetric flow rate would be 60 cubic feet in one minute, or 1 cubic foot per second.

Liquids

• Liquid volumetric flow rates are somewhat easier to understand because of the basic non-compressibility of liquids compared to gases, although most liquid volumetric expressions are also calculated at standard conditions because of slight variations in mass as temperature changes.

Units Conversion

• Standard conversion formulas allow cubic feet per minute to be converted to gallons per minute, liters per minute, per hour or per second. Small units for trace amounts of gas used in laboratories and hospitals would be cubic centimeters per minute. Similarly, milliliters apply to metric liquid flow rates, while liters per minute apply to larger flows. There are real time calculators right on the web that will take any gas or liquid flow units and convert them to any other with the push of a button.

Why Different Conventions

• Many are confused with volumetric and mass flows. Mass flow expressions tend to be used in scientific applications, although not always, and volumetric expressions prevail in commercial applications like buying a quart of milk.