Air Supply Ducts

In the HVAC field, air distribution systems are used to supply cold/hot air to various spaces to keep the occupants comfortable and/or to keep equipment operational. Fresh air shall also be provided through the air distribution system to provide appropriate ventilation levels, in order to alleviate carbon dioxide (CO2) levels.


The method in which air is routed throughout a building is through the use of ducts, which can be constructed of metal, plastic or fiberglass. As a minimally competent engineer, one should be able to accomplish the following:

1) Determine the velocity in ducts.
2) Determine the pressure drop in ducts.
3) Size the duct based on required velocity or pressure drop.

Darcy Weisbach Equation

The equation used to determine the pressure drop in ducts is the Darcy Weisbach Equation.

Darcy Weisbach Equation for the Mechanical PE Exam

Converting to more commonly used HVAC and Refrigeration units:

Darcy Weisbach Equation for the Mechanical PE Exam

Although this equation is the governing equation for determining pressure drop, it is most often not used in the HVAC and Refrigeration field. In this field, airflow pressure drop calculations through are completed through the use of Friction Charts. Airflows, pressures, elevations and duct construction NOT normally encountered in the HVAC and Refrigeration should use the Darcy Equation.

Determining Velocity in Ducts for Pressure Calculations.

In the HVAC and Refrigeration field airflow is typically measured in cubic feet per minute or CFM. But velocity is the term that is required in determining the pressure drop of the air flow through a duct. In order to determine the velocity, the area of the duct must be found. Finding the area of ducts is a simple calculation for circular ducts, which are shown below.

Darcy Weisbach Equation for the Mechanical PE Exam

For rectangular and oval ducts, the area CANNOT be calculated as shown below. The rectangular and oval duct dimensions MUST FIRST be converted to Equivalent Diameter. Remember, that the pressure loss calculations require a circular shape.

Darcy Weisbach Equation for the Mechanical PE Exam

Determining Diameter of Duct

The Friction Charts and the Darcy Equation are typically a function of duct diameter. Thus no calculations are necessary for a circular duct. However, rectangular and oval ducts must be converted to an equivalent diameter circular duct before the equation can be properly completed. The equations for determining equivalent diameters are shown below.

Darcy Weisbach Equation for the Mechanical PE Exam

A quicker way to determine equivalent diameter is to use the Equivalent Diameter Tables for Rectangular and Oval ducts shown in ASHRAE Fundaments.

Determining Pressure Drop in Ducts

Once the equivalent diameter of the duct is found and the CFM is known, then simply refer to the Friction Charts for Ducts and simply read the pressure drop. This process is detailed more in Fan Sizing, later on in this section.