The Hazen-Williams equation is an empirical formula used to calculate pressure drop in water systems, widely adopted in HVAC design for its simplicity and accuracy at typical system velocities. It requires less computation than theoretical methods while providing reliable results for chilled water and condenser water applications.
Formula and Application
The equation accounts for pipe diameter, flow rate, length, and roughness coefficient (C-factor), ranging from 140 for new copper to 100 for aged steel pipes. Results are typically accurate within 10% for turbulent flow at velocities between 2-10 feet per second. ASHRAE standards recommend Hazen-Williams for quick design calculations in water systems.
Practical Use
HVAC designers prefer Hazen-Williams for preliminary sizing because it requires fewer inputs than Darcy-Weisbach and nomographs are readily available. Standard tables provide instant pressure drop values for common pipe sizes and flow rates. However, it becomes less accurate below 1 foot per second or above 10 feet per second, where Darcy-Weisbach provides better precision for extreme conditions.