ABSTRACT
The design procedure for air duct systems is explained. The principles are shown from the use of Bernoulli’s equation for straight ducts, changes in duct cross-section, flows through branches, grilles and fans, through to a spreadsheet that can be used for duct routes. Both supply and extract air duct systems are used. The spreadsheet can be used for individual air ducts and systems having up to five branches. It can be expanded by the user to accommodate larger systems once the methods are understood. Node numbers are used throughout the duct systems, and these can easily be changed by the user for applications. The air temperature can be changed for each section of duct. Air pressure terms and methods of measurement are explained. Detailed pressure changes at duct fittings and fans are calculated. The worksheet calculates all the total and static air pressures throughout the duct system and in the ventilated room or space. The index duct route and the fan pressure rise are automatically selected. There is no need to refer to reference data, tables or charts to be able to calculate the correct sizes of air ducts, because sufficient data for many applications is on the worksheet. Data on fittings’ velocity pressure loss factors can be entered from other sources.
