Introduction

This chapter is adapted mainly from ASHRAE Fundamentals Handbook (2001). Air ducts deliver conditioned air from HVAC units to the space to be conditioned, and, depending on the design, may also bring fresh and recirculated air to the units. Proper design of ducts facilitates distribution of air that is balanced throughout the conditioned space, thereby avoiding areas of under supply as well as over supply. Improper design can additionally lead to excessive noise, discomfort, and adverse health effects. The ducts should be designed so that a proper balance between high flow rates, which cause noise and vibration, and low flow rates, which cause buildup of contaminants in the ducts and pose health risks, is achieved. Interior duct lining and outside insulation can also be used to reduce noise transmitted to the interior space.

Pressure Head and Energy Equation. A given fluid mass has three components of mechanical energy: kinetic energy (due to motion), potential energy (due to gravity and height), and pressure energy (due to fluid pressure). As this fluid mass travels, the total mechanical energy does not stay conserved along the path of flow and is dissipated due to energy losses. These losses are categorized as two different types: major losses due to presence of friction introduced by duct walls, and the so-called minor losses due to sudden expansions/contractions and fluid mixing in joints, junctions, fittings, equipment, inlets/outlets, and the like. The major friction losses are due to the momentum exchange between molecules and between...