Air Conditioning Duct Design Principles to Minimize Energy Losses

Bad duct design can increase fan energy. Circulation of air in ducted system is dependent on static pressure. High static pressure reduces volume of air flow. Reduction in air volume creates loss of cooling, even if main plant is operating satisfactorily.

Straight runs of ducts put minimum resistance to air flow. Duct sections are fabricated keeping in view air velocity and pressure drop through a duct section. High air velocity creates noise and vibration in ducted system. Velocity close to main air handling unit is kept less than 1500 fpm. Out of air handling unit rooms in main ducts, optimum velocity ranges provide noiseless air distribution and minimum duct costs.

Duct section can be worked out when volume flow of air is known and velocity in each section is determined. Duct cross sectional area is calculated by dividing volume of air flow with velocity of air. Round ducts are smooth and easy to fabricate. Wherever space allows, round ducts are installed. Rectangular ducts are used where there is a limited space. These can be fabricated in any section. According to available space, height or width may be changed. These are easy to fit in ceiling spaces.

Pressure drop in ducts is another consideration for selection of ducted system. Flexible ducts put maximum pressure drop. Fans require more energy with flexible duct systems. These are used in smaller lengths for terminal connections. Flexible duct may be used in areas where round, oval or rectangular ducts are difficult to install.

Undue bends and transformations are avoided in ducts. Bends increase pressure drop. Full radius elbows are used for change of direction. Turning vanes inside reduce pressure drop. Dampers and control components are installed in accessible location. Main dampers are located in fan plenum inside AHU room. This area is accessible without causing any disturbance to building operations. Dampers in occupied areas are provided with ceiling access panels. Fan motor power depends on air volume circulation and total pressure drop in the longest run. Duct frictional losses are added starting from fan to far off air terminal. Keeping pressure drop low throughout ducting network saves air circulation energy.