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## Objectives

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**Objectives**- Finish Duct design - Learn about fans and fan curves**Pressures**• Static pressure • Velocity pressure • Total pressure – sum of the two above**Total and static pressure drops are proportional to square**of velocity Plot of pressure drop vs. volumetric flow rate (or velocity) is called system characteristic Duct Design**Non-circular Ducts**• Parallel concept to wetted perimeter**Dynamic losses**• Losses associated with • Changes in velocity • Obstructions • Bends • Fittings and transitions • Two methods • Equivalent length and loss coefficients**Loss Coefficients**ΔPt = CoPv,0**Example 18.7**• Determine total pressure drop from 0 to 4**Fans**• Driving force to move air in buildings • Raise pressure and produce flow • Two main types: • Centrifugal • Axial**Relevant Fan Parameters**• Total pressure rise • Static pressure rise • Power requirement • Efficiency • Note ηtot does not account for motor efficiency**Fan Laws**• “Approximate relationships” • Given two aerodynamically similar fans, can extrapolate performance data • Be very careful**Question**• How does fan power consumption vary with volumetric air flow rate?**Summary of Air Distribution Systems**• Select diffuser based on ADPI • Given a layout of duct system • Calculate total pressure drop • Identify fittings contributing most to pressure drop • Deal with non-circular ducts • Use fitting loss coefficient or equal length method • Select the fan based on fan and duct curve