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Objectives - Learn about fans and fan curves
- Solve examples related to duct design - Learn about fans and fan curves
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Frictional Losses
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Non-circular Ducts Parallel concept to wetted perimeter
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Dynamic losses Losses associated with Two methods
Changes in velocity Obstructions Bends Fittings and transitions Two methods Equivalent length and loss coefficients
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Loss Coefficients ΔPt = CoPv,0
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Electrical Resistance Analogy
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Example 18.7 Determine total pressure drop from 0 to 4
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Conversion Between Methods
Se Example 18.8 and compare with 18.7
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Fans Driving force to move air in buildings
Raise pressure and produce flow Two main types: Centrifugal Axial
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Relevant Fan Parameters
Total pressure rise Static pressure rise Power requirement Efficiency Note ηtot does not account for motor efficiency
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Balance Point
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Fan Performance
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Centrifugal fans
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Axial Fans
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Fan Laws “Approximate relationships”
Given two aerodynamically similar fans, can extrapolate performance data Be very careful
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Question How does fan power consumption vary with volumetric air flow rate?
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Controlling Air Flow
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Dampers
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Effect on Fan Curve
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VAV Operation
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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
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