Objectives Finish Duct design Learn about fans and
Objectives - Finish Duct design - Learn about fans and fan curves
Pressures • Static pressure • Velocity pressure • Total pressure – sum of the two above
Relationship Between Static and Total Pressure
Duct Design • 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
System Characteristic
Electrical Resistance Analogy
Frictional Losses
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
ΔPt = Co. Pv, 0 Loss Coefficients
Example 18. 7 • Determine total pressure drop from 0 to 4
Conversion Between Methods
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
Balance Point
Fan Performance
Centrifugal fans
Axial Fans
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?
Controlling Air Flow
Dampers
Effect on Fan Curve
VAV Operation
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
- Slides: 30