Objectives Describe room distribution basics Select diffusers Supply and return duct sizing

Pressures Static pressure – pressure not due to velocity Velocity pressure – pressure due to velocity Total pressure – sum of 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 Two methods Changes in velocity Obstructions Bends Fittings and transitions Two methods Equivalent length and loss coefficients

Loss Coefficients ΔPt = CoPv,0

ΔPt = CoPv,0

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

Duct Layout Overview Layout and select diffusers Use maximum air flow rates Locate air handling equipment and do all connections Space requirements Aesthetics Acoustics Assign fittings

Duct Layout (cont.) Find pressure drop data for all components and duct fittings Complete rough sizing using either Equal friction Static regain method Complete second sizing Check sound levels Access for maintenance

Summary 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 Next class: fans, duct layout, other equipment