Objectives Air Distribution Systems -Diffuser selection -Duct design

Air Distribution System Design Describe room distribution basics Select diffusers Supply and return duct sizing

Forced driven air flow Diffusers Linear diffusers Grill (side wall) diffusers Horizontal one side Vertical

Diffusers types swirl diffusers wall or ceiling floor Valve diffuser ceiling diffuser

Coanda effect on ceiling and walls Coanda effect

Diffusers Perforated ceiling diffuserJet nozzle diffuser Square conical ceiling diffuser Round conical ceiling diffuser Wall diffuser unitSwirl diffuser Floor diffuser Auditorium diffuser DV diffuser External louvre Smoke damper Linear slot diffuser

Low mixing Diffusers Displacement ventilation

18.7

Diffuser Selection Procedure Select and locate diffusers, divide airflow amongst diffusers V = maximum volumetric flow rate (m 3 /s, ft 3 /min) Q tot = total design load (W, BTU/hr) Q sen = sensible design load (W, BTU/hr ) ρ = air density (kg/m 3, lbm/ft 3 ) Δt = temperature difference between supply and return air (°C, °F) Δh = enthalpy difference between supply and return air (J/kg, BTU/lbm)

Find Characteristic Length (L)

Indicator of Air Distribution Quality ADPI = air distribution performance index Fraction of locations that meet criteria: -3 °F < EDT < 2 °F or -1.5 °C < EDT < 1 °C Where, EDT = effective draft temperature Function of V and Δt (Eqn 18.1) EDT=(t local -t average )-M(V local -V average ), M=7 °C/(m/s) ADPI considers ONLY thermal comfort (not IAQ)

Ideal and Reasonable Throws

Select Register Pick throw, volumetric flow from register catalog Check noise, pressure drop

Summary of Diffuser Design Procedure 1)Find Q sensible total for the space 2)Select type and number of diffusers 3)Find V for each diffuser 4)Find characteristic length 5)Select the diffuser from the manufacturer data

Example 18.3 Q tot = 38.4 kBTU/hr Δh = 9.5 BTU/lbm a omission in text

Pressures Static pressure Velocity pressure Total pressure – sum of the two above

Relationship Between Static and Total Pressure

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

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

Loss Coefficients ΔP t = C o P v,0

Example 18.7 Determine total pressure drop from 0 to 4

Conversion Between Methods

Reading asignement Chapter (including 18.4)