Objectives Duct Design AND Diffuser Selection Relationship Between

Objectives • Duct Design AND Diffuser Selection

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

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

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

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

Diffusers » Perforated ceiling diffuser » Wall diffuser unit » Linear slot diffuser » Jet nozzle diffuser » Swirl diffuser » DV diffuser » Round conical ceiling diffuser » Floor diffuser » External louvre » http: //www. titus-hvac. com/techzone/ » http: //www. halton. com/halton/cms. nsf/www/diffusers » Square conical ceiling diffuser » Auditorium diffuser » Smoke damper

Low mixing Diffusers Displacement ventilation

Diffuser Selection Procedure » V = maximum volumetric flow rate (m 3/s, ft 3/min) » Qtot = total design load (W, BTU/hr) » Qsen = 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) • Select and locate diffusers, divide airflow amongst diffusers

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=(tlocal-taverage)-M(Vlocal-Vaverage) , 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) 2) 3) 4) 5) Find Q sensible total for the space Select type and number of diffusers Find V for each diffuser Find characteristic length Select the diffuser from the manufacturer data

Reading asignement • Chapter 18 • 18. 1 -18. 4 (including 18. 4)