Objectives Air Distribution Systems Diffuser selection Duct design

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 Vertical Horizontal one side Grill (side wall) diffusers

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

Coanda effect on ceiling and walls Coanda effect https: //www. youtube. com/watch? v=Av. Lwq. RCb. GKY

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

18. 7

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 http: //www. titus-hvac. com/utility/getfile 2. aspx? fileid=8967

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

Example 18. 3 • Qtot = 38. 4 k. BTU/hr • Δh = 9. 5 BTU/lbma omission in text

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

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