Objectives Finish heat exchangers Air Distribution Systems Diffuser

Objectives - Finish heat exchangers - Air Distribution Systems - Diffuser selection - Duct design - fluid dynamics review

Fin Efficiency • Assume entire fin is at fin base temperature • Maximum possible heat transfer • Perfect fin • Efficiency is ratio of actual heat transfer to perfect case • Non-dimensional parameter

Heat exchanger performance (11. 3) • NTU – absolute sizing (# of transfer units) • ε – relative sizing (effectiveness) Criteria NTU ε cr P RP

Example problem AHU For the problem 9 HW assignment # 2 (process in AHU) calculate: a) Effectiveness of the cooling coil b) Uo. Ao value for the CC Inlet water temperature into CC is coil is 45ºF M OA CC CC (mcp)w steam RA tc, in=45ºF Qcc=195600 Btu/h t. M=81ºF t. CC=55ºF

Summary • Calculate efficiency of extended surface • Add thermal resistances in series • If you know temperatures • Calculate R and P to get F, ε, NTU • Might be iterative • If you know ε, NTU • Calculate R, P and get F, temps

Reading Assignment • Chapter 11 - From 11. 1 -11. 7

Analysis of Moist Coils 1. Redo fin theory 2. Energy balance on fin surface, water film, air Introduce Lewis Number 3. Digression – approximate enthalpy 4. Redo fin analysis for cooling/ dehumidification (t → h)

Overall Heat Transfer Coefficients • Very parallel procedure to dry coil problem • U-values now influenced by condensation • See Example 11. 6 for details

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

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

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)