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 P RP cr

Example problem AHU M OA CC CC RA tc,in=45ºF Qcc=195600Btu/h tM=81ºF For the problem 9 HW assignment # 2 (process in AHU) calculate: a) Effectiveness of the cooling coil b) UoAo value for the CC Inlet water temperature into CC is coil is 45ºF OA CC CC (mcp)w steam RA tc,in=45ºF Qcc=195600Btu/h tM=81ºF tCC=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 Redo fin theory Energy balance on fin surface, water film, air Introduce Lewis Number Digression – approximate enthalpy 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 Grill (side wall) diffusers Linear diffusers Vertical Horizontal one side

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

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

Low mixing Diffusers Displacement ventilation

18.7

Diffuser Selection Procedure Select and locate diffusers, divide airflow amongst diffusers V = maximum volumetric flow rate (m3/s, ft3/min) Qtot = total design load (W, BTU/hr) Qsen = sensible design load (W, BTU/hr) ρ = air density (kg/m3, lbm/ft3) 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=(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 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 kBTU/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 Two methods Changes in velocity Obstructions Bends Fittings and transitions Two methods Equivalent length and loss coefficients

Loss Coefficients ΔPt = CoPv,0

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)