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Objectives

Objectives. Finish with Exchangers - Start Air Distribution Systems - Diffuser selection. 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.

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Objectives

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  1. Objectives • Finish with Exchangers - Start Air Distribution Systems - Diffuser selection

  2. 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

  3. Heat Transfer tP,o From the pipe and fins we will find t tF,m

  4. Resistance model • Q = U0A0Δtm • Often neglect conduction through tube walls • Often add fouling coefficients

  5. Heat exchanger performance (Book section 11.3) • NTU – absolute sizing (# of transfer units) • ε – relative sizing (effectiveness)

  6. 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

  7. Example Heat Exchangers Sensible and Enthalpy Wheels

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

  9. L = characteristic length (m, ft) g = acceleration due to gravity (m/s2, ft/min2) T = absolute temperature (K, °R) β = 1/T (1/K, 1/ °R) v = kinematic viscosity (m2/s, ft2/min) Designing Room Airflow • Very complex problem • Pumped flow, buoyant flow (or mixed flow) • What non-dimensional parameters govern each regime? • Archimedes number = Ar = gβLΔT/v2

  10. Computational Fluid Dynamics

  11. CFD in Air Distribution Design Contaminant concentration in a kitchen

  12. Buoyancy driven flow:Example of airflow in a stairway Heater (radiator)

  13. Forced driven air flowDiffusers Grill (side wall) diffusers Linear diffusers Vertical Horizontal one side

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

  15. Low mixing Diffusers Displacement ventilation

  16. 18.7

  17. 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) Diffuser Selection Procedure • Select and locate diffusers, divide airflow amongst diffusers

  18. Find Characteristic Length (L)

  19. Indicator of Air DistributionQuality • 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)

  20. Ideal and Reasonable Throws

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

  22. 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

  23. Example 18.3 • Qtot = 38.4 kBTU/hr • Δh = 9.5 BTU/lbma Note omission in text

  24. Reading asignement • Chapter 18 • 18.1-18.2 (including 18.2)

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