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Objectives

Objectives. Analyze Rotary Heat Exchangers Use Psychometric chart - Start Air Distribution Systems - Diffuser selection. Sensible and Enthalpy Wheels . http://doas-radiant.psu.edu/4428.pdf. Example of Dual Wheel System . Psychrometrics of sensible and enthalpy wheel.

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Objectives

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  1. Objectives • Analyze Rotary Heat Exchangers • Use Psychometric chart - Start Air Distribution Systems - Diffuser selection

  2. Sensible and Enthalpy Wheels http://doas-radiant.psu.edu/4428.pdf

  3. Example of Dual Wheel System

  4. Psychrometrics of sensible and enthalpy wheel Example NMS Building HVAC system: Fans Enthalpy wheel Exhaust air Sensible wheel D A C B 1 2 Return air form building Fresh air Filters Hot and fresh air to building 7 Preheater 6 5 4 3 Heating coil Cooling coil Cool and fresh air to building

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

  6. 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

  7. Computational Fluid Dynamics

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

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

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

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

  12. Low mixing Diffusers Displacement ventilation

  13. 18.7

  14. 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

  15. Find Characteristic Length (L)

  16. 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)

  17. Ideal and Reasonable Throws

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

  19. 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

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

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

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