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

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

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

Heat exchanger performance (Book section 11.3) NTU – absolute sizing (# of transfer units) ε – relative sizing (effectiveness) Criteria NTU ε P RP cr

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

Example Heat Exchangers Sensible and Enthalpy Wheels

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

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 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)

Computational Fluid Dynamics

CFD in Air Distribution Design Contaminant concentration in a kitchen

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

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

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 Note omission in text

Reading asignement Chapter 18 18.1-18.2 (including 18.2)