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

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

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

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

9. 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

10. Example Heat Exchangers Sensible and Enthalpy Wheels

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

12. 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)

13. Computational Fluid Dynamics

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

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

16. Forced driven air flow Diffusers
Grill (side wall)
diffusers
Linear diffusers
Vertical
Horizontal one side

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

18. Diffuser flow https://www.youtube.com/watch?v=LFgJn75LhGc
Near Human Body flow
Diffuser flow

19. Low mixing Diffusers Displacement ventilation

22. 18.7

23. 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)

24. Find Characteristic Length (L)

25. 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)

26. Ideal and Reasonable Throws

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

28. 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

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

30. Reading asignement
Chapter 18
(including 18.2)