1. Objectives Finish heat exchangers Air Distribution Systems
Diffuser selection
Duct design
fluid dynamics review

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

5. 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

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. Reading Assignment
Chapter 11
- From

8. 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)

10. Overall Heat Transfer Coefficients
Very parallel procedure to dry coil problem
U-values now influenced by condensation
See Example 11.6 for details

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

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

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

14. 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

15. Low mixing Diffusers Displacement ventilation

18. 18.7

19. 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)

20. Find Characteristic Length (L)

21. 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)

22. Ideal and Reasonable Throws

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

24. 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

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

26. Pressures Static pressure Velocity pressure
Total pressure – sum of the two above

27. Relationship Between Static and Total Pressure

28. 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

29. System Characteristic

30. Electrical Resistance Analogy

31. Frictional Losses

32. Non-circular Ducts
Parallel concept to wetted perimeter

33. Dynamic losses Losses associated with Two methods
Changes in velocity
Obstructions
Bends
Fittings and transitions
Two methods
Equivalent length and loss coefficients

34. Loss Coefficients
ΔPt = CoPv,0

36. Example 18.7
Determine total pressure drop from 0 to 4

37. Conversion Between Methods

38. Reading asignement
Chapter 18
(including 18.4)