1. Objectives Finish with Heat exchangers
Learn about Specifics of VAV and DOAS systems

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

4. 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

5. Reading Assignment
Chapter 11
- From

6. 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)

7. Energy and mass balances
Steady-state energy equation on air
Energy balance on water
Mass balance on water

9. 4. Fin analysis for wet fins
Heat conduction only occurs in y-direction through water film

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. Wet Surface Heat Transfer
If you know dry surface heat transfer
Reynolds number changes – empirical relationships
Approximate wet-surface
Does a wet or a dry coil have higher or lower heat exchange?
Does a wet or a dry coil have higher or lower pressure drop?

12. HW4(posted on the website)
Homework Assignment # 4 Pat A
DUE: April 9, 2013
Problem 11.1 (page 326)
Problem 11.7 (page 327)
Designs Problem
For the problem #9 from the HW assignment 2 (design process in AHU) calculate the needed a) Effectiveness of the cooling coil b) UoAo value for the CC Assume Inlet water temperature into CC is coil is 45ºF
Homework Assignment # 4 Pat B
DUE: April 16, 2013
Problem 18.13
Problem 18.16
Problem 18.19
Problem 18.20
Problem 18.27

13. Sensible and Enthalpy wheels

14. VAV Example One system Multiple Spaces/Zones
- VAV box for each space/zone

15. VAV System Terms & Equations (you can find them in ASHRAE Standard 62)
Fraction of minimum outdoor air in space/zone supply air
Xi = VOAi / VSAi
Critical space
Space with maximum Xi , Z=max(Xi)
Fraction of outdoor air in system supply air
X = sum(VOA ) / sum(VSA)
Required Fraction of outdoor air:
Y=X/(1+X-Z)

16. Other Issues Related to VAV
Reheaters (sizing and control)
Fan control
Minimum supply air
Occupancy sensors

17. DOAS System

18. DOAS with multi-split systems
Fresh air?

19. DOAS fresh air configurations

20. DOAS fresh air configurations

21. Issues Related to DOAS Split of sensible and latent load
Selection of hydronic system
Winter vs. summer operation
Winter operation with DX systems (heat pump)