1. Objectives Finish heat exchanger Heat exchanger performance
Dry HX vs. Vet heat exchanger
Example

2. Heat exchangers Air-liquid Tube heat exchanger Air-air
Plate heat exchanger

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

7. Example Following HW2 problem
AHU M
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 RA
tc,in=45ºF
Qcc=…..Btu/h
tM=81ºF
tCC=55ºF

8. 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

9. 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. 1. Redo Fin Theory
Same result

11. 2. Energy and mass balances
Steady-state energy equation on air
Energy balance on water
Mass balance on water
Rewrite energy balance on water surface
Reintroduce hg0 (enthalpy of sat. water vapor at 0 °C or °F)

13. 3. Define heat-transfer coefficient for wet surface 4
3. Define heat-transfer coefficient for wet surface Fin analysis for wet fins
Heat conduction only occurs in y-direction through water film

14. Overview of Procedure
Same approach as for dry fin with addition of conduction through water film
Define “fictitous moist air enthalpy” define at water surface temperature
Define heat-transfer coefficient
Develop new governing equation

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

16. Approximate Expression for Mean Enthalpy Difference
h1 enthalpy of entering air stream
h2 enthalpy of leaving air stream
hs,R,1 fictitious enthalpy of saturated air at entering refrigerant temp.
hs,R,2 fictitious enthalpy of saturated air at leaving refrigerant temp.

17. Wet Surface Heat Transfer
Approximate dry and wet wet-surface area and apply relevant equations
Reynolds number changes – empirical relationships
Questions:
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?

18. Enthalpy wheel

19. Psychrometrics of Enthalpy Wheel