1. Objective Heat Exchangers Learn about different types
Define Heat Exchanger Effectiveness (ε)
Analyze how geometry affects ε
Solve some examples

2. Heat Exchanger Effectiveness (ε)
C=mcp
Mass flow rate
Specific capacity of fluid
THin
TCout
THout
TCin
Location B
Location A

3. Air-Liquid Heat Exchangers
Coil Extended Surfaces Compact Heat Exchangers
Fins added to refrigerant tubes
Important parameters for heat exchange?

4. What about compact heat exchangers?
Geometry is very complex
Assume flat circular-plate fin

5. Overall Heat Transfer Q = U0A0Δtm Mean temperature difference
Transfer Coefficient
Mean temperature
difference

6. Heat Exchangers Parallel flow Counterflow Crossflow
Ref: Incropera & Dewitt (2002)

7. Heat Exchanger Analysis - Δtm

8. Heat Exchanger Analysis - Δtm
Counterflow
For parallel flow is the same or

9. Counterflow Heat Exchangers
Important parameters:

10. What about crossflow heat exchangers?
Δtm= F·Δtm,cf
Correction
factor
Δt for
counterflow
Derivation of F is in the book:
………

11. Example:
Calculate Δtm for the residential heat recovery system if : mcp,hot= 0.8· mc p,cold
th,i=72 ºF, tc,i=32 ºF
For ε = 0.5 → th,o=52 ºF, th,i=48 ºF → R=1.25, P=0.4 → F=0.89
Δtm,cf=(20-16)/ln(20/16)=17.9 ºF, Δtm=17.9 ·0.89=15.9 ºF

12. Overall Heat Transfer
Q = U0A0Δtm
Need to find this

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

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

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

17. 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

20. 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