1. Objective
Heat Exchangers
Define Heat Exchanger Effectiveness (ε)

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

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

4. Counterflow Heat Exchangers
Important parameters:

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

6. 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

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

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

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

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

12. 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

15. 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