Objective Heat Exchangers Define Heat Exchanger Effectiveness (ε)

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

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

Counterflow Heat Exchangers Important parameters:

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

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

Overall Heat Transfer Q = U0A0Δtm Need to find this

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

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

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

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

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