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

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

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

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

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

Heat Exchanger Analysis - Δtm

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

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