Objective

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Objective

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

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Heat Exchanger Effectiveness (e)
Cmcp
Mass flow rate
Specific capacity of fluid
THin
TCout
THout
TCin
Location B
Location A
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Air-Liquid Heat Exchangers
Coil Extended Surfaces Compact Heat Exchangers

• Fins added to refrigerant tubes
• Important parameters for heat exchange?

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What about compact heat exchangers?

• Geometry is very complex
• Assume flat circular-plate fin

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Overall Heat Transfer

• Q U0A0?tm
• Overall Heat
• Transfer Coefficient

Mean temperature difference
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Heat Exchangers

• Parallel flow
• Counterflow
• Crossflow

Ref Incropera Dewitt (2002)
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Heat Exchanger Analysis - ?tm
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Heat Exchanger Analysis - ?tm
Counterflow
For parallel flow is the same
or
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Counterflow Heat Exchangers
Important parameters
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What about crossflow heat exchangers?

• ?tm F?tm,cf

Correction factor
?t for counterflow
Derivation of F is in the book
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• Example
• Calculate ?tm for the residential heat recovery
  system if mcp,hot 0.8 mc p,cold
• th,i72 ºF, tc,i32 ºF
• For e 0.5 ? th,o52 ºF, th,i48 ºF ? R1.25,
  P0.4 ? F0.89
• ?tm,cf(20-16)/ln(20/16)1
  7.9 ºF, ?tm17.9 0.8915.9 ºF

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Overall Heat Transfer

• Q U0A0?tm

Need to find this
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Heat Transfer
From the pipe and fins we will find
t

• tP,o

tF,m
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Resistance model

• Q U0A0?tm
• Often neglect conduction through tube walls
• Often add fouling coefficients

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Heat exchanger performance (Book section 11.3)

• NTU absolute sizing ( of transfer units)
• e relative sizing (effectiveness)

Criteria
NTU
e P RP
cr
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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

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Summary

• Calculate efficiency of extended surface
• Add thermal resistances in series
• If you know temperatures
• Calculate R and P to get F, e, NTU
• Might be iterative
• If you know e, NTU
• Calculate R,P and get F, temps