Filed Trip is Tomorrow at 9 am

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Filed Trip is Tomorrow at 9 am
https//www.google.com/maps/place/SaintEdwardsUn
iversity/_at_30.2273026,-97.7529731,1095m/data!3m1!1
e3!4m2!3m1!1s0x8644b492ae61201b0x1142c282cbe51336

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Objectives

• Continue with heat exchangers (ch.11)

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Coil Extended Surfaces Compact Heat Exchangers

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

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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
Q U0A0?tm
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What about crossflow heat exchangers?

• ?tm F?tm,cf

Correction factor
?t for counterflow
Derivation of F is in the text book
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Overall Heat Transfer

• Q U0A0?tm

Need to find this
AP,o
AF
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Heat Transfer

• From hot fluid to pipe
• Through the wall
• From the pipe and fins

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Resistance model

• Q U0A0?tm
• From eq. 1, 2, and 3
• We can often neglect conduction through pipe
  walls
• Sometime more important to add fouling
  coefficients

R Internal
R cond-Pipe
R External
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Example
The air to air heat exchanger in the heat
recovery system from previous example has flow
rate of fresh air of 200 cfm.

• With given
• Calculate the needed area of heat exchanger A0?

Solution Q mcp,cold ?tcold mcp,hot ?thot
U0A0?tm From heat exchanger side Q
U0A0?tm ? A0 Q/ U0?tm U0 1/(RInternalRCond
RFinRExternal) (1/100.00201/10) 4.95
Btu/hsfF ?tm 16.5 F From air side Q
mcp,cold ?tcold
200cfm60min/h0.075lb/cf0.24Btu/lbF16 3456
Btu/h Then A0 3456 / (4.9516.5) 42 sf
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For Air-Liquid Heat Exchanger we need 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

tF,m
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Fin Theory
k conductivity of material
hc,o convection coefficient
pLL(hc,o /ky)0.5
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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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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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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

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Reading Assignment

• Chapter 11
• - From 11.1-11.7

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Analysis of Moist Coils

• Redo fin theory
• Energy balance on fin surface, water film, air
• Introduce Lewis Number
• Digression approximate enthalpy
• Redo fin analysis for cooling/ dehumidification
  (t ? h)

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Energy and mass balances

• Steady-state energy equation on air
• Energy balance on water
• Mass balance on water

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