Download - Objectives
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Objectives
• Finish heat exchanger • Heat exchanger performance• Dry HX vs. Vet heat exchanger • Example
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Air-liquid Tube heat exchanger
Plate heat exchanger
Heat exchangers
Air-air
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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 (11.3)
• NTU – absolute sizing (# of transfer units)• ε – relative sizing (effectiveness)
Criteria
NTU
ε P RP
cr
hphcpc cmcm ,, hphcpc cmcm ,,
cpc
oocmAU
,
hph
cpc
cmcm
,
,
hph
oocmAU
,
cpc
hph
cmcm
,
,
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ExampleFollowing HW2 problem
For the problem 9 HW assignment # 2 (process in AHU) calculate:a) Effectiveness of the cooling coilb) UoAo value for the CCInlet water temperature into CC is coil is 45ºF
AHU
M CC
RA
OA
Qcc=…..Btu/htM=81ºFtCC=55ºF
CC
(mcp)w
tc,in=45ºF
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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, ε, NTU• Might be iterative
• If you know ε, NTU• Calculate R,P and get F, temps
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Analysis of Moist Coils
1. Redo fin theory 2. Energy balance on fin
surface, water film, airIntroduce Lewis Number
3. Digression – approximate enthalpy
4. Redo fin analysis for cooling/ dehumidification (t → h)
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1. Redo Fin Theory
• Same result pLpLtanh
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2. Energy and mass balances
• Steady-state energy equation on air• Energy balance on water• Mass balance on water• Rewrite energy balance on water surface
• Reintroduce hg0 (enthalpy of sat. water vapor at 0 °C or °F)
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3. Define heat-transfer coefficient for wet surface
4. Fin analysis for wet finsHeat conduction only occurs in y-direction through water film
w
w
ocw
pa
ky
hbcwoh
,
1,
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Overview of Procedure
1. Same approach as for dry fin with addition of conduction through water film
2. Define “fictitous moist air enthalpy” define at water surface temperature
3. Define heat-transfer coefficient
4. Develop new governing equation
Lx
xΔhΔh
xΔhΔh
ykh
xΔh
BFFFF
F
woF
@0
0@
dd
dd
,2
2,
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Overall Heat Transfer Coefficients
• Very parallel procedure to dry coil problem• U-values now influenced by condensation• See Example 11.6 for details
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Approximate Expression for Mean Enthalpy Difference
1,,2
2,,1
1,,22,,1
lnrs
rs
rsrsm
hhhh
hhhhh
h1 enthalpy of entering air stream
h2 enthalpy of leaving air stream
hs,R,1 fictitious enthalpy of saturated air at entering refrigerant temp.
hs,R,2 fictitious enthalpy of saturated air at leaving refrigerant temp.
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Wet Surface Heat Transfer
• Approximate dry and wet wet-surface area and apply relevant equations
Reynolds number changes – empirical relationships
Questions:• Does a wet or a dry coil have higher or lower heat
exchange?• Does a wet or a dry coil have higher or lower pressure
drop?