Lecture 20 Applied Heat Transfer 2 CM3110 11/18/2019
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ยฉ Faith A. Morrison, Michigan Tech U.
CM3110 Transport IPart II: Heat Transfer
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Heat Transfer with Phase Change Evaporators and Condensers
Professor Faith Morrison
Department of Chemical EngineeringMichigan Technological University
ยฉ Faith A. Morrison, Michigan Tech U.
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See also Incropera et al., Fundamentals of Heat and Mass Transfer, 6th edition, 2007, Chapter 10, Boiling and Condensation
Heat Transfer with Phase Change
Heat Transfer with Phase Change
1. forced convectionโข laminarโข turbulent
2. natural convectionNewtonโs law of cooling
๐๐ด
โ ๐ ๐
So far we have discussed heat transfer at a boundary due to convection in the fluid.
Lecture 20 Applied Heat Transfer 2 CM3110 11/18/2019
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Heat Transfer with Phase Change
1. forced convectionโข laminarโข turbulent
2. natural convection3. phase change
When a phase change takes place, the temperature on one side is CONSTANT, but the presence of boiling/condensing fluids produces heat transfer.
โข Important in evaporation, distillationโข LARGE hโขItโs important to know in which regime you operateโขEach regime has different correlations
ยฉ Faith A. Morrison, Michigan Tech U.
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Newtonโs law of cooling
๐๐ด
โ ๐ ๐
So far we have discussed heat transfer at a boundary due to convection in the fluid.
Heat Transfer with Phase Change
vapor phase
liquid phase
satTBoiling
resistance to heat transfer
At low ฮ๐, few bubbles form, and heat transfer is by natural convection.
As ฮ๐ increases, more bubbles form, increasing convection (flow) in the liquid phase, increasing h.
heat
ยฉ Faith A. Morrison, Michigan Tech U.
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๐ ๐
Heat Transfer with Phase Changeฮ๐ ๐ ๐
Lecture 20 Applied Heat Transfer 2 CM3110 11/18/2019
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vapor phase
liquid phase
satT Film Boiling
resistance to heat transfer
As ฮ๐ increases further, a film appears and grows, increasing resistance to heat transfer
heat
ยฉ Faith A. Morrison, Michigan Tech U.
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๐ ๐
Heat Transfer with Phase Changeฮ๐ ๐ ๐
Geankoplis 4th ed, p284
natu
ral
conv
ectio
n
nucl
eate
boi
ling
tran
sitio
n bo
iling
film
boi
ling
Regimes of Heat Transfer during Boiling
25.0T
radiation becomes important
There are correlations
for h for each regime.
ยฉ Faith A. Morrison, Michigan Tech U.
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Heat Transfer with Phase Changeฮ๐ ๐ ๐
ฮ๐ ๐ ๐
โexcess temperatureโ
Lecture 20 Applied Heat Transfer 2 CM3110 11/18/2019
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ยฉ Faith A. Morrison, Michigan Tech U.
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Heat Transfer with Phase Change
Because of high heat transfer rates and โmany engineering
devices operate in this regime
Geankoplis 4th ed, p284
There are correlations for h for each regime.
ยฉ Faith A. Morrison, Michigan Tech U.
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For example: Nucleate boiling, horizontal surfaces
โ 1043 ฮT๐๐ด
16
โ 5.56 ฮT 16๐๐ด
240
ฮ๐ ๐พ๐๐ด
๐๐๐
โ๐๐ ๐พ
Equations good for
these units:
Regimes of Heat Transfer during Boiling
Heat Transfer with Phase Changeฮ๐ ๐ ๐
Lecture 20 Applied Heat Transfer 2 CM3110 11/18/2019
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Geankoplis 4th ed, p285
There are correlations for h for each regime.
ยฉ Faith A. Morrison, Michigan Tech U.
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For example: Nucleate boiling, vertical surfaces
โ 537 ฮT๐๐ด
3
โ 7.95 ฮT 3๐๐ด
63
ฮ๐ ๐พ๐๐ด
๐๐๐
โ๐๐ ๐พ
Equations good for
these units:
Regimes of Heat Transfer during Boiling
Heat Transfer with Phase Changeฮ๐ ๐ ๐
Geankoplis 4th ed, p285
There are correlations for h for each regime.
ยฉ Faith A. Morrison, Michigan Tech U.
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For example: Nucleate boiling, forced convection inside tubes
โ 2.55ฮ๐ ๐
ฮ๐ ๐พ๐๐ด
๐๐๐
โ๐๐ ๐พ
๐ ๐๐๐
Equations good for
these units:
Regimes of Heat Transfer during Boiling
Heat Transfer with Phase Changeฮ๐ ๐ ๐
Lecture 20 Applied Heat Transfer 2 CM3110 11/18/2019
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Geankoplis 4th ed, p285
There are correlations for h for each regime.
ยฉ Faith A. Morrison, Michigan Tech U.
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For example: Film boiling, horizontal tubes
โ 0.62๐ ๐ ๐ ๐ ๐ ฮ๐ป ๐ 0.4๐ถ , ฮ๐
๐ท๐ ฮ๐
ฮ๐ ๐ ๐ ๐พ
โ๐๐ ๐พ
Equations good for these units:
๐
๐ ,๐๐๐๐
ฮ๐ป๐ฝ๐๐
๐ท ๐
๐ ๐๐ ๐ ๐ ๐/๐
๐๐ ๐
2(All material properties at the film temperature)
Regimes of Heat Transfer during Boiling
Heat Transfer with Phase Changeฮ๐ ๐ ๐
vapor phase
condensed phase
satT
heat
ยฉ Faith A. Morrison, Michigan Tech U.
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Condensation
At low ฮ๐, few droplets form,
As ฮ๐ increases, more droplets form, increasing convection (flow).
Vertical plates; horizontal tubes important
๐ ๐
Heat Transfer with Phase Change-Condensationฮ๐ ๐ ๐
Lecture 20 Applied Heat Transfer 2 CM3110 11/18/2019
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vapor phase
condensed phase
satTsatTT
Dropwise Condensation
heatโขhigh hโขhard to maintainโขnot used in practice
ยฉ Faith A. Morrison, Michigan Tech U.
13There are correlations for h for each regime.
Heat Transfer with Phase Change-Condensationฮ๐ ๐ ๐
vapor phase
condensed phase
satTsatTT
Film Condensation
heat
โขfilm reduces hโขvery stableโขoften used
ยฉ Faith A. Morrison, Michigan Tech U.
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heat
There are correlations for h for each regime.
Heat Transfer with Phase Change-Condensationฮ๐ ๐ ๐
Lecture 20 Applied Heat Transfer 2 CM3110 11/18/2019
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Geankoplis 4th ed, p289
Regimes of Heat Transfer during Condensation
There are correlations for h for each regime.
ยฉ Faith A. Morrison, Michigan Tech U.
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For example: Film condensation, vertical surfaces, laminar flow
๐๐ขโ๐ฟ๐
1.13๐ ๐ ๐ ๐ฮ๐ป ๐ ๐ฟ
๐ ๐ ฮ๐
ฮ๐ ๐ ๐ ๐พ
โ๐๐ ๐พ
๐๐๐๐
Equations good for these units:
๐
๐ ,๐๐๐๐
ฮ๐ป๐ฝ๐๐
๐ฟ ๐
๐ ๐๐ ๐ ๐ ๐/๐
๐๐ ๐
2(All material properties at the film temperature)
Re4๐๐๐ท๐
1800
Heat Transfer with Phase Change-Condensation
Geankoplis 4th ed, p289
There are correlations for h for each regime.
ยฉ Faith A. Morrison, Michigan Tech U.
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For example: Film condensation, vertical surfaces, turbulent flow
๐๐ขโ๐ฟ๐
0.0077๐ ๐L
๐Re .
โ๐๐ ๐พ
๐๐๐๐
Equations good for these units:
๐๐๐๐
๐ฟ ๐
๐๐๐
๐ ๐๐ ๐ ๐ ๐/๐
๐๐ ๐
2(All material properties at the film temperature)
Re4๐๐๐ท๐
1800
Regimes of Heat Transfer during Condensation
Heat Transfer with Phase Change-Condensation
Lecture 20 Applied Heat Transfer 2 CM3110 11/18/2019
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Geankoplis 4th ed, p285
There are correlations for h for each regime.
ยฉ Faith A. Morrison, Michigan Tech U.
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For example: Film condensation, outside horizontal cylinders, laminar flow
๐๐ขโ๐ฟ๐
0.725๐ ๐ ๐ ๐ฮ๐ป ๐ ๐ท
๐๐ ๐ ฮ๐
ฮ๐ ๐ ๐ ๐พ
โ๐๐ ๐พ
๐๐๐๐
๐ ๐พ
Equations good for these units:
๐
๐ ,๐๐๐๐
ฮ๐ป๐ฝ๐๐
๐ท ๐
๐ ๐๐ ๐ ๐ ๐/๐
๐๐ ๐
2(All material properties at the film temperature)
Re4๐๐๐ท๐
1800
Regimes of Heat Transfer during Condensation
Heat Transfer with Phase Change-Condensationฮ๐ ๐ ๐
Evaporators
hot fluid (steam)
boiling liquor
steamsatT ,
liquorsatT ,
large heat-transfer resistance on liquor-side
hl
hs
negligible heat-transfer resistance due to wall
Issues:โขviscosity, mโขscale formationโขliquor velocityโขtime in evaporator
ยฉ Faith A. Morrison, Michigan Tech U.
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heat
Applied Heat Transfer with Phase ChangeโEvaporators
Lecture 20 Applied Heat Transfer 2 CM3110 11/18/2019
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steam
horizontal-tube evaporator
vertical-tube evaporator
โขsteam in tubesโขliquor on outsideโขinexpensive, but poor liquid circulationโขgood for non-depositing, low ๐fluids
โขliquor in tubesโขsteam on outsideโขliquid circulates by natural convection
steamsteam
Geankoplis, p492
downcomer
โขhydrostatic head in tubes prevents boiling in tubes
ยฉ Faith A. Morrison, Michigan Tech U.
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Applied Heat Transfer with Phase ChangeโEvaporators
steam
condensate
long-tube vertical evaporator
forced-circulation evaporator
โขliquor in tubesโขsteam on outsideโขliquid circulates by natural convectionโขsingle passโขhigh liquid velocities
โขliquor in tubesโขsteam on outsideโขliquid circulates by forced convectionโขgood for high ๐ fluids
Geankoplis, p492ยฉ Faith A. Morrison, Michigan Tech U.
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Applied Heat Transfer with Phase ChangeโEvaporators
Lecture 20 Applied Heat Transfer 2 CM3110 11/18/2019
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non-boiling zone
boiling, 2-phase zone
Long-tube vertical evaporators
g
ยฉ Faith A. Morrison, Michigan Tech U.
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For discussion of how to estimate ๐, see Geankoplis 4th
edition, p533
Decreasing ๏ฟฝฬ ๏ฟฝ
Increasing ๐ฃ
Applied Heat Transfer with Phase ChangeโEvaporators
Greater efficiency may be obtained by operating several evaporators in series:
Multiple-Effect Evaporation
ยฉ Faith A. Morrison, Michigan Tech U.
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1P 2P 3P
1T 2T 3T
fT
sT
Applied Heat Transfer with Phase ChangeโEvaporators
Lecture 20 Applied Heat Transfer 2 CM3110 11/18/2019
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ยฉ Faith A. Morrison, Michigan Tech U.
Applied Heat Transfer with Phase ChangeโEvaporators
Norbert Rillieux: Inventor of Multiple-Effect Evaporation
steam steam condensate
feed
vapor
concentrated liquor
bT
bT
bT
bT
fT
sT
Single-Effect Evaporators
ยฉ Faith A. Morrison, Michigan Tech U.
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Applied Heat Transfer with Phase ChangeโEvaporators
Lecture 20 Applied Heat Transfer 2 CM3110 11/18/2019
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Multiple-Effect Evaporators
For each effect the vapor product becomes the source of heat for the subsequent effect 321
321TTTPPP
1P 2P 3P
1T 2T 3T
fT
sT
ยฉ Faith A. Morrison, Michigan Tech U.
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pressure, temperature decrease
Applied Heat Transfer with Phase ChangeโEvaporators
3T
3T
fT
sT
3T
1P 2P 3P
1T 2T 3T
fT
sT
3TTUAQ s
323
212
11
TTUAqTTUAqTTUAq s
3TTUAqQ si
Compare Single- and Multiple-Effect Evaporators
same capacity = same amount of heat transferred(but we did not have to pay for it all = more efficient)
ยฉ Faith A. Morrison, Michigan Tech U.
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Applied Heat Transfer with Phase ChangeโEvaporators
Lecture 20 Applied Heat Transfer 2 CM3110 11/18/2019
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Boiling/Evaporation
โข Industrially we operate either in nucleate boiling or film boiling regimes
โข There are different correlations for each regime and for different geometries
Nucleate boiling, horizontal surfaces Nucleate boiling, vertical surfaces Nucleate boiling, forced convection Film boiling, horizontal tube
Condensation
โข Industrially we operate in film condensation
โข There are different correlations for different geometries
Vertical surfaces, laminar or turbulent Outside stack of horizontal cylinders
Evaporators โ designed with the boiling regimes in mind
ยฉ Faith A. Morrison, Michigan Tech U.
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Summary
Heat Transfer with Phase Change
ยฉ Faith A. Morrison, Michigan Tech U.28
Next:
โข Heat transfer with phase changeโข Evaporatorsโข Radiationโข DONE
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