Heat Transfer with Phase Changefmorriso/cm310/lectures/2019 heat lecture 20 old heat 09.pdfFilm...

Lecture 20 Applied Heat Transfer 2 CM3110 11/18/2019

1

© Faith A. Morrison, Michigan Tech U.

CM3110 Transport IPart II: Heat Transfer

1

Heat Transfer with Phase Change Evaporators and Condensers

Professor Faith Morrison

Department of Chemical EngineeringMichigan Technological University


2

See also Incropera et al., Fundamentals of Heat and Mass Transfer, 6th edition, 2007, Chapter 10, Boiling and Condensation

Heat Transfer with Phase Change


1. forced convection• laminar• turbulent

2. natural convectionNewton’s law of cooling

𝑞𝐴

ℎ 𝑇 𝑇

So far we have discussed heat transfer at a boundary due to convection in the fluid.


2


1. forced convection• laminar• 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.

• Important in evaporation, distillation• LARGE h•It’s important to know in which regime you operate•Each regime has different correlations


3

Newton’s law of cooling

𝑞𝐴

ℎ 𝑇 𝑇

So far we have discussed heat transfer at a boundary due to convection in the fluid.


vapor phase

liquid phase

satTBoiling

resistance to heat transfer

At low Δ𝑇, few bubbles form, and heat transfer is by natural convection.

As Δ𝑇 increases, more bubbles form, increasing convection (flow) in the liquid phase, increasing h.

heat


4

𝑇 𝑇

Heat Transfer with Phase ChangeΔ𝑇 𝑇 𝑇


3

vapor phase

liquid phase

satT Film Boiling

resistance to heat transfer

As Δ𝑇 increases further, a film appears and grows, increasing resistance to heat transfer

heat


5

𝑇 𝑇


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.


6


Δ𝑇 𝑇 𝑇

“excess temperature”


4


7


Because of high heat transfer rates and ℎmany engineering

devices operate in this regime


There are correlations for h for each regime.


8

For example: Nucleate boiling, horizontal surfaces

ℎ 1043 ΔT𝑞𝐴

16

ℎ 5.56 ΔT 16𝑞𝐴

240

Δ𝑇 𝐾𝑞𝐴

𝑘𝑊𝑚

ℎ𝑊𝑚 𝐾

Equations good for

these units:




5




9

For example: Nucleate boiling, vertical surfaces

ℎ 537 ΔT𝑞𝐴

3

ℎ 7.95 ΔT 3𝑞𝐴

63

Δ𝑇 𝐾𝑞𝐴

𝑘𝑊𝑚

ℎ𝑊𝑚 𝐾

Equations good for

these units:






10

For example: Nucleate boiling, forced convection inside tubes

ℎ 2.55Δ𝑇 𝑒

Δ𝑇 𝐾𝑞𝐴

𝑘𝑊𝑚

ℎ𝑊𝑚 𝐾

𝑝 𝑘𝑃𝑎

Equations good for

these units:




6




11

For example: Film boiling, horizontal tubes

ℎ 0.62𝑘 𝜌 𝜌 𝜌 𝑔 Δ𝐻 𝑇 0.4𝐶 , Δ𝑇

𝐷𝜇 Δ𝑇

Δ𝑇 𝑇 𝑇 𝐾

ℎ𝑊𝑚 𝐾

Equations good for these units:

𝑘

𝜌 ,𝜌𝑘𝑔𝑚

Δ𝐻𝐽𝑘𝑔

𝐷 𝑚

𝜇 𝑃𝑎 𝑠𝑔 𝑚/𝑠

𝑇𝑇 𝑇

2(All material properties at the film temperature)



vapor phase

condensed phase

satT

heat


12

Condensation

At low Δ𝑇, few droplets form,

As Δ𝑇 increases, more droplets form, increasing convection (flow).

Vertical plates; horizontal tubes important

𝑇 𝑇

Heat Transfer with Phase Change-CondensationΔ𝑇 𝑇 𝑇


7

vapor phase

condensed phase

satTsatTT

Dropwise Condensation

heat•high h•hard to maintain•not used in practice


13There are correlations for h for each regime.


vapor phase

condensed phase

satTsatTT

Film Condensation

heat

•film reduces h•very stable•often used


14

heat




8


Regimes of Heat Transfer during Condensation



15

For example: Film condensation, vertical surfaces, laminar flow

𝑁𝑢ℎ𝐿𝑘

1.13𝜌 𝜌 𝜌 𝑔Δ𝐻 𝑇 𝐿

𝜇 𝑘 Δ𝑇


ℎ𝑊𝑚 𝐾

𝑚𝑘𝑔𝑠


𝑘


Δ𝐻𝐽𝑘𝑔

𝐿 𝑚


𝑇𝑇 𝑇


Re4𝑚𝜋𝐷𝜇

1800

Heat Transfer with Phase Change-Condensation




16

For example: Film condensation, vertical surfaces, turbulent flow

𝑁𝑢ℎ𝐿𝑘

0.0077𝜌 𝑔L

𝜇Re .

ℎ𝑊𝑚 𝐾

𝑚𝑘𝑔𝑠


𝜌𝑘𝑔𝑚

𝐿 𝑚

𝑘𝑊𝑚


𝑇𝑇 𝑇


Re4𝑚𝜋𝐷𝜇

1800


Heat Transfer with Phase Change-Condensation


9




17

For example: Film condensation, outside horizontal cylinders, laminar flow

𝑁𝑢ℎ𝐿𝑘

0.725𝜌 𝜌 𝜌 𝑔Δ𝐻 𝑇 𝐷

𝑁𝜇 𝑘 Δ𝑇


ℎ𝑊𝑚 𝐾

𝑚𝑘𝑔𝑠

𝑇 𝐾


𝑘


Δ𝐻𝐽𝑘𝑔

𝐷 𝑚


𝑇𝑇 𝑇


Re4𝑚𝜋𝐷𝜇

1800



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:•viscosity, m•scale formation•liquor velocity•time in evaporator


18

heat

Applied Heat Transfer with Phase Change—Evaporators


10

steam

horizontal-tube evaporator

vertical-tube evaporator

•steam in tubes•liquor on outside•inexpensive, but poor liquid circulation•good for non-depositing, low 𝜇fluids

•liquor in tubes•steam on outside•liquid circulates by natural convection

steamsteam

Geankoplis, p492

downcomer

•hydrostatic head in tubes prevents boiling in tubes


19


steam

condensate

long-tube vertical evaporator

forced-circulation evaporator

•liquor in tubes•steam on outside•liquid circulates by natural convection•single pass•high liquid velocities

•liquor in tubes•steam on outside•liquid circulates by forced convection•good for high 𝜇 fluids

Geankoplis, p492© Faith A. Morrison, Michigan Tech U.

20



11

non-boiling zone

boiling, 2-phase zone

Long-tube vertical evaporators

g


21

For discussion of how to estimate 𝑈, see Geankoplis 4th

edition, p533

Decreasing �̅�

Increasing 𝑣


Greater efficiency may be obtained by operating several evaporators in series:

Multiple-Effect Evaporation


22

1P 2P 3P

1T 2T 3T

fT

sT



12



Norbert Rillieux: Inventor of Multiple-Effect Evaporation

steam steam condensate

feed

vapor

concentrated liquor

bT

bT

bT

bT

fT

sT

Single-Effect Evaporators


24



13

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


25(made white sugar possible)

pressure, temperature decrease


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)


26(used in sugar production, for example)



14

Boiling/Evaporation

• Industrially we operate either in nucleate boiling or film boiling regimes

• 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

• Industrially we operate in film condensation

• There are different correlations for different geometries

Vertical surfaces, laminar or turbulent Outside stack of horizontal cylinders

Evaporators – designed with the boiling regimes in mind


27

Summary


© Faith A. Morrison, Michigan Tech U.28

Next:

• Heat transfer with phase change• Evaporators• Radiation• DONE

Heat Transfer with Phase Changefmorriso/cm310/lectures/2019 heat lecture 20 old heat 09.pdfFilm...

Documents

Transcript of Heat Transfer with Phase Changefmorriso/cm310/lectures/2019 heat lecture 20 old heat 09.pdfFilm...