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Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..
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Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Jan 05, 2016

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Page 1: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Theory of Heat Exchanging

P M V SubbaraoProfessor

Mechanical Engineering Department

I I T Delhi

Understanding of Qualities of An Ideal Heat Exchangers…..

Page 2: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Theoretical Base for Working of Heat Exchangers

• A Simple adiabatic Heat Exchanger model.

phihhot cTm ,, ,

pciccold cTm ,, ,

phehhot cTm ,, ,

pceccold cTm ,, ,

Hot fluid Cold fluid

hT

cT

Page 3: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Electrical Analogy of A Finite Heat Exchanger

Hot fluid Cold fluid

hT

cT

Page 4: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Basic Information for Overall Resistance/Conductance

Page 5: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Heat Transfer between two fluids separated by finite thick surface wall

Page 6: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

First Level Engineering Compromise: Area

chgainiceccpcold TTLUDQTTcm 11,,,

Energy Balance:

2211 DUDU

chlossehihhphot TTLUDQTTcm 22,,,

1

2

2

1

D

U

D

U

Page 7: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Second Level Engineering Compromise: Teffective

Mean Temperature Difference

• A Simple adiabatic Heat Exchanger model.

phihhot cTm ,, ,

pciccold cTm ,, ,

phehhot cTm ,, ,

pceccold cTm ,, ,

Page 8: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Second Level Engineering Compromise: T

Page 9: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Simple Counter Flow Heat Exchangers

cpcold

hphot

cm

cmC

,

,

1C

Page 10: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Simple Counter Flow Heat Exchangers: C < 1

Page 11: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Simple Parallel Flow Heat Exchangers

Page 12: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Theoretical Base for Working of Heat Exchangers

• For an Infinitesimal Heat communication between cold and hot.

• A Simple adiabatic Heat Exchanger model.

phihhot cTm ,, ,

pciccold cTm ,, ,

phehhot cTm ,, ,

pceccold cTm ,, ,

Page 13: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Third Level Engineering Compromise: variation of U

U

Page 14: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

How many "zones" are involved in A HX?

Page 15: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Various Simple Zones (HX)

Page 16: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

How many "zones" are involved in A HX?

• "Zones" can best be defined as regimes of phase changes where the overall heat transfer coefficient (U) will vary. 

• Using T-Q (Temperature-Heat) diagrams are the best way to pinpoint zones. 

• The system is defined as co-current or countercurrent and the diagram is constructed. 

• The diagram on the left illustrates the use of T-Q diagrams. • These diagrams should accompany your basic (input-

output) diagram of the heat exchanger.   • Defining zones is one of the most important aspects of heat

exchanger design. 

Page 17: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Role of SG in Rankine Cycle

Page 18: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

Gas to Phase Changing Fluid Heat Exchange

Liquid Heating

Page 19: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

DPNL

Platen SHT

R

RHTR

LTSH

Economiser

APH ESP ID Fan

drum

Furnace

BCWpump

Bottom ash

stack

screentubes

Thermal Structure of A Boiler Furnace

Page 20: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

dido Di

TubularFlow

Annular Flow

Overall heat transfer coefficient of a used HX, based on outside area:

ofowallfi

i

out

ii

outoutsideOverall

hRRR

AA

hAA

U11

1,

A Simple Heat Exchanger

Page 21: Theory of Heat Exchanging P M V Subbarao Professor Mechanical Engineering Department I I T Delhi Understanding of Qualities of An Ideal Heat Exchangers…..

owall

ii

outoutsideOverall

hR

hAA

U11

1,

i

owall d

d

kR ln

2

1

Overall heat transfer coefficient of a new/cleaned HX, based on outside area:

Thermal resistance of any annular solid structure:

MeanoutDPHX TUAQ