Short Notes: Heat Exchangers | Short Notes for Mechanical Engineering PDF Download

 Page 1


Heat Exchangers 
Heat Exchanger: A heat exchanger is an equipment where heat energy is 
transferred from a hot fluid to a colder fluid. The transfer of heat energy between 
the two fluids could be carried out:
• either by direct mixing of the two fluids and the mixed fluids leave at an 
intermediate temperature determined from the principles of conservation of 
energy, or
• by transmission through a wall separating the two fluids. The former types are 
called direct contact heat exchangers such as water cooling towers and jet 
condensers. The latter types are called regenerators, recuperator surface 
exchangers.
A heat exchanger is a piece of equipment built for efficient heat transfer from one 
medium to another. Boilers and condensers in thermal power plants are the 
example of large industrial heat exchangers. Heat exchangers are also abundant in 
chemical and process industries.
Heat exchangers are generally classified according to the relative directions of hot 
and cold fluids: •
• Parallel Flow - the hot and cold fluids flow in the same direction.
• Counter Flow - the two fluids flow through the pipe but in opposite directions.
• Cross-flow - A cross-flow heat exchanger has the two fluid streams flowing at 
right angles to each other.
• Condenser and Evaporator - In a condenser, the condensing fluid temperature 
remains almost constant throughout the exchanger and temperature of the 
colder fluid gradually increases from the inlet to the exit.
• Compact Heat Exchangers - these devices have close arrays of finned tubes 
or plates and are typically used when atleast one of the fluids is a gas.
Page 2


Heat Exchangers 
Heat Exchanger: A heat exchanger is an equipment where heat energy is 
transferred from a hot fluid to a colder fluid. The transfer of heat energy between 
the two fluids could be carried out:
• either by direct mixing of the two fluids and the mixed fluids leave at an 
intermediate temperature determined from the principles of conservation of 
energy, or
• by transmission through a wall separating the two fluids. The former types are 
called direct contact heat exchangers such as water cooling towers and jet 
condensers. The latter types are called regenerators, recuperator surface 
exchangers.
A heat exchanger is a piece of equipment built for efficient heat transfer from one 
medium to another. Boilers and condensers in thermal power plants are the 
example of large industrial heat exchangers. Heat exchangers are also abundant in 
chemical and process industries.
Heat exchangers are generally classified according to the relative directions of hot 
and cold fluids: •
• Parallel Flow - the hot and cold fluids flow in the same direction.
• Counter Flow - the two fluids flow through the pipe but in opposite directions.
• Cross-flow - A cross-flow heat exchanger has the two fluid streams flowing at 
right angles to each other.
• Condenser and Evaporator - In a condenser, the condensing fluid temperature 
remains almost constant throughout the exchanger and temperature of the 
colder fluid gradually increases from the inlet to the exit.
• Compact Heat Exchangers - these devices have close arrays of finned tubes 
or plates and are typically used when atleast one of the fluids is a gas.
Fouling Factor (Rf): The phenomenon of rust formation and deposition of fluid 
impurities on surface which responsible to increase thermal resistance with a 
corresponding drop in the performance of the heat exchange equipment.
' 4 k r - ' cltir
where, v = Overall heat transfer coefficient.
Logarithmic Mean Temperature Difference (LMTD)
LMTD is used to determine the temperature driving force for heat transfer in flow 
systems, most notably in heat exchangers. LMTD is a logarithmic average of the 
temperature difference between the hot and cold streams at each end of the 
exchanger.
Q >
^ H o t fluid
< D
k .
3
(0 4
W
\ ^ H o t fluid
fO
K
| A
E
V < 1
,c \
-------- 1 B
''''C old fluid °2
Cold fluid tC B
c2
Temp
Length 
Parallel flow
erature distribution for parallel
Length 
Counter flow
and counter flow heat exchanger
Heat transfer,
O = UA e'-- = UA6_
V
log,
0,
LMTD
< ? _ =
6;
lo?- l
3 = K - k
£ = K. ~ t,.
If the heat capacity of two fluid equal 
dm = 6i ~
Where, 07 = Temperature difference between two streams at end A 
02 = Temperature difference between two streams at end B . 
dm = Logarithmic mean temperature difference
Capacity Ratio: The capacity ratio is defined as the ratio of the minimum to 
maximum capacity rate.
Capacity ratio c = me, where c = Specific heat
I f mhck> m ece, R =
m hch
Page 3


Heat Exchangers 
Heat Exchanger: A heat exchanger is an equipment where heat energy is 
transferred from a hot fluid to a colder fluid. The transfer of heat energy between 
the two fluids could be carried out:
• either by direct mixing of the two fluids and the mixed fluids leave at an 
intermediate temperature determined from the principles of conservation of 
energy, or
• by transmission through a wall separating the two fluids. The former types are 
called direct contact heat exchangers such as water cooling towers and jet 
condensers. The latter types are called regenerators, recuperator surface 
exchangers.
A heat exchanger is a piece of equipment built for efficient heat transfer from one 
medium to another. Boilers and condensers in thermal power plants are the 
example of large industrial heat exchangers. Heat exchangers are also abundant in 
chemical and process industries.
Heat exchangers are generally classified according to the relative directions of hot 
and cold fluids: •
• Parallel Flow - the hot and cold fluids flow in the same direction.
• Counter Flow - the two fluids flow through the pipe but in opposite directions.
• Cross-flow - A cross-flow heat exchanger has the two fluid streams flowing at 
right angles to each other.
• Condenser and Evaporator - In a condenser, the condensing fluid temperature 
remains almost constant throughout the exchanger and temperature of the 
colder fluid gradually increases from the inlet to the exit.
• Compact Heat Exchangers - these devices have close arrays of finned tubes 
or plates and are typically used when atleast one of the fluids is a gas.
Fouling Factor (Rf): The phenomenon of rust formation and deposition of fluid 
impurities on surface which responsible to increase thermal resistance with a 
corresponding drop in the performance of the heat exchange equipment.
' 4 k r - ' cltir
where, v = Overall heat transfer coefficient.
Logarithmic Mean Temperature Difference (LMTD)
LMTD is used to determine the temperature driving force for heat transfer in flow 
systems, most notably in heat exchangers. LMTD is a logarithmic average of the 
temperature difference between the hot and cold streams at each end of the 
exchanger.
Q >
^ H o t fluid
< D
k .
3
(0 4
W
\ ^ H o t fluid
fO
K
| A
E
V < 1
,c \
-------- 1 B
''''C old fluid °2
Cold fluid tC B
c2
Temp
Length 
Parallel flow
erature distribution for parallel
Length 
Counter flow
and counter flow heat exchanger
Heat transfer,
O = UA e'-- = UA6_
V
log,
0,
LMTD
< ? _ =
6;
lo?- l
3 = K - k
£ = K. ~ t,.
If the heat capacity of two fluid equal 
dm = 6i ~
Where, 07 = Temperature difference between two streams at end A 
02 = Temperature difference between two streams at end B . 
dm = Logarithmic mean temperature difference
Capacity Ratio: The capacity ratio is defined as the ratio of the minimum to 
maximum capacity rate.
Capacity ratio c = me, where c = Specific heat
I f mhck> m ece, R =
m hch
I f mkck< m cc(. R =
»h c -.
Effectiveness of Heat Exchanger: The effectiveness of a heat exchanger is defined 
as the ratio of the energy actually transferred to the maximum theoretical energy 
transfer.
e =
(Q,.,) actual heat transfer_____
(Qi t j k )maximum possible heat transfer
Q o c : = mkC hG\
= ~ 0 
0n.x=C n in ^ - f< I)
If mcC n < m^Ch = > C m in - mccc
m cC ' K ~ rc) = f> : - I
mc C < _rfl) \ -*<l
If mccn < rrihC h = > cm in = mh C h
=> s** =
-h .)
«* C > . ({K
Where, Ch = Heat capacity rates for hot fluids 
Cc = Heat capacity rates for cold fluids
t. t.t andt
\ < 1
are the temperature streams for hot and cold fluids 
m/, = Mass of hot fluid 
mc = Mass of cold fluid
Number of Transfer Units (NTU): NTU is a measure of the size of heat exchanger, it 
provides some indication of the size of the heat exchanger.
N T U = ™
Where, U = Overall heat transfer coefficient
A = Surface area
C m in = Minimum capacity rate
If mhC h < mccc = > C m in = rn^Cc
UA
>.VTU =
m,ct
If m^Ch < mccc = > cm in - rrihC h
=¦ y r u =
m, c ._
Effectiveness for Parallel Flow Heat Exchanger: It is given by
_ l-e x p [— JV7T/(1 + £)]
l + £
Page 4


Heat Exchangers 
Heat Exchanger: A heat exchanger is an equipment where heat energy is 
transferred from a hot fluid to a colder fluid. The transfer of heat energy between 
the two fluids could be carried out:
• either by direct mixing of the two fluids and the mixed fluids leave at an 
intermediate temperature determined from the principles of conservation of 
energy, or
• by transmission through a wall separating the two fluids. The former types are 
called direct contact heat exchangers such as water cooling towers and jet 
condensers. The latter types are called regenerators, recuperator surface 
exchangers.
A heat exchanger is a piece of equipment built for efficient heat transfer from one 
medium to another. Boilers and condensers in thermal power plants are the 
example of large industrial heat exchangers. Heat exchangers are also abundant in 
chemical and process industries.
Heat exchangers are generally classified according to the relative directions of hot 
and cold fluids: •
• Parallel Flow - the hot and cold fluids flow in the same direction.
• Counter Flow - the two fluids flow through the pipe but in opposite directions.
• Cross-flow - A cross-flow heat exchanger has the two fluid streams flowing at 
right angles to each other.
• Condenser and Evaporator - In a condenser, the condensing fluid temperature 
remains almost constant throughout the exchanger and temperature of the 
colder fluid gradually increases from the inlet to the exit.
• Compact Heat Exchangers - these devices have close arrays of finned tubes 
or plates and are typically used when atleast one of the fluids is a gas.
Fouling Factor (Rf): The phenomenon of rust formation and deposition of fluid 
impurities on surface which responsible to increase thermal resistance with a 
corresponding drop in the performance of the heat exchange equipment.
' 4 k r - ' cltir
where, v = Overall heat transfer coefficient.
Logarithmic Mean Temperature Difference (LMTD)
LMTD is used to determine the temperature driving force for heat transfer in flow 
systems, most notably in heat exchangers. LMTD is a logarithmic average of the 
temperature difference between the hot and cold streams at each end of the 
exchanger.
Q >
^ H o t fluid
< D
k .
3
(0 4
W
\ ^ H o t fluid
fO
K
| A
E
V < 1
,c \
-------- 1 B
''''C old fluid °2
Cold fluid tC B
c2
Temp
Length 
Parallel flow
erature distribution for parallel
Length 
Counter flow
and counter flow heat exchanger
Heat transfer,
O = UA e'-- = UA6_
V
log,
0,
LMTD
< ? _ =
6;
lo?- l
3 = K - k
£ = K. ~ t,.
If the heat capacity of two fluid equal 
dm = 6i ~
Where, 07 = Temperature difference between two streams at end A 
02 = Temperature difference between two streams at end B . 
dm = Logarithmic mean temperature difference
Capacity Ratio: The capacity ratio is defined as the ratio of the minimum to 
maximum capacity rate.
Capacity ratio c = me, where c = Specific heat
I f mhck> m ece, R =
m hch
I f mkck< m cc(. R =
»h c -.
Effectiveness of Heat Exchanger: The effectiveness of a heat exchanger is defined 
as the ratio of the energy actually transferred to the maximum theoretical energy 
transfer.
e =
(Q,.,) actual heat transfer_____
(Qi t j k )maximum possible heat transfer
Q o c : = mkC hG\
= ~ 0 
0n.x=C n in ^ - f< I)
If mcC n < m^Ch = > C m in - mccc
m cC ' K ~ rc) = f> : - I
mc C < _rfl) \ -*<l
If mccn < rrihC h = > cm in = mh C h
=> s** =
-h .)
«* C > . ({K
Where, Ch = Heat capacity rates for hot fluids 
Cc = Heat capacity rates for cold fluids
t. t.t andt
\ < 1
are the temperature streams for hot and cold fluids 
m/, = Mass of hot fluid 
mc = Mass of cold fluid
Number of Transfer Units (NTU): NTU is a measure of the size of heat exchanger, it 
provides some indication of the size of the heat exchanger.
N T U = ™
Where, U = Overall heat transfer coefficient
A = Surface area
C m in = Minimum capacity rate
If mhC h < mccc = > C m in = rn^Cc
UA
>.VTU =
m,ct
If m^Ch < mccc = > cm in - rrihC h
=¦ y r u =
m, c ._
Effectiveness for Parallel Flow Heat Exchanger: It is given by
_ l-e x p [— JV7T/(1 + £)]
l + £
Where,
Effectiveness for the Counter Flow Heat Capacity: It is given by
1 -e\p [-JV T L T ( l - ^ ) ]
1 — Re\p[— XTU ( 1 - R ) ]
Heat Exchanger Effectiveness Relation: 
Concentric tube:
Parallel flow:
1 —e x p '— N (1 + R i]
_____ Li____ p — r- r
Counter flow:
1 — e x p N (l — R n r 
1 —R exp N (l —R)] ’
<1
c = N /( l + N) forR = 1
Cross flow (single pass): 
Both fluids unmixed:
€ = 1 — exp U R)fN ) jexp I — R (N)°'"S! — 1J
Cm a x mixed , Cm jn unmixed: 
c = (l/R ,1 —exp j — R (1—exp(— N )iJ| 
Cm in mixed, Cm a x unmixed:
€ = 1 - exp | - R _1 {l - exp ( — RN)}]
All exchangers (R = 0):
€ = 1 — exp (— N )