Heat Transfer Su Yongkang
School of Mechanical Engineering
# 1
HEAT TRANSFER
CHAPTER 11
Heat Exchangers
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 2
Heat Exchangers,LMTD Method
Where we’ve been ……
So far have focused on detailed heat transfer
analysis of specific conditions,such as external
heat transfer coefficient
Where we’re going:
Investigate methods for larger system level
analysis that combine all these modes of heat
transfer in heat exchangers
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 3
Heat Exchangers,LMTD Method
KEY POINTS THIS LECTURE
Types of heat exchangers,advantages and
disadvantages
Overall heat transfer coefficient,concept of
fouling factor
Log mean temperature difference
Application of LMTD to heat exchanger
analysis
Text book sections,§ 11.1 – 11.3
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 4
Heat Exchanger Types
inAT,
inAT,outAT,
inBT,
outBT,
outAT,
inBT,
outBT,
inAT,outAT,
inBT,
outBT,
Example:
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 5
Heat Exchanger Types (Cont’d)
outBT,
s id e ) ( s h e ll,inBT
s id e ) ( tu b e,inAT
outAT,
Shell and Tube:
s id e ) ( tu b e,inAT
outAT,
s id e ) ( s h e ll,inBT
outBT,
Shell and Tube:
Shell and Tube:
s id e ) ( s h e ll,inBT
outBT,
side) (tube,inA
T
outAT,
Shell and Tube,
(common in chemical process industry)
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 6
Heat Exchanger Types (Cont’d)
Shell and Tube:
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 7
Heat Exchanger Types (Cont’d)
Plate and Frame
Series of plates with flow channels embossed in
them,
The two fluids are guided through alternating
rows of the plates
Advantages,__________________________
Application pictured,Electrocoat paint in
automotive assembly plant
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 8
Heat Exchanger Types (Cont’d)
Plate and Fin
Dense array of plates that guide alternating
channels of fluids (typically air)
Series of fins connect the plates and greatly
increase the heat transfer area
Advantage,very large heat transfer surface area
per unit volume,
One common application,Aircraft
environmental control systems
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 9
inAT,outAT,
inBT,
outBT,
Overall heat transfer coefficient for HX
Recall from earlier the overall thermal resistance
concept:
Types of resistances involved with heat
exchangers (covered in previous sessions)
– Cold side internal convection
– Cold side fouling factor
– Conduction through wall
– Hot side external convection (smooth wall
or may involve fins)
– Hot side fouling factor
Review how
these were
calculated
hoho
hf
w
co
cf
co
hhcc
hAA
R
R
A
R
hA
AUAUUA
)(
1
)()()(
1
111
,,
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 10
Analysis of heat transfer
Total heat transfer rate is found through energy
balance,regardless of the HX type or flow path
For hot fluid,Energy balance:
Define
For the cold fluid:
(Note,no minus sign,-” in this equation,
since heat flow in)
hhphh
hhhphhhph
o u tin
dTcmdq
dqdTTcmTcm
EE
,
,,
inhotT,
incoldT,
outhotT,
outcoldT,q
Tcm p?
dq
dTTcm p
f l u i d )h o t f o r r a t ec a p a c i t y ( H e a t
ccc dTCdq?
hhh dTCdq
hhph Ccm?,?
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 11
Analysis of heat transfer (Cont’d)
Energy balance gives:
For the entire flow length
A convenient way to compute the heat transfer is
from the mean temperature difference between
the hot and cold fluids
Next,
Evaluation of?Tm different for parallel and
counter flow
dqdqdq hc
mTUAq ch TTT
hhcc dTCdTCdq
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 12
Analysis of parallel flow heat transfer
Parallel flow heat exchanger
At any location along the heat exchanger
Where:
So:
Integrating from the inlet to the outlet
inhotT,
incoldT,
outhotT,
outcoldT,dq TU d Adq
hhhhphh
c
cccccpcc
dTCdTcmdq
C
dqdTdTCdTcmdq
,
,
chch dTdTTdsoTTT,,D e f i n e
T
Td
CC
U d A
CC
T-d
TU d Adq
ch
ch
11
11
Eq,11.13
ch CC
UATT 11ln
1
2
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 13
Analysis of parallel flow heat transfer (Cont’d)
From the overall energy balance,total heat transfer:
thus:
Combining
For parallel flow:
inco u tcco u thinhh
inco u tccpcco u thinhhphh
TTCqTTCqso
TTcmqTTcmqq
,,,,
,,,,,,
an d,
incoutccouthinhh TT
qC
TT
qC
,,,,
a n d
ln,,,,
outin
incoutcouthinh
i
o
TT
q
UA
q
TT
q
TT
UA
T
T
Eq,11.14
lmTUAq
o utin
o utin
TT
TT
/lnT lm o utco utho
incinhin
TTT
TTT
,,
,,
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 14
Analysis of parallel flow heat transfer (Cont’d)
Temperature profile for parallel flow:
dqiT?
oT?
hdT
cdT
T?
In Out
Fig 11.7
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 15
Analysis of counter flow heat transfer
For counter flow:
Temperature profile for counter flow:
dqi
T?
oT?
hdT
cdT
T?
In Out
Fig 11.8
o utin
o utin
TT
TT
/lnT lm incouthout
outcinhin
TTT
TTT
,,
,,
PFl m,CFl m,TT
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 16
Typical use of the LMTD method:
Given:
Need to cool a certain mass flow rate of fluid A
from TA,i to TA,o using the fluid B at TB,i
Find:
Design / size the heat exchanger
Solution Method:
Use the overall energy balance to find
Select the heat exchanger type (based on the
other project needs,available resources,size and
weight considerations,etc.,etc.)
Select tube diameters and types of heat transfer
surfaces (fins,no fins,etc.)
Use to determine the needed
heat exchanger heat transfer area (? length)
hh TCq
lmTUqA /
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 17
Special cases
For a condensing vapor
For an evaporating liquid
What if Ch = Cc in a counter-flow HX?
ch CCo r
x
T
In Out
x
T
In Out
ch CCo r
x
T
In Out
CondT
EvapT
21 TT
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 18
Multipass and cross-flow heat exchangers
The equations are the same.
lmTUAq
hhcc dTCdTCdq
outin
outin
TT
TT
/lnT CFl m,incouthout
outcinhin
TTT
TTT
,,
,,
CFlmlm TFT,
Counter-flow conditions
To find F,please refer to the figures 11.10-13.
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 19
Typical Example
E11.1 ( textbook,pp619)
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 20
Heat Exchangers,LMTD Method
KEY POINTS THIS LECTURE
Various types of heat exchangers that are
commonly used in industry and product designs,
Understanding of when to consider using each
type.
Defined the fluid heat capacity:
Log mean temperature difference introduced again
Temperature distribution parallel vs,counterflow
f l u i d )h o t f o r r a t ec a p a c i t y ( H e a t,hphh cmC
ln
i
o
ino u t
L M T D
T
T
TTT
ocoho
icihi
TTT
TTT
,.
,.
,f l o w P a r a l l el
icoho
ocihi
TTT
TTT
,.
,.
,f l o wC o u n t er
dqiT?
oT?
hdT
cdT
T?dqiT? oT?
hdT
cdT
T?
Parallel Counterflow
School of Mechanical Engineering
# 1
HEAT TRANSFER
CHAPTER 11
Heat Exchangers
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 2
Heat Exchangers,LMTD Method
Where we’ve been ……
So far have focused on detailed heat transfer
analysis of specific conditions,such as external
heat transfer coefficient
Where we’re going:
Investigate methods for larger system level
analysis that combine all these modes of heat
transfer in heat exchangers
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 3
Heat Exchangers,LMTD Method
KEY POINTS THIS LECTURE
Types of heat exchangers,advantages and
disadvantages
Overall heat transfer coefficient,concept of
fouling factor
Log mean temperature difference
Application of LMTD to heat exchanger
analysis
Text book sections,§ 11.1 – 11.3
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 4
Heat Exchanger Types
inAT,
inAT,outAT,
inBT,
outBT,
outAT,
inBT,
outBT,
inAT,outAT,
inBT,
outBT,
Example:
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 5
Heat Exchanger Types (Cont’d)
outBT,
s id e ) ( s h e ll,inBT
s id e ) ( tu b e,inAT
outAT,
Shell and Tube:
s id e ) ( tu b e,inAT
outAT,
s id e ) ( s h e ll,inBT
outBT,
Shell and Tube:
Shell and Tube:
s id e ) ( s h e ll,inBT
outBT,
side) (tube,inA
T
outAT,
Shell and Tube,
(common in chemical process industry)
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 6
Heat Exchanger Types (Cont’d)
Shell and Tube:
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 7
Heat Exchanger Types (Cont’d)
Plate and Frame
Series of plates with flow channels embossed in
them,
The two fluids are guided through alternating
rows of the plates
Advantages,__________________________
Application pictured,Electrocoat paint in
automotive assembly plant
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 8
Heat Exchanger Types (Cont’d)
Plate and Fin
Dense array of plates that guide alternating
channels of fluids (typically air)
Series of fins connect the plates and greatly
increase the heat transfer area
Advantage,very large heat transfer surface area
per unit volume,
One common application,Aircraft
environmental control systems
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 9
inAT,outAT,
inBT,
outBT,
Overall heat transfer coefficient for HX
Recall from earlier the overall thermal resistance
concept:
Types of resistances involved with heat
exchangers (covered in previous sessions)
– Cold side internal convection
– Cold side fouling factor
– Conduction through wall
– Hot side external convection (smooth wall
or may involve fins)
– Hot side fouling factor
Review how
these were
calculated
hoho
hf
w
co
cf
co
hhcc
hAA
R
R
A
R
hA
AUAUUA
)(
1
)()()(
1
111
,,
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 10
Analysis of heat transfer
Total heat transfer rate is found through energy
balance,regardless of the HX type or flow path
For hot fluid,Energy balance:
Define
For the cold fluid:
(Note,no minus sign,-” in this equation,
since heat flow in)
hhphh
hhhphhhph
o u tin
dTcmdq
dqdTTcmTcm
EE
,
,,
inhotT,
incoldT,
outhotT,
outcoldT,q
Tcm p?
dq
dTTcm p
f l u i d )h o t f o r r a t ec a p a c i t y ( H e a t
ccc dTCdq?
hhh dTCdq
hhph Ccm?,?
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 11
Analysis of heat transfer (Cont’d)
Energy balance gives:
For the entire flow length
A convenient way to compute the heat transfer is
from the mean temperature difference between
the hot and cold fluids
Next,
Evaluation of?Tm different for parallel and
counter flow
dqdqdq hc
mTUAq ch TTT
hhcc dTCdTCdq
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 12
Analysis of parallel flow heat transfer
Parallel flow heat exchanger
At any location along the heat exchanger
Where:
So:
Integrating from the inlet to the outlet
inhotT,
incoldT,
outhotT,
outcoldT,dq TU d Adq
hhhhphh
c
cccccpcc
dTCdTcmdq
C
dqdTdTCdTcmdq
,
,
chch dTdTTdsoTTT,,D e f i n e
T
Td
CC
U d A
CC
T-d
TU d Adq
ch
ch
11
11
Eq,11.13
ch CC
UATT 11ln
1
2
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 13
Analysis of parallel flow heat transfer (Cont’d)
From the overall energy balance,total heat transfer:
thus:
Combining
For parallel flow:
inco u tcco u thinhh
inco u tccpcco u thinhhphh
TTCqTTCqso
TTcmqTTcmqq
,,,,
,,,,,,
an d,
incoutccouthinhh TT
qC
TT
qC
,,,,
a n d
ln,,,,
outin
incoutcouthinh
i
o
TT
q
UA
q
TT
q
TT
UA
T
T
Eq,11.14
lmTUAq
o utin
o utin
TT
TT
/lnT lm o utco utho
incinhin
TTT
TTT
,,
,,
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 14
Analysis of parallel flow heat transfer (Cont’d)
Temperature profile for parallel flow:
dqiT?
oT?
hdT
cdT
T?
In Out
Fig 11.7
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 15
Analysis of counter flow heat transfer
For counter flow:
Temperature profile for counter flow:
dqi
T?
oT?
hdT
cdT
T?
In Out
Fig 11.8
o utin
o utin
TT
TT
/lnT lm incouthout
outcinhin
TTT
TTT
,,
,,
PFl m,CFl m,TT
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 16
Typical use of the LMTD method:
Given:
Need to cool a certain mass flow rate of fluid A
from TA,i to TA,o using the fluid B at TB,i
Find:
Design / size the heat exchanger
Solution Method:
Use the overall energy balance to find
Select the heat exchanger type (based on the
other project needs,available resources,size and
weight considerations,etc.,etc.)
Select tube diameters and types of heat transfer
surfaces (fins,no fins,etc.)
Use to determine the needed
heat exchanger heat transfer area (? length)
hh TCq
lmTUqA /
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 17
Special cases
For a condensing vapor
For an evaporating liquid
What if Ch = Cc in a counter-flow HX?
ch CCo r
x
T
In Out
x
T
In Out
ch CCo r
x
T
In Out
CondT
EvapT
21 TT
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 18
Multipass and cross-flow heat exchangers
The equations are the same.
lmTUAq
hhcc dTCdTCdq
outin
outin
TT
TT
/lnT CFl m,incouthout
outcinhin
TTT
TTT
,,
,,
CFlmlm TFT,
Counter-flow conditions
To find F,please refer to the figures 11.10-13.
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 19
Typical Example
E11.1 ( textbook,pp619)
Heat Transfer Su Yongkang
School of Mechanical Engineering
# 20
Heat Exchangers,LMTD Method
KEY POINTS THIS LECTURE
Various types of heat exchangers that are
commonly used in industry and product designs,
Understanding of when to consider using each
type.
Defined the fluid heat capacity:
Log mean temperature difference introduced again
Temperature distribution parallel vs,counterflow
f l u i d )h o t f o r r a t ec a p a c i t y ( H e a t,hphh cmC
ln
i
o
ino u t
L M T D
T
T
TTT
ocoho
icihi
TTT
TTT
,.
,.
,f l o w P a r a l l el
icoho
ocihi
TTT
TTT
,.
,.
,f l o wC o u n t er
dqiT?
oT?
hdT
cdT
T?dqiT? oT?
hdT
cdT
T?
Parallel Counterflow
