5.1 shell-and-tube heat exchangers - homepageshomepages.wmich.edu/~leehs/me539/section 5.4...
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41
5.1 Shell-and-Tube Heat Exchangers
The most common type of heat exchanger in industrial applications is shell-and-tube heat
exchangers. The exchangers exhibit more than 65% of the market share with a variety of
design experiences of about 100 years. Shell-and tube heat exchangers provide typically
the surface area density ranging from 50 to 500 m2/m
3 and are easily cleaned. The design
codes and standards are available in the TEMA (1999)-Tubular Exchanger Manufacturers
Association. A simple exchanger, which involves one shell and one pass, is shown in
Figure 5.18.
Shell inlet
Shell outletTube inlet
Tube outlet
Baffles Endchannel
TubeShell
Shell sheet
Figure 5.18 Schematic of one-shell one-pass (1-1) shell-and-tube heat exchanger.
Baffles
In Figure 5.18, baffles are placed within the shell of the heat exchanger firstly to support
the tubes, preventing tube vibration and sagging, and secondly to direct the flow to have a
higher heat transfer coefficient. The distance between two baffles is baffle spacing.
Multiple Passes
Shell-and-tube heat exchangers can have multiple passes, such as 1-1, 1-2, 1-4, 1-6, and
1-8 exchangers, where the first number denotes the number of the shells and the second
number denotes the number of passes. An odd number of tube passes is seldom used
except the 1-1 exchanger. A 1-2 shell-and-tube heat exchanger is illustrated in Figure
5.19.
42
Shell inlet
Shell outletTube inlet
Tube outlet
Baffles
Endchannel
Passpartition
TubeShell
Figure 5.19 Schematic of one-shell two-pass (1-2) shell-and-tube heat exchanger.
Lt
Ds
B
Figure 5.20 Dimensions of 1-1 shell-and-tube heat exchanger
Dimensions of Shell-and-Tube Heat Exchanger
Some of the following dimensions are pictured in Figure 5.20.
L = tube length
tN = number of tube
pN = number of pass
sD = Shell inside diameter
bN = number of baffle
B = baffle spacing
The baffle spacing is obtained
1+=
b
t
N
LB (5.128)
43
Shell-Side Tube Layout
Figure 5.21 shows a cross section of both a square and triangular pitch layouts. The tube
pitch tP and the clearance tC between adjacent tubes are both defined. Equation (5.30) of
the equivalent diameter is rewritten here for convenience
heated
c
eP
AD
4= (5.129)
From Figure 5.21, the equivalent diameter for the square pitch layout is
( )o
ote
d
dPD
ππ 44
22 −= (5.130a)
From Figure 5.21, the equivalent diameter for the triangular pitch layout is
2
84
34
22
o
ot
ed
dP
Dπ
π
−
= (5.130b)
The cross flow area of the shell cA is defined as
T
tsc
P
BCDA = (5.131)
Pt
di
do
FlowPt
(a) (b)
do
di
CtCt
Figure 5.21 (a) Square-pitch layout, (b) triangular-pitch layout.
The diameter ratio dr is defined by
i
or
d
dd = (5.132)
Some diameter ratios for nominal pipe sizes are illustrated in Table C.6 in Appendix C.
The tube pitch ratio Pr is defined by
44
o
tr
d
PP = (5.133)
The tube clearance Ct is obtained from Figure 5.21.
ott dPC −= (5.134)
The number of tube Nt can be predicted in fair approximation with the shell inside
diameter Ds.
( )ShadeArea
DCTPN s
t
42π= (5.135)
where CTP is the tube count constant that accounts for the incomplete coverage of the
shell diameter by the tubes, due to necessary clearance between the shell and the outer
tube circle and tube omissions due to tube pass lanes for multiple pass design [1].
CTP = 0.93 for one-pass exchanger
CTP = 0.9 for two-pass exchanger (5.136)
CTP = 0.85 for three-pass exchanger
2
tPCLShadeArea ⋅= (5.137)
where CL is the tube layout constant.
CL = 1 for square-pitch layout (5.138)
CL = sin(60°) = 0.866 for triangular-pitch layout
Plugging Equation (5.137) into (5.135) gives
22
2
2
2
44 or
s
t
st
dP
D
CL
CTP
P
D
CL
CTPN
=
=ππ
(5.139)
Table 5.1 Summary of shell-and-tube heat exchangers
Description Equation
Basic Equations ( )oip TTcmq 1111 −= & (5.140)
( )iop TTcmq 2222 −= & (5.141)
Heat transfer areas of the
inner and outer surfaces
of an inner pipe
LNdA tii ⋅⋅⋅= π
LNdA too ⋅⋅⋅= π
(5.142a)
(5.142b)
45
Overall Heat Transfer
Coefficient
oo
i
o
ii
o
o
AhkL
d
d
Ah
AU
1
2
ln1
1
+
+
=
π
(5.143)
Tube side
Reynolds number µµρ
c
iim
DA
dmdu &==Re
p
tic
N
NdA
4
2π=
(5.144)
(5.144a)
Laminar flow
(Re < 2300)
14.03
1
PrRe86.1
==
s
i
f
i
DL
d
k
hdNu
µµ
0.48 < Pr < 16,700
0.0044 < ( )sµµ < 9.75
Use 66.3=DNu if 66.3<DNu
(5.145)
Turbulent flow
(Re > 2300)
Friction factor
( )( )( ) ( )1Pr2/7.121
Pr1000Re2/3221 −+
−==
f
f
k
hdNu D
f
i
D
6105Re3000 ×<< D [4]
2000Pr5.0 ≤≤
( )( ) 228.3Reln58.1
−−= Df
(5.146)
(5.147)
Shell side
Square pitch layout
(Figure 5.21) ( )
o
ote
d
dPD
ππ 44
22 −=
(5.148a)
Triangular pitch layout
(Figure 5.21)
2
84
34
22
o
ot
ed
dP
Dπ
π
−
=
(5.148b)
Cross flow area
t
tsc
P
BCDA =
(5.149)
Reynolds number µµρ
c
eem
DA
DmDu &==Re
(5.150)
46
Nusselt number
14.0
3155.0 PrRe36.0
==
sf
eo
k
DhNu
µµ
2000 <Re < 1 x 106
(5.151)
εεεε-NTU Method
Heat transfer unit
(NTU)
( )minp
oo
cm
AUNTU
&=
(5.152)
Capacity ratio ( )( )
max
min
p
p
rcm
cmC
&
&
=
(5.153)
Effectiveness
One shell (2, 4,.. passes)
( ) ( )[ ]( )[ ]212
1
212
1212
1exp1
1exp1112
+−−
+−++++=
r
rrr
CNTU
CNTUCCε
pNNTUNTU =1
(5.154)
Heat transfer unit (NTU) ( )
+−
+−=−
1
1ln1
212
E
ECNTU r
where ( )
( ) 2121
12
r
r
C
CE
+
+−=
ε
(5.155)
Effectiveness ( )( )
( ) ( )( )( )( ) ( )iip
iop
iip
oip
TTcm
TTcm
TTcm
TTcm
q
q
21min
2222
21min
1111
max −
−=
−
−==
&
&
&
&ε
(5.156)
Heat transfer rate ( ) ( )iip TTcmq 21min−= &ε (5.157)
Tube Side Pressure
Drop
Pressure drop 2v
2
114 ⋅
+
⋅=∆ ρp
i
t Nd
LfP
(5.158)
Laminar flow Df Re16= (5.159)
Turbulent flow ( )( ) 228.3Reln58.1
−−= Df
(5.160)
Shell Side Pressure
Drop
48
Example 5.2 Miniature Shell-and-Tube Heat Exchanger
A miniature shell-and-tube heat exchanger is designed to cool engine oil in an engine
with the engine coolant (50% ethylene glycol). The engine oil at a flow rate of 0.23 kg/s
enters the exchanger at 120°C and leaves at 115°C. The 50% ethylene glycol at a rate of
0.47 kg/s enters at 90°C. The tube material is Cr alloy (kw = 42.7 W/mK). Fouling factors
of 0.176x10-3 m
2K/W for engine oil and 0.353x10
-3 m
2K/W for 50% ethylene glycol are
specified. Route the engine oil through the tubes. The permissible maximum pressure
drop on each side is 10 kPa. The volume of the exchanger is required to be minimized.
Since the exchanger is custom designed, the tube size can be smaller than NPS 1/8 (DN 6
mm) that is the smallest size in Table C.6 in Appendix C, wherein the tube pitch ratio of
1.25 and the diameter ratio of 1.3 can be applied. Design the shell-and-tube heat
exchanger.
Figure E5.2.1 Shell and tube heat exchanger
MathCAD format solution:
Design concept is to develop a MathCAD modeling for a miniature shell-and-tube heat
exchanger and then seek the solution by iterating the calculations by varying the
parameters to satisfy the design requirements. It is reminded that the design requirements
are the engine oil outlet temperature less than 115°C and the pressure drop less than 10
kPa in each side of the fluids.
The properties of engine oil and ethylene glycol are obtained using the average
temperatures from Table C.5 in Appendix C.
Toil120°C 115°C+( )
2117.5 °C⋅=:= Tcool
90°C 100°C+( )
295 °C⋅=:=
(E5.2.1)
49
Engine oil (subscript 1)-tube side 50% Ethylene glycol (subscript 2)-shell side
ρ1 828kg
m3
:= ρ2 1020kg
m3
:=
(E5.2.2)
cp1 2307J
kg K⋅:= cp2 3650
J
kg K⋅:=
k1 0.135W
mK⋅:= k2 0.442
W
mK⋅:=
µ1 1.027102−
⋅N s⋅
m2
:= µ2 0.08 102−
⋅N s⋅
m2
:=
Pr1 175:= Pr2 6.6:=
The thermal conductivity for the tube material (Chromium alloy) is given
kw 42.7W
m K⋅:=
(E5.2.3)
Given information:
The inlet temperatures are given as
T1i 120°C:= T2i 90°C:= (E5.2.4)
The mass flow rates are given as
mdot1 0.23kg
s:= mdot2 0.47
kg
s:=
(E.5.2.5)
The fouling factors for engine oil and 50% ethylene glycol are given as
Rfi 0.176103−
⋅m2K⋅
W:= Rfo 0.35310
3−⋅
m2K⋅
W:=
(E5.2.6)
Design requirement:
The engine oil outlet temperature must be less than 115°C.
T1o 115°C≤ (E5.2.7)
The pressure drop on each side must be
∆P 10kPa≤ (E5.2.8)
50
Design parameters to be sought by iterations
Initially, estimate the following boxed parameters and then iterate the calculations with
different values toward the design requirements.
Ds 2.0in:= Shell inside diameter Ds 50.8mm= (E5.2.9)
Lt 15in:= Tube length Lt 381mm= (E5.2.10)
do1
8in:= Tube outside diameter do 3.175mm⋅=
(E5.2.11)
The diameter ratio (dr = do/di) is given as suggested in the problem description.
dr 1.3:= di1
dr
do⋅:= di 2.442mm⋅=
(E5.2.12)
The tube pitch ratio (Pr = Pt/do) is given as suggested in the problem description.
Pr 1.25:= (E5.2.13)
The tube pitch is then obtained from Equation (5.133).
Pt Pr do⋅:= (E5.2.14)
The baffle spacing is assumed and may be iterated, and the baffle number from Equation
(5.128) is defined.
B8
8in:= B 25.4mm=
(E5.2.15)
Nb
Lt
B1−:=
Nb 14= (E5.2.16)
The number of passes is defined by
Np 1:= (E5.2.17)
The tube clearance Ct is obtained from Figure 5.21 as
Ct Pt do−:= Ct 0.794mm⋅= (E5.2.18)
51
From Equation (5.136), the tube count calculation constants (CTP) up to three-passes are
given
CTP 0.93 Np 1if
0.9 Np 2if
0.85 otherwise
:=
(E5.2.19)
From Equation (5.138), the tube layout constant (CL) for a triangular-pitch layout is
given by
CL 0.866:= (E5.2.20)
The number of tubes Nt is estimated using Equation (5.139) and rounded off in practice.
Note that the number of tubes in the shell inside diameter defined earlier indicates the
compactness of a miniature exchanger. A 253-tube exchanger in a 2.25-inch shell outside
diameter is commercially available for a 2-inch shell diameter.
Ntube Ds do, Pr, ( ) π
4
CTP
CL
⋅Ds
2
Pr2do
2⋅
⋅:= Ntube Ds do, Pr, ( ) 138.189=
(E5.2.21)
Nt round Ntube Ds do, Pr, ( )( ):= Nt 138= (E5.2.22)
Tube side (Engine oil)
The crossflow area, velocity and Reynolds number are defined as
Ac1
π di2
⋅
4
Nt
Np
⋅:= Ac1 6.465 104−
× m2
=
(E5.2.23)
v1
mdot1
ρ1 Ac1⋅:= v1 0.43
m
s=
(E5.2.24)
Re1
ρ1 v1⋅ di⋅
µ1:= Re1 84.603=
(E5.2.25)
The Reynolds number indicates very laminar flow. The velocity in the tubes appears
acceptable when considering a reasonable range of 0.5 – 1.0 m/s in Table 5.4 for the
engine oil.
52
The friction factor is determined automatically whether it is either laminar or turbulent
using the following program as
f ReD( ) 1.58 ln ReD( )⋅ 3.28−( ) 2−ReD 2300>if
16
ReD
otherwise
:=
(E5.2.26)
The Nusselt number for turbulent or laminar flow is defined using Equations (5.145) and
(5.146) with assuming that µ changes moderately with temperature. The convection heat
transfer coefficient is then obtained.
NuD Dh Lt, ReD, Pr, ( )f ReD( )
2
ReD 1000−( ) Pr⋅
1 12.7f ReD( )
2
0.5
⋅ Pr
2
31−
⋅+
⋅ ReD 2300>if
1.86Dh ReD⋅ Pr⋅
Lt
1
3
⋅ otherwise
:=
(E5.2.27)
Nu1 NuD di Lt, Re1, Pr1, ( ):= Nu1 8.484= (E5.2.28)
h1
Nu1 k1⋅
di
:= h1 468.972W
m2K⋅
⋅=
(E5.2.29)
Shell side (50% ethylene glycol)
The free-flow area is obtained using Equation (5.131) and the velocity in the shell is also
calculated
Ac2
Ds Ct⋅ B⋅
Pt
:= Ac2 2.581 104−
× m2
=
(E5.2.30)
v2
mdot2
ρ2 Ac2⋅:= v2 1.786
m
s=
(E5.2.31)
53
The velocity of 1.786 m/s in the shell is acceptable because the reasonable range of 1.2 –
2.4 m/s for the similar fluid shows in Table 5.4. The equivalent diameter for a triangular
pitch is given in Equation (5.148b) as
De 4
Pt2
3⋅
4
π do2
⋅
8−
π do⋅
2
⋅:= De 2.295mm⋅=
(E5.2.32)
Re2
ρ2 v2⋅ De⋅
µ2:= Re2 5.225 10
3×=
(E5.2.33)
The Nusselt number is given in Equation (5.152) and the heat transfer coefficient is
obtained.
Nu2 0.36 Re20.55
⋅ Pr2
1
3⋅:=
(E5.2.34)
h2
Nu2 k2⋅
De
:= h2 1.442 104
×W
m2K⋅
⋅=
(E5.2.35)
The total heat transfer areas for both fluids are obtained as
Ai π di⋅ Lt⋅ Nt⋅:= Ai 0.403m2
= (E5.2.36)
Ao π do⋅ Lt⋅ Nt⋅:= Ao 0.524m2
⋅= (E5.2.37)
The overall heat transfer coefficient is calculated using Equation (5.143) with the fouling
factors as
Uo
1
Ao
1
h1 Ai⋅
Rfi
Ai
+
lndo
di
2 π⋅ kw⋅ Lt⋅+
Rfo
Ao
+1
h2 Ao⋅+
:= Uo 209.677W
m2K⋅
⋅=
(E5.2.38)
54
ε -NTU method
The heat capacities for both fluids are defined and then the minimum and maximum heat
capacities are obtained using the MathCAD built-in functions as
C1 mdot1 cp1⋅:= C1 530.61W
K⋅=
(E5.2.39)
C2 mdot2 cp2⋅:= C2 1.716 103
×W
K⋅=
(E5.2.40)
Cmin min C1 C2, ( ):= Cmax max C1 C2, ( ):= (E5.2.41)
The heat capacity ratio is defined as
Cr
Cmin
Cmax
:= Cr 0.309=
(E5.2.42)
The number of transfer unit is defined as
NTUUo Ao⋅
Cmin
:= NTU 0.207=
(E5.2.43)
The effectiveness for shell-and-tube heat exchanger is give using Equation (5.154) as
NTU1NTU
Np
:=
(E5.2.44a)
εhx 2 1 Cr+ 1 Cr2
+
0.5 1 exp NTU1− 1 Cr2
+
0.5
⋅
+
1 exp NTU1− 1 Cr2
+
0.5
⋅
−
⋅+
1−
⋅:= εhx 0.182=
Since (E5.2.44b)
Using Equation (5.156), the effectiveness is expressed as
εhxq
qmax
C1 T1i T1o−( )⋅
Cmin T1i T2i−( )⋅
C2 T2o T2i−( )⋅
Cmin T1i T2i−( )⋅ (E5.2.45)
The outlet temperatures are rewritten for comparison with the outlet temperatures.
55
T1i 120 °C⋅= T2i 90 °C⋅=
T1o T1i εhx
Cmin
C1
⋅ T1i T2i−( )⋅−:= T1o 114.544°C⋅=
(E5.2.46)
T2o T2i εhx
Cmin
C2
⋅ T1i T2i−( )⋅+:= T2o 91.687°C⋅=
(E5.2.47)
The engine oil outlet temperature of 114.544°C is close enough to the requirement of
105°C. The heat transfer rate is obtained
q εhx Cmin⋅ T1i T2i−( )⋅:= q 2.895 103
× W= (E5.2.48)
The pressure drops for both fluids are obtained using Equations (5.158) and (5.161) as
∆P 1 4f Re1( ) Lt⋅
di
1+
⋅ Np⋅1
2⋅ ρ1⋅ v1
2⋅:= ∆P 1 9.325kPa⋅=
(E5.2.49)
∆P 2 f Re2( )Ds
De
⋅ Nb 1+( ) 1
2⋅ ρ2⋅ v2
2⋅:= ∆P 2 5.141kPa⋅=
(E5.2.50)
Both the pressure drops calculated are less than the requirement of 10 kPa. The iteration
between Equations (E5.2.9) and (E5.2.46) is terminated. The surface density β for the engine oil side is obtained using the relationship of the heat transfer area over the volume
of the exchanger.
β1
Ao
π Ds2
⋅
4
Lt⋅
:= β1 679.134m2
m3
⋅=
(E5.2.51)
Summary of the design of the miniature shell-and-tube heat exchanger
Given information
T1i 120 °C⋅= engine oil inlet temperature
T2i 90 °C⋅= 50% ethylene glycol inlet temperature
kg
56
mdot1 0.23kg
s= mass flow rate of engine oil
mdot2 0.47kg
s= mass flow rate of 50% ethylene glycol
Rfi 1.76 104−
× m2 K
W⋅⋅= fouling factor of engine oil
Rfo 3.53 104−
× m2 K
W⋅⋅= fouling factor of 50% ethylene glycol
Requirements for the exchanger
T1o 115°C≤ Engine outlet temperature
∆P 1 10kPa≤ Pressure drop on both sides
Design obtained
Np 1= number of passes
Ds 50.8 mm⋅= shell inside diameter Ds 2in=
do 3.175mm⋅= tube outer diameter
di 2.442mm⋅= tube inner diameter
Lt 381 mm⋅= tube length Lt 15in=
Nt 138= number of tube
Ct 0.794mm⋅= tube clearance
B 25.4mm⋅= baffle spacing B 1in=
Nb 14= number of baffle
T1o 114.544°C⋅= engine oil outlet temperature
T2o 91.687°C⋅= 50% ethylene glycol outlet temperature
q 2.895kW⋅= heat transfer rate
β1 679m2
m3
⋅= surface density
∆P 1 9.325kPa⋅= pressure drop for engine oil
57
∆P 2 5.141kPa⋅= pressure drop for 50% ethylene glycol
The design satisfies the requirements.
Problems (corrected)
Shell-and-Tube Heat Exchanger
5.3 A miniature shell-and-tube heat exchanger is designed to cool glycerin with cold
water. The glycerin at a flow rate of 0.25 kg/s enters the exchanger at 60°C and leaves
at 50°C. The water at a rate of 0.54 kg/s enters at 18°C, which is shown in Figure
P5.3. The tube material is Cr alloy (kw = 60.5 W/mK). Fouling factors of 0.253x10-3
m2K/W for water and 0.335x10
-3 m
2K/W for glycerin are specified. Route the
glycerin through the tubes. The permissible maximum pressure drop on each side is
30 kPa. The volume of the exchanger is required to be minimized. Since the
exchanger is custom designed, the tube size may be smaller than NPS 1/8 (DN 6 mm)
that is the smallest size in Table C.6 in Appendix C, wherein the tube pitch ratio of
1.25 and the diameter ratio of 1.3 can be applied. Design the shell-and-tube heat
exchanger.
Figure P5.3 Shell-and tube heat exchanger