Thursday 10 March 2011

DESIGN CALCULATIONS FOR SHELL & TUBE EXCHANGERS



Heat Exchanger (E-2)
Heat Balance
T1 (360°F) ® T2 (275°F)
t2 (120°F) ¬ t1 (86°F)
Mass flow rate of oil          =          0.59 Btu/lb°F
Heat duty, Q                             =          58060 ´ 0.59 ´ 85
                                       =          2908224 Btu/hr
Mass flow rate of water           =          29376 lbs/hr
Corrected LMTD                   =          182°F
Av. Temp. of oil            =          318°F
Av. Temp. of water                 =          103°F
Assumed UD                         =          40 Btu/hr ft2 °F
Area required A             =          Q/UD LMTD=          2.98 ´ 106/40 ´ 182
                                                                   =          400 ft2
No. of tubes required          =          400/12 ´ 0.2618          =          127


Specification of Shell and tubes
Shell Side                                                    Tube Side
ID                         =          19.25 inch             No. of tubes          =          132
Baffle spacing           =          12in                       Length        =          12 ft
Passes                             =          1                  Tube OD & BWG          = 1 in, 12
                                                                   Pitch            =          1.25 in2

Calculation for heat transfer area
Shell Side, oil
Tube Side, Water
D
0.0825ft
D
0.065 ft
as
0.3208 ft2
a’t
0.479 in2
Gs
W/as= 19756 lbft2
a
0.2196 ft2
NRe
5610
Gt
133770 lb/hr ft2
IH
40
NRe
4618
NPr
2.9
IH
18
ho
90.2 Btu/hr ft2 °F
NPr
1.517


hi
154 Btu/hr ft2°F


hio
121 Btu/hr ft2°F
Uc calculated              =          51.67 Btu/hr ft2°F
Area available           =          132 ´ 12 ´ 0.2618          =          415ft2
UD calculated              =          2.98 ´ 106 / 182 ´ 415 = 38.39 Btu/hr ft2 °F
Dirt factor, Rd      =          (UC-UD)/UC ´ UD           =           0.006 °F ft2 hr/Btu

PRESSURE DROP CALCULATION
Shell Side, oil
Tube Side, Water
NRe
5610
NRe
4618
f
0.0025
f
0.00035
s
0.89
s
1
Ds
1.6ft
Pt
0.002Psi
N+1
12L/B=12
Pr
0.02Psi
DPs
0.5Psi
DP
0.022Psi
Heat exchanger
Heat Balance
T1 (360°F) ® T2 (341°F)
t2 (120°F) ¬ t1 (77°F)
Mass flow rate of oil               =          616906 lbs/hr
Sp heat of oil                                    =          0.54 Btu/lb °F
Heat duty,Q                              =          6.21 ´ 106 Btu/hr
Mass flow rate of water                     =          144418 lbs/hr
Corrected LMTD                             =          252°F
Av. Temp. of oil                     =          351°F
Av. Temp. of water                           =          98.5°F
Assumed UD                                   =          50 Btu/hr ft2 °F
No. of tubes required                 =          157

Specification of Shell and tubes
Shell Side                                                    Tube Side
ID                         =          21.25 inch             No. of tubes          =          166
Baffle spacing           =          18in                       Length        =          12 ft
Passes                             =          1                            Tube OD & BWG          = 1 in,12
                                                                   Pitch            =          1.25 in2
Calculation for heat transfer area
Shell Side, oil
Tube Side, Water
D
0.0825ft
D
0.065 ft
as
0.531 ft2
a’t
0.479 in2
Gs
W/as= 1161781 lbft2
a
0.276 ft2
NRe
44006
Gt
523256lb/hr ft2
IH
130
NRe
1.8 ´ 104
NPr
2.67
IH
68
ho
269 Btu/hr ft2 °F
NPr
1.72


hi
663 Btu/hr ft2°F


hio
518 Btu/hr ft2°F
Uc calculated              =          177 Btu/hr ft2°F
Area available           =          166 ´ 12 ´ 0.2618          =          521ft2
UD calculated              =          47.2 Btu/hr ft2 °F
Dirt factor, Rd      =          (UC-UD)/UC ´ UD           =           0.003 °F ft2 hr/Btu

PRESSURE DROP CALCULATION
Shell Side, oil
Tube Side, Water
NRe
44006
NRe
1.8´104
f
0.0016
f
0.00023
s
0.89
S
1
Ds
1.77ft
Pt
0.44Psi
N+1
12L/B=8
Pr
0.096Psi
DPs
8.25Psi
DP
0.7Psi
E-4 Design
T1 (182°F) ® T2 (127°F)
t2 (122°F) ¬ t1 (77°F)
Wwater                    =          149280 lbs/hr
Wair              =          729806 lbs/hr
Heat duty, Q          =          8.2 ´ 106 Btu/hr
Approach temperature          =          50°F
Cooling range            =          55°F
Overall coefficient, UD     =          80 Btu/hr ft2°F
Required surface                  =          210 ft2/(MM Btu/hr)
Total surface for heat transfer          =          1724 ft2
Bay width                        =          16 ft
Tube length for 16 ft bay          =          24 ft
Fanned section length / Bay Width           =          1.5
Fan installed           =          1
Plot area                =          384 ft2
Total horsepower for fan          =          28.8 h.p.
Remarks 1 fan is employed; 30 h.p. driver is used

E-5 Design
T1 (127°F) ® T2 (86°F)
t2 (122°F) ¬ t1 (77°F)
Wwater                    =          106630 lbs/hr
Wair              =          388600 lbs/hr
Heat duty, Q          =          4.37 ´ 106 Btu/hr
Approach temperature          =          9°F
Cooling range            =          55°F
Overall coefficient, UD     =          80 Btu/hr ft2°F
Required surface                  =          210 ft2/(MM Btu/hr)
Total surface for heat transfer          =          903 ft2
Bay width                        =          12 ft
Tube length for 16 ft bay          =          16 ft
Fanned section length / Bay Width           =          1.3
Fan installed           =          1
Plot area                =          192 ft2
Total horsepower for fan          =          14.4h.p.
Remarks 1 fan is employed; 15 h.p. driver is used

NOMENCLATURE


as = Area of shell
a//t = Flow area
at = Flow area per tube
B = Baffle Spacing
C = Clearance b/w tubes
Cp = Specific Heat Capacity
D = Inside diameter of tubes
De = Equivalent diameter of shell
Ds = Inside diameter of shell
f= Friction Factor
Gt = Tube side mass velocity
Gs = Shell side mass velocity
hi , ho = Inside and Outside film coefficient
hio = Value of hi when referred to the tube ,OD,Btu/hr.ft2.oF

ID = Internal Diameter

L = Tube Length,ft

LMTD = Log Mean Temperature Difference
m= Mass flow rate n = Number of passes
OD = Outer Diameter
PT = Tube Pitch
DP, DPt, DPr = Total, tube, return pressure drop
DPs = Pressure drop of shell
Q = Heat Flow
Rd = Combined dirt factor
Re = Reynolds number, dimensionless
s= Specific Gravity
UC,UD = Clean and Design overall coefficient
w = Mass Flow Rate of water
m = Viscosity
f = Viscosity ratio, (m/mw) 0.14

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