1ST EFFECT
Selection: Long Tube Vertical Falling Film Evaporator
Mode of feed supply is Forward Feed.
DESIGN:
If uD1 , uD2 , uD3 are the corresponding overall heat transfer coefficients and A1, A2 and A3 are the areas of heat transfer required, then we may write
Effect 1: Q1 = ms λs = UD1A1(Ts-T1)
Effect 2: Q2 = ms1 λs1 = UD2A2(T1-T2)
Effect 3: Q3 = ms2 λs2 = UD3A3(T2-T3)
Assumptions:
- Equal heat transfer rates in all effects. From this rate of evaporation is same in all effects.
- Equal areas of heat transfer for all effects.
From assumption (1);
Q1 = Q2 = Q3
UD1A1ΔT1 = UD2A2ΔT2 = UD3A3ΔT3
A1 = A2 = A3
UD1 ΔT1 = UD2 ΔT2 = UD3 ΔT3
Assume values of UD1 and UD2 and UD3
From these ΔT1 , ΔT2 and ΔT3 ; select the pressures of three effects. Then calculate values of UD1 ,UD2 and UD3
To select the pressure of evaporators is an iterative procedure. The final iteration results are given here;
| | Pressure (kPa) | ΔP (kPa) | Steam or vapor temperature (K) | λ (kJ/kg) |
| Steam chest, 1st effect | 2026.5 | ------- | ts = 486 | λ s = 1886 |
| Steam chest, 2nd effect | 962 | 1064.5 | t1 = 451 | λ 1 = 2020 |
| Steam chest, 3rd effect | 304 | 658 | t2 = 401 | λ 2 = 2179 |
| Vapor to condenser | 35.46 | 268.54 | T3 = 346 | λ 3 = 2326 |
ts1 = 455 K C1 = 4.21 kJ/kg
ts2 =405 K C2 = 4.033kJ/kg
ts3 = 350 K Cf = 4.244 kJ/kg
wf = 129286 kg/hr wt = 86773 kg/hr
weg = 25508 kg/hr
by solving Equations (1), (2), (3) and (4); putting above values and values from table.
w1 = 30652 kg/hr
w2 = 22194 kg/hr
w3 = 33927 kg/hr
ws = 27931 kg/hr
Now to calculate Area
Let’s assume that
UD1 = 2985 kJ/hr.m2 K
UD2 = 2304 kJ/hr.m2 K
UD3 = 1649 kJ/hr.m2 K
A1= 569 m2
A2= 584 m2
A3= 575 m2
As we assumed that area of each effect is same. So we use the area of each effect, average of these three values.
A = (A1 + A2 + A3)/3
= (569+584+575)/3
= 576 m2
For number of tubes N
A=NDL∏
¾ “O.D, 14 B.W.G tubes,
L = 9.1435 m (30 ft)
Using No. of Units = 5
STAGE-1
N= 208
STAGE-2
N= 213
STAGE-3
N=210
Using Square Pitch Arrangement
Pitch Range = (1.25 -1.50) x Tube Dia
Pitch = 1.25 x 19.05
= 23.81mm
Clearance = 1/4 x Tube Dia
= 4.76mm
Area of Tube = 567mm2
Effect – 1
N = 208
Area of Single Tube = 567mm2
Shell Area = 208*567
= 117943.312mm2
= 0.117m2
A = πD2 / 4
Shell Diameter = 0.387m
Effect – 2:
N= 213
Area of Single Tube = 567mm2
Shell Area = 213*567
= 120771mm2
= 0.121m2
Shell Diameter = 0.393 m
Effect – 3
N = 210
Area of Single Tube = 567mm2
Shell Area = 210 x 567
= 119070mm2
= 0.119 m2
Shell Diameter = 0.389 m
Using down Take Area is 10% of Shell Area
Effect – 1
Down Take Area = 0.1 x 0.117
= 0.0117 m2
Area = A = πD2/4
Down Take Diameter = 0.122 m
Effect – 2
Down Take Area = 0.1 x 0.121
= 0.0121 m2
Area = A = πD2/4
Down Take Diameter = 0.124 m
Effect – 3
Down Take Area = 0.1 x 0.119
= 0.0119 m2
Area = A = πD2/4
Down Take Diameter = 0.123 m
Size of Vapor Space = Tube Length
Tube Length = 9.14 m
Vapor Space Size = 9.14 m
Shell Length = 18.28 m
CORRECTED SHELL DIAMETER & AREA
Effect – 1
Shell Dia = Shell Dia + Down Take Dia
= 0.387+0.122
= 0.509 m
Shell Area = л/4 D2
Shell Area = 21 m2
Effect – 2
Shell Dia = Shell Dia + Down Take Dia
= 0.393+0.124
= 5.134 m
Shell Area = л/4 D2 = 20.691 m2
Effect – 3
Shell Dia = Shell Dia + Down Take Dia
= 0.389+0.0119
= 5.11 m
Shell Area = л/4 D2
Shell Area = 20.51 m2
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