Page 118 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

4.7 Design illustration 115

Substituting the values in Eq. (3.3), the overall heat transfer coefficient is

1 1 0:00018 0:0254 4 0:0254
þ 1:1 10 þ
U 1389:0236 0:01859 8020:8236 0:01859
¼ 0:00018 þ
þ
2
m K
3
¼ 1:4262 10
W
or U ¼ 701:15 W
2 , which is almost same as the assumed value. So design is Ok (no over design).
m K
Using Kern’s method for pressure drop calculation, Dp t for tube-side fluid can be obtained from
Eq. (4.23) where f t can be obtained from Eq. (3.16a) for turbulent flow in tubes (with 5% tolerance) as
0:32 0:125 3
0:32
f t ¼ 0:0014 þ 0:125=ðReÞ ¼ 0:0014 þ ¼ 5:16 10
ð57031:24Þ
Thus,
3
1 4 5:16 10 6 2
2 4f t L p N p 2
t t
Dp t ¼ r u þ 4N p ¼ 0:5 992:22 ð1:985Þ þ 8
2 D i 0:01859
2 2
¼ 302.124 kPa ¼ 0.425 kg=cm < 0:7kg=cm limit
6
For shell-side fluid, Re s ¼ 30862:344 and since 400 < Re s 10 , f from Eq.(4.25) is
f ¼ exp f0:576 0:19 lnðRe s Þg ¼ exp f0:576 0:19 lnð30862:344Þg ¼ 0:2484.
L 6
¼ 24:5z25 and
For baffle spacing B ¼ 0:2445, number of baffles N b ¼ ¼
B 0:2445
N b þ 1 ¼ 26.
Since for two tube passes nozzles are on opposite side and ðN b þ1Þ is odd, we consider N b þ 1 ¼ 27.
This gives pressure drop from Eq. (4.24) as
2 2
f s G ðN b þ 1ÞD s 0:2484 ð453:858Þ 27 0:489
s
Dp s ¼ ¼ ¼ 25912:134
2r De 2 710 0:01836
s
2 2
Pa ¼ 0.26 kg cm < 0.7 kg cm .
So the pressure drop for both tube and shell side is within the maximum allowable pressure drop
specified in the problem.
For D s ¼ 489 mm, the minimum bundle to shell clearance for easy pull out of bundle in case of
fixed tube is 10 mm from Table 4.11 and from Table 4.18, nominal nozzle diameter is 101.6 mm (4 ).
00
From Table 4.19, the nozzles are fabricated from Schedule 80 pipe with a corrosion allowance of
3 mm.
p 2 3 2
ð0:1016Þ ¼ 8:107 10 m
4
Nozzle cross sectional area A nozzle ¼
And velocity of entering shell-side fluid through nozzle
m s 10:8472
¼ 1:8844 m s.
u s;in ¼ ¼ 3
r A nozzle 710 8:107 10
s
2
kg
This gives r s;in u 2 s;in ¼ 710 ð1:8844Þ ¼ 2521:287 ms 2.
This is slightly higher than the limit for noncorrosive, nonabrasive, single-phase fluid, 2230 kg
ms 2
impingement. However, for this design, impingement baffle is not included as the limit is exceeded
only by a small value.
We adopt single segmental baffle with 25% cut. As per IS 4503, from Table 4.14 for D s > 400 and
700 and distance between adjacent baffles > 150 and 300, baffle thickness is 4 mm.