Page 69 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

3.4 Design illustration 65

Solution

Caustic inlet, 80 C, / Caustic exit, 40 C

2000 kg/hr (0.5556 kg/s) T c,avg ¼ 60 C(Following properties are at 60 C)

C p;c ¼ 3983:2kJ=kg.K
m c ¼ 5:8 10 4 Pa.s
3
r c ¼ 1055 kg=m
k c ¼ 0:688 W=m.K
2
R d;c ¼ 0:00035 m .K=W

Pr c ¼ C p;c m c k c ¼ 3:3579
CW exit,33 C ) CW inlet.33 C

T w,avg ¼ 39 C(Following properties are at 39 C)

C p;w ¼ 4185 kJ=kg.K
m w ¼ 6:65 10 4 Pa.s
3
r w ¼ 1000 kg=m
k w ¼ 0:6541 W=m.K
2
R d;w ¼ 0:00018 m :K=W

Pr w ¼ C p;w m w k w ¼ 4:254

DT 1 ¼ 80 45 ¼ 35 C LMTD ¼ 17.3974 C DT 2 ¼ 40 33 ¼ 7 C

4
Q ¼ m c :C p;c : T c;in T c;out ¼ 8.8515 10 W
m c ¼ 0:5555; m w ¼ Q = ðC p;w ðT w;out T w;in Þ¼ 0.5555 kg=s
LMTD ¼ 17.3974 C.

We choose 40 sch., 1.5 x2 ND double-pipe heat exchanger made from steel pipes. Maximum
00
00
2
pressure in the system being 6 kg/cm (g) (w75 psig), we choose all fittings with a 150 lbs rating. For
this, size of piping,
D i ¼ 34.98 mm ¼ 0.03498 m; ¼ 52.48 mm ¼ 0.05248 m; ¼ 42.1 mm ¼ 0.0421 m
2 4 2 4m 2
A i ¼ pD =4 ¼ 9.6101 10 m ; ¼ 7.7106 10
i
Since A i > A o , the higher flow of water is considered for the inner pipe
V i ¼ m w =ðr :A i Þ¼ 1:8375 m=s; V o ¼ m c =ðr :A o Þ¼ 0:6829 m=s
w c
2 2
G i ¼ m w =ðA i Þ¼ 1834:5kg= m :s ; G o ¼ m o =ðA o Þ¼ 720:5116 kg= m :s
P r;t ¼ P r;w ¼ 4:254; P r;o ¼ P r;c ¼ 3:3579

Re t ¼ D i :V i :r =m ¼ 96497; Re o ¼ D i;o D o :V o :r =m ¼ 12; 895
c
c
w
w
0:8 0:33
o
t
t
Assuming f ¼ 1 and f ¼ 1, Nu t ¼ 0:027:ðRe t Þ :ðPr t Þ :f ¼ 423:1341;