Page 34 - Design and Operation of Heat Exchangers and their Networks
P. 34
Basic thermal design theory for heat exchangers 21
h i 3
3
3
Nu 3 ¼ 3:66 +0:7 +1:077 RePrd=xÞ 1=3 0:7
ð
x,T
" # 3
1=6
1 2 1=2
+ ð RePrd=xÞ (2.18)
2 1 + 22Pr
In Fig. 2.3, these equations are compared with the equation of Stephan
(Baehr, 1960)
1:33
0:0677 RePrd=LÞ
ð
Nu T ¼ 3:66 + (2.19)
0:83
1+0:1Pr Red=LÞ
ð
and the equation of Churchill and Ozoe (1973b)
n o 3=8
8=9
Nu x,T ¼ 5:357 1 + π=388ð½ ÞRePrd=x
2 0 1 4=3 33=8
ð π=284ÞRePrd=x
6 B C 7
1+ @4 A 5
n o 1=2 n o 3=4
2=3 8=9
ð
1+ Pr=0:0468Þ 1+ π=388ÞRePrd=x
½
ð
1:7 (2.20)
6
The maximum deviation among them is 6.5% in 0.1<RePrd/L<10 and
0.7 Pr 10.
10 3
Nu T , Eq. (2.17) (Gnielinski, 1989, 2010a, 2013a)
Nu T , Eq. (2.19) (Baehr, 1960)
Nu x,T , Eq. (2.18) (Gnielinski, 2010a, 2013a)
Nu x,T , Eq. (2.20) (Churchill and Ozoe, 1973)
10 2 Pr = 0.7
Nu T , Nu x,T Nu T
10 Pr = 10
Nu x,T
1
0.1 1 10 10 2 10 3 10 4 10 5 10 6
RePrd/x
Fig. 2.3 Local and mean Nusselt number Nu x,T and Nu T for thermally and
hydrodynamically developing laminar flow.
