Page 233 - Design and Operation of Heat Exchangers and their Networks

222 Design and operation of heat exchangers and their networks

s fs,h h fs,h
σ h ¼
s f,h h f,h + N fl,c h f,c +2δ p =N fl,h
0:001724 0:009384
¼ ¼ 0:3923
½
ð
0:00187 0:00953 + 9 0:00953 + 2 0:0008Þ=8Š
(5.104)
s fs,c h fs,c
σ c ¼
s f,c h f,c +2δ p + N fl,h h f,h =N fl,c
0:001724 0:009384
¼ ¼ 0:4414
ð
0:00187 0:00953 + 2 0:0008 + 8 0:00953=9Þ
(5.105)

(3) Calculation of heat transfer coefficients and pressure drops
We use the correlations of Joshi and Webb (1987) to calculate the j and f
factors. The mass fluxes of hot and cold fluids can be given as follows:

0:8962 0:001724
_ m h s fs,h 2
G 2ðÞ,h ¼ ¼ ¼ 15:95kg=m s
A c,h s fs,h δ f,h 0:06139 0:001724 0:000146
_ m c s fs,c 0:8296 0:001724 2
G 2ðÞ,c ¼ ¼ ¼ 11:71kg=m s
A c,c s fs,c δ f,c 0:07739 0:001724 0:000146

The Reynolds number in their correlation is defined by Eq. (3.266):

15:95 0:002614
G 2ðÞ,h d h2ðÞ,h
Re 2ðÞ,h ¼ ¼ ¼ 1560
μ h 2:672 10 5
G 2ðÞ,c d h2ðÞ,c 11:71 0:002614
Re 2ðÞ,c ¼ ¼ ¼ 1759
μ c 1:741 10 5
The critical Reynolds number indicating the flow transition from
laminar to turbulent is expressed by Eq. (3.274) as

1:23 0:58
ð
∗ 257 l s,h =s fs,h Þ ð δ f,h =l s,h Þ d h2ðÞ,h
Re ¼ p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2 ðÞ,h
δ f,h +1:328 l s,h d h2ðÞ,h =Re 2ðÞ,h
1:23 0:58
ð
ð
257 0:0063=0:001724Þ 0:000146=0:0063Þ 0:002614
¼ p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
0:000146 + 1:328 0:0063 0:002614=1560
¼ 1320
1:23 ð δ f,c =l s,c Þ 0:58
257 l s,c =s fs,c Þ
ð
d h2ðÞ,c
Re ∗ ¼ p ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 1358
2 ðÞ,c
δ f,c +1:328 l s,c d h2ðÞ,c =Re 2ðÞ,c