Page 322 - Mechanical Engineers' Handbook (Volume 4)

3 Rating Methods 311

Gravity-Controlled Flow. The vapor shear force on the condensate is small compared to the
gravity force, so condensate drains by gravity. This condition can be expected, according to
Ref. 18, when J 0.5. Under gravity-controlled conditions, the condensate film heat-transfer
g
coefficient is calculated as follows:
Fh (34)
h cƒ gN
The term h is the heat-transfer coefficient from the well-known Nusselt derivation, given
N
in Ref. 14 as
Horizontal Tubes
h 0.725 k ( )g 0.25
3
v
l
l
l
N
(T T )D (35)
l
s
w
where latent heat.
Vertical Tubes
h 1.1k ( )g 0.33
l
v
l
N
l
Re c (36)
2
l
4W
Re c (37)
c
D l
The term F in Eq. (34) is a correction for condensate loading, and depends on the exchanger
g
geometry. 14
On horizontal X-type tube bundles
F N 1/6 (38)
rv
g
(Ref. 12), where N number of tubes in a vertical row.
rv
On baffled tube bundles (owing to turbulence)
F 1.0 (frequent practice) (39)
g
In horizontal tubes
F 1 0.75
g 0.667 (from Ref. 14) (40)
1 (1/y 1)( / )
v
l
or
F 0.8 (from Ref. 18) (41)
g
Inside or outside vertical tubes
F 0.73 Re 0.11 (rippled film region) (42)
c
g
or
F 0.021 Re 0.58 Pr 0.33 (turbulent film region) (43)
c
g
Use higher value of Eq. (42) or (43).
For quick hand calculations, the gravity-controlled flow equations may be used for h ,
cƒ
and will usually give conservative results.