Page 153 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10D 5/30/2001 4:30 PM Page 117

Heat Transfer 117

Condensate layer, outside of tube, t 2

Scale thickness, resistance, ri
Warm or condensing
side of tube Stagnant film of water, t 7
Metal wall t (bulk)
thickness = L 8
W
t
2
Coolant side, cool
t , t
1 V
t
4
∆
m
t 2
Direction of t
heat flow 6
Process ∆
Saturated Overall, t – t 8
1
vapor,
t or t V
1
(bulk)
t , t 5
W
Water in
turbulent motion
t 3 (or coolant), t
Condensing side 8
scale, dry t 4

Figure 10-66. Condensing vapors on cooling metal (or other) wall (also see Figure 10-28). Note that t 4 and t 5 are wall temperatures and may be
essentially equal to t w wall. This illustration is not for vertical tube, but represents the condensing/cooling mechanism.

where
t d o for vertical tube (perimeter), ft
k l liquid thermal conductivity, Btu/(hr) (ft)(°F)
d o tube outside diameter, ft
l liquid density, lb/ft 3
mass rate of flow of condensate from lowest point on
v vapor density, lb/ft 3
condensing surface divided by the breadth (unit latent heat of vaporization, Btu/lb
perimeter), lb/(hr) (ft). For a vertical tube:
g acceleration of gravity, ft/(sec) (sec)
w/ D.
L tube length, ft
G condensate loading for horizontal tubes, lb/(hr) (ft)
T sat saturation temperature, °F
G condensate loading for vertical tubes, lb/(hr) (ft).
T w surface temperature, °F
l liquid viscosity, lb/(ft) (hr)
70
82
McAdams and Kern both suggest the same relationship
for condensation on the outside of vertical tubes:
4G¿ o
For 6 2,000 (10-73)
2 1>3
1 f 2 14G¿2 1>3 t
3 2
h c 1.47 (10-71) k f f g 1>3 k f tg N t D o 1>2
3 2
3 2
1k f f g2 1 f 2 h cm 0.945c d 0.945c d (10-73A)
f G o ¿ f W
8
g acceleration of gravity, 4.17 10 , ft/(hr) (hr) W
G o ¿ , lb.>hr. 1linear foot2 (10-73B)
N t D o
Bell 172 suggests the relation:
For 4 G o / f 2,000 (reference 82),the following equa-
3
3k l l 1 l v 2 g4 1>4 tion is usually applicable to long tubes and high flow rates;
h c 0.943 (10-72)
3 l L1T sat T w 24 the average film coefficient: