Page 156 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10D 5/30/2001 4:31 PM Page 119

Heat Transfer 119

B

C
D

A

Figure 10-67B. Correlation of McAdams representing the condensing film coefficient on the outside of vertical tubes, integrated for the entire
82
tube length. This represents the streamline transition and turbulent flow conditions for Prandtl numbers 1 and 5. Do not extrapolate Prandtl
numbers, Pr f , beyond 5. (Used by permission: Engineering Data Book II ©1984, Wolverine Tube, Inc.)

3 2
k f f g 1>3 4W 0.4
h cm 0.0077a b a b (10-74)
2
f f D o
(10-78)
N Re,f 2 '> f
For steam at atmospheric pressure and t from 10°F–
150°F: 82 The critical Reynolds Number of 2,100 corresponds to
4,000 4 / f of 4,200 for horizontal tube. 82
h cm (10-75)
1>4
L t 1>3
where ' w>L, per horizontal tube, lb> 1ft2 1hr2
L tube length, ft The thickness of the film 94A for
t t sv t w (temperature of saturation of dew point— Reynolds Number 2,100 13 ' > g2 1>3
2
temperature of tube wall surface), °F.
For steam at atmospheric pressure, 82 the average film
Horizontal Tube Bundle 70 coefficient is
See Figures 10-67A and 10-67B. 5,800
70
For single pure vapors Kern recommends the following, h cm 1>4 1>3 (10-79)
1N v D o ¿2 1 t m 2
due to the splashing of condensed liquid (outside) from
horizontal tubes as it drips/splashes to and off of the lower
where
tubes in the bundle: N v number of rows of tubes in a vertical tier
D o tube O.D., in.
W t m (t v –t w )/2, °F
G– , lb>1hr21linear ft2; see Figure 10-67A (10-76)
2>3 t v temperature of vapor, °F
LN t
t w temperature of tube wall, °F
Then the heat transfer for condensation is 70, 82 on a hori- h cm average value of condensing film coefficient, Btu/hr
2
zontal bundle: (ft ) (°F), for vertical rows of horizontal tubes
k f thermal conductivity at film temperature, Btu/(hr)
2
2 1>3
1 f 2 1.5 14G–2 1>3 (ft ) (°F/ft)
(10-77) 3
h cm 3 2 f density lb/ft at film temperature, t f
1k f f g2 1 f 2
g acceleration of gravity, ft/(hr) (hr) 4.17 10 8
f viscosity at film, lb/(ft) (hr) centipoise 2.42
The preceding equation automatically allows for the
lb/ (ft) (hr)
effect of the number of vertical rows of horizontal tubes as W flow rate, lb/hr, condensate
70
82
proposed by Kern and cited later in this discussion. The D o outside diameter of tubes, ft
flow should be streamlined (laminar) flow, with a Reynolds N t total number of tubes in bundle used for
82
Number of 1,800— 2,100 for the condensation, see Figures condensation
10-67A and 10-67B. L tube length, ft, straight