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122 Applied Process Design for Chemical and Petrochemical Plants

2. Determine the number and size of tubes required to the number of tubes in the assumed unit when the effect
calculate this area. Also set the number of tube passes of any new shell diameter should be reviewed; otherwise the
and tube pitch. unit should perform satisfactorily.
3. Determine the inside film coefficient by methods pre-
viously outlined for convection. Subcooling
4. Estimate film temperature of fluid on the outside of
tubes and determine t across condensing film. The literature is limited on design data/correlations for
5. For vertical tubes, determine condensate loading G o subcooling condensed liquids in or on vertical or horizontal
(Equation 10-73B). For these charts, G o (viscosity in condenser units. Certain analytical logic can be used to exam-
centipoise, at film temperature) is limited to 1,090. ine what is taking place and the corresponding heat transfer
6. For horizontal tubes, use Figure 10-68 to determine the functions can be used to establish the relations to break
equivalent number of condensate streams, n s , based on the desired unit into its components (in terms of heat trans-
the total number in a circular bundle, N t . fer) and combine them to develop a single condensing/
(a) Then calculate condensate loading (horizontal subcooling unit. Also see reference 70.
tubes):
Subcooling Condensate Outside Vertical Tubes
(10-86)
G = W/Ln s
The area concerned with the subcooling only can be eval-
Note that this varies from the form used in the Kern uated using a film coefficient calculated from Figure 10-54
relation. for liquids outside tubes. This assumes that the liquid being
(b) Determine the average number of tubes, N a : cooled is held in the area around the tube by a level control
or pipe seal, allowing drainage at the rate it builds up and
(10-87) covering a portion of the tubes.
N a = N t /n s
Care should be used in determining the temperatures
4/3 that prevail at tube inlet and outlet, as well as the shell side
(c) Determine turbulence correction factor C N from
Figure 10-69. For compounds other than those in and out for the subcooling portion. This becomes partic-
shown, select the nearest type for reference and ularly tedious for multipass units.
evaluate C N . To obtain more conservative results,
4/3 Subcooling Inside Vertical Tubes
reduce the value of C N but never to less than 1.0.
7. Film coefficients: h cm 173
(a) For vertical tubes, use Figure 10-70 or 10-71 and cor- Colburn et. al. conducted some fundamental studies
responding scales for compounds at t f and G (as using organic liquids; they developed the subcooling coefficient
defined for use with these charts). when R e 2100:
(b) For horizontal tubes, use Figure 10-70 or 10-71 and 2 2
1k C p 2 14 2 1>3
corresponding scales for compounds at t f and G (as h s 7.5 c d (10-90)
defined for use with these charts). 1 f 2 1 f 2
8. Evaluate the overall clean coefficient, U c .
which is used to calculate the area required, and then this
9. Check the assumed temperature drop across the con-
area is added to an earlier calculated condensing area.
densate film, t.
where
t 1U c >h cm 2 1t h t c 2 (10-88)
h s subcooling film coefficient, pcu/(hr) (ft ) (°F)
2
k thermal conductivity, Btu/(hr) (ft)(°F)
If these values are not in good agreement, reassume
liquid density, lb/ft 3
and recalculate.
C p heat capacity of condensate, pcu/(lb) (°C)
10. Calculate the overall fouling coefficient, adding the condensate rate per unit periphery, lb/(hr) (ft)
appropriate fouled factors to clean, U c . f viscosity of condensate at average film temperature,
11. Determine the required surface area: lb/(hr) (ft)
2
2
Note: 1.0 Btu/(ft ) (hr)(°F) 1.0 pcu/(hr) (ft )(°C)
A Q>U t (10-89)
If this surface area is slightly less than that assumed for the Subcooling Condensate Outside Horizontal Tubes
unit, say 10—20%, the unit should be acceptable. If the
required area is larger, the new number of same length can Subcooling in horizontal condensers is accomplished by a
be determined by the ratio of required area/assumed area liquid seal on the liquid outlet or by a baffle, which dams the