Page 166 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 155

-DOWNCOMER AREA (ONE SIDE). '/o TOWER AREA

Figure 8-101. Qualitative effect of liquid and vapor loads on bubble
cap tray performance. Used by permission, Bolles, W. L., Pet. Pro-
cessing, Feb. thru May (1 956).
40 50 60 70 80 90
WEIR LENGTH, 70 TOWER DIAMETER

Figure 8-1 00. Segmental downcomer design chart. Used by permis- Dimensions
sion, Bolles, W. L. Pet. Processing; Feb. thru May (1 956).
The most popular and perhaps most adaptable siLe is
about 4 in. O.D. (3% in. I.D.). The 3-in. and Gin. are also in
Turndown Ratio common use for the smaller and larger diameter towers,
although it is not necessary to change cap size with change
Turndown can be applied to all types/styles of tray
in tower diameter. For a given active cap area the cost of
columns; however, it is more relevant to sieve and valve installing the smaller diameter caps is l0-15% greater than
trays. The generally accepted explanation of turndown is the 4in., while the larger (and fewer number) &in. caps
as follows [199] (also see Figure 8-101): are about 15% cheaper. There is usually less waste tray area
Turndown ratio is the ratio of the maximum allowable
vapor rate at or near flooding conditions (rates) to the with the 3-in. caps than the 4in. or Gin. caps. Table 8-17.
For the sake of standardization and developing a feel on
minimum vapor rate when weeping or liquid leakage the part of the designer for the effect of various design
becomes significant; it may be termed the minimum allow- variables on tray performance, the 4in. cap (or %in. or
able vapor velocity [ 193, 199, 2001.
4X-in.) caps are recommended as good general purpose
1. For bubble cap trays, this ratio is approximately 10: 1. units. This means that any application from a 2.5-ft dia. to
2. For sieve trays, this ratio is approximately 2-3:l. 10-ft dia. tower is first evaluated using the 4in. cap. If
3. For valve trays, this ratio is approximately 4-5:l. there are points of poor performance, cap sizes can be

Bubble Caps
Table 8- 17
Although there are many styles and dimensions of caps General Purpose Guide for Pressed Cap Size Selection
in use, the round bell shaped bubble cap is quite practical ~ ~ ~~~~
and efficient. It is recommended as a good basis for the Tower Diameter, General Purpose Possible Alternate,
Ft
In.
Size, I.D., In.
contacting requirements. This selection does not infer ~~ ~
that other contacting caps are not acceptable, in fact many 2.5, 3 37 2%; 3; 3%
are in use in the chemical and petroleum industry. Their 3.5, 4 3A 3, .?A
design criteria is limited to the proprietary knowledge of 5, 6, 7, 8, 9, 10 37 6
the manufacturer and not available to the designer. ~~ 11, 12, 13, 14 G ~~ ?d ~
~~~