Chemical Engineering July 2020 - 31

A flexible system
With a condensate pot, a flexible
system can be devised. One can
hook the steam control either to
the valve in the steam line or to the
level valve in the condensate line.
The condensate drum needs to be
designed so that it is about as tall
as the reboiler and ends a few feet
below the reboiler.
Condensate pots
Figures 2 and 3 depict different condensate
pot arrangements. Some of
the main considerations in their design
and operation are the following:
Consideration No. 1. Condensate
drums may be vertical or horizontal.
Consideration No. 2. A pressureequalizing
line must be provided. This
is a small line (often 1-in. dia.) connecting
the top of the reboiler with the
top of the condensate pot. Without
this line, it will be impossible to maintain
a steady pressure and level in the
condensate pot. Setting the equalizing
line must consider the pressure
balance. One horizontal thermosiphon
reboiler (Figure 7a) experienced
deficient steam flow, leading to poor
stripping in the tower [28]. The measured
steam-side pressure drop was
3 ft of water, making the condensate
level in the reboiler at least 3 ft higher
than in the condensate pot. This was
enough to flood some of the reboiler
tubes. The flooded height increases
with the pressure drop, which in
turn increases with the square of the
steam flowrate. With horizontal thermosiphons,
even a short submerged
height can lose considerable condensation
area. It was recommended
[12,28] to connect the equalizing line
to the valve immediately below the
lower pass partition baffle (Figure 7b).
engineers wishing to use the valve
in the steam line (Figure 7a) should
closely watch the pressure balance
and pot level and elevation [3].
Consideration No. 3. In the Figure
2 and Figure 3c arrangements, the
condensate in the condensate pot
is subcooled. The subcooled condensate
has a vapor pressure that
may be an order of magnitude lower
than the steam pressure. If this subcooled
condensate reaches the
liquid surface in the pot, the drum
pressure will dive, causing a rush of
vapor from the equalizing line, which
the equalizing line may not be able
to supply. Steam from the equalizing
line may collapse onto the subcooled
surface, causing instability,
inability to control the condensate
pot level and hammering. One such
case study was described [29] and
the author is familiar with others.
To prevent this, it is important to
feed the subcooled liquid near the
bottom of the drum or into the drum
outlet line and to prevent it from rising
to the surface. Figure 8 shows
the design and shielding baffle [29]
used to solve the hammering and instability
issue described above.
Consideration No. 4. Arrangements
2a, 2b, and 3c require a sufficient
surge volume in the condensate pot
in order to prevent reboiler level variations
from flooding or draining the
pot. In arrangement 3a (with or without
a pump) and 3b, a lower volume
is adequate. Here, the main consideration
is providing sufficient seal
height to avoid vapor breakthrough.
In this arrangement, liquid level is
kept below the reboiler bottom.
Consideration No. 5. Bertram [30]
has discussed sizing condensate
pots. The paper presents a sizing
chart and a few examples, but no
firm guidelines. Based on the author's
interpretation [1] of the information
presented by Bertram, the
following guidelines can be inferred:
For all arrangements depicted in
Figures 2 and 3):
* The maximum recommended liquid
velocity
through a vertical
drum is generally a function of
the condensate valve size, as
shown in the box below:
Condensate control
valve size, in.
1
1½-3
>4
Maximum liquid
velocity, gal/min per ft2
25
37
50
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Chemical Engineering July 2020

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