Page 334 - Mechanical Engineers' Handbook (Volume 4)

4 Common Operational Problems 323

the vaporization process or (2) the local temperature exceeds that for which a liquid phase
can exist. 32 Methods of estimating the maximum design heat flux are given in Section 3.3,
and the subject of critical heat flux is covered in great detail in Ref. 27. However, in most
cases where failures have occurred, especially for shellside vaporizers, the problem has been
caused by local liquid deficiency, owing to lack of attention to flow distribution considera-
tions.

4.6 Instability
The instability referred to here is the massive large-scale type in which the fluid surging is
of such violence as to at least disrupt operations, if not to cause actual physical damage.
One version is the boiling instability seen in vertical tubeside thermosiphon reboilers at low
operating pressure and high heat flux. This effect is discussed and analyzed by Blumenkrantz
56
and Taborek. It is caused when the vapor acceleration loss exceeds the driving head, pro-
ducing temporary flow stoppage or backflow, followed by surging in a periodic cycle. This
type of instability can always be eliminated by using more frictional resistance, a valve or
orifice, in the reboiler feed line. As described in Ref. 32, instability normally only occurs at
low reduced pressures, and normally will not occur if design heat flux is less than the
maximum value calculated from Eq. (55).
Another type of massive instability is seen for oversized horizontal tubeside pure com-
ponent condensers. When more surface is available than needed, condensate begins to sub-
cool and accumulate in the downstream end of the tubes until so much heat-transfer surface
has been blanketed by condensate that there is not enough remaining to condense the in-
coming vapor. At this point the condensate is blown out of the tube by the increasing pressure
and the process is repeated. This effect does not occur in vertical condensers since the
condensate can drain out of the tubes by gravity. This problem can sometimes be controlled
by plugging tubes or injecting inert gas, and can always be eliminated by taking a small
amount of excess vapor out of the main condenser to a small vertical backup condenser.

4.7 Inadequate Venting, Drainage, or Blowdown
For proper operation of condensers it is always necessary to provide for venting of noncon-
densables. Even so-called pure components will contain trace amounts of noncondensables
that will eventually build up sufficiently to severely limit performance unless vented. Vents
should always be in the vapor space near the condensate exit nozzle. If the noncondensable
vent is on the accumulator after the condenser, it is important to ensure that the condensate
nozzle and piping are large enough to provide unrestricted flow of noncondensables to the
accumulator. In general, it is safer to provide vent nozzles directly on the condenser.
If condensate nozzles are too small, condensate can accumulate in the condenser. It is
recommended that these nozzles be large enough to permit weir-type drainage (with a gas
core in the center of the pipe) rather than to have a full pipe of liquid. Standard weir
formulas 57 can be used to size the condensate nozzle. A rule of thumb used in industry is
that the liquid velocity in the condensate piping, based on total pipe cross section, should
not exceed 3 ft/sec (0.9 m/sec).
The problem of inadequate blowdown in vaporizers is similar to the problem of inad-
equate venting for condensers. Especially with kettle-type units, trace amounts of heavy,
high-boiling, or nonboiling components can accumulate, not only promoting fouling but also
increasing the effective boiling range of the mixture, thereby decreasing the MTD as well
as the effective heat-transfer coefficient. Therefore, means of continuous or at least periodic