Page 333 - Mechanical Engineers' Handbook (Volume 4)

322 Heat Exchangers, Vaporizers, Condensers

condenser to a backup condenser or if the number of fins per inch on bottom rows is less
than on top rows to counteract the difference in temperature driving force.
For multipass tubeside condensers, or tubeside condensers in series, the vapor and liquid
tend to separate in the headers with liquid running in the lower tubes. The fraction of tubes
filled with liquid tends to be greater at higher pressures. In most cases the effect of this
separation on the overall condenser heat-transfer coefficient is not serious. However, for
multicomponent mixtures the effect on the temperature profile will be such as to decrease
the MTD. For such cases, the temperature profile should be calculated by the differential
flash procedure, Section 3.2. In general, because of unpredictable effects, entering a pass
header with two phases should be avoided when possible.

4.4 Temperature Pinch

When the hot and cold streams reach approximately the same temperature in a heat ex-
changer, heat transfer stops. This condition is referred to as a temperature pinch. For shellside
single-phase flow, unexpected temperature pinches can be the result of excessive bypassing
and leakage combined with a low MTD and possibly a temperature cross. An additional
factor, ‘‘temperature profile distortion factor,’’ is needed as a correction to the normal F
factor to account for this effect. 11,13 However, if good design practices are followed with
respect to shellside geometry, this effect normally can be avoided.
In condensation of multicomponent mixtures, unexpected temperature pinches can occur
in cases where the condensation curve is not properly calculated, especially when the true
curve happens to be of type III in Fig. 15. This can happen when separation of liquid
containing heavy components occurs, as mentioned above, and also when the condensing
mixture has immiscible liquid phases with more than one dew point. 20 In addition, con-
densing mixtures with large desuperheating and subcooling zones can produce temperature
pinches and must be carefully analyzed. In critical cases it is safer and may even be more
effective to do desuperheating, condensing, and subcooling in separate heat exchangers. This
is especially true of subcooling. 3
Reboilers can also suffer from temperature-pinch problems in cases of wide boiling
mixtures and inadequate liquid recirculation. Especially for poorly designed thermosiphon
reboilers, with the circulation rate is less than expected, the temperature rise across the
reboiler will be too high and a temperature pinch may result. This happens most often when
the reboiler exit piping is too small and consumes an unexpectedly large amount of pressure
drop. This problem normally can be avoided if the friction and momentum pressure drop in
the exit piping is limited to less than 30% of the total driving head and the exit vapor fraction
is limited to less than 0.25 for wide boiling range mixtures. For other recommendations, see
Ref. 25.

4.5 Critical Heat Flux in Vaporizers
Owing to a general tendency to use lower temperature differences for energy conservation,
critical heat flux problems are not now frequently seen in the process industries. However,
for waste heat boilers, where the heating medium is usually a very hot fluid, surpassing the
critical heat flux is a major cause of tube failure. The critical heat flux is that flux (Q/A )
o
above which the boiling process departs from the nucleate or convective boiling regimes and
a vapor film begins to blanket the surface, causing a severe rise in surface temperature,
approaching the temperature of the heating medium. This effect can be caused by either of
two mechanisms: (1) flow of liquid to the hot surface is impeded and is insufficient to supply