Page 349 - Fluid mechanics, heat transfer, and mass transfer

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HEAT TRANSFER EQUIPMENT INVOLVING
PHASE TRANSFER

11.1 Condensers 331 & External nuclei in the inlet vapor will enhance fog
11.2 Reboilers 336 formation.
11.3 Evaporation and Evaporators 347
& Not desirable. Fog formation hinders condensation
11.3.1 Multiple Effect Evaporators 361
process. The tiny droplets in a fog hinder movement
11.3.2 Evaporator Performance 364
of vapors toward the condensing surfaces. These
11.3.3 Auxiliary Equipment 368
droplets tend not to collect on the condensing sur-
11.3.4 Utilities 370
faces and may be lost through the condenser vent
unless special precautions are taken for removal.
Usual demisters will not capture fog droplets.
& Carry over of liquid droplets out of the condenser
may give rise to plumes of fog outside the vent
systems with environmentally objectionable effects.
11.1 CONDENSERS
. What is the basic reason for fog formation in a
. What are the advantages and limitations of direct condenser?
contact condensers? & Fogging occurs in a condenser when mass transfer is
& The main advantage of a direct contact condenser is slower than heat transfer.
that there are no metal wall and fouling resistances & The design must provide sufficient time for mass
involved with the consequence of high heat transfer transfer to occur.
rates compared to indirect contact condensers. & A high DT with noncondensables present or a wide
& Environmental considerations preclude use of direct range of molecular weights can produce a fog.
contact condensers as the effluent needs to be freed & The high DT gives a high driving force for heat
from contaminants, requirement of treatment of ef- transfer. The driving force for mass transfer, how-
fluents before discharge to permissible levels. ever, is limited to the concentration driving force
& Because of the above limitation these are not favored (DY) between the composition of the condensable
any more, except in cases of vapors containing solids component in the gas phase and the composition
that would foul heat transfer surfaces of surface in equilibrium with the liquid at the tube wall
condensers very quickly. temperature.
. In condensation of vapors from a noncondensablegas, is & The mass transfer driving force (DY ) thus has a limit.
fog formation a desirable phenomena? Explain. The DT driving force can, under certain conditions,
& If bulktemperature of thegas falls below dew point of increase to the point where heat transfer completely
vapors, fog formation can take place. Droplet sizes in outstrips mass transfer, which produces fogging.
a fog are in the range of 1–20 mm. . “In condensers, external nuclei in the inlet vapors will
& Fog can be formed in a condenser when the ratio of enhance fog formation.” What are the sources of such
noncondensable to condensable vapor is high and the nuclei?
temperature difference is high. & Solids in air.

Fluid Mechanics, Heat Transfer, and Mass Transfer: Chemical Engineering Practice, By K. S. N. Raju
Copyright Ó 2011 John Wiley & Sons, Inc.

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