Page 353 - Fluid mechanics, heat transfer, and mass transfer

CONDENSERS 335

redissolving in the condensate which accelerates but larger diameter unit, which is generally more
corrosion either in the evaporator or of the conden- expensive than a long, thin unit.
sate return system. & Headroom limitations and permissible condenser
& Equipment with tube side condensation is more tube length should take into consideration the
easily vented than on shell side condensation, pro- distance beyond the tube sheets of the condenser
vided condensate is not permitted to flood the heat required to permit removing tubes for replacement.
transfer surface. & Condensers must be mounted so that liquid level on
. Give examples of condensation involving presence of the hot well is sufficiently high to permit removal by
noncondensable gases. condensate pumps, without cavitation effects.
& Dehumidification in an air conditioning system in & For air conditioning installations, the liquid level
which water vapor is partially condensed from the should be 1.5 m above the inlet to the condensate
humid air. pump and preferably 0.3 or 0.6 m higher.
& Refinery and chemical plant distillation units, over- & This permits an economical pump selection.
head condensers often involve condensable vapors in . In a well-insulated steam heater, process fluid being
the presence of gases. heated is on the tube side and steam is on the shell side.
& Power plant condensers involve noncondensable air The insulation on the shell is taken out during mainte-
derived from feed waters as dissolved gas. nance work and the plant restarted without insulation?
& Condensers used in evaporator systems. What happens?
. How is corrosion of boiler tubes minimized? & Condensation of steam occurs due to (i) heat transfer
to tube side fluid and (ii) heat losses to atmosphere as
& By removal of dissolved oxygen in feed water and
a consequence of insulation being removed.
maintaining alkaline conditions.
& Additional condensation due to loss of insulation
& By keeping surfaces clean.
might result in flooding of the condenser as provision
& By counteracting effect of corrosive gases in steam
for condensate drainage may not be enough to re-
and condensate systems with chemical treatment.
move condensate at the rate at which it forms (e.g.,
& By avoiding absorption of CO 2 in water as it forms
insufficient steam trap capacity).
corrosive carbonic acid.
➢ Some of the tubes are submerged in the accumu-
. In a steam-generating system, what are the possibilities
lated condensate.
for air leakage into it? How are the air leakage effects
➢ The submerged tube surfaces are not available for
minimized?
condensation heat transfer.
& Air enters a steam-generating system at points
➢ Net effect is reduction of heat transfer area for
around the condenser due to the vacuum generated
condensation.
within the system. Typical air entry points are the
& Heat transfer coefficients for these tubes will be far
expansion joints between the turbine and condenser,
less than those for condensation.
turbine and pump seals, etc.
& Structural failures in the condenser shell or other & Tube fluid outlet temperatures will come down.
mechanical problems can increase air leakage, some- & In other words, performance of the heater will be
times very rapidly, which cannot be handled by the adversely affected.
normal air removal systems. In such cases air forms a . Why a vacuum breaker is used on shell and tube heat
film on the heat transfer surfaces resulting drastic exchangers that are utilizing steam as the heating
reduction in heat transfer rates. medium?
& Most condensers in power plants are equipped with & During plant shutdown, absence of a vacuum
one or more air removal compartments, in which breaker on the condensing side of a heat exchanger
mechanical vacuum is applied to remove the gases can create vacuum on account of condensation of
that enter the condenser. vapors in the vapor space. Vacuum breaker avoids
. What are the considerations involved in the layout of such a situation and prevents accidental ingress of
condensers? any undesirable fluids into the space, contaminating
& Space considerations often influence the cost of a the condensates, necessitating treatment before
condenser. disposal.
& For example, if the surface condenser is limited in & In the absence of a vacuum breaker, condensation on
tube length, a multipass design may be required in shutdown creates vacuum in the exchanger. The
place of a single-pass design. This results in a short, exchanger might not have been designed towithstand