Deaerator

  Packed Column Type Deaerator Design

Fluid Technologies packed column type deaerators are of the counterflow design, utilizing a circular packing enclosure (If
Necessary) and stainless steel packing to achieve the most efficient and structurally sound deaerator manufactured today.

This design is not only the basic design for all mechanical degasification processes, including deaeration, decarbonation
(C02 removal) and the removal of any other gas from a solution which follows or can be made to follow Henry's Law, but
also the basic design for all types of process towers used in any adsorption or desorption process.

This design is also the basic design used for most "Direct Contact" type heat exchange equipment.

In a counterflow design, the solution to be degasified, water, is introduced at the top of a column, a packing enclosure, or
space above the packing (*as in this case) of a packed column type deaerator where an enclosure is not required, by
means of spray valves or pipes. The water is sprayed downward into an atmosphere which is devoid of the gas which is to
be removed from the water, steam in a deaerator, or which contains the gas to be removed, but at a lower partial pressure
than the partial pressure of the gas in the solution. The spraying action breaks the surface tension of the water. This
action, and the direct contact of the water with the steam, allows the water to be heated to nearly the saturation
temperature of the steam making up the surrounding atmosphere, and accomplishes the first requirement of gas removal
by the use of a scrubbing vapor, mechanical separation, “ebullition”, which allows the gas or gasses being removed, by
the oxygen and carbon dioxide in the case of a deaerator, to escape or "diffuse" from the water, to the steam atmosphere
surrounding them.

Diffusion is the second requirement of gas removal by mechanical separation, but cannot be accomplished without
ebullition first taking place. As indicated above, nearly all the steam is condensed by coming in "direct" contact with the
incoming water, heating the water to nearly its saturation temperature, just as in a direct contact condenser. The small
amount of steam that is not condensed is vented to the atmosphere, carrying the gasses which have diffused from the
water with it.

This section of the deaerator is called the internal, direct contact preheater and vent condenser section, and it is here that
the majority (90-95%) of the non-condensable gasses are released from the water, and the water is heated to nearly the
saturation temperature of the incoming steam.

As the steam flows upward through packing, the water flows downward. The design of the packing being used forces the
steam and water to come into continuous contact, constantly accomplishing both of the functions required for gas removal,
ebullition and diffusion.

The water flowing through the packing does not flow in a constant stream, but disperses (breaks up) into droplets as it
flows in and through the packing, creating a continual state of "ebullition" as it flows downward, exposing the droplets to
the steam flowing upward through the packing. This allows any gases not removed in the internal direct contact preheater
and vent condenser section of the unit to "diffuse" to the scrubbing steam passing through it. In addition, it exposes these
droplets to the steam to complete the heating of the water to approximately the exact temperature corresponding to the
saturated temperature of the steam.

Very little steam, if any, is condensed in the packings, since the water is heated to nearly the saturation temperature of the
steam in the direct contact preheater section of the deaerator.

Our circular enclosure, if an enclosure is used, allows us to design our deaerators with hydraulic packing loads usually
around 173#/hr./sq.inch (50 gpm/sq.ft.), as compared to tray type deaerators which are limited to tray loads of usually
between 90-120 #/hr./sq.inch. per square inch, maximum.

This gives you an idea of the efficiency of a packed column deaerator, and the savings which you can realize by using this
type of unit instead of a tray unit.

Due to the higher packing loads, overall deaerator shell diameters with this type of unit are considerably less than those
used by deaerators having square or rectangular tray compartments.

Also, circular enclosures, if an enclosure is used, allow for uniform distribution of the water and steam throughout the unit
at all loads, since there are no "dead" spaces, corners, which allow for channeling of water and steam.

These corners also tend to run "dry" at low loads. These dry areas can also create corrosion problems, which are These
discussed in the proposal pages, "Enclosure Fabrication and Design.”
discussed in the proposal pages, "Enclosure Fabrication and Design.”
These corners also tend to run "dry" at low loads. These dry areas can also create corrosion problems, which are
discussed in the proposal pages, "Enclosure Fabrication and Design.”

The water, after it completes its flow through the packing, has an oxygen content of 7.0 ppb or less, and a free carbon
dioxide content of zero.

Enclosure Fabrication and  Design

As you know, the most common failures in deaerators occur during transient loads, or load re-jection, when the square or
rectangular tray compartments of deaerators implode due to the vacuum being pulled on the deaerator top due to the loss
of positive steam pressure in the unit. In addition, the square or rectangular tray compartments are susceptible to failure
and corro-sion from and during normal operations, due to their method of fabrication. These types of failures will not occur
with the design of Fluid Technologies packing enclosure, if one is used in a packed column type deaerator.

Enclosures are not really required in this type of deaerator, since the deaerator top, having a much smaller diameter than
a typical tray type deaerator, can usually be fabricated from stainless steel, thereby eliminating the requirement for any
type of enclosure.

During fabrication of the square or rectangular tray compartments, the sides are either formed from four (4) separate
sheets welded together, or from two (2) pieces, with each piece formed to make two (2) sides, which are then welded
together. During this process, considerable forces are invoked in the material. In addition to this, there is very little
reinforcement added to the thin compartment walls, even during normal operation, the compartment is subject to "tin
canning" or flexing, due to both temperature and pressure changes. Eventually, the forces ap-plied during both fabrication
and operation cause the welds holding the square or rectangular together to fail, exposing the carbon steel shell of the
unit to non-condensable gases, and sub-sequent corrosion.

In addition, this method of fabrication makes the stainless steel more susceptible to stress cor-rosion cracking. These
problems and failures will not occur with Fluid Technologies circular enclosure.

All Fluid Technologies deaerators, whether packed column type or our round tray type are equipped with a circular
enclosure constructed of stainless steel as specified, and having a minimum thickness of 1/8”. The fabrication of the
circular enclosure invokes few stresses upon the material of manufacture (there is no bending or cutting on a brake) and
there are no edges welded together at 90-degree angles. Flexing of the material during operation is virtually im-possible,
due to the support rings welded either externally or internally in the enclosure, de-pending on whether the unit is equipped
with packing or trays, respectively.

The circular tray enclosure is equipped either externally or internally with support rings, nor-mally 1/4" X 2" flat bar,
constructed of stainless steel. These support rings reinforce the enclo-sure to such an extent that it is the strongest
enclosure in the industry.

No other deaerator manufacturer can make this statement.

The support rings strengthen the enclosure; just as stiffening rings do on a vessel designed for full vacuum. In most
cases, these rings actually reinforce the enclosure to an extent that the en-closure itself is capable of withstanding a full
vacuum.

This will prevent the failure of the enclosure should it be subjected to a loss of steam pressure.

The circular enclosure also allows for even distribution of water and steam throughout the en-closure, packing and trays,
which results in better process results from the deaerator, and help to prevent corrosion which can occur in the stainless
steel of square or rectangular compart-ments.

In square or rectangular compartments, corner areas may tend to run "dry", particularly at lower load conditions. These
dry areas and the welds in these areas are susceptible to stress corrosion cracking, especially when the incoming water is
high in chloride, as the chlorides can be deposited and trapped in the corner, where corrosion will be accelerated due to
these deposits and the high temperature of the "dry" metal.

Again, since there are no corners in the circular enclosure, and only one weld holding the en-closure together, as
compared to either two (2) or four (4) in most tray type deaerators, the cir-cular enclosure is not susceptible to this type of
corrosion.

It should be noted that, in general, and until now, pressurized, tray type deaerator's have been the only type of
degasification equipment utilizing an internal compartment, and having this compartment in the shape of a square or a
rectangle. This was done primarily to promote trays of a proprietary design, since the deaerator manufacturer's did not
want to use typical process packing or trays, which could be replaced, if necessary, without being purchased directly from
them.

This led to many tray and internal designs which are neither conducive to handling a liquid and gas process in general,
nor to deaerator operation in today's steam plants.


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