Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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ONCE-THROUGH EVAPORATOR FOR A STEAM GENERATOR
Cross-Reference to Related Applications
This application derives and claims priority from U.S. Provisional
Application Serial No. 60/416,083, filed October 4, 2002.
Technical Field
This invention relates in general to steam generators and, more
particularly, to an evaporator for a steam generator and tubing for such an
evaporator.
Background Art
Steam finds widespread use in industry, perhaps the most important
of these uses being the generation of electrical power. Typically, hot gases,
in many instances generated by combustion, pass through a steam
generator which converts water into superheated steam. Representative of
these installations are heat recovery steam generators (HRSGs) which are
used to extract heat from the hot gases discharged by gas turbines that
drive electrical generators. The heat extracted produces steam which
passes on to a steam turbine that powers another electrical generator.
The typical steam generator, aside from a duct through which the hot
gases pass, in its most basic form, includes three additional components
namely, a superheater, an evaporator, and an economizer or feedwater
heater arranged in that order with respect to the flow of gases in the duct.
The water flows in the opposite direction, that is through the economizer
where it is heated, but remains a liquid, then through the evaporator where it
is converted into mostly saturated steam, and then through the superheater
where the saturated steam becomes superheated steam.
Evaporators come in two basic configurations - the circulation type
and the once-through type - each with its own advantages and
disadvantages. Both have an array of tubes in the duct through which the
hot gases pass.
In the circulation type, the tubes reside in a circuit with a steam drum
that is above the tubes. The drum contains water which flows from the
drum, through a downcomer, and then into the tubes where some of it is
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converted into steam, but the steam exists as bubbles within the water, and
is returned through a riser into the steam drum. Here the steam, which is
saturated, separates from the liquid water and passes on to the
superheater. It is replaced by feedwater which is supplied to the drum. The
tubes of a circulation evaporator remain wet all the time - that is to say,
liquid water exists against their interior surfaces throughout. This promotes
good heat transfer. It also maintains the tubes at relatively moderate
temperatures, thus eliminating the need for high temperatures alloys in the
tubing.
But circulation evaporators have their detractions. Perhaps the
greatest of these is the expense attributable to steam drums, large
downcomers, and headers to supply water to their tubes. Moreover, the
reservoirs of water contained in them require time to bring up the boiling
temperature, so the start-up time for a circulation evaporator is extended.
Once-through evaporators do not require downcomers or drums and
are less expensive to manufacture. Moreover, the only stored water in them
resides in the tubes themselves and the supply header from which the tubes
extend. This enables a once-through evaporator to be brought to operating
conditions more rapidly than a natural circulation evaporator. However, a
once-through evaporator must completely convert the water into steam, so
that only steam escapes from its tubes and flows on to the superheater. No
liquid water should leave the evaporator. The evaporator relies on a
feedwater pump located upstream in the water circuit to circulate water
through it at a controlled rate - a rate that if correct allows the steam to
leave in a saturated or a slightly superheated condition.
Thus, in a once-through evaporator the tube walls nearest to the
water inlet run wet as in a circulation type evaporator, because these ends
of the tube see only liquid water. But farther on in the tubes the water turns
into a mist and then into saturated steam. In the mist flow regime, water is
sheered from the interior surfaces of the tube walls, so the mist exists in
cores which extend through the centers of the tubes. The walls around
these cores run dry. This produces higher temperature in the tube walls and
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less efficient heat transfer. The higher temperatures may require metals
that are better able to withstand these temperatures or, in other words, a
resort to expensive high alloy steels.
Brief Description of Drawings
Figure 1 is a schematic sectional view of a steam generator
equipped with a once-through evaporator constructed in accordance with
and embodying the present invention;
Figure 2 is a perspective view of the evaporator;
Figure 3 is a sectional view taken along line 3-3 of Fig. 2;
Figure 4 is a fragmentary sectional view of the end of one of the
evaporator tubes showing a twisted tape anchored in the tube;
Figure 5 is a fragmentary sectional view similar to Fig. 4, but rotated
90°; and
Figure 6 is a fragmentary view of one of the evaporator tubes,
partially cut away and in section, showing the flow in the tube.
Best Mode for Carrying Out the Invention
Referring now to the drawings, a steam generator A (Fig. 1 ) basically
includes a duct 2 having an inlet end 4 and a discharge end 6. The inlet end
4 is connected to a source of hot gases, such as a gas turbine or an
incinerator, and those gases flow through the duct 12, leaving it at the
discharged end 6. In addition, a steam generator A includes a superheater
12, an evaporator 14, and a feedwater heater or economizer 16 arranged in
the duct 2 in that order from the inlet end 4 of the outlet end 6. Thus, the
hot
gases flow first through the superheater 12, then through the evaporator 14,
and finally through the economizer 16. Water flows in the opposite
direction. To this end, the economizer 16 is connected to a feedwater pump
18 which delivers feedwater to the economizer 16. It extracts heat from the
hot gases and transfers that heat to the liquid water that flows through it,
thereby elevating the temperature of the water, but the water remains a
liquid. Leaving the economizer 16, the liquid water then flows to the
evaporator 14 through which it passes. The evaporator 14 converts the
water to steam, mostly saturated steam. The steam flows into the
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superheater 12 which raises its temperature, transforming it into
superheated steam that may be used to power a turbine or in some
industrial process or even to heat a building. The superheater 12,
evaporator 14, and economizer 16 are basically tube banks. The
evaporator 14 operates on the once-through principle. Actually, the steam
generator A may have more than one evaporator 14.
The evaporator 14 includes (Fig. 2) a supply header 26, a discharge
header 28 and tubes 30 which extend between the two headers 26 and 28.
The supply header 26 has an inlet port 32 that is connected to the
economizer 16 and receives heated water from the economizer 16 - indeed,
water which is delivered to it under the head produced by the pump 18. The
discharge header 26 has outlet ports 34 which are connected to the
superheater 12, and through the ports 34 steam, that is saturated or slightly
superheated, is directed to the superheater 12. The tubes 30 have fins 36
which facilitate the extraction of heat from the gases flowing through the
duct 2.
Within the tubes 30 the heated water from supply header 26 is
converted into the steam which collects within the discharge header 28 and
then passes on to the superheater 12. Thus, the portion of each tube 30
that is closest to the supply header 20 contains liquid water, while the
portion that is closest to the discharge header 28 contains steam that is
saturated and perhaps even slightly superheated. In the intermediate
portion of each tube 30 the liquid water undergoes the change of phase and
becomes steam. Here the water boils, becoming a mist or a mixture of
water and saturated steam. Further along the mist becomes saturated
steam, and finally the saturated steam may become superheated steam,
albeit only slightly superheated. The superheated region of the tube 30, if
indeed there is superheated steam, is quite short. The tubes 30 are formed
from carbon steel or chrome-moly steel.
Each tube 30 contains a helical tape 40 (Figs. 3-5) which extends
from its inlet and, that is its end which is connected to the supply header
26,
through the regions in which the mist exists. The width of each tape 30 is
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slightly less than the inside diameter of the tube 30 through which it
extends,
so that the tape 40 can be inserted into or withdrawn from the tube 30
without interference from the tube 30 itself. Preferably, the width of each
tape 40 should be about 1/16 inches smaller than the inside diameter of its
tube 30, at least for a tube having a 2 inch inside diameter. The tape 40 is
twisted multiple times between its ends, so that its edges form helices that
lie along the inside surface of the tube 30. Indeed, a full 360° twist
of the
tape 40 should occur within a distance amounting to a length to diameter of
5 to 25. For example, for a tube 30 having a 2 inch inside diameter and a
length to diameter ratio of 5 for the twist in its tape 40, a full 360°
twist of the
tape 40 will occur in 10 inches of the tube 40. That end of the tape 40 that
resides at the inlet of the tube 30 is fitted with an anchor bar 42 that
extends
transversely across like inlet end of the tube 32. The bar 42 is welded to the
end of the tube 30 and to the tape 40, thus anchoring the tape 40 with its
tube 30. The tapes 40 are formed from a metal that can withstand the
temperatures associated with slightly superheated steam and are further
compatible with the metal of the tube 30 in the sense electrolytic reactions
are minimized. Stainless steel is suitable when the tubes 30 are carbon
steel.
In the operation of the steam generator A, hot gases flowing through
the duct 2 pass over the tubes of the superheater 12, the evaporator 14 and
the economizer 16 in that order and at each undergo a reduction in
temperature. The feedwater pump 18 forces water into and through the
economizer 16 where the water extracts heat from the gases that flow over
the tubes of the economizer 16. The temperature of the water rises, but the
water remains in the liquid phase. Under the head produced by the pump
18, the water flows from the economizer 16 into the supply header 26 of the
evaporator 14 and then into the tubes 30 of the evaporator 14. Within the
tubes 30, the water encounters even higher temperatures derived from the
gases passing through the duct 2. Indeed, the gases passing through the
evaporator 14 elevate the temperature of the tubes 30 high enough to
convert the water in the tubes 30 to steam. The water, initially upon entering
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the tubes 30, remains in the liquid phase, but as it flows through the tubes
30 it begins to boil, producing a mist. The tapes 40 extend through the
region of mist flow and produce a good measure of turbulence in the mist as
it flows on toward the discharge header 28. The turbulence brings the mist,
that is to say the water particles, against the inside surfaces of the tubes
30
(Fig. 6), thereby effecting better and more efficient transfer of heat between
the gases flowing over the tubes 30 and the mist in the tubes 30. This
further protects the tubes 30 from overheating. Were it not for the tapes 40,
the mist would tend to remain in the center of the tubes 30 and would be
surrounded by saturated or superheated steam along the interior surfaces of
the tubes 30, thus causing the tubes 30 in the regions of the mist to operate
at higher temperatures. As the mist in the tubes 30 flows on and
approaches the discharge header 28 it transforms into saturated steam and
may even change to superheated steam, albeit only slightly superheated.
But the regions of the tubes 30 that see only superheated steam are short
and are maintained at relatively moderate temperatures by reason of heat
conducted from them to the regions occupied by the mist and the liquid
water.
In lieu of anchoring the tapes 40 to the tubes 30 at the supply header
26, they may be anchored at the discharge header 28, in which event they
will extend toward the supply header 26. The tapes 40 may extend the full
lengths of the tubes 30 through which they pass or only through the regions
of mist flow. The evaporator 14 in lieu of having its tubes 30 arranged in a
single bank, may have them organized in multiple banks.
