U.S. patent application number 10/677443 was filed with the patent office on 2004-04-15 for once-through evaporator for a steam generator.
Invention is credited to Schroeder, Joseph E..
Application Number | 20040069244 10/677443 |
Document ID | / |
Family ID | 32093813 |
Filed Date | 2004-04-15 |
United States Patent
Application |
20040069244 |
Kind Code |
A1 |
Schroeder, Joseph E. |
April 15, 2004 |
Once-through evaporator for a steam generator
Abstract
A steam generator has a once-through evaporator which converts
liquid water into steam in tubes over which hot gases flow. Each
tube contains a metal tape which is twisted into a helical
configuration to induce turbulence in the mist produced by the
boiling, and the turbulence insures that the mist wets the inside
surfaces of the tubes, thus producing good heat transfer and
moderate temperatures in the tubes.
Inventors: |
Schroeder, Joseph E.;
(Union, MO) |
Correspondence
Address: |
POLSTER, LIEDER, WOODRUFF & LUCCHESI
12412 POWERSCOURT DRIVE SUITE 200
ST. LOUIS
MO
63131-3615
US
|
Family ID: |
32093813 |
Appl. No.: |
10/677443 |
Filed: |
October 2, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60416083 |
Oct 4, 2002 |
|
|
|
Current U.S.
Class: |
122/406.4 |
Current CPC
Class: |
F22B 1/1815 20130101;
F22B 29/06 20130101; F22B 37/18 20130101 |
Class at
Publication: |
122/406.4 |
International
Class: |
F22D 007/00 |
Claims
What is claimed is:
1. A once-through evaporator for a steam generator, said evaporator
comprising: a supply header for receiving liquid water; a discharge
header spaced from the supply header for receiving steam; tubes
extending between and connected to the supply and discharge
headers, so that water from the supply header may flow toward the
discharge header and be converted to steam by heat to which the
tubes are subjected; and tapes in at least some of the tubes for
inducing turbulence in a mist that is produced in such tubes as the
water converted to steam in such tubes.
2. An evaporator according to claim 1 wherein each tape is twisted
such that its edges form helices that lie along the interior
surfaces of the tubes in which they lie.
3. An evaporator according to claim 2 wherein each tape has a
length to diameter for a 360.degree. twist of about 5 to 25.
4. An evaporator according to claim 1 wherein the tape is anchored
at one end of the tube through which it extends.
5. An evaporator according to claim 2 and further comprising a bar
extending transversely across each tube that contains a tape at the
end of the tube at which it is anchored; wherein the bar is
attached to the tube across which it extends; and wherein the tape
for the tube is secured to the bar.
6. An evaporator according to claim 4 wherein each tape is anchored
to the tube through which it extends at that end of the tube which
is at the supply header.
7. An evaporator according to claim 2 wherein the width of each
tape is less than the inside diameter of the tube through which it
extends.
8. In a steam generator including a duct through which hot gases
pass, a superheater and an economizer located in the duct, with the
superheater being located upstream from the economizer with respect
to the flow of the gases, a pump for delivering liquid water to the
economizer, an improved once-through evaporator located in the duct
between the superheater and the evaporator and being connected to
the economizer and to the superheater such that water from the
economizer flows into the evaporator which converts it into a mist
flow and then into steam that is directed into the superheater
where it leaves as superheated steam, said evaporator comprising:
tubes which lie within the duct so that the hot gases pass over
them; and a twisted tape located within each tube in the region of
the mist flow.
9. The combination according to claim 8 wherein the tapes are
twisted such that their edges form helices that lie along the
inside surfaces of the tubes.
10. The combination according to claim 8 wherein each tape is
anchored at one end of the tube through which it extends.
11. The combination according to claim 10 and further comprising a
bar extended across and is secured to each tube at the end at which
the tape is anchored; and wherein the twisted tape in that tube is
attached to the bar.
12. The combination according to claim 8 wherein the liquid water
within each tube transforms into a mist and then into saturated
steam; and wherein the tape for the tube lies at least within the
region of the mist.
13. The combination according to claim 12 wherein the twisted tape
in each tube extends from the inlet and through at least the region
of the tube in which the mist exists.
14. For use in a once-through evaporator, the combination
comprising: a tube having an inlet end and an outlet end; and a
twisted tape located within the tube and having helical edges that
lie along the inside surfaces of the tube.
15. The combination according to claim 14 wherein the tape is
anchored to the tube at one the ends of the tube.
16. The combination according to claim 14 wherein the tape
possesses a length to diameter for a full 360.degree. twist of 5 to
25.
17. The combination according to claim 14 wherein the width of the
tape for each tube is slightly less than the inside diameter of the
tube.
18. The combination according to claim 14 and further comprising
water within one end of the tube and steam in the other end and a
mist flow region between the water and the steam, and wherein the
tape lies within the mist flow region.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application derives and claims priority from U.S.
Provisional Application Serial No. 60/416,083, filed Oct. 4,
2002.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not Applicable
BACKGROUND OF THE INVENTION
[0003] This invention relates in general to steam generators and,
more particularly, to an evaporator for a steam generator and
tubing for such an evaporator.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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 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
circulations 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.
[0008] 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.
[0009] 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.
[0010] 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 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 THE SEVERAL VIEWS OF THE DRAWINGS
[0011] FIG. 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;
[0012] FIG. 2 is a perspective view of the evaporator;
[0013] FIG. 3 is a sectional view taken along line 3-3 of FIG.
2;
[0014] FIG. 4 is a fragmentary sectional view of the end of one of
the evaporator tubes showing a twisted tape anchored in the
tube;
[0015] FIG. 5 is a fragmentary sectional view similar to FIG. 4,
but rotated 90.degree.; and
[0016] FIG. 6 is a fragmentary view of one of the evaporator tubes,
partially cut away and in section, showing the flow in the
tube.
DETAILED DESCRIPTION OF INVENTION
[0017] 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 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.
[0018] 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.
[0019] 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.
[0020] 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 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 {fraction (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.degree. 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.degree. 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.
[0021] 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 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.
[0022] 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.
* * * * *