DUAL PATH PARALLEL SUPERHEATER
FIELD AND BACKGROUND OF INVENTION
The present invention relates generally to methods and devices for effectively
increasing the delivery of steam in a controlled and efficient manner.
It is commonly required that temperature and/or steam flow (capacity) of an
existing boiler be increased. Pressure drop across the superheater increases as the
steaming capacity increases. High pressure drop is often the limiting factor for a
capacity increase. As a result, the complete superheater regularly needs to be replaced
to provide a lower pressure drop.
In a typical scenario, an operator requires that steam flow be increased (e.g.,
543.4kpph). Standard practice is to arrange the superheater such that there is only one
path by which steam can become superheated. In order to superheat at the increased
rate of steam, additional surface is added. Figure 1 hereof shows a typical prior art
arrangement 10 for a single-path series superheater, in a new surface 12 is added to an
existing surface 14 to process the increased capacity. There is a provided a drum 16 for
delivering steam to surfaces 12 and 14 and a turbine 18 for ultimately receiving steam
from surfaces 12 and 14
Table 1, below, predicted steam temperatures and pressures at the locations
as defined by Figure 1.
Table 1: Typical Prior Art Arrangement – Steam Temperature and Pressure
Profile
Desired outlet pressure is 1300 psig and desired outlet temperature is 900 ºF.
To control steam temperature there are spray attemperators at two interstage
locations, the first between locations B and C and the second between positions D and
E. The prior art arrangement is predicted to make full steam temperature with a total of
49ºF of spray attemperation. However, the arrangement does not achieve the target
outlet pressure of 1300 psig. The best achievable outlet pressure is only 1236 psig.
The traditional remedy for this is to increase the number of parallel steam flow paths in
the existing surface. This requires the replacement of all the existing superheater
tubes, superheater headers, roof seals, etc. and often requires that sootblower cavities
be relocated.
Thus, there is a need for increased steaming rate without the need for
replacement of the existing superheater.
SUMMARY OF INVENTION
A first aspect of the present invention is drawn to a dual-path parallel
superheater, comprising a drum adapted to deliver steam, the drum comprising a drum
outlet, the drum outlet opening to separate first and second paths for steam, a first
surface adapted to receive steam from said drum, and defining the first path for
superheating said steam and to deliver steam in a direction of a steam receiving
apparatus and a second surface adapted to receive steam from the drum and defining
the second path for superheating steam and to deliver steam in said direction of a
steam receiving apparatus. The second path is located at a position substantially
parallel to said first position. Each of said first path and said second path has a spray
attemperator at an interstage location thereof. The first and second paths re-combine at
a superheater outlet, the first surface and the second surface each comprise their own
respective superheating surface, and the first and second paths remain separate
between the drum outlet and the superheater outlet for separate superheating of steam
traversing each path to full steam temperature before re-combining. The arrangement
further comprises a combined path positioned to deliver a single quantity of
superheated steam from said superheater outlet toward the steam receiving apparatus.
[0009a] A second aspect of the present invention provides a dual-path parallel
superheater, comprising: a drum, said drum adapted to deliver steam; a steam receiving
apparatus opposite said drum; a first surface adapted to receive steam from said drum,
to provide a first path for superheating said steam and to deliver steam in a direction of
said steam receiving apparatus; a second surface adapted to receive steam from said
drum, to provide a second path for superheating steam and to deliver steam in said
direction of said steam receiving apparatus, said second path located at a position
substantially parallel to said first path; and wherein the first surface and the second
surface each comprise their own respective superheating surfaces, and wherein the first
and second paths remain separate between the drum outlet and a superheater outlet for
separate superheating of steam traversing each path to full steam temperature; wherein
said first surface and said second surface are arranged such that steam delivered from
said first path and steam delivered from said second path combine to so that a single
quantity of steam is delivered in said direction of said steam receiving apparatus.
[0009b] A third aspect of the present invention provides a dual-path parallel
superheater, comprising: a drum, adapted to deliver steam; a first surface adapted to
receive steam from said drum and to provide a first path for superheating steam; a
second surface adapted to receive steam from said drum and to provide a second path
for superheating steam, said second path located at a position parallel to said first path;
wherein the first surface and the second surface each comprise separate respective
superheating surfaces, wherein each of said first path and said second path has a spray
attemperator at an interstage location thereof; and wherein the arrangement is shaped
so that steam delivered from said first path and steam delivered from said second path
are heated separately, and combine after achieving full steam temperature to form a
single quantity of steam.
[0009c] A fourth aspect of the present invention provides a dual-path parallel
superheater, comprising: a drum, adapted to deliver steam; a first surface adapted to
receive steam from said drum and to provide a first path for superheating steam; a
second surface adapted to receive steam from said drum and to provide a second path
for superheating steam, said second path located at a position parallel to said first path;
and wherein the arrangement is shaped so that steam delivered from said first path and
steam delivered from said second path are heated separately and combine after heating
to form a single quantity of steam.
Described herein is a system and method in which steam is divided into two
paths at the drum outlet. One path is defined by existing superheater surface and the
other by new surface overhanging the furnace. Each path is independently controlled
with spray attemperation and independently achieves full steam temperature. The
streams are re-combined to a single path at the superheater outlet. The present dual-
path parallel superheater (“DPPS”) allows for an increased steaming rate without
requiring the replacement of the existing superheater.
[0010a] A fifth aspect of the present invention provides a method of superheating
steam, comprising: providing a drum; providing a steam receiving apparatus; providing
a first surface defining a first path adapted for receiving and delivering steam; providing
a second surface defining a second path adapted for receiving and delivering steam,
said second path arranged substantially parallel to said first path; wherein said first
surface and said second surface each comprise separate respective superheating
surfaces, delivering a first quantity of steam from said drum in the direction of said first
path; delivering a second quantity of steam from said drum in the direction of said
second path; superheating said first quantity of steam along said first path; superheating
said second quantity of steam along said second path; delivering said first quantity of
steam from said first path in the direction of said steam receiving apparatus; delivering
said second quantity of steam from said second path in the direction of said steam
receiving apparatus; wherein the first quantity of steam and the second quantity of
steam are maintained separately during superheating; wherein said first quantity of
steam is delivered from said first path such that it mixes with said steam from said
second path, such that said first quantity of steam delivered from said first path and said
second quantity of steam delivered from said second path recombine to form a single
third quantity for delivery to said steam receiving apparatus.
[0010b] A fifth aspect of the present invention provides a method of superheating
steam, comprising: providing a drum; providing a first surface defining a first path
adapted for receiving and delivering steam; providing a second surface defining a
second path adapted for receiving and delivering steam, the second path arranged
substantially parallel to said first path; wherein said first surface and said second
surface each comprise separate respective superheating surfaces; delivering a first
quantity of steam from said drum in the direction of said first path; delivering a second
quantity of steam from said drum in the direction of said second path; superheating said
first quantity of steam along said first path; superheating said second quantity of steam
along said second path; wherein the first quantity of steam and the second quantity of
steam are maintained separately during superheating; delivering said first quantity of
steam away from said first path; and delivering said second quantity of steam away
from said second path; wherein said first quantity of steam from said first path is
delivered such that it mixes with said second quantity of steam from said second path.
The various features of novelty which characterize the invention are pointed
out with particularity in the claims annexed to and forming part of this disclosure. For a
better understanding of the present invention, and the operating advantages attained by
its use, reference is made to the accompanying drawings and descriptive matter,
forming a part of this disclosure, in which a preferred embodiment of the invention is
illustrated.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings, forming a part of this specification, and in
which like reference numbers are used to refer to the same or functionally similar
elements:
is a schematic view of a prior art single path series superheater; and
is a schematic view of the present dual path parallel superheater.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference to the the dual path parallel superheater (“DPPS”)
according to the present invention is shown, the superheater arranged such that there
are two parallel paths by which steam becomes superheated. shows the DPPS
arrangement, in which a new surface 22 is added to the original surface 24 to process
increased capacity. As in the prior art structure, there is a provided a drum 30 for
delivering steam to surfaces 22 and 24 and a steam receiving apparatus 32 such as a
turbine for ultimately receiving steam from surfaces 22 and 24.
Table 2 below shows predicted steam temperatures and pressures at the
locations A1-A4 and B1-B4, defined in
Table 2: Steam Temperature and Pressure Profile for the present DPPS:
Desired outlet pressure is 1300 psig and desired outlet temperature is
900ºF.
reflects two paths: Path A, marked by locations A1-A4, and Path B,
marked by locations B1-B4. To control steam temperature, each path has a spray
attemperator 26, 28 at one interstage location.
As shown in Path A, including locations A1-A4, is arranged in a
side by side orientation in order to utilize interstage spray 26 while only requiring that
one new bank be installed. The interstage spray attemperator 26 is located between
positions A2 and A3. The attemperator 26 controls steam temperature and combats
high metal temperatures inherent to low steam flow.
The tubes in the Path A bank may be made of a steel compound such as
SA213-T22, a plurality of rows of stainless steel tubes may be employed in the outlet
legs. Additionally, the side by side design of the present invention minimizes the
amount of new heating surface required because hot steam is reintroduced to the front
of the furnace, where the flue gas is hottest.
Path B, including locations B1-B4, reuses the unit’s existing superheater
surface and existing interstage spray 28 location between positions B2 and B3. The
interstage spray 28 controls steam temperature and combats high metal temperatures
inherent to low steam flow. Similar to Path A, metals in the Path B banks may be made
of materials well-known to those of skill in the art. The exception is the outlet rows of
the Path B primary superheater: These rows generally require replacement with
stainless steel tubes.
Both Path A and Path B achieve full steam temperature independently.
Path A has 41ºF of spray margin and Path B has a 61ºF of spray margin. After being
controlled to the same temperature, steam from Path A and Path B recombine to form a
single outlet.
The parallel paths A and B are designed for the same pressure drop. This
can be accomplished initially by under drilling headers in the new surface or installing
orificed Dutchman in the existing surface. Under drilling headers and the installation of
orificed Dutchmen are techniques known to those of skill in the art. However, as the
unit becomes dirty, and spray flow changes, the pressure loss in each line may change.
As a means of control, a trim valve may be installed in at least one of the lines. With the
ability to dynamically adjust pressure drop, steam flow is enabled to remain as designed
in each path. Thereby, steam temperature and pressure can also be maintained as
designed.
The present invention offers numerous advantages. The present invention is
for industrial boilers undergoing capacity increases. When steaming rate increases the
amount of pressure drop between the drum and superheater outlet increases. If the
newly-desired steaming rate is high enough, a new superheater with additional flow
paths is required to maintain outlet pressure. A new surface is required regardless of
the existing superheater condition. As a result, operators are often forced to scrap
tubes before they reach end-of-life, or, abandon their projects all together due to high
project costs. The present DPPS allows for increased steam flow without replacing
existing surface.
Operators continuously strive to get as much as possible from existing
equipment before investing in replacements. This is especially true when the existing
equipment is in good operating condition. The present invention provides cost savings
to operators through the re-use of the existing surface. The present invention allows
satisfaction of an increased steam demand at a lower cost than traditional solutions.
The present invention may be applied to many surface different arrangements, offering
flexibility in its application.
The present DPPS arrangement may be applied to several boiler types,
including but not limited to, process recovery in the paper industry, stirling power
boilers, waste-to-energy applications, and biomass combustion technologies.
A comparison of Table 1 and Table 2, above, shows that the present DPPS
allows an increased steam flow to be controlled to a target steam temperature while
maintaining the desired outlet pressure.
Under increased flow conditions the DPPS design provides ability to re-use
existing superheater surface without lowering outlet pressure; ability to reach full steam
temperature with less heating surface than prior art designs; and ability to control
pressure drop across each steam path.
Alternative methods for processing an increased flow condition include
allowing outlet pressure to decrease and removing the existing superheater (tubes,
headers, roof seals, etc.) and installing new surface with additional parallel flow paths.
In another alternative, all or a portion of capacity increases may be derived
from increases in operating temperature. In these embodiments the method described
herein may further be used to maintain a desired pressure drop while maintaining a
desired superheater outlet temperature. While specific embodiments and/or details of
the invention have been shown and described above to illustrate the application of the
principles of the invention, it is understood that this invention may be embodied as more
fully described in the claims, or as otherwise known by those skilled in the art, including
any and all equivalents, without departing from such principles.
The term ‘comprising’ as used in this specification and claims means
‘consisting at least in part of’. When interpreting statements in this specification and
claims which include the term ‘comprising’, other features besides the features prefaced
by this term in each statement can also be present. Related terms such as ‘comprise’
and ‘comprised’ are to be interpreted in a similar manner.