U.S. patent application number 13/521688 was filed with the patent office on 2012-12-13 for process and apparatus for heating feedwater in a heat recovery steam generator.
This patent application is currently assigned to NOOTER/ERIKSEN, INC.. Invention is credited to Yuri M. Rechtman.
Application Number | 20120312019 13/521688 |
Document ID | / |
Family ID | 44320187 |
Filed Date | 2012-12-13 |
United States Patent
Application |
20120312019 |
Kind Code |
A1 |
Rechtman; Yuri M. |
December 13, 2012 |
PROCESS AND APPARATUS FOR HEATING FEEDWATER IN A HEAT RECOVERY
STEAM GENERATOR
Abstract
A feedwater heater (14) in a heat recovery steam generator (A,B)
lies within a flow of hot exhaust gas. The feedwater heater (14)
converts subcooled feedwater into saturated feedwater water, the
temperature of which is only lightly above the acid dew point
temperature of the exhaust gas so that corrosive acids do not
condense on coils (18) of the feedwater heater (14). Yet the
temperature of the saturated feedwater lies significantly below the
temperature of the exhaust gas at the coils (18), so that the coils
(18) operate efficiently and require minimal surface area. Pumps
(26, 28, 30) elevate the pressure of the saturated feedwater and
direct it into an economizer (64, 90) where, owing to the increase
in pressure, the water is again subcooled. The economizer (64, 90)
elevates the temperature still further and delivers the higher
pressure feedwater to evaporators (34, 70, 78) that convert it into
saturated steam that flows on to the superheaters (50, 78, 84).
Higher pressure pegging stem admitted to the feedwater heater (14)
controls the pressure--and temperature--of saturated steam and
water in the feedwater heater (14).
Inventors: |
Rechtman; Yuri M.; (Fenton,
MO) |
Assignee: |
NOOTER/ERIKSEN, INC.
Fenton
MO
|
Family ID: |
44320187 |
Appl. No.: |
13/521688 |
Filed: |
January 31, 2011 |
PCT Filed: |
January 31, 2011 |
PCT NO: |
PCT/US11/23110 |
371 Date: |
July 11, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61300222 |
Feb 1, 2010 |
|
|
|
Current U.S.
Class: |
60/645 ; 60/659;
60/670 |
Current CPC
Class: |
F22B 35/007 20130101;
F22B 1/1815 20130101; F22B 37/025 20130101; F22D 1/02 20130101 |
Class at
Publication: |
60/645 ; 60/670;
60/659 |
International
Class: |
F01K 13/00 20060101
F01K013/00; F01K 1/00 20060101 F01K001/00 |
Claims
1. In an HRSG for extracting heat from an exhaust gas that flows
through the HRSG and has an acid dew point temperature, and for
utilizing that heat to convert subcooled feedwater into steam, the
improvement comprising: a feedwater heater located in the flow of
the exhaust gas, the feedwater heater converting subcooled
feedwater at a low temperature into saturated steam and saturated
water, with the temperature of the saturated water being above the
acid dew point temperature of the exhaust gas; a feedwater pump
receiving water from the feedwater heater and elevating the
pressure of the water; and an economizer located in the flow of the
exhaust gas upstream from the feedwater heater and connected to the
feedwater pump for receiving feedwater at an elevated pressure from
the pump and elevating the temperature of that feedwater.
2. The combination according to claim 1 wherein the feed water
heater comprises: a steam drum into which the subcooled feedwater
is directed; a coil located below the steam drum within the flow of
the exhaust gas and at its lower and upper ends communicating with
the steam drum such that water from the steam drum circulates
through coil where some of the water is converted to saturated
steam; and a feedwater discharge line connecting the steam drum and
the feedwater pump.
3. The combination according to claim 2 wherein the steam drum is
connected to a source of pegging steam for controlling the pressure
in the steam drum.
4. The combination according to claim 2 and further comprising: an
initial pump that receives subcooled water at a low pressure and
temperature; and a feedwater line connecting the initial pump and
the drum of the feedwater heater for directing feedwater from the
initial pump into the drum.
5. The combination according to claim 4 and further comprising an
evaporator including a coil located in the flow of exhaust gas
upstream from the coil of the feedwater heater and receiving heated
feedwater at an elevated pressure produced by the feedwater pump
and producing saturated steam; and a steam discharge line through
which saturated the steam escapes.
6. The combination according to claim 5 wherein the evaporator
further includes a steam drum with which the coil of the evaporator
at its upper and lower ends communicates, so that water from the
steam drum circulates through the coil and some of it transforms
into saturated steam that flows into the steam drum.
7. The combination according to claim 6 and further comprising: a
pegging line that opens into the steam drum of the feedwater heater
and communicates with the steam drum of the evaporator; and a
pegging valve in the pegging line for controlling the pressure of
steam admitted to the steam drum of the feedwater heater through
the pegging line.
8. The combination according to claim 5 and further comprising a
superheater located in the flow of the exhaust gas upstream from
the coil of the evaporator and connected to the discharge line of
the evaporator for converting saturated steam received from the
evaporator into superheated steam.
9. The combination according to claim 5 wherein the coil of the
evaporator is located between the economizer and the coil of the
feedwater heater in the flow of the exhaust gas.
10. The combination according to claim 5 wherein the economizer is
located between the coil of the evaporator and the coil of the
feedwater heater in the flow of the exhaust gas, and the feedwater
pump forces feedwater from the feedwater heater through the
economizer and into the evaporator.
11. The combination according to claim 5 and further comprising
another evaporator located in the flow of the exhaust gas upstream
from the economizer for receiving heated feedwater from the
economizer and converting it into saturated steam.
12. The combination according to claim 1 wherein the feedwater
heater produces saturated water, the temperature of which exceeds
the acid dew point temperature by no more than about 15.degree.
F.
13. A process for furnishing liquid water at an elevated
temperature and pressure to an evaporator located in a flow of
exhaust gas having an acid dew point temperature, said process
comprising: extracting heat from the flow of exhaust gas to heat
subcooled feedwater to a saturation temperature above the acid dew
point temperature so as to provide saturated feedwater; elevating
the pressure of the saturated feedwater to produce higher pressure
feedwater; and upstream from the extraction of heat to create
saturated feedwater, extracting more heat from the flow of exhaust
gas to heat the higher pressure feedwater to a higher
temperature.
14. The process according to claim 13 and further comprising
upstream from the extraction of heat to heat the higher pressure
feedwater extracting more heat from the flow of the exhaust gas to
convert the higher pressure feedwater into saturated steam.
15. The process according to claim 13 wherein extracting heat from
the exhaust gas to heat subcooled water comprises: introducing the
subcooled water into a steam drum; and circulating the water from
the steam drum through a coil located in the flow of the exhaust
gas such that some of the water converts into saturated steam and
both saturated steam and saturated water flow into and occupy the
steam drum.
16. The process according to claim 15 and further comprising
controlling the temperature of the saturated steam and water in the
feedwater heater by controlling the pressure of the steam in the
steam drum.
17. The process according to claim 15 and further comprising:
converting the higher pressure feedwater into higher pressure
saturated steam; and controlling the temperature of the saturated
water in the steam drum by subjecting it to the higher pressure
saturated steam.
18. The process according to claim 15 wherein the temperature of
the saturated steam and water in the steam drum does not exceed the
acid dew point temperature by more than about 15.degree. F.
Description
RELATED APPLICATION
[0001] This application derives priority from and otherwise claims
the benefit of U.S. provisional application 61/300,222 filed 1 Feb.
2010, which application is incorporated herein by reference.
TECHNICAL FIELD
[0002] The invention relates to boilers and more particularly to a
heat recovery steam generators having improved feedwater
heating.
BACKGROUND ART
[0003] Boilers designed to convert liquid water into steam by
extracting energy from hot gases have become more efficient over
the years, and much of this efficiency derives from extracting heat
from the gases at lower temperatures--temperatures at which the
gases might otherwise be exhausted to the atmosphere. But this
increased efficiency has created its own problems that if left
unaddressed can result in corrosion of the low temperature surfaces
of the boilers.
[0004] Heat recovery steam generators (HRSGs) represent an
important class of high efficiency boilers. The typical HRSG
operates in a system that includes a gas turbine that drives an
electrical generator. The turbine discharges exhaust gas at an
elevated temperature, and this gas flows into the HRSG which
extracts heat from it to convert subcooled liquid water into
superheated steam, usually at several pressures. The steam powers a
steam turbine which in turn drives another electrical generator.
The HRSG has multiple banks of coils, the last of which in the
direction of the gas flow often forms part of a feedwater heater.
It receives condensate that is derived from low pressure steam
discharged by the steam turbine and elevates the temperature of the
water before the water is discharged into one or more evaporators
that convert it into saturated steam. Superheaters in turn convert
the saturated steam to superheated steam that powers the steam
turbine.
[0005] The combustion of a fossil fuel, such as natural gas, fuel
oil or coal, produces the hot exhaust gas that powers the gas
turbine and flow through the HRSG. By the time the hot gas reaches
the feedwater heater at the back end of the HRSG its temperature is
quite low, but it should not be so low that acids condense on the
heating surfaces of the feedwater heater. After all, the combustion
produces primary carbon dioxide and water in the vapor phase, but
the gas will also include traces of sulfur in the form of sulfur
dioxide and trioxide. Those compounds will combine with water to
produce sulfuric acid which is highly corrosive. As long as the
temperatures of the heating surfaces remain above the acid dew
point temperature of the exhaust gas, SO.sub.2 and SO.sub.3 pass
through the HSRG without harmful effects. But if any surface drops
to a temperature below the acid dew point temperature, sulfuric
acid will condense on that surface and corrode it, and the
vulnerable surfaces exist on the feedwater heater.
[0006] Dew point temperatures vary depending on the fuel that is
consumed. For natural gas the temperature of the heating surfaces
should not fall below about 140.degree. F. For most fuel oils it
should not fall below about 235.degree. F.
[0007] The condensate that is pumped to an HRSG to be converted
into saturated steam will typically arrive at the feedwater heater
at about 100.degree. F. But if directed through the feedwater
heater at that temperature, sulfuric acid will condense on the
downstream surfaces of the feedwater heater. To maintain all of the
surfaces of the feedwater heater, which surfaces are typically
coils, above the acid dew point temperature, in some HRSGs some of
the low-temperature feedwater is diverted directly to the first
evaporator at a bypass (FIG. 1). This reduces the load on the
feedwater heater. Typically, some of the heated water that has
passed through the coils of the feedwater heater, instead of
flowing to the first evaporator, is re-circulated to mix with the
cooler condensate, so that the temperature of the water entering
the coils exceeds the acid dew point temperature. The bypass water,
when employed, does not achieve the benefit of an initial
temperature rise in the feedwater heater and decreases the
efficiency of the HRSG. The re-circulation requires a
re-circulation pump and valve and further requires bringing the
feedwater that is discharged to a temperature higher than otherwise
may be necessary. That too decreases efficiency. Moreover, the
temperature of the exhaust gas as it passes into the feedwater
heater is often not much greater than the temperature of the water
leaving the feedwater heater, and as a consequence the feedwater
heater must contain a rather large and expensive grouping of coils.
The large feedwater heater coupled with the bypass results in a
significant pressure drop across them, and this imposes a
substantial load on the feedwater pump.
[0008] The conventional procedure for addressing the condensation
of acid, that is to say a feedwater heater with re-circulation, and
perhaps a bypass as well, works reasonably well where the exhaust
gas derives from the combustion of natural gas, which has a
relatively low acid dew point temperature on the order of
140.degree. F. With a low acid dew point temperature, the feedwater
heater can see a relatively large temperature differential between
the exhaust gas as it flows through the feedwater heater and the
feedwater in the coils of the heater, so the coils need not present
an excessively large surface area. However, when dew point
temperature is higher, such as at 230.degree. F. for exhaust gas
derived from some fuel oils, a large temperature differential is
not available at the feedwater heater (it becomes very tight--FIG.
1A). As a consequence, the feedwater heater requires a large bundle
of coils, making a conventional feedwater heater expensive in its
own right and also requiring a significant head from the condensate
pump just to force water through it and into the economizers and
low pressure evaporator beyond it.
[0009] Apart from that, most HRSGs produce superheated steam at
three pressure levels--low pressure (Ip), intermediate pressure
(ip) and high pressure (hp). The feedwater heater typically
discharges some of the heated feedwater directly into a low
pressure evaporator, so the condensate pump must not only overcome
the head required to force the feedwater through the feedwater
heater, but also the pressure at which the Ip evaporator operates.
The remainder of the feedwater goes to several ip and hp pumps
which force it through an economizer to elevate its temperature
still further so that it is better suited for ip and hp evaporators
located still further upstream in the flow of exhaust gases.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a schematic view of a feedwater heater and low
pressure evaporator and economizer as used in HRSGs of the prior
art;
[0011] FIG. 1A is a graphical representation of the temperature
differential between the feedwater in and exhaust gas flowing
through a feedwater heater and economizer of the prior art;
[0012] FIG. 2 is a schematic view showing a heat recovery steam
generator having an improved feedwater heater, economizer, and pump
arrangement, all constructed in accordance with and embodying the
present invention;
[0013] FIG. 2A is a graphical representation of the temperature
differential between the feedwater in and the exhaust gas flowing
through the feedwater heater and economizer of the HRSG depicted
FIG. 2;
[0014] FIG. 3 is a schematic view of an alternative heat recovery
steam generator embodying the present invention; and
[0015] FIG. 3A is a graphical representation of the temperature
differential between the feedwater in and exhaust gas flowing
through the feedwater heater and economizer of the HRSG depicted in
FIG. 3.
BEST MODES FOR CARRYING THE INVENTION
[0016] Referring now to the drawings, a heat recovery steam
generator (HRSG) A (FIG. 2) that extracts heat from a hot gas flow
provides superheated steam at several pressure levels. That steam
may be directed to a steam turbine to power it. After passing
through the turbine the steam discharges at a lower pressure and
temperature and is condensed into subcooled liquid water which is
circulated back to the HRSG A to again be converted into
superheated steam.
[0017] The HRSG A includes (FIG. 2) a housing 2 that is basically a
duct having an inlet 4 and an outlet 6. The HRSG A also includes a
series of heat exchangers contained within the housing 2, and their
functions are to a large measure described by their names. In
addition, the HRSG A includes pumps, valves, and lines or conduits
connecting the heat exchangers, pumps and valves together into the
functioning HRSG A. Hot exhaust gas derived from the combustion of
a fossil fuel enters the housing 2 at its inlet 4, passes through
the several heat exchangers, which extract heat from it, and is
discharged at the outlet 6. Typically, the exhaust gas represents
the exhaust of a gas turbine that burns natural gas or fuel oil or
even coal. It may enter the housing 2 at between 900.degree. F. and
1200.degree. F. The combustion produces primarily carbon dioxide
and water. But fossil fuels invariably contain trace amounts of
sulfur, so the combustion also produces small amounts of sulfur
dioxide and sulfur trioxide. To prevent this combination from
condensing into sulfuric acid on surfaces of the heat exchangers,
those surfaces must be maintained above the acid dew point
temperature for the exhaust gas. Sulfuric acid, of course, is
highly corrosive and will attack most metals, including the metals
from which the heat exchangers are formed. The water flows through
the HRSG A generally in the direction opposite to the direction in
which the hot exhaust gas flows, so the heat exchangers most
vulnerable are those located at the back end, that is to say, the
heat exchangers located downstream in the flow of the exhaust gas.
This holds particularly true when the exhaust gas has a high acid
dew point, such as the gas derived from the combustion of fuel oil.
Surfaces that this gas encounters should be maintained above at
least 230.degree..
[0018] Actually, the heat exchangers that are most vulnerable are
those that heat the incoming water, called feedwater, so that the
subcooled feedwater water is more efficiently converted into steam.
Notwithstanding the relatively high temperatures at which these
heat exchangers are capable of operating, they maintain very
effective temperature differentials between the liquid water in
them and the hot exhaust gas passing over them. This is achieved by
dividing the heating of the feedwater into two components--an
evaporative component and a sensible component. In the evaporative
component the temperature of the feedwater remains constant. In the
sensible component the temperature of the feedwater rises from the
constant temperature in the evaporative component.
[0019] Beginning at the back end of the HRSG A, an initial or
condensate pump 10 pumps liquid water--typically subcooled
condensate from a steam turbine--into a feedwater line 12 that
leads to an evaporative feedwater heater 14 where the evaporative
component of the heating occurs. In contrast to a feedwater heater
in a conventional HRSG, which includes a large bank of coils and a
re-circulation pump, the feedwater heater 14 has a steam drum 16
and a small bank of coils 18 below the steam drum 16. The steam
drum 16 is connected to the lower ends of coils 18 through a
downcomer 20 that enables liquid water to flow from the steam drum
16 into the lower ends of the coils 18. The upper ends of the coils
communicate with the lower region of the steam drum 16 through
risers 22. Within the coils 18 some of the water transforms into
saturated steam, and the coils 18 discharge liquid water and
saturated steam into the steam drum 16 through the risers 22. The
water circulates by natural convection through the drum 16, the
downcomer 20, the coils 18 and the risers 22 at a constant
temperature, which is the saturation temperature for the steam and
water, although a pump may be provided to assist the natural
convection.
[0020] The steam drum 16 contains saturated liquid water and above
the liquid water saturated steam. The liquid water must exist at
the minimum temperature for the coils 18 to which it is directed
through the downcomer 20, and that temperature is usually slightly
above the acid dew point temperature. When fuel oil produces the
exhaust gas, that the dew point temperature is often 230.degree. F.
but could be higher. The condensate will enter the steam drum 16 at
a relatively low temperature, typically about 100.degree. F., and
there it condenses the saturated steam and mixes with the saturated
water in the steam drum 16. Of course, it undergoes a rise in
temperature--indeed, instantaneously to the minimum temperature of
the water discharged into the downcomer 20 and thence to the coils
18. The hot exhaust gas passing through the coils 18 elevates the
energy of the liquid water in the coils 18 and converts some of it
to saturated steam, much like a circulation-type evaporator. The
pressure of the steam and liquid water in the steam drum 16
controls the temperature of that saturated water and steam.
[0021] Some of the liquid water in the steam drum 16 re-circulates
through the feedwater heater 14. However, the volume of the water
in the steam drum 16--remains generally constant, so while some
water re-circulates, more water is displaced by the incoming
feedwater introduced through the feedwater line 12. The displaced
water flows out of the drum 16 through a discharge line 24 that
leads to three feedwater pumps--a low pressure (Ip) pump 26, an
intermediate pressure (ip) pump 28, and a high pressure (hp) pump
30. Actually, two or even all three of the pumps 26, 28, 30 may be
combined into a single pump having multiple stages or discharges.
All three of the pumps 26, 28, 30 discharge liquid water at nearly
the same temperature, which is essentially the temperature of the
saturated water in the steam drum 16 of the feedwater heater 14,
although they discharge at progressively higher pressures and thus
the water again becomes subcooled.
[0022] The Ip pump 26 delivers liquid water through a low pressure
supply line 32 to a low pressure (Ip) evaporator 34 which,
operating on the natural circulation principle, converts that water
into saturated team. To this end, the Ip evaporator 34 has a steam
drum 36 into which the supply line 32 opens. In addition, it has
coils 38 located below the drum 36, a downcomer 40 leading from the
drum 36 to the lower ends of the coils 38, and risers 42 leading
from the upper ends of the coils 38 to the steam drum 36. The Ip
evaporator 34 produces saturated steam at a pressure and
temperature greater than the pressure and temperature at which the
feedwater heater 14 operates. The saturated steam A leaves the Ip
evaporator 34 through a discharge line 44 that extends away from
the top of the steam drum 36.
[0023] In the operation of the Ip evaporator 34 subcooled liquid
water enters the steam drum 36 through the Ip supply line 32. There
the water mixes with saturated steam--it being the product of the
Ip evaporator 34 as a consequence of liquid water entering the
downcomer 40 and flowing to the bottom of the coils 38. Upon rising
through the coils 38 some of the water converts to saturated steam
that enters the steam drum 36 along with the water that remains in
the liquid phase. The temperature of both exceeds the temperature
of the water entering the steam drum 36 at the supply line 32 from
the Ip pump 26, and that temperature is essentially the temperature
of the water discharged by the feedwater heater 14. The saturated
steam leaves the steam drum 36 through the discharge line 44, while
the liquid water re-circulates by natural convection through the
downcomer 40.
[0024] The discharge line 44 directs the saturated steam to a low
pressure (Ip) superheater 50 located upstream in the flow of
exhaust gas from the feedwater heater 14 and the evaporator 34. It
converts the saturated steam into superheated steam. The
superheated steam leaves the Ip superheater 50 through an Ip steam
line 52 that may lead to the low pressure stage of a steam turbine.
The steam line 52 controls the pressure at which the Ip evaporator
34 operates.
[0025] The discharge line 44 leading away from the steam drum 36 of
the Ip evaporator 34 also connects with the steam drum 16 of the
feedwater heater 14 through a pegging line 54 and pegging valve 56
in the line 54. Owing to the Ip pump 26 interposed between the
feedwater heater 14 and the Ip evaporator 34, the pressure in the
steam drum 36 of the latter exceeds the pressure in the steam drum
16 of the former. The pegging valve 56 admits the higher pressure
steam from the Ip evaporator 34 into the steam drum 16 of the
feedwater heater 14 such that the feedwater heater 14 operates at a
desired pressure. And that pressure correlates with a saturation
temperature that exceeds the dew point temperature of the exhaust
gas as it flows through the coils 18 of the evaporator--indeed, as
it flows through the last or downstream of the coils 18. A pressure
sensor monitors the pressure of the water in the steam drum 16 of
the feedwater heater 14 and produces a signal to which the pegging
valve 56 responds--opening and closing so that the saturation
temperature in the steam drum 16 remains at the desired level. The
temperature in the steam drum 16 should of course be above the acid
dew point temperature, but should not exceed it by more than about
15.degree. F. and preferably by no more than about 5.degree. F. to
achieve maximum efficiency in the HRSG A.
[0026] The ip pump 28 and the hp pump 30 deliver water heated by
the feedwater heater 14, but now subcooled owing to the increase in
pressure, to an economizer 64 which heats both the ip and hp water
in separate coils to still higher temperatures, yet the ip and hp
water remains in the liquid phase. The economizer 64 provides the
sensible component of the heating. Preferably the coils for the ip
and hp steam are located side by side in the duct 2 so that each
encounter exhaust gas at the same temperature, although they may be
located one ahead of the other. The economizer 64 discharges the
liquid water into two discharge lines--an ip discharge line 66 and
an hp discharge line 68.
[0027] The ip discharge line 66 leads to a combined unit 70 that
serves as an ip evaporator and an initial ip superheater, it being
located upstream in the flow of exhaust gas from the economizer 64.
The ip combined unit 70, by extracting heat from the exhaust gas,
produces saturated steam and later superheated steam that leaves
through an ip line 72. The hp discharge line 68 leads to another hp
economizer 74 located upstream from the ip combined unit 70. The
economizer 74 heats the liquid water still hotter and discharges it
through an hp line 76.
[0028] Both the ip line 72 and the hp line 76 connect with and feed
a combined hp evaporator and ip superheater unit 78, there being in
the unit 78 separate coils for the hp water-steam and ip
superheated steam. The unit 78 discharges superheated steam at an
intermediate pressure through an ip steam line 80 that may lead to
a steam turbine. The unit 78 also delivers saturated steam to an hp
connecting line 82. It leads to an hp superheater 84 that converts
the saturated steam into superheated steam. The superheater 84
discharges the superheated steam through an hp steam line 86 that
may lead to the steam turbine.
[0029] In the operation of the HRSG A, hot exhaust gas enters the
housing 2 at its inlet 4 and flows continuously through it to the
outlet 6, at which it may be discharged to the atmosphere. In so
doing the gas encounters the hp superheater 84, the hp evaporator
and ip superheater unit 80, the hp economizer 74, the combined ip
evaporator and superheater unit 70, the Ip superheater 50, the hp
and ip economizer 64, the Ip evaporator 34, and the feedwater
heater 14 in that order. Feedwater, which may be a condensate at a
temperature of 100.degree. F. or lower, enters the HRSG A at the
condensate pump 10 which forces it through the feedwater line 12
into the steam drum 16 of the feedwater heater 14 where it
undergoes an instantaneous rise in temperature (FIG. 2A). Actually,
the head overcome by the pump 10 is minimal and could be 5 psi or
lower, inasmuch as the feedwater heater 14 operates at a low
pressure and relies on convection to circulate water--and
steam--through it. The drum 16 already contains some liquid water
as do the coils 18 below the drum 16. Moreover, the drum 16
contains saturated steam above the water in it, and the pressure at
which that steam exists determines the temperature of the liquid
water, which is the saturation temperature of the steam and water
at that pressure. That temperature, which is constant, should
exceed the acid dew point temperature of the exhaust gas, but
preferably not by more than about 5.degree. F. The liquid water in
the steam drum 16 flows through the downcomer 20 to the lower ends
of the coils 18 through which it rises. The heat extracted from the
exhaust gas as it flows over the coils 18 converts some of the
liquid water into saturated steam which rises through the coils 18
and risers 22 along with the remaining water. Both enter the steam
drum 16. As with a natural circulation evaporator, the water
circulates through the feedwater heater 14 by convection, natural
or forced, and it remains at a constant temperature. The incoming
feedwater introduced into the steam drum 16 through the feedwater
line 12, displaces water from the drum 16, and that displaced water
flows through the discharge line 24 to the three pumps 26, 28, 30
at an elevated temperature that exceeds the dew point temperature
of the exhaust gas, but at the low pressure at which the feedwater
heater 14 operates. Apart from that, the incoming feedwater, while
undergoing an instantaneous rise in temperature, condenses the
saturated steam within the steam drum 16, thus preventing an excess
of steam in the drum 16. In any event, steam does not escape from
the drum 16; it simply reverts to saturated liquid water. The
feedwater heater 14 provides the evaporative component for heating
the feedwater.
[0030] The Ip pump 26 delivers the water at a higher pressure, so
it is again subcooled, through the Ip supply line 32 to the steam
drum 36 of the Ip evaporator 34 where it mixes with higher
temperature water already in the steam drum 36, water which is
located below saturated steam that also occupies the steam drum 36.
The water leaves the steam drum 36 through the downcomer 40 which
directs it into the lower ends of the coils 38. Being subjected to
the exhaust gas at a higher temperature than the coils 18 of the
feedwater heater 14, the coils 38 of the Ip evaporator 34 convert
some of the higher pressure water into saturated steam which along
with the remainder of the water flows upwardly into the steam drum
36, with the steam occupying the upper regions of the drum 36.
Owing to the higher pressure in the evaporator 34, the saturated
steam and water in its steam drum 36 exist at a higher temperature
than the water and steam in the feedwater heater 14. Again the
circulation is by natural convection, although the evaporator 34
may have a pump assist. From the steam drum 36 the saturated steam
flows through the discharge line 44 to the Ip superheater 50. Being
located upstream from the Ip evaporator 34 in the flow of exhaust
gas, the Ip superheater 50 sees temperatures higher than the Ip
evaporator 34--indeed, temperatures high enough to convert the
saturated steam into superheated steam. That steam escapes through
the Ip steam line 52.
[0031] Some of the Ip saturated steam that leaves steam drum 36 of
the Ip evaporator 34 serves as pegging steam for controlling the
pressure and temperature of the saturated steam in the steam drum
16 of feedwater heater 14 and likewise the temperature of the
liquid water in the steam drum 16--and that is the water that
circulates through the downcomer 20 and into the coils 18 of the
feedwater heater 14
[0032] The higher pressure pegging steam flows through the pegging
line 54, it being discharged into the steam drum 16 of the
feedwater heater 14 at a pressure determined by the pegging valve
56. That pressure is such that the saturation temperature of the
steam and water in the steam drum 16 is slightly above the acid dew
point temperature of the exhaust gas at the feedwater heater
14.
[0033] The ip pump 28 delivers water that has been heated at the
feedwater heater 14, discharging it at an intermediate pressure
higher than the pressure produced by the Ip pump. That subcooled
water flows through the combined economizer 64 where it is elevated
to a higher temperature. It then flows to the combined ip
evaporator and ip superheater unit 70 from which it leaves as
superheated steam. That superheated steam undergoes a further
elevation in temperature at the ip coils of the combined hp
evaporator and ip superheater unit 78, from which it is discharged
into the ip steam line 86. The economizer 64 provides the sensible
component for heating the feedwater.
[0034] The hp pump 30 also delivers water heated at the feedwater
heater 14, it being directed in a subcooled condition into the
combined economizer 64 where its temperature is elevated, and
thence into the hp coils of the combined hp evaporator and ip
superheater unit 78 where it is converted to saturated steam. The
saturated steam flows into the hp superheater 84 where it becomes
superheated steam. That steam leaves through the hp steam line
86.
[0035] The evaporative feedwater heater 14 with its evaporative
component combined with the economizer 64 with its sensible
component produces wider temperature differentials between the
exhaust gas flowing over those heat exchangers and the liquid water
flowing through them than could be achieved by a conventional
feedwater heater, even one operating with an upstream economizer.
In this regard, the temperature of the water in the feedwater
heater 14 remains constant and does not rise with the higher
temperatures at its end upstream in the flow of exhaust gas. As a
consequence, a relatively large temperature differential exists at
the upstream end and further downstream as well (FIG. 2A).
Moreover, the water flows on to the economizer 64 at this lower
temperature, so the coils at the downstream end of the economizer
64, reference being to the flow of the exhaust gas, also see a
significant temperature differential between the gas following over
those coils and the feedwater in them.
[0036] By way of example--and example only--exhaust gas derived
from the combustion of fuel oil and having an acid dew point
temperature of 230.degree. F., approaches the combined economizer
64 in the housing 2 at 600.degree. F. The economizer 64 extracts
enough heat to lower the temperature to 400.degree. F., and at that
temperature it flows into the Ip evaporator 34 which extracts more
heat, causing the exhaust gas to flow on to the feedwater heater 14
at 300.degree. F. The feedwater heater 14 extracts still more heat
from the exhaust gas, enough to reduce its temperature to
245.degree. F., and at that temperature the exhaust gas leaves the
housing 2 through the outlet 4. The condensate pump 10 delivers
500,000 lb/hr of feedwater to the steam drum 16 of the evaporator
14 at 100.degree. F., and this requires only enough head to
overcome whatever pressure the Ip evaporator 34 produces in the
interior of the drum 16 which may be as low as 8 psig. Indeed, that
pressure is sufficient to keep the saturation temperature of the
steam and water in the steam drum 16 at 235.degree. F. And the
saturated water leaves the drum 16 at that temperature and enters
the coils 18 where it remains at 235.degree. F. while some of it is
converted to steam. Since the temperature of the water in the coils
18 does not rise, a substantial temperature differential exists
between the water and exhaust gas. Being at a temperature above the
acid dew point temperature of the exhaust gas, the coils 18 do not
experience condensation of acid on their surfaces. Saturated water
also leaves the steam drum 16 through the discharge line 24 at
235.degree. F., flowing into the pumps 26, 28, 30 at that
temperature. The Ip pump 26 directs 100,000 lb/hr to the steam drum
36 of the Ip evaporator 34 at 235.degree. F. The Ip evaporator 34
converts that water to saturated steam, discharging 100,000 lb/hr.
through the discharge line 44 at 350.degree. F. The ip pump 28 and
hp pump 30 deliver the remaining 400,000 lb/hr through the supply
lines 60 and 62, again at 235.degree. F., to the combined
economizer 64 which extracts more heat--indeed quite efficiently
because the liquid water enters the economizer 64 at about
235.degree. F., producing a large temperature differential between
the exhaust gas and feedwater at the downstream end of the
economizer and farther upstream as well.
[0037] The HRSG A equipped with the feedwater heater 14 and
companion economizer 64 heats a large volume of feedwater without
requiring the re-circulation and bypass utilized by a feedwater
heater for a conventional HRSG. Moreover, the feedwater heater 14
heats the feedwater with a relatively small bank of coils 18, since
the water in the coils 18 remains at the saturation temperature,
and a relatively large temperature differential exists between the
water in those coils 18 and the exhaust gas flowing over them. The
water circulates through the coils 18 by natural or forced
convection, so the condensate pump 10 need not overcome whatever
resistance the coils 18 may otherwise impose on the flow. Since the
coils 18 see a substantial drop in the temperature of the exhaust
gas as it flows across them, they are highly efficient. Upon
leaving the feedwater heater 14, the liquid water at the relatively
low saturation temperature in the heater 14 flows on to the
economizer 64 through the ip supply line 60 and hp supply line 62,
so that at the downstream end of the economizer 64, another
substantial temperature differential exists.
[0038] In contrast, the feedwater heater of a conventional HRSG
will normally have a large bank of coils designed to extract heat
over a relatively small temperature differential in the flow of
exhaust gas, and those coils offer significant resistance to the
flow of feedwater. The condensate pump must overcome that
resistance. Indeed, the feedwater heater 14, owing to the absence
of re-circulation in the heater 14 itself, need not heat the
feedwater to as high a temperature as a conventional feedwater
heater. Moreover, the temperature differential between the
feedwater at the coils and upstream economizer of a conventional
feedwater heater is not as great as seen by the feedwater heater 14
and economizer 64 (compare FIG. 1A and FIG. 2A).
[0039] The components operating under the pressures developed by
the ip pump 28 and the hp pump 30 may vary from that described in
the foregoing text and depicted in the drawings. Moreover, one of
the pumps 28 or 30 and the components it services may be eliminated
altogether, producing an HRSG that delivers superheated steam at
two pressures. Also, the pegging steam supplied to the drum 16 of
the feedwater 14 may come from a source of pressurized steam other
than the drum 36 of the Ip evaporator 34. While the evaporator 34
and the evaporative components of the combined units 70 and 80 are
depicted and described as natural circulation evaporators, they may
take the form of pump-assisted circulation evaporators or even
once-through evaporators.
[0040] An alternative HRSG B (FIG. 3) closely resembles the HRSG A
and as such includes the same feedwater heater 14 and Ip evaporator
34. However, the three pumps 26. 28, 30 feed another combined
economizer 90 located between the feedwater heater 14 and the Ip
evaporator 34 in the flow of the exhaust gas As such liquid water
enters the economizer 90 at the saturation temperature of the water
discharged by the feedwater heater, so a substantial temperature
differential exists between the water in the combined economizer 90
and the exhaust gas flowing through the economizer 90 (FIG. 3A).
This likewise contrasts with the heating of feedwater in a
conventional HRSG (compare FIG. 1A and FIG. 3A). The economizer 90
thus requires a relatively small bundle of coils for each of the
Ip, ip and hp water. Also, the feedwater heater 14 may include a
deaerator 92 into which the feedwater line 12 and pegging line 54
open, so that the feedwater and pegging steam flow into the
deaerator 92. It in turn communicates with the steam drum 16
through a connecting line 94.
* * * * *