FIELD OF INVENTION
The present invention generally relates to a co generation or a combined cycle
power plant. In particular the invention relates to a Heat recovery steam
generator (HRSG) system in a combined cycle/ cogeneration plant, adaptable to
downstream of Gas Turbine, which employs a condensate/ makeup water heater
which are being placed as a last stage heat transfer section of the heat recovery ,
steam generator system. The provision of the condensate pre-heater or makeup
water heater increases the overall heat recovery from the HRSG. The present
invention further relates to a method of enhancing the performance of the
condensate pre heater or makeup water heater by cooling the exhaust gases
below the dew point. More particularly, the present invention relates to a water
pre-heater device in a heat recovery steam generator system for enhancing the
performance of HRSG.
BACKGROUND OF THE INVENTION
It is known that combined or cogeneration power plants which convert fossil fuel
energy into electrical energy is efficient in comparison to coal fired power plants
and they are relatively easy and inexpensive to construct. In a typical combined
cycle /cogeneration plant gaseous /liquid fuels are fired in gas turbine and HRSG.
The exhaust gases leave the gas turbine at about 600 degree and contain a
significant source of energy. To recover this energy, the typical combined cycle
or cogeneration plant has a heat recovery steam generator through which the
hot exhaust gases is caused pass to produce steam for the process or for use in
steam turbine. The exhaust gases leave the HRSG between 100 to 200 degree.
The steam generated in the HRSG passes through the steam turbine and then
into a

condenser where it is converted into liquid water at a relatively low temperatures
of around 30 to 50 degc. This low temperature water enters the water pre heater
of HRSG where it gets heated to a near-feed-water temperature. To enhance the
performance of HRSGs, water pre heaters are generally installed as the last
stage heat recovery section in the Heat Recovery Steam Generators. These water
pre heaters generally operate at low metal temperatures, thereby increasing
their susceptibility to corrosion by acid gases .In order to deter the formation of
sulfuric acid, these water heaters are designed to recover limited quantity of heat ,
energy in the exhaust of the gas turbine and the water enters the water heaters
at a temperature higher than acid dew point temperatures. Hence the extent of
heat recovery is dictated by the amount of sulphur present in the fuels being
fired in the Gas turbines and in the supplementary firing system of the HRSGs.
Whenever any of the fossil fuels containing sulfur are fired in Gas turbine or in
the supplementary firing system of the Heat Recovery Steam Generator, sulfur
dioxide, and to a small extent sulfur trioxide, are formed in addition to CO2 and
water vapour. The SO3 combines with water vapor in the flue gas of the gas
turbine exhaust to form sulfuric acid and condenses on the last stage heat
transfer sections, which are at or below the dew point of the acid gas. This could
lead to corrosion and destruction of the last stage heat recovery surfaces or
water heaters, which are made up of metallic surfaces. However with the
reduction in exit gas temperature the thermal efficiency of the equipment is
increased.

The prior art concept of protecting the water heater or back end heat transfer
surfaces from dew point corrosion is by maintaining a higher cold end
temperature by adopting a recirculation system as depicted in Fig 1. In this
configuration a recirculation pump draws off some of the outlet heated water
from water heater outlet and mixed with the inlet colder condensate or makeup
water. The temperature of the mixed water entering the water heater is elevated
to a level higher than the dew point temperature to avoid corrosion of the cold
end surfaces.

US 6508206 B1 dated January 21, 2003 (Fig2) discloses a system for maintaining
a higher water inlet temperature entering the cold end sections by initially
heating the cold water in an external water heat exchanger (1), and using an
outlet(4), the heated water from one of the parallel sections(3) located inside
the HRSG, the cooled water is then fed to an inlet(5) of a second parallel heat
transfer section(6) located inside the HRSG for further heating, thus maintaining
wall temperatures higher than dew point temperatures as depicted in Fig 2.In
this arrangement the water heater has two sections(3&6) which are located in
parallel in so far as the flow of exhaust gases is concerned. As far as the water
flow is concerned the preheated water flow is in series which enters at a bottom
(2) of the first parallel section gets heated by the flue gases, then enters the
external heat exchanger where it gets cooled to the original entry temperature of
the first section,thereafter enters a bottom(5) of the second parallel section
located inside the HRSG and gets heated by the flue gases to the final required
temperature at the outlet(7).
The major disadvantages of the prior art is that the heat recovery potential is
limited by the amount of sulphur content in the fuel being fired in GT or HRSG.
Another disadvantage of the prior art is that the reduction in exit gas

temperature is not possible beyond a limit, which limits the thermal efficiency of
the equipment. Further there is a possibility of production of sulphuric acid as a
by-product by condensation of acid gases.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a water preheater
device to reduce the exit gas temperature in HRSG, which eliminates the
disadvantages of prior art.
Another object of the present invention is to propose a water pre heater device
in HRSG for increasing the efficiency of the HRSG, which is compact and easy to
operate.
A still another object of the present invention is to propose a water pre heater
device in HRSG which facilitates condensing the acid gases in HRSG.
A further object of the present invention is to propose a Iwater heater device in
HRSG, which is environment friendly and does not allow exit of pollutant gases
to the atmosphere.
A still further object of the present invention is to propose a water pre heater

device in HRSG, which enables production of sulphuric acid as a by-product.
Yet another object of the present invention is to propose a water pre heater
device in HRSG, which enables protection of the last-stage heat transfer surfaces
from cold back end corrosion.

SUMMARY OF THE INVENTION
Accordingly, there is provided a water pre heater device for reducing the exit gas
temperature in a heat recovery steam generator (HRSG) system, the HRSG being
flowably connected to a gas turbine via a transition duct acting as a passage for
exhaust gas from the gas turbine, the HRSG comprising: a super heater formed
of a plurality of tubes; an evaporator consisting of a plurality of configured
tubes; an economizer adjacently located to the evaporator; and a water pre
heater/makeup water heater adjacently located to the economizer, the exhaust
gases from the economizer/condensate pre heater/makeup water heater emitting
via a stack, the water pre heater device comprising an inlet water entry line
from condenser ; a set of finned or bare tubes coated with corrosion resistant
material along with a water outlet line, and disposed as a last stage heat
transfer section in the HRSG
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig-1 is a schematic presentation of a water pre heater in HRSG as per prior art.
Fig-2 is a schematic presentation of a water pre heater in HRSG as per prior art.
Fig-3 is a schematic view of a power system that incorporates an HRSG provided
with corrosion resistant condensing water heater constructed in accordance with
the present invention.
Fig-4 is a schematic view of the water pre heater of HRSG of the present
invention.

DETAILED DESCRIPTION OF A PREFERRRED EMBODIMANT OF THE
INVENTION
Figure 3 shows a Gas turbine (10) discharging exhausting gases at a high
temperature of around 600 degc through an inlet transition duct (11) of the
complete Heat recovery steam generator (19). The system comprises a Gas
Turbine (10) connected by a inlet transition duct (11) to a heat recovery steam
generator (19). The HRSG (19) has a plurality of devices arranged in the
following order namely a super heater (12), an evaporator (13), an economizer
(14) and a condensate water pre heater (15). The exhaust gases leave the
economizer/condensate water pre heater of HRSG (19) through and then via a
stack (20). The steam from the superheater (12) is delivered to a steam turbine
(18) or to process headers for electric power generation or process use. The
steam turbine (18) discharges steam at a low temperature/pressure into a
condenser (17) where it gets condensed into condensate. The condenser (17) is
connected to a condensate pump (16), which delivers the water into the water
pre heater (15) of the HRSG (19) This is heated by the exhausted gases and
delivers the preheated water to a deaerator (not shown) or to the required
system.
According to the present invention, as shown in Figure 3, exhaust gases from the
Gas turbine entering the HRSG (19) at a higher temperature before the last
stage heat recovery section namely corrosion resistant coated water pre heater
device (15) and the condensate entering into the water pre heater (15) from the
condensate /makeup water pump (16) at a lower temperature in the range of
30 to 60 degc which results in the metal temperatures falling below the acid dew
point of the gases and reduces the gas temperature leaving the HRSG (19) . The
finned orjjare tubes^of the water pre heater and'headers are coated with any of
the corrosion resistant polymer or metal or refractory based materials based on
the^ " " " " "~

level of concentration of the sulphuric acid formation for protecting the HRSG
(15). The coated water pre heater (15) of the HRSG (19) reduces the gas
temperature leaving the HRSG which increases the efficiency of the overall
system, reduces the atmospheric pollution and further facilitates generation of
sulphuric acid as a by product.
Referring to figure 4, in another aspect of the present invention, the thickness of
the protective coating of the tubes (22) aogLheaders_(21) of the water pre heater
can be varied for obtaining the required conductivity as well as corrosion
protection.
According to the present invention, the method of coating on the water pre
heater finned/bare tubes (22) and headers (21) can be any one of the types of
spraying or painting or dip processing based on the coating material and
functional performance requirements.
Another advantage of the present invention with the application of a refractory
based protective coating, is that the outside surface metal temperature of the
finned tubes (22) of the water pre heater (15) can be increased which
comparatively reduces the condensation of acid gases on the water heater with
marginal reduction in efficiency as compared to Metal or polymer based coating
materials.
A further advantage of the present invention with the corrosion resistant
precoated tubes (22) and headers (21) of the water pre heater is that it allows
condensation of acid gases into sulphuric acid as a by product.

A still further advantage of the present invention with the corrosion resistant
precoated tubes (22) and headers (21) of the water pre heater is that, the
coating can be on either bare or finned tubes.
The coating is applied with required finish, which eliminates sticking of the soot
on the tubes (22) and headers (21) of the waterpreheater (15).
A still another advantage of the present invention with the corrosion resistant
precoated water pre heaters (15), is that the Tubes and headers (22&21)
eliminate the use of recirculation pump of Fig 1 of the prior art.
Yet another advantage of the present invention with the coated water pre heater
(15), is that the, tubes and headers (22&21) eliminates the use of external
water-to-water heat exchanger of Fig 2 of the prior art.
The water pre heater (15) device consisting of tubes and headers (22,21) are
coated with any of the corrosion resistant polymer or metal or refractory based
materials based on the level of concentration of the sulphuric acid formation.
Yet further advantage of the present invention with the coated water pre heater
(15),is that the performance of the coating can be varied by selecting coating
material with good permeance property. Permeance determines the rate and the
extent to which a particular coating will be penetrated by the exposure medium.
Materials with lowest permeance will offer the best corrosion protection.
Thus, the invention provides a device, which facilitates achieving higher
efficiency of power plant and reduces the life cycle cost of the plant.

WE CLAIM
1. A water pre-heater (15) device in a HRSG system, the system (19) having a
super heater (12), an evaporator (13), an economizer (14) and at least one
condensate water pre-heater (15) connected to a gas turbine (10) through an
exhaust gas duct (11) through which combustion gas flows and gets entry to at
least one HRSG (19) for heat recovery, and combustion gas further passing
through a stack (20), the device comprising;
- a plurality of tubes (22) connected between a plurality of headers (21), an inlet
pipe (20) supplying cold water to the pre-heater and an outlet pipe (23)
supplying hot water using the exhaust gases, wherein the plurality of tubes
(22) and the headers (21) are coated with coating materials that facilitates
formation of H2S04 in the exhaust gases at low temperature as well as
increases the efficiency of equipment by enhancing increased heat recovery.
2. The device as claimed in claim 1, wherein the coating material comprises
corrosion resistant polymer or metal or refractory based material selected
according to the concentration level of sulphuric acid formation.
3. The device as claimed in claim 1 wherein the tubes (22) can be either bare
or finned tubes.

4. The device (15) as claimed in claim 1, wherein the thickness of the protective
coating of the water pre-heater tubes (22) and header (21) is variable.
5. The device (15) as claimed in claim 1 or 4, wherein the coating on the water
pre-heater finned/ bare tubes (22) and headers (21) is achieved by any one of
the process like spraying or painting or dip processing based on the coating
material and functional performance requirements.

ABSTRACT

A water pre-heater device in a heat recovery steam generator system for
enhancing the performance of HRSG
There is provided a water pre-heater device for reducing the exist gas temperature in
a heat recovery gas generator (HRGS) 19, having super heater (12), evaporator (13),
an economizer (14), and condense water pre-heater (15) connected to a gas turbine
(10) through an exhaust duct (11) through which combustion gas flows and gets
entry to a HRSG (19) system for heat recovery and combustion gas passes through
the stack (20), the device comprising ; plurality of coated tubes (22) connected
between headers (21), the inlet pipe supplying cold water to the pre-heater and outlet
pipe (23) supplying hot water using the exhaust gases, wherein the plurality of tubes
(22) and headers (21) are coated with coated materials.