FIELD OF THE INVENTION
The present invention relates to a device for smooth change over from gas turbine (GT) mode
to fresh air force draft (FD) mode with reduction of back pressure to the gas turbine. More
particularly, the present invention relates to a device of integration of heat recovery steam
generation (HRSG) system with induced draft fan arranged downstream of HRSG causing
reduction of backpressure to the gas turbine and for smooth change over from GT mode to
fresh air force draft (FD) mode or fresh air firing mode in combined cycle and cogeneration
system reducing the change over time.
BACKGROUND OF THE INVENTION
It is known in conventional combined or cogeneration power plants the heat recovery steam
generators are placed in between the Gas turbine exhaust gas outlet and main stack with no
devices present at the outlet of heat recovery steam generators.
The prior art of reducing the back pressure to the turbine is by increasing the free gas area in
the HRSG heat transfer sections and thereby reducing the exhaust gas velocity through the
tubes resulting in larger HRSG.

Another prior of patent no PN 200 500 44859 dated 03.03.05 discloses
integrating the HRSC with forced draft (FD) fan supplying fresh air to HRSG
supplementary firing system burners for uninterrupted steam generation,
even in cases where the Gas turbines are tripped. The disadvantages of
the prior art for change over from GT mode to FD mode do not result in
smooth change over. The disadvantages of the prior art devices for
reducing the back pressure to gas turbine are that they require bulky and
cumbersome HRSG leading to high cost towards manufacturing operation
and maintenance. Further the backpressure cannot be reduced below a
certain level.
It is therefore an object of the present invention to propose a device in
HRSG for reducing back pressure of the Gas turbine, which eliminates the
disadvantages of prior art
Another object of the present Invention is to propose a device in HRSG for
reducing back pressure of the Gas turbine, which is compact and easy to
operate.

A still another object of the present invention is to propose a device in HRSG which facilitates in

avoiding leakage of exhaust gases to the atmosphere.
A further another object of the present invention is to propose a device in HRSG which
eliminates the damage to gas turbine due to back pressure.
A still further object of the present invention is to propose a device in HRSG which facilitates
smooth change over from Gas turbine mode to fresh air firing mode.
SUMMARY OF THE INVENTION
Accordingly, there is provided a device for inducing draft in a heat recovery steam generator
(HRSG) system, the HRSG being flowable connected to a gas turbine via a transition duct acting
as a passage for exhaust gas from the gas turbine, the HRSG comprising: (i) a super heater
formed of a plurality of tubes, (ii) an evaporator consisting of a plurality of configured tubes; (iii)
an economizer adjacently located to the evaporator; and (iv) a condensate 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 through a device
called induced draft fan. The device comprising an uptake duct, a set of blades

along with shaft and an outlet duct disposed in between the last heat
transfer section of HRSG and stack.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig-1, is a schematic presentation of a HRSG disposed behind the gas
turbine, with the GT Exhaust gases leaving the HRSG entering the stack
directly as per the prior art
Fig-2, is a schematic representation of the present Invention with an
Induced draft fan in the downstream section of HRSG.
Fig-3, is a schematic representation of an variant of the present invention
with an induced draft fan in the downstream section of HRSG and an forced
draft fan before duct burner of the of an HRSG.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
Figure 1 shows a schematic arrangement of HRSG system. The system
generally comprises a Gas Turbine (1) connected by a transition duct (2) to
a heat recovery steam generator (3). The HRSG (3) has a plurality of
devices arranged in the following order namely a superheater, evaporator,

an economizer and condensate preheater. The exhaust gases leave the
econmizer/condensate preheater of HRSG (3) through an outlet duct (4)
to stack (7).
According to the present invention, as indicated in Fig 2, the GT (1)
exhaust gases leaving the HRSG (3) Is passed through an outlet duct (4) to
the inlet of an Induced draft fan (5). Further the outlet of the Induced draft
fan is connected by a duct (6) to stack (7). The induced draft fan (5)
disposed downstream of the HRSG (3) reduces the back pressure Imposed
on the Gas turbine (1) which reduces the possible damage to the gas
turbine (1) and further facilitates compact HRSG (3) construction and
higher gas turbine (1) power output
Referring to figure 3, in another aspect of the present invention, a
combination of a forced draft fan (8) Installed before the duct burner of a
supplementary fired HRSG (3) and an induced draft fan (5) disposed
downstream of the HRSG (3) to facilitate changeover from GT (1) mode to
Fresh Air firing mode and vice-a-versa, which further reduces the change
over time from GT (1) mode to Fresh air firing mode. The combustion air
duct (9) is provided to supply fresh air required for combustion during the
fresh air firing mode i.e. which the GTG is not operating.

In-an advance development of the present invention the ID Fan will be kept running under all
the GTG & HRSG operating loads and the augmenting air duct (11) will be kept open to achieve
smooth and fast change over operation. A single or multiple augmenting air ducts are provided

with dampers for control of air.
Since the induced draft fan (5) will always be running, the change over from GT mode to fresh
air firing mode will be smooth with reduced change over time from GT mode to fresh air firing
mode.
Since Induced fans are present, during GT trip, the openings in the duct (11) will allow air to be
sucked for the burner 10 and steam generation can continue without FD fans(8).
A further advantage of development of the present Invention with an addition of an induced
draft fan (5) downstream of HRSG (3) the leakages of exhaust gases from the HRSG (3) to
atmosphere improper duct connections are eliminated.
According to the invention, the Heat recovery steam generator (3) disposed behind gas turbine
(1) with an induced draft fan (5) device consists of Gas turbine (1); gas turbine exhaust ducting
(2); Heat recovery steam generator (3); HRSG outlet duct (4); downstream induced draft fan
(5); Induced outlet duct (6); the main stack (7);
The exhaust gases from the gas turbine (1) through a GT exhaust duct (2) and through a
considerable number of upstream heat transfer sections in HRSG (3) and HRSG outlet duct (4)
before reaching the induced draft fan device (5) which facilitates in reducing and eliminating the
exhaust gas leakages from HRSG (3) to atmosphere.

The induced draft fan (5) is arranged downstream of the HRSG (3) and is
disposed between HRSG (3) and stack (7), which reduces the back
pressure to the Gas Turbine.
The induced draft fan (5) is sized for either no back pressure to gas turbine
or for reduced back pressure to the gas turbine (1).
The induced draft fan (5) device is arranged downstream of the HRSG (3)
and is disposed between HRSG (3) and stack (7), which eliminates separate
scanner air/seal air fans for supplying cooling air to scanners and soot
blowers in cogeneration application.
In another aspect of the invention, a forced draft fan (8) is arranged
upstream of HRSG (3) of supplementary fired HRSG (3) and an induced
draft fan (5) arranged downstream of HRSG (3), which further facilitates
smooth change over from GT (1) to fresh air firing mode and vice versa.
The induced draft fan (5) system device arranged between HRSG (3) and
stack (7) can be either one or multiple numbers.
Thus, the invention provides a system which achieves reduced back
pressure to gas turbine (1) and eliminates the exhaust gas leakages from
HRSG. (3).

The invention for reducing the back pressure to the gas turbine (1) make
the HRSG (3) compact and easy to operate and maintain.
Thus, the Invention provides a system which facilitates smooth change over
from GT mode to FD mode

WE CLAIM
1. A device for smooth change over from gas turbine (GT) mode to fresh air force draft (FD)
mode with reduction of back pressure to the gas turbine comprising:
a gas turbine (1) having an exhaust gas duct (2) for flow of combustion gas for entering
to a heat recovery steam generator (HRSG) (3) for heat recovery;
Characterized in that,
a HRSG outlet duct (4) disposed for carrying the exhaust gas to induced draft fan (5)
disposed for reducing and eliminating the exhaust gas leakages from HRSG (3) to
atmosphere and to facilitate changeover from GT (1) mode to fresh air force draft mode
and vice versa when a force draft fan (8) connected with combustion air duct (9) for
supply of fresh air required for combustion during the fresh air force draft mode further
facilitates smooth change over from GT (1) to fresh air force draft mode and vice versa,
wherein the induced draft fan (5) is disposed in the system for sucking air continuously
through the openings in the duct (11) for the burner (10) for continuous steam
generation without FD fans (8).
2. The device as claimed in claim 1, wherein the induced draft fan (5) is disposed between
the HRSG (3) and stack (7) in the basic combined cycle power systems with gas turbine
(1) exhaust gas duct (2), HRSG (3), HRSG outlet duct (6), stack (7) being disposed
downstream of induced draft fan (5).

3. The device as claimed in claim 1, wherein the induced draft fan (5) is disposed
downstream of HRSG (3) and between HRSG (3) and stack (7).
4. The device as claimed in claim 1, wherein the forced draft fan (8) is disposed upstream of
HRSG (3).
5. The device as claimed in claim 1, wherein the augmenting air duct (11) is disposed
before the burner (10) for allowing the sucked air through the opening for the burner
(10).
5. The device as claimed in claim 1, wherein a single or multiple augmenting air ducts are
provided with dampers for control of air.
7. The device as claimed in claim 1, wherein one or more induced draft fans may be
accommodated.

A DEVICE FOR SMOOTH CHANGE OVER FROM GAS TURBINE (GT) MODE TO FRESH AIR
FORCE DRAFT ( FD) MODE WITH REDUCTION OF BACK PRESSURE TO THE GAS TURBINE
A device for smooth change over from gas turbine (GT) mode to fresh air force draft (FD) mode
with reduction of back pressure to the gas turbine consists of a gas turbine (1) having an
exhaust gas duct (2) for flow of combustion gas for entering to a heat recovery steam generator
(HRSG) (3) for heat recovery when a HRSG outlet duct (4) disposed for carrying the exhaust gas
to induced draft fan (5) disposed for reducing and eliminating the exhaust gas leakages from
HRSG (3) to atmosphere and to facilitate changeover from GT (1) mode to fresh air force draft
mode and vice versa when a force draft fan (8) connected with combustion air duct (9) for
supply of fresh air required for combustion during the fresh air force draft mode (FD) further
facilitates smooth change over from GT (1) to fresh air force draft mode (FD) and vic
versa.