FORM 2
The Patent Act 1970
(39 of 1970)
&
The Patent Rules, 2005

COMPLETE SPECIFICATION
(SEE SECTION 10 AND RULE 13)

TITLE OF THE INVENTION

“MODULE IN HEAT RECOVERY STEAM GENERATOR”
APPLICANTS:

Name Nationality Address
BHARAT HEAVY ELECTRICALS LIMITED Indian BHEL HOUSE, SIRI FORT, NEW DELHI – 110049, INDIA., with one of its manufacturing units at
High Pressure Boiler Plant, Tiruchirapalli - 620014, Tamil Nadu

The following specification particularly describes and ascertains the nature of this invention and the manner in which it is to be performed:-

The present invention generally relates to heat exchangers, and more specifically relates to Heat Recovery Steam Generators (HRSG).

Gas turbine power plants produce exhaust gas. The exhaust gas is at very high temperature and has enormous heat potential. This heat is recovered from exhaust gas using a HRSG. HRSG is a heat exchanger that recovers heat from the stream of the exhaust gas to produce steam. This steam can be used in equipment that converts the energy in the steam into mechanical energy. Generally, the steam is used to drive one or more steam turbines.

HRSG essentially comprises Evaporator, Superheater and Economizer sections which are mounted inside a duct through which the exhaust gas is made to pass. However, an additional Reheater assembly can also be included in HRSG. Accordingly in each of the sections water is converted into steam or already existing steam is further heated using the heat in the exhaust gas.

Each of the above mentioned sections comprises of multiple modules comprising tubes. Each module comprises of a top header, a bottom header and tubes. Tubes are welded between top header and bottom header to form a structure wherein the tubes are parallel to each other. Headers of different modules in one section are connected to enable fluid communication between adjacent modules in one section. The module is mounted in the duct by suspending the module using multiple supports.

Currently HRSGs are used behind Fr.9E Gas Turbines. Fr.9E is a lower capacity gas turbine which as per ISO rating generates 126 MW electricity. When HRSG is used behind such gas turbines, the Pressure in the superheater is about 78.2Kg/Cm2, temperature of the superheater is about 514 Degree Celsius and Steam flow in the superheater is at the rate of 187.1 Tons Per Hour. Further, currently module height is around 17.9 meters with header having huge diameter of about 219.1 mm. However, having such design if found to be inefficient for HRGSs which are mounted behind Fr.9Fa Gas Turbine, which are also known as Advanced class gas turbine. The Fr.9Fa gas turbines generate 255 MW electricity as per ISO rating. The HRSG is subjected to operating parameters which are far more stressing, thus making the currently known design undesirable. To address this issue, headers of huge diameter are being used, however, such huge diameter headers may not be readily available and the design of module using huge diameter header leads to bulkier modules which makes the modules unsafe and expensive in certain situations.

In light of the forgoing discussion, there is a need for an optimized design for module in HRSG.

STATEMENT OF INVENTION
An object is to provide an optimized design for a Heat Recovery Steam Generator (HRSG).
Another object is to provide a module in HRSG which is configured behind Fr.9Fa Gas Turbine.
Yet another object is to provide HRSG which can provide desired output parameters.
In view of the foregoing, an embodiment herein provides a Heat Recovery Steam Generator (HRSG) for recovering heat from exhaust gas. The HRSG comprises a module, wherein the module comprises a top header, a bottom header and plurality of tubes. The top header is of diameter 127 mm and thickness 28mm. The plurality of tubes are welded between the top header and the bottom header, thereby forming the module having height of about 23 meters, whereby the tubes are placed parallelly to each other at intervals along the longitudinal axis of the top header and the bottom header; and at least two support assembly attached to the top header, thereby holding the assembly against a roof of a duct from which the exhaust gas is passed.

Another embodiment provides method of assembling a module in a Heat Recovery Steam Generator (HRSG) for recovering heat from exhaust gas. The method comprises, welding plurality of tubes between a top header of diameter 127 mm and thickness 28mm and bottom header, thereby forming a module having height of about 23 meters.
These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.

BRIEF DESCRIPTION OF DRAWINGS
The embodiments herein will be better understood from the following description with reference to the drawings, in which:
FIG. 1 illustrates a simplified Heat Recovery Steam Generator, in accordance with an embodiment; and
FIG. 2 illustrates a module in a Heat Recovery Steam Generator, in accordance with an embodiment.

DETAILED DESCRIPTION
The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
The embodiments herein provide a Heat Recovery Steam Generator (HRSG). Referring now to the drawings, and more particularly to FIGS. 1 through 2, where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments.
FIG. 1 illustrates a simplified Heat Recovery Steam Generator 100, in accordance with an embodiment. The HRSG 100 includes a duct 102 into which exhaust gas is made to pass in the direction indicated by arrow 110. The exhaust gas is from a gas turbine (not shown). The exhaust gas is at very high temperature and has enormous heat potential. This heat is recovered from exhaust gas using tubes 108, top headers 104 and bottom headers 106. Assembly of tubes 108 with top headers 104 and bottom headers 106 form Evaporator, Superheater and Economizer sections. In an embodiment, the assembly could also form reheaters. In each of the sections water is converted into steam or already existing steam is further heated using the heat in the exhaust gas. Each section may have multiple modules.
A single module 202 in HRSG 100 is illustrated in FIG. 2, in accordance with an embodiment is illustrated. Module 202 comprises of a top header 104a, a bottom header 106a, and plurality of tubes 1081a, 10811a and so on (hereinafter referred to as 108a). The tubes 108 are welded between top header 104a and bottom header 106a. The tubes 108 are placed parallelly to each other along the longitudinal axis 208 of top header 104a and longitudinal axis 210 of bottom header 210. Further, two support assemblies 1141a and 11411a are attached to the top header 104a. In an embodiment, the support assemblies 1141a and 11411a are welded to the top header 104a. The support assemblies 1141a and 11411a enable holding the module 202 against roof 112 (illustrated in FIG. 1) of the duct 102. In an embodiment, the module 202 is also provided with supports 116a to the bottom header.
Such individual modules 202 are arranged along the length and width of the duct 102 to form the sections of the HRSG 100. Top header 104 and bottom header 106 of each module is connected to the top header 104 and bottom header 106 of the module next to it to enable fluid communication between the headers and the tubes 108. In an embodiment, the bottom headers 106 are also similarly connected.
In an embodiment, the top header 104 diameter is about 127mm and the thickness is about 28mm. Further, the tubes 108 have a diameter of about 38.1 mm and a thickness of about 3.6mm. Further, the length of the tubes 108 is chosen such that the height of the module202 is about 23 meters. In an embodiment, the HRSG 100 with the above module 202 design is configured to receive exhaust gas from a Fr 9Fa Gas Turbine. Further, in an embodiment each module 202 comprises 44 tubes 108. The different sections of the HRSG 100 comprising, such as superheater, reheater, evaporator and economizer which are made using multiple modules 202, are operated under parameters provided in Table 1.

Table 1
S.No Description Temperature(DegC) Pressure(Kg/cm^2) Max. Inducing Stress(Mpa) Allowable Stress(Mpa)
01 Superheater 590 158 57.61 60
02 Reheater 590 40 43.57 69.2
03 Evaporator 360 166.5 58.57 134.7
04 Economiser 345 188 56.43 134.7

The superheaters are exposed to a temperature of about 590 degree Celsius and pressure of about 158 Kg/cm2. Further the maximum inducing stress is 57.61 Mpa which is within the allowable stress of 60 Mpa.
Further, the reheaters are exposed to a temperature of about 590 degree Celsius and pressure of about 40 Kg/cm2. Further the maximum inducing stress is 43.57 Mpa which is within the allowable stress of 69.2 Mpa.
The evaporators are exposed to a temperature of about 360 degree Celsius and pressure of about 166.5 Kg/cm2. Further the maximum inducing stress is 58.57 Mpa which is within the allowable stress of 134.7 Mpa.
The economizers are exposed to a temperature of about 345 degree Celsius and pressure of about 188 Kg/cm2. Further the maximum inducing stress is 56.43 Mpa which is within the allowable stress of 134.7 Mpa.
The comparison between the maximum inducing stress and the allowable stress for various sections clearly shows that the design of the module is optimized to function safely when subjected to operating conditions as mentioned in the table above.

The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the appended claims.

We claim:
1. A Heat Recovery Steam Generator (HRSG) for recovering heat from exhaust gas, the HRSG comprising a module comprising:
a top header of diameter 127 mm and thickness 28mm;
a bottom header;
plurality of tubes, wherein the plurality of tubes are welded between the top header and the bottom header, thereby forming the module having height of about 23 meters, whereby the tubes are placed parallelly to each other at intervals along the longitudinal axis of the top header and the bottom header; and
at least two support assembly attached to the top header, thereby holding the module against a roof of a duct from which the exhaust gas is passed.
2. The HRSG according to claim 1, wherein the HRSG is configured to receive the exhaust gas from an advanced class gas turbine.
3. The HRSG according to claim 1, wherein the advanced class gas turbine is a Fr 9Fa Gas Turbine.
4. The HRSG according to claim 1, wherein the tubes have a diameter of about 38.1 mm.
5. The HRSG according to claim 1, wherein the tubes have a thickness of about 3.6 mm.
6. The HRSG according the claim 1, further comprising multiple modules, wherein the top header and bottom header of each of the modules are connected to the top header and bottom header of other modules, thereby establishing fluid communication between headers and the pipes in the modules.
7. A method of assembling a module in a Heat Recovery Steam Generator (HRSG) for recovering heat from exhaust gas, the method comprising, welding plurality of tubes between a top header of diameter 127 mm and thickness 28mm and bottom header, thereby forming a module having height of about 23 meters.
8. A Heat Recovery Steam Generator (HRSG) for recovering heat from exhaust gas substantially as herein above described in the specification with reference to the accompanying drawings.
9. A method of assembling a module in a Heat Recovery Steam Generator (HRSG) for recovering heat from exhaust gas substantially as herein above described in the specification with reference to the accompanying drawings.