FORM-2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2006
COMPLETE
Specification
(See Section 10 and Rule 13)
TWO PASS FIRE TUBE BOILER WITH INTEGRATED HEAT
RECOVERY SYSTEM
THERMAX LIMITED
an Indian Company
of D-13, MIDC Industrial Area, R.D. Aga Road, Chinchwad, Pune - 19,
Maharashtra, India
The following specification particularly describes the invention and the manner in which
it is to be performed.

FIELD OF THE INVENTION
The present invention relates to the field of fire tube boilers.
Particularly, the present invention relates to two pass fire tube boilers and
feed water heaters.
DEFINITIONS OF TERMS USED IN THE SPECIFICATION
The term "pass" used in the specification means the passage of heating media through a mass of liquid, typically water to be heated. The pass may include one or more tubes. In case of multiple passes the tubes may be turned to return through the mass of liquid or independent sets of tubes may be provided. Therefore, typically in a three pass system the tube or set of tubes may turn twice through the mass of the liquid to be heated before being discharged.
The term "Induced Draft" used in the specification is the forced movement of-air or gases typically caused by the suction created by the inlet side of a fan.
These definitions are in addition to those expressed in the art.
BACKGROUND OF THE INVENTION & PRIOR ART
A fire-tube boiler is a type of boiler in which hot gases produced from combustion pass through one or more tubes placed inside the boiler shell.

The heat energy of the flue gases is transferred to the water contained inside the boiler through the tubes by thermal convection for heating the water and ultimately creating steam.
Most common small boilers are three pass fire tube boilers with a furnace and two convective pass placed inside a water drum. These boilers have high water holdup. Higher water holdup leads to higher startup time and higher heat loss in the operating cycle of a boiler. These types of boilers are less efficient for a process, which requires steam only during a limited duration of the day. These boilers have low furnace volume for combustion as only a portion of the furnace volume over the grate is available for combustion. This leads to poor combustion and higher incomplete combustion loss. Currently, this challenge is mostly addressed by using a hybrid configuration of the boiler with an external combustion chamber, a water cooled furnace and a drum with convective fire tubes but this is a costly solution and requires more space as the combustion chamber is placed external to the drum with convective fire tubes.
Therefore, there is felt a need for a compact fire tube boiler that overcomes the drawbacks of the prior art and can be used effectively with minimum water holdup in applications which require steam only during a limited duration of the day.

OBJECTS OF THE INVENTION
An object of the present invention is to provide a compact fire tube boiler.
Another object of the present invention is to provide a fire tube boiler with minimum water holdup.
Yet another object of the present invention is to provide a fire tube boiler which requires minimum startup time for heating a fluid.
Still another object of the present invention is to provide a fire tube boiler having minimum heat loss in the operating cycle of the boiler.
One more object of the present invention is to provide a fire tube boiler having high furnace volume for combustion.
Still one more object of the present invention is to provide a fire tube boiler with minimum incomplete combustion loss.
Yet one more object of the present invention is to provide a fire tube boiler with minimum emission.
An additional object of the present invention is to provide a fire tube boiler with integrated heat recovery system and deaeration system.

Still additional object of the present invention is to provide a compact steam generation system with lower incomplete combustion losses and higher efficiency.
SUMMARY OF THE INVENTION
In accordance with the present invention a boiler system is provided, said boiler system comprising:
• a fire-tube boiler assembly adapted to provide steam, said fire-tube boiler assembly comprising:
■ a furnace adapted to combust a fuel and generate hot flue gases;
■ a refractory arch for containment of the flames produced during combustion of the fuel within said furnace;
■ a shell adapted to surround said furnace to contain steam, hot feed water, or steam-water mixture;
■ a reversal chamber provided within said boiler assembly at one of its operative ends adapted to collect the hot flue gases from said furnace and transfer to a first set of heat exchanger tubes;
■ a set of tube plates provided on the lateral faces of said shell interconnecting said furnace and said shell and adapted to define an enclosed region as a steam drum between said shell and said furnace;
■ the first set of heat exchanger tubes disposed within said steam drum adapted to receive the hot flue gases, transfer the heat of the hot flue gases to the hot feed water, and

convert the hot feed water to steam, the hot flue gases becoming partially heat-extracted flue gases; and
a steam separator device provided on said shell, said
steam separator device comprising: a steam separator
adapted to eliminate moisture from the steam received
therein from said steam drum, a first port adapted to
release the steam, and a second port adapted to discharge
air during boiler startup;
a feed water tank assembly provided on the operative top of said fire-tube boiler assembly and adapted to provide hot feed water, said feed water tank assembly comprising:
a feed water tank adapted to store cold feed water;
an inlet operatively connected to said first set of heat
exchanger tubes and adapted to receive the partially heat-
extracted flue gases;
a second set of heat exchanger tubes housed inside said
feed water tank and adapted to receive the partially heat-
extracted flue gases via said inlet, wherein the partially
heat-extracted flue gases transfer heat to the cold feed
water, to provide hot feed water and become cooled flue
gases;
inducing means to induce the flow of the partially heat-
extracted flue gases from said first set of heat exchanger
tubes through said second set of heat exchanger tubes;

a vent pipe provided at the operative top of said feed
water tank for discharging the evaporated steam and air
from said feed water tank;
conduit means to lead the hot feed water to the steam
drum; and
an inlet port adapted to receive cold feed water in said
feed water tank; and
• a support frame adapted to hold said first assembly and said second assembly.
Typically, in accordance with the present invention, said furnace is semi-cylindrical in shape.
In accordance with the present invention, the fuel combusted in said furnace is selected from wood, coal, rice husk, coconut shell, and the like.
Preferably, in accordance with the present invention, said shell has a dome shape and is placed around said furnace.
Additionally, in accordance with the present invention, the cross-section of said steam separator is larger than the cross-section of the outlet pipe of said first port.
In accordance with the present invention, a smoke chamber is provided to receive the partially heat-extracted gases from said first set of heat exchanger

tubes and adapted to transfer the partially heat-extracted gases to said second set of heat exchanger tubes.
Preferably, in accordance with the present invention, said inlet is provided on one of the faces of said feed water tank.
Typically, in accordance with the present invention, said inducing means are provided opposite to said inlet.
Additionally, in accordance with the present invention, said steam drum houses therein said first set of heat exchanger tubes or said first and said second set of heat exchanger tubes, with said reversal chamber integrated within said steam drum.
Preferably, in accordance with the present invention, said shell is curved at the bottom for joining with said furnace.
In accordance with the present invention, a method for generating steam in a boiler system is provided, said method comprising the following steps: (i) combusting a fuel in a furnace to generate hot flue gases; (ii) containing hot feed water in a shell which is provided surrounding said furnace, a set of tube plates are provided on the lateral faces of said shell to define an enclosed region as a steam drum;

(iii) receiving the hot flue gases through a first set of heat exchanger
tubes, wherein said first set of heat exchanger tubes are disposed
within said steam drum; (iv) vaporizing the hot feed water by extracting heat from the hot flue
gases to generate steam, the hot flue gases becoming partially
heat-extracted flue gases; (v) releasing the steam through a steam separator to eliminate any
moisture; (vi) receiving the partially heat-extracted flue gases via an inlet into a
second set of heat exchanger tubes, wherein said second set of
heat exchanger tubes are housed inside a feed water tank
containing coid feed water; (vii) inducing the flow of the partially heat-extracted flue gases
through said second set of heat exchanger tubes by using
inducing means; (viii) transferring the heat in the partially heat-extracted flue gases to
the cold feed water to generate hot feed water, the partially heat-extracted gases becoming cooled flue gases which are
subsequently discharged; (ix) discharging vapors from said feed water tank through a vent pipe
which is operatively connected at the top of said feed water tank;
and (x) feeding the hot feed water from said feed water tank to said
steam drum by conduit means provided therein.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention will now be described with the help of the accompanying drawings, in which,
Figure 1 illustrates an isometric view of the boiler system, in accordance with the present invention;
Figure 2 illustrates a schematic diagram of the fire-tube boiler assembly of the boiler system, in accordance with the present invention;
Figure 3 illustrates an isometric view showing the steam drum, the shell, the furnace, the Tube plate, and the first set of heat Exchanger tubes of the fire-tube boiler assembly, in accordance with the present invention;
Figure 4 illustrates yet another isometric view of the steam drum showing the steam separator, in accordance with the present invention;
Figure 5 illustrates an isometric view of the feed water tank assembly of the boiler system, in accordance with the present invention; and
Figure 6 illustrates the fire-tube boiler assembly showing the hot flue gases path, in accordance with the present invention.

DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention will now be described with reference to the accompanying drawings which do not limit the scope and ambit of the invention. The description provided is purely by way of example and illustration.
The present invention envisages a boiler system for generating steam which is compact and efficient with high furnace volume and provides minimum water hold up, minimum start-up time, minimum heat loss, minimum incomplete combustion losses, and reduced emissions, thus, overcoming the drawbacks enlisted in the prior art. The boiler system of the present invention, illustrated in Figure 1 and generally represented by numeral 100, mainly comprising a fire-tube boiler assembly 102 which is adapted to generate steam by transferring heat of the hot flue gases produced in the furnace 106 to the hot feed water by using a first set of heat transfer tubes 112 placed within boiler assembly 102 and a feed water tank assembly 104 which is adapted to generate hot feed water using the partially heat-extracted flue gases from the boiler assembly 102, the hot feed water is subsequently fed to the fire-tube boiler assembly 102 for vaporizing. The boiler assembly 102 and the feed water tank assembly 104 are supported by a support frame 108, wherein the feed water tank assembly 104 is placed at the operative top of the boiler assembly 102 which provides for the compactness of the boiler system 100.

Figure 1 & 2 illustrate the fire-tube boiler assembly 102 comprising the furnace 106 which is typically semi-cylindrical in shape and is placed over a combustor 110, this arrangement provides for a good furnace volume for combustion. The furnace 106 is adapted to receive hot combustion gases from the combustor 110 wherein a solid fuel selected from coal, wood, rice husk, coconut shells, and the like, is combusted. The hot gases from the furnace are passed through a first set of heat exchanger tubes, illustrated in Figure 3 by numeral 112, to generate steam. A refractory arch, illustrated in Figure 6 by numeral 114, is provided in the fiirnace 106 to contain the flames produced during the combustion of the fuel within the furnace 106. A shell 116 is provided adequately surrounding the furnace 106, wherein the shell 116 forms a dome-like enclosure around the furnace 106 to contain steam, hot feed water, or steam-water mixture. A reversal chamber 118 is provided with the boiler assembly 102 at one of its operative ends, to collect the hot flue gases from the furnace and transfer them to the first set of heat exchanger tubes 112. The lateral faces of the steam drum 116 are fitted with a set of tube plates 120 which interconnect the furnace 106 and the shell 116 such as to define an enclosed region for a steam drum 122 between the shell 116 and the furnace 106. The first set of heat exchanger tubes 112 are placed within the steam drum 122, as shown in Figure 3.
The hot gases from the furnace 106 are received through the first set of heat exchanger tubes 112, where heat from the hot gases is transferred by convection to the hot feed water contained in the steam drum 122, thus vaporizing the hot feed water to generate steam which is contained in the steam drum 122, and the hot gases becoming partially heat-extracted gases

which are collected in a smoke chamber 124. The heat transfer surface area cannot be increased to cool the hot gases sufficiently before entering the boiler containing the first set of heat exchanger tubes 112 due to the minimum water holdup constraint.
Figure 6 shows the flue gas path in the fire-tube boiler assembly 102. In small furnaces, there is a possibility that the flames produced during combustion touch the boiler assembly tube plates causing overheating and tube plate cracking. The small furnaces can also lead to higher carryover of the fine particulate, which can lead to fouling of the first set of heat exchanger tubes 112. To prevent the first set of heat exchanger tubes 112 from fouling and to protect the set of tube plates 120 from getting exposed to the flames in the combustor 110, the refractory arches 114 are provided. These refractory arches also help to increase the residence time and settling particulate to reduce emission level. Referring to Figure 6, the direction arrow 152 indicates the entry of air in the combustor 110, where the hot flue gases are produced. The hot flue gases follow the direction arrows 154 and 156 leading them to the reversal chamber 118, indicated by the direction arrow 158. The direction arrows 160 and 162 indicate the entry and exit of the hot flue gases and the partially heat-extracted flue gases through the first set of heat exchanger tubes 112, respectively. The partially heat-extracted flue gases are received in the smoke chamber 124 as indicated by direction arrow 162. The partially heat-extracted flue gases exit the boiler assembly 102 through the smoke chamber 124 as indicated by direction arrow 164.

A steam separator device 126 is provided at the operative top of the steam drum 122 to release the steam from the steam drum 122. The steam separator device 126 comprises a steam separator 128 for eliminating moisture or water droplets present in the steam. The steam separator device 126 further comprises an air vent pipe 130 (second port) for releasing the air and a steam outlet pipe 132 (first port) for discharging steam. A higher steam space for steam separation cannot be provided due to minimum water holdup constraint. Poor steam quality and higher water level fluctuation may occur due to less steam space. The steam separator 128 is a pipe with a larger diameter to provide additional height to reduce the steam carry over, with a perforated plate for the separation of moisture from steam. The larger diameter pipe also helps to reduce the steam velocity near the steam outlet pipe (first port) 132 to reduce localized swelling. The problem of poor steam quality and higher water level fluctuation is prevented with the help of the steam separator device 126. The steam separator device 126 consists of a steam separator 128 having a larger cross-section than the cross section of the steam outlet pipe 132 (refer Figure 4). The larger cross-section of the steam separator 128 helps in reducing the effect of a sudden change of the steam requirement on the water level fluctuation steam drum 122 and to achieve better steam quality due to lower velocity near steam separation zone. Only the first set of heat exchanger tubes 112 are provided in the steam drum 122 of the fire-tube boiler assembly 102 due to the minimum water holdup constraint. But, the one pass configuration may lead to higher stack temperature. The problem of higher stack temperature is prevented by providing the second set of heat transfer tubes in the feed water tank

assembly 104 for heating cold feed water. The feed water tank assembly 104 is integrated with feed water heating to reduce water evaporation.
The feed water tank assembly 104 comprises a feed water tank 134 containing cold feed water. The partially heat-extracted gases from the first set of heat exchanger tubes 112 are received through the smoke chamber 124 in the second set of heat transfer tubes 140 placed in feed water tank assembly 104 at an inlet 136. The inlet 136 is typically provided on one of the side faces of the feed water tank 134. A second set of heat exchanger tubes, illustrated in Figure 5 by numeral 140, are provided housed inside the feed water tank 134. These second set of heat exchanger tubes 140 receive the partially heat-extracted gases via the inlet 136. The partially heat-extracted gases passing through the second set of heat exchanger tubes 140 are available at the pressure lower than the atmospheric pressure. Therefore, inducing means 142 are provided to induce the flow of the partially heat-extracted gases through the second set of heat exchanger tubes 140. The inducing means 142 are preferably provided on the operative top of the first assembly 102 opposite to the inlet 136 in operative connection to the feed water tank 134 at an opening 144. This arrangement further helps in making the boiler system 100 compact and easy to operate, having a high combustion volume, high efficiency, while providing less water hold up, less emissions, and less energy losses.
The remaining heat from the partially heat-extracted gases is extracted in the cold feed water contained in the feed water tank 134 to generate hot feed water, the partially heat extracted gases becoming cooled gases. A vent pipe

146 is typically provided at the operative top of the feed water tank 134 for discharging the evaporated steam and vapors from feed water tank 134. The hot feed water produced in the feed water tank 134 is carried by conduit means 148 to the steam drum 122. Further, an inlet port 150 is provided at the operative top of the feed water tank 134 to provide cold feed water in the feed water tank 134. The feed water heater is integrated with the feed water tank 134 wherein the second set of heat exchanger tubes 140 are placed inside the feed water tank 134 due to the possibility of water evaporation in a external feed water heater working on natural circulation as the water exit temperature from the boiler is very high and also due to space constraints.
The hotter system 100 as explained above and illustrated in the Figure 1 -6 is very compact, ready to use, has high combustion volume leading to higher efficiency and less emission and has very low water holdup leading to less startup time and heat loss. This concept can also be extended to high capacity boilers with three-pass configuration.
TRIAL RESULTS
Trial Result 1
Table 1: Following table presents the key performance indicators like boiler & water pre heater (WPH) outlet temperature, boiler pressure and water consumption with time for load and efficiency analysis when wood logs were used.

Time Boiler outlet (deg.C) WPH Outlet (deg.C) Pressure
(kg/cm^2
) Pump
3:07:10 283 184 3.625 stops
3:23:00 288 189 5.000 starts
3:25:16 280 185 4.000 stops
3:41:08 282 195 5.000 starts
3:43:25 284 194 3.750 stops
3:57:35 305 200 5.000 starts
3:59:58 301 198 3.750 stops
4:11:24 301 199 5.000 starts
4:13:42 294 196 4.000 stops
4:27:09 308 198 5.125 starts
4:29:50 328 208 4.500 stops
4:41:16 323 203 5.325 starts
4:43:57 314 200 4.000 stops
4:55:40 319 216 5.325 starts
4:58:14 312 215 4.500 stops
5:09 323 208 5.500 starts
5:11 320 203 4.500 stops

Flue gas composition has been also observed during the complete cycle of
trial.
Flue gas composition (%), when combustor is completely fed:
02 =10
CO = 117ppm
C02= 10.42
Flue gas composition (%), after 10 minutes of feeding:
02 =11.3
CO = 265 ppm
C02 = 9.19
Efficiency based on average 02 = 74 %

Trial Result 2
Table 2: Same trial has been conducted with smaller size of wood.

Time Boiler outlet (deg.C) WPH Outlet (deg.C) Pressure
(kg/cm^2
) Pump
2:55:38 PM 293 189 4.125 stops
3:09:05 PM 298 194 5.500 starts
3:11:05 PM 290 190 4.500 stops
3:24:02 PM 292 200 5.500 starts
3:26:00 PM 294 199 4.250 stops
3:37:20 PM 315 205 5.500 starts
3:39:20 PM 311 203 4.250 stops
3:49:55 PM 311 204 5.500 starts
3:51:50 PM 304 201 4.500 stops
4:02:50 PM 318 203 5.625 starts
4:05:00 PM 338 213 5.000 stops
4:16:20 PM 333 208 5.825 starts
4:17:40 PM 324 205 4.500 stops
4:27:20 PM 329 221 5.825 starts
4:29:10 PM 322 220 5.000 stops

Flue gas composition has been observed as follows.
Flue gas composition (%), when combustor is completely fed:
02 9.5
CO = 117ppm
C02=11
Flue gas composition (%), after 10 minutes of feeding: 02 = 10.5 CO = 265 ppm CO2=10.2
Efficiency based on average 02 = 74 %

Trial Result 3
Table 3: The similar trial has been conducted with other fuel like coal.

Time Boiler outlet (deg.C) WPH Outlet
(deg.C) Pressure (kg/cm^2) Pump
1:58:00 318 201 5.750
2:03:42 299 192 4.750 starts
2:05:53 267 178 3.000 stops
2:17:25 337 215 4.750 starts
2:19:41 325 210 3.500 stops
2:29:05 342 218 4.750 starts
2:31:15 301 197 3.250 stops
2:41:18 337 221 5.000 starts
2:43:36 300 195 3.000 stops
2:54:10 322 214 4.750 starts
2:56:15 300 202 3.000 stops
3:07:30 307 203 4.500 starts
3:09:49 290 192 4.000 stops

Flue gas composition has been observed as follows.
Flue gas composition (%), when combustor is completely fed:
02 =6
CO = 132 ppm
C02= 14.14
Flue gas composition (%), after 10 minutes offending: 02 = 11 CO = 70 ppm C02 = 9.43
GCV of coal = 4500 kcal/kg Efficiency based on average 02 = 77 %

TECHNICAL ADVANCEMENTS
A two pass fire tube boiler with integrated heat recovery system as described in the present invention has several technical advantages including but not limited to the realization of:
• a compact fire tube boiler;
• a fire tube boiler with minimum water holdup;
• a fire tube boiler which requires minimum startup time for heating a fluid;
• a fire tube boiler having minimum heat loss in the operating cycle of the boiler;
• a fire tube boiler having high furnace volume for combustion;
• a fire tube boiler with minimum incomplete combustion loss;
• a fire tube boiler with integrated heat recovery system and deaeration system; and
• a fire tube boiler with minimum emission.
The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the invention, unless there is a statement in the specification specific to the contrary.
In view of the wide variety of embodiments to which the principles of the present invention can be applied, it should be understood that the illustrated

embodiments are exemplary only. While considerable emphasis has been placed herein on the particular features of this invention, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principle of the invention. These and other modifications in the nature of the invention or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.

We Claim:
1. A boiler system comprising:
• a fire-tube boiler assembly adapted to provide steam, said fire-tube boiler assembly comprising:
■ a furnace adapted to combust a fuel and generate hot flue gases;
■ a refractory arch for containment of the flames produced during combustion of the fuel within said furnace;
■ a shell adapted to surround said furnace to contain steam, hot feed water, or steam-water mixture;
■ a reversal chamber provided within said boiler assembly at one of its operative ends adapted to collect the hot flue gases from said furnace and transfer to a first set of heat exchanger tubes;
■ a set of tube plates provided on the lateral faces of said shell interconnecting said furnace and said shell and adapted to define an enclosed region as a steam drum between said shell and said furnace;
■ the first set of heat exchanger tubes disposed within said steam drum adapted to receive the hot flue gases, transfer the heat of the hot flue gases to the hot feed water, and convert the hot feed water to steam, the hot flue gases becoming partially heat-extracted flue gases; and
■ a steam separator device provided on said shell, said steam separator device comprising: a steam separator

adapted to eliminate moisture from the steam received
therein from said steam drum, a first port adapted to
release the steam, and a second port adapted to discharge
air during boiler startup;
• a feed water tank assembly provided on the operative top of
said fire-tube boiler assembly and adapted to provide hot feed
water, said feed water tank assembly comprising:
■ a feed water tank adapted to store cold feed water;
■ an inlet operatively connected to said first set of heat exchanger tubes and adapted to receive the partially heat-extracted flue gases;
■ a second set of heat exchanger tubes housed inside said feed water tank and adapted to receive the partially heat-extracted flue gases via said inlet, wherein the partially heat-extracted flue gases transfer heat to the cold feed water, to provide hot feed water and become cooled flue gases;
■ inducing means to induce the flow of the partially heat-extracted flue gases from said first set of heat exchanger tubes through said second set of heat exchanger tubes;
■ a vent pipe provided at the operative top of said feed water tank for discharging the evaporated steam and air from said feed water tank;
■ conduit means to lead the hot feed water to the steam drum; and

■ an inlet port adapted to receive cold feed water in said feed water tank; and • a support frame adapted to hold said first assembly and said second assembly.
2. The boiler system as claimed in claim 1, wherein said furnace is semi-cylindrical in shape.
3. The boiler system as claimed in claim 1, wherein the fuel combusted in said furnace is selected from wood, coal, rice husk, coconut shell, and the like.
4. The boiler system as claimed in claim 1, wherein said shell has a dome shape and is placed around said furnace.
5. The boiler system as claimed in claim 1, wherein the cross-section of said steam separator is larger than the cross-section of the outlet pipe of said first port.
6. The boiler system as claimed in claim 1, wherein a smoke chamber is provided to receive the partially heat-extracted gases from said first set of heat exchanger tubes and adapted to transfer the partially heat-extracted gases to said second set of heat exchanger tubes.
7. The boiler system as claimed in claim 1, wherein said inlet is provided on one of the faces of said feed water tank.

8. The boiler system as claimed in claim 7, wherein said inducing means are provided opposite to said inlet.
9. The boiler system as claimed in claim 1, wherein said steam drum houses therein said first set of heat exchanger tubes or said first and said second set of heat exchanger tubes, with said reversal chamber integrated within said steam drum.
10.The boiler system as claimed in claim 1, wherein said shell is curved at the bottom for joining with said furnace.
11.A method for generating steam in a boiler system, said method comprising the following steps:
(i) combusting a fuel in a furnace to generate hot flue gases;
(ii) containing hot feed water in a shell which is provided
surrounding said furnace, a set of tube plates are provided on the
lateral faces of said shell to define an enclosed region as a steam
drum; (iii) receiving the hot flue gases through a first set of heat exchanger
tubes, wherein said first set of heat exchanger tubes are disposed
within said steam drum; (iv) vaporizing the hot feed water by extracting heat from the hot flue
gases to generate steam, the hot flue gases becoming partially
heat-extracted flue gases; (v) releasing the steam through a steam separator to eliminate any
moisture;

(vi) receiving the partially heat-extracted flue gases via an inlet into a second set of heat exchanger tubes, wherein said second set of heat exchanger tubes are housed inside a feed water tank containing cold feed water;
(vii) inducing the flow of the partially heat-extracted flue gases through said second set of heat exchanger tubes by using inducing means;
(viii) transferring the heat in the partially heat-extracted flue gases to the cold feed water to generate hot feed water, the partially heat-extracted gases becoming cooled flue gases which are subsequently discharged;
(ix) discharging vapors from said feed water tank through a vent pipe which is operatively connected at the top of said feed water tank; and
(x) feeding the hot feed water from said feed water tank to said steam drum by conduit means provided therein.