THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2006
COMPLETE
Specification
(See Section 10 and Rule 13) A COMPACT FIRE TUBE BOILER
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 INVENTION
The present invention relates to the field of fire tube boilers.
Particularly, the present invention relates to configuration of fire tube
boilers.
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 term "mmWC" used in the specification means millimeters of water column and is used to define the pressure drop in the boiler.
The term "creep failure" used in the specification means the slow deformation of the material and successive failure under the continuous exposure of higher temperature and higher stress.
The term "Larson-Miller parameter (LMP)" used in the specification is a means of predicting the operating life of a material as a function of time, temperature and stress using a correlative approach based on the Arrhenius rate equation.
The term "Finite Element Method (FEM)" used in the specification is a numerical technique for finding approximate solutions of partial differential equations (PDE) and integral equations.

The term "convective tubes" used in the specification mean the tubes conveying heat transfer media there through and facilitating heat transfer.
BACKGROUND
Fire tube boilers are widely used in industries for generating hot water or steam. Typically, a fire tube boiler comprises a housing/shell containing therein a furnace for producing flue gases, a supply of water, and a plurality of tubes for carrying the flue gases there through, the tubes being provided in communication with the water and the water being in heat-exchange relationship with the flue gases.
GB 2068513 discloses a fire tube boiler comprising a shell containing a first flue and an additional flue, wherein the additional flue is provided for obtaining improved boiler efficiency over a single-pass boiler. The additional flue is a wide tube comprising a plurality of tubes having a relatively large diameter, which extend transversely towards the longitudinal extend of the wide tube. The fire tube boiler of GB 2068513 aims at extracting substantial heat from the flue gases before discharging the gases through a stack thus providing enhanced boiler efficiency.
GB 853816 discloses a cylindrical fire tube boiler having three or more horizontally extending gas passes, the first and second pass communicate via a first gas chamber disposed within an extension of the boiler shell and the third pass communicates through a second gas chamber also disposed within the extension of the boiler shell. The gas chambers are provided serially, where the first chamber is provided with a door to the boiler and the second

chamber is provided with a door to the first chamber. The fire tube boiler of GB 853816 provides improved efficiency and easy access to the fire tubes for cleaning purposes.
US 3153429 discloses a typical oil-fired horizontal return tube boiler having a shell, a furnace, and return tubes. Additionally, nozzles having flanges, a bore, and helical grooves are provided at the entrance of the return tubes for inducing the flue gas flow through the length of the return tubes. The disclosure in US 3153429 aims at enhancing the efficiency of a boiler.
GB 644469 discloses a horizontal multi tubular steam boiler wherein the fire tubes are disposed below or above the furnace flues and additional cylindrical metal return flue is provided in the water space. The flue gases from the furnace are first passed through the fire tubes and then passed through the cylindrical return flue. The metal return flue comprises an air heater therein. The disclosure in GB 644469 aims at recovering optimal heat from the flue gases generated in the furnace.
From the above listed prior art, it is apparent that most of the developments in the fire tube boilers primarily concentrate on enhancing the boiler efficiency and/or controlling the emissions. However, with the growing need for miniaturization, compactness of a boiler is also as important as its effectiveness and environmental friendliness. One of the major challenges in developing a compact boiler is the need for an even more effective utilization of the heat transfer surface, where effectiveness of the heat transfer surface is a function of the heat transfer coefficient.

OBJECT 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 that effectively utilizes high pressure gas for the combustion of fuel.
Still another object of the present invention is to provide a fire tube boiler having low combustion volume.
Yet another object of the present invention is to provide a fire tube boiler which can be used at high flue gas velocities in the convective pass.
One more object of the present invention is to provide a fire tube boiler with low tube plate temperature.
Still one more object of the present invention is to provide a fire tube boiler which provides excellent heat transfer rate with small heat transfer area.
Yet one more object of the present invention is to provide a fire tube boiler in which low flue gas temperature is maintained at the entry of the convective tubes.
An additional object of the present invention is to provide a fire tube boiler in which reduced stresses are developed in the convective tube plates.

SUMMARY OF THE INVENTION
In accordance with the present invention, is provided a fire tube boiler comprising at least three heat transfer zones:
■ a first heat transfer zone in which 30 - 35 % of heat is transferred to a fluid to be boiled, wherein said first heat transfer zone comprises a furnace which is adapted to receive flue gases having temperature in the range of 450 - 600 °C and pressure in the range of 500 - 2000 mmWC and further adapted to elevate the temperature of the flue gases to 1300 - 1400 °C;
■ a third heat transfer zone in which 55 - 60 % of heat is transferred to the fluid to be boiled, wherein said third heat transfer zone comprises a plurality of convective tubes adapted to receive flue gases at a temperature in the range of 900 - 1000 °C and pressure in the range of 300-1500mmWC;
■ a second heat transfer zone interspersed between said first heat transfer zone and said third heat transfer zone, said second heat transfer zone principally adapted to transfer the flue gases from said first heat transfer zone to said third heat transfer zone, said second heat transfer zone further adapted to transfer 10 - 15 % of heat to the fluid to be boiled, wherein said second heat transfer zone comprises a plurality of convective tubes adapted to receive the flue gases from said first heat transfer zone at a temperature in the range of 1300 - 1400 °C and pressure in the range of 500 - 2000 mmWC; and

■ wherein, the ratio of diameter of the convective tubes in said third heat transfer zone to the convective tubes in said second heat transfer zone is in the range of 1:2 - 1: 3, and the ratio of the heat transfer area of the convective tubes in said third heat transfer zone to the heat transfer area convective tubes in said second heat transfer zone is in the range of 2:1 — 4:1.
Typically, in accordance with the present invention, an external reversal chamber is disposed between said second heat transfer zone and said third heat transfer zone along an operative exterior wall of said boiler.
Preferably, in accordance with the present invention, an internal reversal chamber is interspersed between said first heat transfer zone and said second heat transfer zone at one of the operative ends of said boiler.
In accordance with the present invention, the arrangement between said first heat transfer zone and said second heat transfer zone is such that the velocity of the flue gases through the convective tubes of said second heat transfer zone is in the range of 60 - 120 m/s.
Additionally, in accordance with the present invention, the arrangement between said second heat transfer zone and said third heat transfer zone is such that the velocity of the flue gases through the convective tubes of said third heat transfer zone is in the range of 80 - 170 m/s.
Typically, in accordance with the present invention, a first set of tube plates is provided to support the plurality of convective tubes in said second heat

transfer zone and define an enclosed region between said first heat transfer zone and said second heat transfer zone.
Preferably, in accordance with the present invention, a second set of tubes plates is provided to support the plurality of convective tubes in said third heat transfer zone and define an enclosed region between said second heat transfer zone and said third heat transfer zone.
Additionally, in accordance with the present invention, said first heat transfer zone is cylindrical, elliptical, or ovular.
Typically, in accordance with the present invention, said external reversal chamber is adapted to be leak-proof.
In accordance with the present invention is provided a method for boiling a fluid in a fire tube boiler, said method comprising the following steps:
a.- receiving flue gases having temperature in the range of 450 -600 °C and pressure in the range of 500 - 2000 mmWC in a first heat transfer zone comprising a furnace;
b. combusting a fuel in said furnace to increase the temperature of
the flue gases in the range of 1300 - 1400 °C and pressure in
the range of 500 - 2000 mmWC;
c. transmitting 30 - 35 % of heat from the hot flue gases in said
first heat transfer zone to a fluid to be boiled;
d. conveying the hot flue gases having temperature in the range of
1300 - 1400 °C and pressure in the range of 500 - 2000

mmWC from said first heat transfer zone to a second heat transfer zone;
e. transferring 10 - 15 % of heat from the hot flue gases in said
second heat transfer zone to the fluid to be boiled;
f. conveying the hot flue gases having temperature in the range of
900 - 1000 °C and pressure in the range of 300 - 1500 mmWC
from said second heat transfer zone to a third heat transfer zone;
and
g. extracting 55 - 60 % of heat from the hot flue gases in said
third heat transfer zone to the fluid to be boiled, to release flue
gases having temperature between 200 - 280°C at the exit of
said third heat transfer zone and generating boiled fluid in said
fire tube boiler.
Preferably, in accordance with the present invention, the method for boiling a fluid in a fire tube boiler includes the step of providing a leak proof external reversal chamber between said second heat transfer zone and said third heat transfer zone.
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 embodiment of the conventional fire tube boiler;

Figure 2 illustrates the temperature profile of a tube plate, under conditions of high flue gas temperature and pressure, in the conventional fire tube boiler;
Figure 3 illustrates a preferred embodiment of the fire tube boiler, in accordance with the present invention;
Figure 4 illustrates the temperature profile of a tube plate, under conditions of high flue gas temperature and pressure, in the fire tube boiler, in accordance with the present invention;
Figure 5 illustrates the stress profile of a tube plate, under conditions of high flue gas temperature and pressure, in the fire tube boiler, in accordance with the present invention; and
Figure 6 illustrates the graph showing the Larson Miller Parameter (LMP) with respect to stress conditions.
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 modified layout of a fire tube boiler which is compact and reduces the boiler foot print by 30%, in comparison with the

conventional fire tube boiler. The fire tube boiler of the present invention effectively utilizes high pressure gas in the combustion of the fuel, receives high pressure flue gases through the plurality of convective tubes at a velocity of 80 - 170 m/s, and overcomes the shortcomings of the conventional fire tube boiler.
Figure 1 illustrates an embodiment of a conventional fire tube boiler, generally represented in Figure 1 by numeral 100. The conventional fire tube boiler 100 primarily comprises a furnace 102, four tube plates 104,106, 108 and 120, a first pass convective tubes 122, a second pass convective tubes 124, a shell/enclosure 126, and a reversal chamber 128. The furnace 102 is placed in the shell 126 and is surrounded by water, enclosed therein with the help of the tubes plates 104, 106, 108 and 120. The shelll26 further encompasses the first pass convective tubes 122 and the second pass convective tubes 124. Typically, fuel oil or natural gas is burnt in the furnace 102 with the help of a burner (not shown in the Figure 1). The hot flue gases produced in the combustion process enter the first pass convective tubes 122 via the reversal chamber 128. The reversal chamber 128 is a cylindrical chamber closed on both sides by the tube plates 104 & 106. The tube plate 104 interconnects the furnace 102 and the reversal chamber 128, wherein the first pass convective tubes 122 are welded on to the tube plate 104. Some of the heat of the hot flue gases generated in the furnace 102 is lost to the cold water contained in the shell 126 surrounding the furnace 102 and the reversal chamber 128. The partially heat-extracted flue gases having temperature between 1000 - 1100 °C enter the first pass convective tubes 122 via the reversal chamber 128 at a velocity of 40 - 50 m/s. The first pass convective tubes 122 and the second pass convective tubes 124 are provided in

communication with the cold water. While passing through the first pass convective tubes 122 the partially heat-extracted flue gases lose a significant amount of heat to the water, thus, heating the water and consequently becoming cooled flue gases. Finally, these cooled flue gases enter the second pass convective tubes 124 via an external reversal chamber (not shown in the Figure) which is operatively connected to the outlet of first pass convective tubes 122. The second pass convective tubes 124 are welded on to the tube plate 108 wherein the tube plates 108 and 120 interconnect the reversal chamber 128 and the shell 126. The left over heat of the cooled flue gases is extracted in the second pass convective tubes 124 by the surrounding heated water to generate steam. The cooled flue gases are then discharged through a boiler stack (not shown in Figure 1). In the conventional fire tube boiler 100 approximately 40 - 45 % of heat from the flue gases is extracted in the furnace 102 and the reversal chamber 128, further 40 - 45 % of the heat is extracted in the first pass convective tubes 122, while remaining 15 - 20 % of heat is extracted in the second pass convective tubes 124. This is the optimum heat transfer distribution obtained using the conventional fire tube boiler 100.
However, the heat transfer coefficient and therefore the effectiveness of the heat transfer surface of the fire tubes can be further improved by increasing the flue gas velocity, high pressure combustion air provides an immense opportunity for the size reduction of the fire tube boiler, as it helps in the reduction of size of the furnace and maximization of the flue gas velocity to reduce the size of the boiler.

Thus, use of high pressure combustion air provides an opportunity for compaction of a fire tube boiler; low combustion volume and high flue gas velocity in the fire tubes provides a compact fire tube boiler having excellent heat transfer even with a reduced heat transfer area. Nonetheless, it is not possible to effectively utilize these high pressure flue gases in the conventional fire tube boiler 100 as shown in Figure 1, to participate in the combustion of the fuel; due to the restriction of flue gas velocity (40 - 50 m/s) and flue gas temperature (1000 - 1100 °C) at the entrance of the fire tubes and the possibility of the tube plate overheating and cracking due to high stress leading to creep failure.
When flue gases having higher temperature and pressure are used for the combustion of fuel in the fire tube boiler; it requires a furnace with reduced diameter and length due to the reduction in flame size. These flue gases will further provide increased flue gas velocity through the fire tubes which improves their heat transfer performance. However, since the conventional fire tube boiler 100 is not adapted to receive flue gases at a velocity above 50 m/s and temperature above 1100 °C; the design of the conventional fire tube boiler 100 can be developed by providing larger furnace diameter and length, to bring the exit temperature of flue gases from the furnace within a safe limit. In addition, in the convective passes, more number of tubes can be employed with low flue gas velocity there through and, thus, reduced heat transfer coefficient, to eliminate the possibility of tube plate overheating. If high pressure and high temperature flue gases are used, the consequent high flue gas velocity will result in tube plate overheating and cracking.

Figure 2 illustrates the temperature profile of the tube plate under conditions of high pressure and temperature in the conventional fire tube boiler 100, evaluated by using the FINITE ELEMENT METHOD (FEM). The temperature profile shows that the temperature of the tube plate is highest at the opening of the fire tubes where the fire tubes are connected to the tube plate and the temperature of the tube plate decreases further away from the fire tubes. Minimum tube plate temperature (250 °C) is recorded at a location farthest from the fire tubes and the furnace, represented by T4 in Figure 2. The region of the tube plate nearest to the fire tubes is heated by the high temperature flue gases to 400 °C (Tl) while the sections away from the fire tubes are cooled by the water contained in the shell. The region of the tube plate between two fire tubes is heated to approx. 320 °C (T2) while the region near the furnace is heated to approx. 300 °C (T3). From, the FEM analysis, it is obtained that the tube plate is exposed to a maximum stress of 310 MPA. At such high temperature and pressure, the tube plate is prone to creep failure.
The present invention not only aims at providing a compact fire tube boiler which effectively utilizes high pressure flue gases, high flue gas velocity through the fire tubes, reduces boiler footprint, and gives substantial energy savings, but also envisions the impediments in fabricating such a boiler and aims at overcoming these impediments.
Figure 3 illustrates a preferred embodiment of the fire tube boiler in accordance with the present invention; the fire tube boiler is represented in Figure 3 by numeral 200. The fire tube boiler 200 comprises a first heat

transfer zone 202 as a radiative heat transfer zone, a second heat transfer zone 206 as the first convective heat transfer zone, and a third heat transfer zone 204 as the second convective heat transfer zone. The three heat transfer zones, viz., 202, 204, and 206 are enclosed in an enclosure or a shell 226, and are adapted to receive hot flue gas as a heat transfer media. The second heat transfer zone 206 is designed in such a manner that it receives the hot flue gas at 60 - 120 m/s velocity and the third heat transfer zone 204 is designed to achieve very high velocity in the range of 80 - 170 m/s. The shell 226 is further provided with a fluid (feed water) supply inlet (not shown in Figure 3) typically located at the operative bottom of the shell 226 away from the combustor (not shown in Figure 3) usually dipped in the fluid. The shell 226 further comprises a steam discharge outlet (not shown in the Figure 3), provided at the operative top of the shell 226 for releasing the steam generated in the fire tube boiler 200. The bottom portion of the shell 226 contains boiling fluid and the top portion contains steam. All heat transfer zones should be necessarily dipped in boiling fluid.
The first heat transfer zone 202 comprises a furnace 212 and the second heat transfer zone 206 and the third heat transfer zone 204 comprise a plurality of convective tubes, referred in Figure 3 by numerals 222 and 224 respectively. In the fire tube boiler 200, approximately 30 - 35 % of the heat from the flue gas is transferred in the first heat transfer zone 202, 55-60% of the heat from the heat transfer media is transferred in the third heat transfer zone 204, and only 10 - 15 % of the heat from the heat transfer media is transferred in the second heat transfer zone 206. The major amount of heat transfer takes place in the plurality of convective tubes 224 of the third heat transfer zone 204 and minimal amount of heat transfer takes place

in the plurality of convective tubes 222 of the second heat transfer zone 206, therefore, the third heat transfer zone 204 is referred to as the main convective pass whereas the second heat transfer zone 206 is referred to as the dummy convective pass. The primary function of the dummy convective pass is to carry the high pressure and high temperature flue gas from the first heat transfer zone 202 to the main convective pass while simultaneously sufficiently cooling the flue gas therein. As heat transfer is not of most importance in the second heat transfer zone 206, the second heat transfer zone 206 is designed such as to provide lower flue gas velocity there through by increasing the diameter of the convective tubes and decreasing their number, thus, also reducing the corresponding tube plate metal temperature.
In the first heat transfer zone 202, 30 - 35 % of the heat from the hot flue gas, typically combustion gases, is extracted by a fluid to be boiled, typically water. The furnace 212 in the first heat transfer zone 202 is adapted to receive flue gases at a temperature in the range of 450 - 600 °C and pressure in the range of 500 - 2000 mmWC. The temperature of the flue gas is further elevated to 1300 - 1400 °C in the furnace 212 by combustion of a fuel while the pressure is maintained. Typically, furnace oil, heavy oil, light oil, or natural gas is used for the fuel. Preferably, the first heat transfer zone 202 is cylindrical, elliptical, or ovular in shape. From the first heat transfer zone 202 the hot flue gas at a temperature in the range of 1300 - 1400 °C is transmitted to the second heat transfer media 206. Optionally, an internal reversal chamber 228 is disposed between the first heat transfer zone 202 and the second heat transfer zone 206. The plurality of convective tubes 222 of the second heat transfer zone 206 can be connected to the internal reversal chamber 228 or can be directly connected to the furnace 212 of the first heat

transfer zone 202. In the absence of the internal reversal chamber 228, the temperature of the flue gas entering the second heat transfer zone 206 will be higher by approximately 100 °C, as the internal reversal chamber 228 will also act as a heat exchange means.
The plurality of convective tubes 222 receive the hot heat transfer media at a temperature between 1300 - 1400 °C and pressure between 500 - 2000 mmWC from the first heat transfer zone 202 at a velocity of 60 - 120 m/s. At this velocity no deposition or accumulation of unburnt particles or dirt happens in the convective tubes 222. The second heat transfer zone 206 is interspersed between the first heat transfer zone 202 and the third heat transfer zone 204 is principally adapted to transport the hot flue gas there from to the third heat transfer zone 204 with a minimal heat transfer in the range of 10-15%.
The plurality of convective tubes 222 are supported in the second heat transfer zone 206 with the help of a first set of tube plates 214 & 216, wherein the first set of tube plate 214 & 216 interconnect the first heat transfer zone 202 and the second heat transfer zone 206 such as to define an enclosed region referred as internal reversal chamber. To control the tube plate temperature, tube diameter of the convective tubes 222 in the second heat transfer zone 206 is significantly increased and flue gas velocity is significantly reduced leading to less heat transfer in the second heat transfer zone 206. A minimal portion of heat is transferred in the second heat transfer zone 206, thus, leaving the major portion of heat to be extracted in the third heat transfer zone 204. From the FEM analysis shown in Figure 2, it is clear that the tube plate temperature is a function of the tube pitch, the heat

transfer media velocity and the heat transfer media temperature. It is observed that the maximum tube plate temperature decreases with an increase in the tube pitch and increases with the flue gas temperature and heat transfer media velocity through the convective tubes. The tube plate temperature is also a function of the convective tube diameter and decreases with an increase in the convective tube diameter. As the flue gas temperature cannot be considerably decreased in the first heat transfer zone 202 due to the compact furnace size, a dummy convective pass, viz., the second heat transfer zone 206 with a larger tube diameter, higher pitch and lower flue gas velocity is used to protect tube plate from getting overheated. Larger tube diameter reduces flue gas velocity and heat transfer area, thus resulting in extraction of only a fraction of heat from the heat transfer media in the second heat transfer zone 206. The diameter of the convective tubes 222 disposed in the second heat transfer zone 206 is in the range of 63.5 - 100 mm.
The hot flue gas exiting the second heat transfer zone 206 has a temperature between 900 - 1000 °C and a pressure between 300 - 1500 mmWC and is received in the third heat transfer zone 204 through an external reversal chamber (not shown in Figure 3), wherein the plurality of convective tubes 224 are connected to the external reversal chamber to accept the heat transfer media. The external reversal chamber is adapted to be leak-proof for safety and to prevent any heat loss, and is located between the second heat transfer zone 206 and the third heat transfer zone 204 along an operative exterior wall of the boiler 200. The plurality of convective tubes 224 of the third heat transfer zone 204 are supported with the help of a second set of tube plates 218 & 220, wherein the tube plate 218 and external reversal chamber are

provided for interconnecting the second heat transfer zone 206 and the third heat transfer zone 204. Since, the third heat transfer zone 204 is the main convective pass where maximum heat from the heat transfer media is to be extracted, the velocity of the heat transfer media must be higher to achieve higher heat transfer coefficient and a greater heat transfer area must be provided to obtain a high heat transfer performance. Further, in the third heat transfer zone 204 the tube plate temperature is not a concern since the flue gas temperature is well within the acceptable limit.
The high velocity is induced by providing convective tubes 224 having a reduced diameter and the number of convective tubes 224 is kept high to provide increased heat transfer area. Typically, the diameter of the convective tubes 224 provided in the third heat transfer zone 204 is in the range of 25 - 44.5 mm. Further, the ratio of diameter of the convective tubes 224 in the third heat transfer zone 204 to the convective tubes 222 in the second heat transfer zone 206 is in the range of 1: 3 - 1: 2; and the ratio of the heat transfer area of the convective tubes 224 in the third heat transfer zone 204 to the heat transfer area of the convective tubes 222 in the second heat transfer zone 206 is in the range of 2:1 -4:1. The hot flue gas is received at a velocity of 80 - 170 m/s through the plurality of convective tubes 224, where 55 - 60 % of heat from the hot heat transfer media is transferred to the fluid to be boiled. At such high velocity, the convective tubes 224 are automatically cleaned of soot, dirt or unburnt -particles, eliminating choking caused by deposition or accumulation of dirt. Therefore, separate frequent cleaning of the convective tubes 224 is not required and the boiler can be operational for a longer duration without halting.

The heat transfer media is cooled to a temperature between 200 - 280 °C before the exit from the third heat transfer zone 204. The left-over heat from the flue gas can be extracted by using an additional convective pass following the main convective pass. Also, the flue gas can be received in a separate heat recovery unit to extract the heat.
The fire tube boiler 200 of the present invention was evaluated using the FEM analysis for the first set of tube plates 214 & 216, as illustrated in Figure 4. The flue gas entry is illustrated by numeral 400. The maximum tube plate temperature was approx. 300 °C (T5) nearest to the convective tubes 222, while further away from the convective tubes 222, the temperature was approx, between 250 °C (T7) - 275 °C (T6). The stress analysis of the first set of tube plates 214 & 216 is illustrated in Figure 5 by numeral 500, which confirms that the tube plate stress is within permissible limits and the maximum stress (135 MPA) on the tube plate is substantially less compared to the maximum tube plate stress (310 MPA) in the conventional boiler 100. The fire tube boiler 200 of the present invention is compact, gives up to 30 % reduced footprints.
TEST RESULTS
Figure 6 illustrates the graph showing the Larson Miller Parameter (LMP) with respect to stress conditions, wherein "X" represents P = T (20 + Log1o t) x 10-3, "Y" represents Stress - 1000 psi, "A" gives the average value, "B" gives the minimum value and "P" represents the plate.

The creep analysis for the tube plate (104 & 106) of the conventional fire tube boiler 100 is given below:
Material of construction of the tube plate: Carbon Steel
LMP = (oF + 460)(C+Log10t)*(10"3) — Eq. (1)
Stress = 310Mpa = 45KPsi Temperature, T = 400° C = 752°F Constant, C = 20
From Figure 6, Larson Miller Parameter (LMP) @ 45Kpsi = 28 (approx.)
Calculating the LMP using Eq (1): 28 = (752+460)(20+Log10t)(10-3) 28 =(1212)(20+Log10t)(10-3) (28000/1212)- 20 = Log10t . Log10t = 3.10231
Therefore, t = 1265.63944 hrs (53 days)
Thus, the above creep analysis confirms a short life of the conventional fire tube boiler 100 when supplied with flue gases having higher pressure and temperature. Thus, to effectively use flue gases having higher pressure and temperature, such that the fire tube boiler has a long life and high reliability, the boiler should be considerably modified.

The creep analysis for the tube plate 214 & 216 of the fire tube boiler 200 is given below:
Material of construction of the tube plate: Carbon Steel
LMP = (°F + 460)(C+Log10t)*(10-3) -- Eq. (1)
Stress = 140MPa = 2 lKPsi Temperature, T = 300° C = 572°F Constant, C = 20
From Figure 6, Larson Miller Parameter (LMP) @21Kpsi = 31 (approx.)
Calculating the LMP using Eq (1): 31 = (572+460) (20+Log101) (103) 31=(1032)(20+Log10t)(10-3) (31000/1032)-20 = LogI0t Log10t= 10.03875969 Therefore, t = 1.09 x 1010 hrs
Thus, the above creep analysis confirms that the fire tube boiler 200 of the present invention is not subject to creep failure when supplied with heat transfer media having pressure between 500 - 2000 mmWC and temperature between 1300- 1400 °C.

TECHNICAL ADVANTAGES
A compact fire tube boiler for effective utilization of high pressure combustion air as described in the present invention has several technical advantages including but not limited to the realization of:
• a fire tube boiler which is very compact;
• a fire tube boiler that can effectively utilize high pressure gas for the combustion of the fuel;
• a fire tube boiler having low combustion volume;
• a fire tube boiler that is able to use high flue gas velocities in the convective pass;
• a fire tube boiler with low convective tube plate temperature;
• a fire tube boiler in which excellent heat transfer rate can be achieved with small heat transfer area;
• a fire tube boiler in which low flue gas temperature is maintained at the entry of the convective tubes; and
• a fire tube boiler in which low stresses are developed in the convective tube plates; and
• a fire tube boiler with varying cross-sectional area of the convective tubes.
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 fire tube boiler comprising at least three heat transfer zones:
■ a first heat transfer zone in which 30 - 35 % of heat is transferred to a fluid to be boiled, wherein said first heat transfer zone comprises a furnace which is adapted to receive flue gases having temperature in the range of 450 - 600 °C and pressure in the range of 500 - 2000 mmWC and further adapted to elevate the temperature of the flue gases to 1300 - 1400 °C;
■ a third heat transfer zone in which 55-60 % of heat is transferred to the fluid to be boiled, wherein said third heat transfer zone comprises a plurality of convective tubes adapted to receive flue gases at a temperature in the range of 900 - 1000 °C and pressure in the range of 300-1500mm WC;
■ a second heat transfer zone interspersed between said first heat transfer zone and said third heat transfer zone, said second heat transfer zone principally adapted to transfer the flue gases from said first heat transfer zone to said third heat transfer zone, said second heat transfer zone further adapted to transfer 10 - 15 % of heat to the fluid to be boiled, wherein said second heat transfer zone comprises a plurality of convective tubes adapted to receive the flue gases from said first heat transfer zone at a temperature in the range of 1300 - 1400 °C and pressure in the range of 500 - 2000 mmWC; and
■ wherein, the ratio of diameter of the convective tubes in said third heat transfer zone to the convective tubes in said second

heat transfer zone is in the range of 1:2 - 1: 3, and the ratio of the heat transfer area of the convective tubes in said third heat transfer zone to the heat transfer area convective tubes in said second heat transfer zone is in the range of 2:1 - 4:1.
2. The fire tube boiler as claimed in claim 1, wherein an external reversal chamber is disposed between said second heat transfer zone and said third heat transfer zone along an operative exterior wall of said boiler.
3. The fire tube boiler as claimed in claim 1, wherein an interna] reversal chamber is interspersed between said first heat transfer zone and said second heat transfer zone at one of the operative ends of said boiler.
4. The fire tube boiler as claimed in claim 1, wherein the arrangement between said first heat transfer zone and said second heat transfer zone is such that the velocity of the flue gases through the convective tubes of said second heat transfer zone is in the range of 60 - 120 m/s.
5. The fire tube boiler as claimed in claim 1, wherein the arrangement between said second heat transfer zone and said third heat transfer zone is such that the velocity of the flue gases through the convective tubes of said third heat transfer zone is in the range of 80 - 170 m/s.
6. The fire tube boiler as claimed in claim 1, wherein a first set of tube plates is provided to support the plurality of convective tubes in said second heat transfer zone and define an enclosed region between said first heat transfer zone and said second heat transfer zone.

7. The fire tube boiler as claimed in claim 1, wherein a second set of tubes plates is provided to support the plurality of convective tubes in said third heat transfer zone and define an enclosed region between said second heat transfer zone and said third heat transfer zone.
8. The fire tube boiler as claimed in claim 1, wherein said'first heat transfer zone is cylindrical, elliptical, or ovular.
9. The fire tube boiler as claimed in claim 2, wherein said external reversal chamber is adapted to be leak-proof.
10.A method for boiling a fluid in a fire tube boiler, said method comprising the following steps:
a. receiving flue gases having temperature in the range of 450 -
600 °C and pressure in the range of 500 - 2000 mmWC in a
first heat transfer zone comprising a furnace;
b. combusting a fuel in said furnace to increase the temperature of
the flue gases in the range of 1300 - 1400 °C and pressure in
the range of 500 - 2000 mmWC;
c. transmitting 30 - 35 % of heat from the hot flue gases in said
first heat transfer zone to a fluid to be boiled;
d. conveying the hot flue gases having temperature in the range of
1300 - 1400 °C and pressure in the range of 500 - 2000
mmWC from said first heat transfer zone to a second heat
transfer zone;

e. transferring 10 - 15 % of heat from the hot flue gases in said
second heat transfer zone to the fluid to be boiled;
f. conveying the hot flue gases having temperature in the range of
900 - 1000 °C and pressure in the range of 300 - 1500 mmWC
from said second heat transfer zone to a third heat transfer zone;
and
g. extracting 55 - 60 % of heat from the hot flue gases in said
third heat transfer zone to the fluid to be boiled, to release flue
gases having temperature between 200 - 280°C at the exit of
said third heat transfer zone and generating boiled fluid in said
fire tube boiler.
1 l.The method for boiling a fluid in a fire tube boiler as claimed in claim 10, which includes the step of providing a leak proof external reversal chamber between said second heat transfer zone and said third heat transfer zone.