FORM-2
THE PATENTS ACT, 19*70
(39 of 1970)
& THE PATENTS RULES, 2006
COMPLETE SPECIFICATION
(See section 10 and rule 13)


A MULTIPLE PASS VERTICAL FIRE TUBE BOILER
THERMAX LTD
an Indian company of D-13, MIDC, Chinchwad Pune-19, 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.
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.
The term "fire-tube boiler" used in the specification is a type of boiler in which hot gases from a fire pass through one or more tubes running through a sealed container of water. The heat energy from the gases passes through the sides of the tubes by thermal conduction, heating the water and ultimately creating steam.
The term "dryness fraction" used in the specification is the ratio of the mass of the steam in the mixture to the mass of the entire mixture. Dryness fraction = mass of steam in mixture / mass of the mixture.
These definitions are in addition to those expressed in the art.

BACKGROUND OF THE INVENTION
Vertical fire tube boilers used in process heating are generally available in two pass configuration due to difficulty in flue gas reversal arrangement. These boilers are not very efficient and have high boiler gas exit temperature. Three pass arrangement for fire tube boilers have not been used commercially successfully because they suffer from the limitation of very higher water holdup leading to higher startup time and higher heat loss in the operating cycle of a boiler. These type of boilers are not efficient for a process which requires steam only during a limited duration of the day. Therefore, conventionally for these applications once through water tube boiler with helical coil configuration is used. But, this configuration has certain limitations like poor steam quality, tube over heating and scaling of convective tubes.
Several vertical fire tube boilers have been disclosed in the prior arts
U.S. Patent Publication No. 4,678,116 issued June 7, 1987, discloses a compact continuous flow energy gas fired water heater/boiler for use in residential, commercial and industrial buildings wherein cold water enters the heater and flows downward through an outer annulus and then rises up through an inner annulus around a central gas flue, and wherein combustion gas flows upward through the central flue and then downward through a set of heat-conductive tubes and finally upwardly through the middle annulus to the exit. Air flow blower provides draft. The burner is capable of burning gas at widely varying rates of gas flow and is regulated by a gas modulator valve approximately proportional to the rate of hot water usage and the force for the gas modulator is generated by an orifice type differential pressure generator in the inlet water pipe. This boiler configuration has a

disadvantage of high water holdup as the annulus exhaust gas chamber is placed with the shell.
U.S. Patent Publication No. 4,579,086 issued Jan 4, 1986, discloses a vertical tube boiler having a plurality of vertical tubes which extend through a water tank and communicate with a combustion chamber and a chamber provided with a flue outlet, the tank being provided with a transverse plate located above the bottom or firing plate of the tank and provided with holes through which the tubes extend with a clearance gap being provided between the periphery of each hole and the outside of the associated tube, through which gaps water flows upwardly to create upward currents in the water within the tank to hold any scale removed from the outside of the tubes in suspension and if such scale drops to the bottom of the tank, then it accumulates on the plate and not on the firing plate thus preventing burning out of the firing plate. This boiler configuration is more suitable for solid fuel combustion and it is having poor heat transfer performance due to single pass configuration.
U.S. Patent Application No. 4,157,078 issued June 5, 1979, discloses a vertical boiler with a cylindrical drum having an upper and lower end wall plate and a concentric flame tube with a burner at top end and two groups of smoke tubes concentrically placed around the furnace between the lower and upper end plate with a water cooled turning chamber extended outside the drum. In this configuration the lower wall is in contact with very high temperature flue gas in the turning chamber, the lower end plate can get heated up as the sediment deposition can take place over it.
Therefore, there is felt a need for a vertical fire tube boiler that overcomes the drawbacks of the prior art and can be used effectively with minimum

water holdup and reduced boiler gas exit temperature with multi pass arrangement of the convective tubes and in processes which requires steam only during a limited duration of the day.
OBJECTS OF THE INVENTION
An object of the present invention is to provide a compact vertical fire tube boiler.
Another object of the present invention is to provide a vertical fire tube boiler with minimum water holdup.
Still one more object of the present invention is to provide a vertical fire tube boiler which is efficient in applications requiring steam only during a limited duration of the day.
Yet another object of the present invention is to provide a vertical fire tube boiler which requires minimum startup time for heating a fluid.
Another object of the present invention is to provide a vertical fire tube boiler having minimum heat loss in the operating cycle of the boiler.
Yet another object of the present invention is to provide a vertical fire tube boiler with less boiler flue gas exit temperature.
Still another object of the present invention is to provide a vertical fire tube boiler with optimum dryness fraction.

Yet another object of the present invention is to provide a vertical fire tube boiler in which heat transfer surface will never get exposed to steam.
Still another object of the present invention is to provide a vertical fire tube boiler with no overheating of convective tubes.
SUMMARY OF THE INVENTION
A multiple pass vertical fire tube boiler comprising:
a) a main shell having a shell wall defining a first space therein;
b) annular shaped perforated top tube plate and bottom tube plate provided on the operative top and the bottom of said main shell;
c) a furnace fitted to said bottom tube plate provided within said first space and spaced apart from said main shell wall and the space between said furnace and said main shell wall defining an enclosure for water and sets of convective tubes;
d) a top shell fitted to said top tube plate defining a second space in fluid communication with said first space, said top shell having a steam outlet for dispensing of steam generated inside said first space;
e) annular reversal chamber fitted on the operative top of said top tube plate with said main shell wall as an outer boundary and wall of said top shell as an inner boundary;
f) a set of first pass convective tubes having inlets opening inside said furnace and passing through said first space and outlets connected to some of said perforations inside said top tube plate for leading hot flue gases from said furnace into said reversal chamber; and

g) a set of second pass convective tubes having inlets in fluid communication with some of said perforations in said top tube plate and outlets in fluid communication with said perforations in said bottom tube plate, said second pass convective tubes providing flow conduits for the flow of flue gases reversed in said reversal chamber and passing through said first space to the exterior of said main shell.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS
The invention will now be briefly described in relation to the accompanying drawings, in which:
Figure 1 illustrates a vertical fire tube boiler showing shell, burner and level controller in accordance with the present invention;
Figure 2 illustrates a furnace and tube pass arrangement in a vertical fire tube boiler in accordance with the present invention;
Figure 3 illustrates tube plates and tubes in a vertical fire tube boiler in accordance with the present invention;
Figure 4 illustrates a vertical fire tube boiler showing shell, reversal chamber and smoke chamber in accordance with the present invention;
Figure 5 illustrates a vertical fire tube boiler showing furnace end plate in accordance with the present invention;
Figure 6 illustrates a vertical fire tube boiler showing smoke chamber and flue gas outlet in accordance with the present invention;

Figure 7 illustrates an alternative embodiment of convective pass arrangement on inner and outer concentric circles of top tube plate in a vertical fire tube boiler;
Figure 8 illustrates further details of arrangement shown in figure 7;
Figure 9 illustrates further details of arrangement shown in figure 7;
Figure 10 illustrates another alternative embodiment of vertical fire tube boiler having flat reversal chamber with straight set of first pass convective tubes and bent set of second pass convective tubes;
Figure 11 illustrates yet another alternative embodiment of vertical fire tube boiler having a truncated cone shaped reversal chamber with straight set of first pass convective tubes and bent set of second pass convective tubes; and
Figure 12 illustrates still another alternative embodiment of two pass vertical fire tube boiler having a truncated cone shaped reversal chamber.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A preferred embodiment will now be described in detail with reference to the accompanying drawings. The preferred embodiment does 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 vertical fire tube boiler to overcome the drawbacks of the prior art.
Figure 1 illustrates a vertical fire tube boiler generally indicated by reference numeral 100. The vertical fire tube boiler 100 comprises a main shell 12, a burner 14, a water level controller 16, and a steam outlet 18.
Figure 2 illustrates a furnace and tube pass arrangement for the vertical fire tube boiler 100. The arrangement comprises a furnace 20, a top shell 22, a furnace tube 24, a set of first pass convective tubes 26 and a set of second pass convective tubes 28.
Figure 3 illustrates tube plates and tubes for the vertical fire tube boiler 100. The arrangement comprises a top end plate 30, a top tube plate 32, and a bottom tube plate 34. The top tube plate 32 and the bottom tube plate 34 are of annular shape.
Figure 4 illustrates a reversal chamber 36, a smoke chamber 38, and shell support 40 provided to the vertical fire tube boiler 100.
Figure 5 shows a furnace end plate 42 of the vertical fire tube boiler 100.
Figure 6 shows a flue gas outlet 44 of the vertical fire tube3 boiler 100.
The construction and structure of the vertical fire tube boiler 100 with respect to figures 1 to 6 can be described as follows:
The vertical fire tube boiler 100 having low boiler gas exit temperature and high efficiency with no chance of tube over heating is achieved by providing two shells (12 and 22) and two tube plates (32 and 34). Typically, the top tube plate 32 and the bottom tube plate 34 are of annular shape. The top tube

plate 32 and the bottom plate 34 are provided on operative top and bottom of the main shell 12 respectively. The top and bottom tube plate 32 and 34 are perforated around their circumference in the form of a circle as shown in figures 2 and 3. The top shell 22 of smaller diameter is provided over the top tube plate 32 and the top end plate 30 is provided on the operative top of top shell 22. The furnace 20 is placed concentric to main shell 12. The bottom tube plate 34 joins the main shell 12 and the furnace 20 defining an enclosure for water and steam. The top portion of the enclosure contains steam and bottom portion of the enclosure contains water. The furnace 20 is perforated at the operative bottom around its circumference as shown in figure 2. The furnace end plate 42 at the topmost portion of the furnace 20 connects furnace tube 24 and the furnace 20. The burner 14 is placed in the furnace tube 24. The furnace tube 24 extends to the top end plate 30. Steam outlet 18 is provided at the top end plate 30 as shown in figure 1. The water level is maintained with the help of the water level controller 16 in such a manner that, the heat transfer surface including the furnace 20 and the set of first pass and second pass convective tubes 26 and 28 will never get exposed to steam. Furnace 20 is kept below the top tube plate 32 and the furnace tube 24 is concentric to the top shell 22 defining an enclosure for steam inside the top shell 22. The top shell 22 provides the required steam space for steam separation. The diameter of furnace tube 24 is less in comparison with furnace 20, this helps in achieving higher steam separation area. As both the tube plates (32 and 34) are of annular shape, the reversal chamber 36 of annular shape is placed over the top tube plate 32 with wall of the main shell 12 as an outer boundary and wall of the top shell 22 as an inner boundary and the smoke chamber 38 is placed below the bottom tube plate 34 with wall of the main shell 12 as an outer boundary and wall of the furnace 20 as

an inner boundary. The flue gas outlet 44 is placed on one side of the smoke chamber 38 as shown in figure 6. The vertical fire tube boiler 100 is mounted on the shell support 40. The enclosure for water inside the main shell 12 and the enclosure for steam inside the top shell 22 are in communication with each other for the passage of steam.
The set of first and second pass convective tubes (26 and 28) of the vertical fire tube boiler 100 are arranged in form of a circle on the perforated top tube plate 32 concentric to the furnace 20 and the main shell 12. Alternate tubes are bent and connected or welded to the perforated holes in the bottom of the furnace 20 and these tubes serve as the set of first pass convective tubes 26 of the boiler. Remaining tubes are straight and connected or welded to the perforated holes of the bottom tube plate 34. These tubes serve as the set of second pass convective tubes 28. The flue gas produced in the furnace 20 enters the set of first pass convective tubes (bent tubes) 26 and reaches to the reversal chamber 36. The flue gases from the reversal chamber 36 enter in to the set of second pass convective tubes 28 and finally reach the smoke chamber 38 and discharges out through the flue gas outlet 44.
Figure 7 illustrates an alternative embodiment of convective pass arrangement on inner and outer concentric circles of top tube plate in a vertical fire tube boiler and is generally indicated by reference numeral 200. Figure 8 and 9 illustrates further details of arrangement shown in figure 7.
The embodiment as shown in figures 7, 8 and 9 is particularly suitable for higher capacity boilers. For the higher capacity boiler, it will be difficult to pack the required heat transfer area (set of first and second pass convective tubes 226 and 228) in one circle. To solve this constraint, the set of first and second pass convective tubes 226 and 228 are arranged in form of two or

more circles 245 on the perforated top tube plate 232 concentric to the furnace 220 where inner bent tubes connected to the furnace 220 act as the set of first pass convective tubes 226 and the outer straight tubes connected to the bottom tube plate 234 act as the set of second pass convective tubes 228.
Figure 10 illustrates another alternative embodiment of a vertical fire tube boiler having flat reversal chamber with straight set of first pass convective tubes and bent set of second pass convective tubes and is generally indicated by reference numeral 300. In this arrangement, the set of first pass convective tubes 326 are straight and the set of second pass convective tubes 328 are bent. The set of first pass convective tubes 326 are connected to a flat reversal chamber 336 at the bottom.
Figure 11 illustrates yet another alternative embodiment of a vertical fire tube boiler having a truncated cone shaped reversal chamber with straight set of first pass convective tubes and bent set of second pass convective tubes and is generally indicated by reference numeral 400. In this arrangement, the set of first pass convective tubes 426 are straight and the set of second pass convective tubes 428 are bent. The set of first pass convective tubes 426 are connected to a truncated cone shaped reversal chamber 446 at the bottom.
Figure 12 illustrates a two pass vertical fire tube boiler, generally indicated by reference numeral 500 which has a truncated cone shaped reversal chamber 546 at the bottom.
The flat reversal chamber 336 as shown in figure 10 will have the problem of overheating due to mud deposition. To overcome this problem, the

truncated cone shaped reversal chambers 446 & 546 as shown in figures 11 and 12 are introduced both in three pass and two pass vertical boilers as the inclined surface of the truncated cone shaped reversal chambers 446 & 546 respectively will not allow mud deposition.
TECHNICAL ADVANCEMENTS
A multiple pass vertical fire tube boiler as described in the present invention has several technical advantages including but not limited to the realization of:
• a compact vertical fire tube boiler;
• a vertical fire tube boiler with minimum water holdup;
• a vertical fire tube boiler which is efficient in applications requiring steam only during a limited duration of the day;
• a vertical fire tube boiler which requires minimum startup time for heating a fluid;
• a vertical fire tube boiler having minimum heat loss in the operating cycle of the boiler;
• a vertical fire tube boiler with less boiler flue gas exit temperature;
• a vertical fire tube boiler with optimum dryness fraction;

• a vertical fire tube boiler in which heat transfer surface will never get exposed to steam; and
• a vertical fire tube boiler with no overheating of convective tubes.
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 principles 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.

. A multiple pass vertical fire tube boiler, said boiler comprising:
a) a main shell having a shell wall defining a first space therein;
b) annular shaped perforated top tube plate and bottom tube plate provided on the operative top and the bottom of said main shell;
c) a furnace fitted to said bottom tube plate provided within said first space and spaced apart from said main shell wall and the space between said furnace and said main shell wall defining an enclosure for water and sets of convective tubes;
d) a top shell fitted to said top tube plate defining a second space in fluid communication with said first space, said top shell having a steam outlet for dispensing of steam generated inside said first space;
e) a reversal chamber fitted on the operative top of said top tube plate with said main shell wall as an outer boundary and wall of said top shell as an inner boundary;
f) a set of first pass convective tubes having inlets opening inside said furnace and passing through said first space and outlets connected to some of said perforations inside said top tube plate for leading hot flue gases from said furnace into said reversal chamber; and
g) a set of second pass convective tubes having inlets in fluid communication with some of said perforations in said top tube plate and outlets in fluid communication with said perforations in said bottom tube plate, said second pass convective tubes providing flow conduits for the flow of flue gases reversed in

said reversal chamber and passing through said first space to the exterior of said main shell.
2. A multiple pass vertical fire tube boiler as claimed in claim 1, wherein at least one set of said perforations are arranged in the form of a circle in said top tube plate.
3. A multiple pass vertical fire tube boiler as claimed in claim 1, wherein at least one set of said perforations are arranged in the form of a circle in said bottom tube plate.
4. A multiple pass vertical boiler as claimed in claim 1 having three passes, wherein individual tubes of said two sets of convective tubes are arranged alternately in the form of a circle in said top tube plate, cylindrically around said furnace.
5. A multiple pass vertical boiler as claimed in claim 1 having three passes, wherein individual tubes of two said sets of convective tubes are arranged alternately in the form of a circle in said bottom tube plate, cylindrically around said furnace.
6. A multiple pass vertical fire tube boiler as claimed in claim 1, wherein two sets of said perforations are arranged in the form of two concentric circles in said top tube plate.
7. A multiple pass vertical fire tube boiler as claimed in claim 1, wherein said set of first pass convective tubes are cylindrically arranged

around said furnace and said set of second pass convective tubes are cylindrically arranged around said set of first pass convective tubes.
8. A multiple pass vertical fire tube boiler as claimed in claim 1, wherein multi sets of said perforations can be arranged in form of a plurality of concentric circles in either said top tube plate or bottom tube plate.
9. A multiple pass vertical fire tube boiler as claimed in claim 1, wherein cross-section of said perforations on said tube plates is a cross-section selected from a group of cross-sections consisting of a circle, square, rectangle, triangle, ellipse, oval, polygonal, trapezium and rhombus.
10.A multiple pass vertical fire tube boiler as claimed in claim 1, wherein cross-section of said first pass and second pass convective tubes is a cross-section selected from a group of cross-sections consisting of a circle, square, rectangle, triangle, ellipse, oval, polygonal, trapezium and rhombus.
11 .A multiple pass vertical fire tube boiler as claimed in claim 1, wherein a smoke chamber is provided below said bottom tube plate with said main shell wall as an outer boundary and wall of said furnace as an inner boundary.
12. A multiple pass vertical fire tube boiler as claimed in claim 1, wherein said set of first pass of convective tubes are straight.

13. A multiple pass vertical fire tube boiler as claimed in claim 1, wherein said set of first pass of convective tubes are bent.
14. A multiple pass vertical fire tube boiler as claimed in claim 1, wherein said set of second pass of convective tubes are straight.

15. A multiple pass vertical fire tube boiler as claimed in claim 1, wherein said set of second pass of convective tubes are bent.
16. A multiple pass vertical fire tube boiler as claimed in claim 1, wherein said reversal chamber is in the form of a truncated cone.
17. A multiple pass vertical fire tube boiler as claimed in claims 1 and 10, wherein said smoke chamber is provided with a flue gas outlet.
18. A multiple pass vertical fire tube boiler as claimed in claim 1, wherein a water level controller is provided in said main shell to maintain the water level in said enclosure.
19. A multiple pass vertical fire tube boiler as claimed in claim 1,
wherein a burner in communication with said furnace is provided for
production of said flue gases.