FIELD OF THE INVENTION:
The present invention relates to a method of modifying a bi-drum boiler, furnace, to achieve higher superheated steam temperature.
BACKGROUND OF THE INVENTION
The prior art oil and gas fired bi-drum boiler, comprises at least two drums connected with bank of tubes and a plurality of superheater coils are arranged within the furnace. With the existing pressure parts arrangement, high superheated steam temperature can be achieved only up to 490°C, thereby failing to achieve higher thermal cycle efficiency. In a typical oil and gas fired boiler, the furnace houses the complete superheater surfaces. Therefore some modifications are required in the prior art bi-drum boilers that includes a new arrangement having a flue gas duct wall, lined with refractory and housing a coil of heat transfer surfaces (LTSH), which enables to produce high temperature superheated steam. By this arrangement, the prior art boiler will achieve high temperature up to 540°C which is not possible in the prior boilers.
US Patent No 1,906,125 teaches a water tube boiler having superposed banks of inclined tubes spaced apart, an interdeck superheater having inlet and outlet headers in the space between said banks, and a plurality of tubes from the steam space of side boiler entering the inlet header from points beyond the ends of said banks, said tubes having removable portions and straight leading from the inlet header to the removable portions exteriorly of the tube bank ends so that the tubes

can be expanded in the inlet header by passing and expander through the parts of the tubes which enter the header.
US Patent No 1,896,241 discloses a boiler having in combination vertically spaced pair of banks of horizontally inclined generating tubes, a row of tubes extending along and crossing the space between said banks, a baffle structure extending in contract with said row from its lower end to a point near said upper bank and thence upwardly through the upper bank so as to separate the first gas pass from the second, said tubes of the baffle supporting row of tubes having two portions, header means near the upper tube bank for connecting said portions, and said baffle having a packing section contacting with the upper face of said header.
US Patent No 1,883,303 claims, in a boiler, a bank of inclined steam generating tubes, a superheater located along one side of the gas path beyond side bank of tubes having its tubes inclined at a greater angle than the tubes of said bank, the superheater tubes adjacent the gas path being spaced apart farther than those remote from said gas path.
US Patent No 1,676,631 claims, in a water tube boiler, a steam and water drum, spaced upper and lower banks of horizontally inclined up flow generating tubes, said banks of tubes terminating at one end in correspondingly. Spaced header means, independent connections from each of said header means to said drum, said connection being constructed and arranged to leave the space between said header means free and unobstructed, and superheater units extending between said spaced header means into the space between said banks.

US Patent No 1,585,528 discloses a water heating and steam generating boiler comprising in combination a furnace grate, a short bank of boiler tubes and headers at each end thereof, through which the furnace gases pass directly upwards, a materially longer bank of tubes and headers at each end thereof and three drums above said banks of tubes, one drum being common to one end of both the short and long bank of tubes whilst the two others are each separately connected to the headers at the opposite ends of the banks of tubes.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method of modifying a bi-drum boiler to achieve higher superheated steam temperature.
Another object of the invention is to propose a method of modifying a bi-drum boiler to achieve higher superheated steam temperature which exhibits higher cycle efficiency.
SUMMARY OF THE INVENTION:
Accordingly, there is provided a method of modifying a bi-drum boiler, the two drums connected with bank of tubes and superheater coils arranged within and outside of the furnace, a plurality of modified pressure parts arranged in the boiler to achieve high superheated steam temperature. This accordingly achieves high superheated steam temperature from the prior art Boiler. The prior boilers generate steam temperatures up to 490°C, which result in low cycle efficiency. According to the invention, a modification.

in the heat transfer surfaces arrangement is made. The modification includes a flue gas duct wall, lined with refractory. The refractory lined wall houses coil of heat transfer surfaces (LTSH), which enables to produce high temperature superheated steam. The saturated steam coming out from the boiler upper drum is allowed to pass through the LTSH then directed to platen and the final superheater. By this arrangement the boiler can achieve high temperature up to 540°C which was not possible in prior art boilers.
This modified method incorporates a refractory lined duct wall to encase the low temperature superheater (LTSH) in-between the boiler bank tubes. Flue gas is diverted to LTSH before leaving the boiler bank tubes.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 – shows a photographic view of a prior art a photographic view of a prior art oil and gas fired boiler.
Figure 2 – shows a block diagram of an improved oil and gas fired boiler according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
A conventional arrangement of typical oil and gas fired boiler with bi-drum concept has two drums (1A and 1B) connected with a bank of tubes (8). This bank of tubes forms a part of an evaporator running parallel to the membrane water wall (5) which forms a closure to the furnace (3). This bank of tubes are formed with a single tube without joints. The tube bank height is restricted by its length.

Joining of the tube banks with the drums are made by expanding the tube to the drum sheet. The operating pressure of the drum is restricted to the sealing tightness of the expanded tubes with the drum sheet, Heat recovering surfaces including an economizer (6) and air preheaters (4) are a part of additional passes of the boiler, which are normally enclosed in a duct lined second pass (7). Presence of in-furnace surfaces named platen (2) and Final (9) forms a part of a superheater.
Newly invented modification method envisages a LTSH heater (10) to provide a solution for the disadvantages of the prior art oil and gas fired boilers. The modified oil and gas fired boiler can serve as an alternate for the prior art bi drum boiler. High temperature super-heated steam is generated with a flow catering to the turbine which interalia enables the boiler to achieve more power. Heat carried by flue gas produced after combustion in the furnace is allowed to pass through a series of heat recovery surfaces which comprise a Platen SH, a Final SH in first pass and the LTSH heater (10) enclosed in refractory lined intermediate pass of the boiler, having refractory lined wall (7A).
The flue gas temperature at the inlet of the superheater should be at least 700o -800oC., then only an optimum heat transfer of the surface can be achieved. But if a superheater is placed after the boiler bank, the gas temperature available will be in order of 450o maximum and because of this low, temperature, a huge surface is required to be provided which is a difficult task for pressure part arrangement. With this proposed modification of the invention, the prior art boiler can be retrofitted or a new boiler can be made with smaller alteration.

Typical flue gas temperatures across the heat transfer surfaces are as follows:

ADVANTAGES
This comparison of this invention with the prior art boilers is given below:


WE CLAIM:
1. A method of modifying a bi-drum boiler to achieve higher superheated steam
temperature, the bi-drum boiler comprising two drums (1A, 1B) connected with a bank of tubes (8) and plurality of superheater coils arranged within and outside the furnace, the tubes (8) forming a part of an evaporator running parallel to the membrane water wall (5) which forms a closure for the furnace (3), heat recovery surfaces including an economizer (6) and air preheaters (4) constituting part of the additional passes of the boiler which are enclosed in a duct lined second pass (7), one each planten SH (2) and a final SH (9) forming a part of said superheater, the modification method comprising the steps of:
- providing before the second pass of the boiler, a low temperature super heater (LTSH) enclosed in a refractory-lined intermediate pass of the boiler;
- disposing the coils of said ( LTSH) along the flue gas path of the boiler;

- allowing the saturated steam exiting from the boiler upper drum to pass through the (LTSH) (10); and
- diverting the saturated steam coming out from the (LTSH) (1) to said planten superheater (2) and the final superheater (9).

2. The method as claimed in claim 1, wherein the boiler is enabled to produce super heated steam with 540°C temperature.