FIELD OF THE INVENTION
The present invention relates to a two-stage reheater for a two-pass boiler to
maintain steam temperature imbalance between the right and left leads of
reheater outlet within the permissible limit.
BACKGROUND OF THE INVENTION
Steam generator is mainly used for producing steam from water by firing
coal/oil/gas fuels. This steam energy is converted to mechanical energy in
Turbine. The generator is coupled to the turbine to generate the electricity.
A Reheater section is mainly used in boiler to reheat the steam taken from the
High pressure (HP) turbine to the rated temperature and the reheated steam is
used in the Intermediate and low pressure (IP/LP) turbines to achieve the rated
power. The location of Reheater in the boiler is shown in Figure 1.
Single stage Reheater with Reheater Front and Reheater Rear section is provided
in boilers to achieve rated Reheater outlet steam temperature. Due to an
unbalance in gas temperatures on account of formation of fire ball in the
combustion chamber, unbalance in the steam temperatures carrying over from
the Reheater Front section to the Reheater Rear section there is an imbalance in
steam temprerature at rheater outlet leads. The Left-Right steam temperature
imbalance is around 20° to 30°C at Reheater outlet leads, which interalia leads
to:
• Higher Reheater spray in one lead affecting the Plant heat rate, and
• Increase in metal temperature in one side which affects the life of tubes &
headers in Reheater outlet sections.

Hence it is imperative that the Reheater steam temperature imbalance between
left and right leads at the Reheater outlet be maintained within the limits of
10°C.
To maintain the Reheater outlet steam temperature in left and right leads well
within the limits of 10°C, a two stage Reheater is introduced with a cross over
links between Reheater Front section and Reheater Rear section, and a plurality
of Siamese headers is provided at Reheater outlet.
According to prior art, a single stage Reheater is provided with Rehear Front and
Reheater Rear section without any intermediate headers between these two
sections (Refer figure 3). The reheater (single stage or two stage reheater after
modification) is positioned in the horizontal pass of the conventional two pass
boiler (Refer figure 1). The steam from High pressure(HP) turbine is getting first
heated in the Reheater Front section. Due to an unbalance generated at gas side
owing to formation of fire ball in the combustion chamber of tangential firing
boilers, the steam temperature produced in the left and right side of the
Reheater Front section outlet differs, that is to say, one of the sides experiences
more flue gas heat and hence more steam temperature compared to the other
side. Hence steam side temperature unbalance occurs in the Reheater Front
section outlet. Since it is a single stage Reheater, the steam side unbalance
experienced in the Reheater Front section is carried over to the Reheater Rear
section (see figure 2). The steam side unbalance carry over from Reheater Front
section to Reheater Rear section. Further, unbalances in flue gas heat in gas
path again increases the unbalance in the steam side due to heat pick up in the
Reheater Rear section. Because of this phenomenon, there is an steam
temperature imbalance of around 20° -30°C at Reheater outlet leads. Reheated
steam temperature at Reheater outlet is controlled primarily by tilt and then by

spray. On account of this steam temperature imbalance of around 20° -30°C at
Reheater outlet leads (R6) [see Figure 3].
Accordingly, differential spray is needed to achieve the rated steam temperature
at the Reheater outlet. This increases the spray quantity which in turn affects the
heat rate of the power plant. This increase in steam temperature at one side also
affects the life of the tubes/headers on that side.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a two-stage reheater in a
two pass tangential fired boiler with cross over links disposed between reheater
front outlet header and reheater rear inlet header to maintain steam temperature
imbalance between the right and left leads of reheater outlet within 10°C.
Another object of the invention is to propose a two-stage reheater with cross
over links disposed between reheater front outlet header and reheater rear inlet
header in a two pass tangential fired boiler to maintain steam temperature
imbalance between the right and left leads of reheater outlet within 10°C, which
reduces imbalance in steam temperature carried-over from first stage re-heater
to the second stage reheater.
A still object of the invention is to propose a two-stage reheater with cross over
links disposed between reheater front outlet header and reheater rear inlet
header in a two pass tangential fired boiler to maintain steam temperature
imbalance between the right and left leads of reheater outlet within 10°C, which
improves the plant heat rate through uniform water spray on both left and right
links of cold reheat lines.

A further object of the invention is to propose a two-stage reheater with cross
over links disposed between reheater front outlet header and reheater rear inlet
header in a two pass tangential fired boiler to maintain steam temperature
imbalance between the right and left leads of reheater outlet within 10°C, which
is capable to increase life of tubes and heaters through maintaining uniform
metal temperatures at left and right side of reheater outlet.
SUMMARY OF THE INVENTION
According to the invention, a two stage Reheater is provided in the horizontal
pass of a two pass tangential fired boiler with cross over links disposed between
Reheater Front outlet header and Reheater Rear inlet header. A plurality of
Siamese headers are located at the Reheater Rear outlet in the boiler.
Accordingly, there is provided a two-stage reheater in a two pass tangential fired
boiler to maintain steam temperature imbalance between the right and left leads
of reheater outlet within 10°C, comprising : a reheater front section having left
side and right side inlet and outlet header; a reheater rear section having left
side and right side inlet and outlet header; the reheater front section outlet
header sustaining temperature imbalance due to gas side unbalance when steam
from a high pressure turbine is first heated; a plurality of criss cross links
provided between the reheater front section and the rear section; and a plurality
of Siamese headers disposed at the outlet of the reheater rear section such that
alternative assemblies of the tubes of reheater rear section are connected to
said Siamese headers.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
FIGURE 1 Schematically shows a two pass conventional boiler.
FIGURE 2 Shows a single stage reheater front section of prior art
experiencing steam-side unbalance.
FIGURE 3 Shows a prior art reheater requiring differential spray of water to
achieve rated steam temperature.
FIGURE 4 Shows an operational diagram of a two stage reheater according to
the invention.
FIGURE 5 Shows a schematic block diagram of the reheater of the present
invention.
DETAIL DESCRIPTION OF THE INVENTION
As shown in figures 4 & 5, a two stage Reheater is provided with Reheater Front
Section and Reheater Rear section. Both the sections are having inlet and outlet
headers. Criss-Cross links(R4 — Figure 4) are provided to connect the Reheater
Front left side outlet header(R3) to Reheater Rear right side inlet header(R5) and
vice versa. The steam from a High pressure (HP) turbine is first heated in the
Reheater Front section (R2). Due to the gas side unbalance on account of the
fire ball formation in the combustion chamber of tangential fired boilers, the
steam temperature produced in the left and right side of the Reheater Front
section outlet differs, that is to say, one of the sides experiences more flue gas
heat and hence more steam temperature compared to the other side. Hence,

steam side temperature unbalance occurs in the Reheater Front section outlet
(R3). Since the Criss-Cross links are provided between the Reheater Front and
Reheater Rear sections, the unbalance experienced in the ReheaterFront section
in one side (for example right side) is directed to the other side in the Reheater
Rear section (left side).This nulifies the unbalance in the pick-up steam
temperature carrying over from reheater fornt section and also due to the
unbalance in flue gas heat in the Reheater Rear section. In addition to that
Siamese headers(R7) are provided at the outlet of the Reheater Rear section.
Alternate assemblies of the Reheater Rear section tubes are connected to the
Siamese headers. The Reheater outlet leads are taken from the centre of the
Siamese headers. Hence the steam temperature at Reheater outlet leads remains
same. To achieve the rated reheater temperature at Reheater Rear section
outlet, uniform spray is required both at left and right side in the Cold Reheat
lines which in turn reduces the spray quantity and improves the heat rate of the
power plant. The life of the tubes and headers at Reheater Rear section outlet
also enhanced.


After modification the maximum unbalance observed in RH outlet is 1 Deg C only
and is within the allowable limits.

The experimental data clearly indicates that in the Improved Reheater system,
the Reheater unbalance is nullified. This is due to the provision of separate
Reheater Front and Reheater Rear section with Criss-Cross arrangement between
Reheater Front outlet and Reheater Rear inlet including provision of Siamese
header at Reheater outlet.
ADVANTAGES OF THE INVENTION
The advantages of the invention -
• To reduce the Steam side unbalance carry over from Reheater Front
section to Reheater Rear section.
• Uniform spray on both left and right links of Cold Reheat lines which
improves the plant heat rate.
• Reheater steam temperature imbalance between left and right leads at
Reheater outlet is maintained within the limits of 10°C.
• Metal temperatures at Left and Right side of Reheater outlet is uniform
which in turn increases the life of tubes & headers.

WE CLAIM :
1. A two-stage reheater in a two pass tangential fired boiler to maintain
steam temperature imbalance between the right and left leads of reheater
outlet within 10°C, comprising :
a reheater front section having left side and right side inlet and outlet
header;
a reheater rear section having left side and right side inlet and outlet
header;
the reheater front section outlet header sustaining temperature imbalance
due to gas side unbalance when steam from a high pressure turbine is
first heated;
a plurality of crisscross links provided between the reheater front section
and the rear section; and
a plurality of Siamese headers disposed at the outlet of the reheater rear
section such that alternative assemblies of the tubes of reheater rear
section are connected to said Siamese headers.
2. The two-stage reheater as claimed in claim 1 wherein the reheater onlet
leads are taken out from the centre of the Siamese headers.

ABSTRACT

The invention relates to a two-stage reheater in a two pass tangential
fired boiler to maintain steam temperature imbalance between the right
and left leads of reheater outlet within 10°C, comprising : a reheater front
section having left side and right side inlet and outlet header; a reheater
rear section having left side and right side inlet and outlet header; the
reheater front section outlet header sustaining temperature imbalance due
to gas side unbalance when steam from a high pressure turbine is first
heated; a plurality of criss cross links provided between the reheater front
section and the rear section; and a plurality of Siamese headers disposed
at the outlet of the reheater rear section such that alternative assemblies
of the tubes of reheater rear section are connected to said Siamese
headers.