FIELD OF THE INVENTION:
The invention relates to Recuperative type Tubular heat exchangers
generally known as Tubular Air Preheaters and more particularly to
an Improved Stepped Block Tubular Air preheaber adaptable to
thermal power boilers.
BACKGROUND OF THE INVENTION:
Tubular Air Preheaters are used in Thermal power boilers / Industrial
applications to transfer the heat from the flue gas leaving the boiler /
furnace to the entering combustion air.
A Tubular Air preheater transfers sensible heat from the flue gas
leaving a boller to the entering combustion air through the tube wall
separating air and gas. The Tubular Air Prehaater consists of a
number of tubes extended at each end into a plurality of tube plates
to form tube banks. The tube banks are enclosed in a steel casing,
which forms a passage for the airflow. Support beams are provided
to strengthen the device. Generally, the gas Is caused to flow Inside
the tubes, the air being allowed to pass over the tubes with a cross
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flow arrangement. Based on the performance requirement, air flow is
allowed in a single pass or in a multi-pass arrangement. Depending
on the heat duty, the Tubular Air Preheater will have one or more
blocks/banks.
The Tubular Air Preheater in a typical Utility boiler consists of three
blocks, These three blocks are arranged one after the other in the
flue gas path, The first block where the gas enters first is called the
hot end block, the second block is called the middie block and the
last block where the gas exits is called the cold end block. All the
three blocks of the Tubular Air Praheater are generally of same size.
The flue gas temperature at the hot end block is high and at the cold
end block is low. As the gas flow area for all the three blocks are
identical, the flue gas velocity is maximum (around 16 M/Sec) at the
hot end block and minimum (around 6 M/Sec) at the cold end block-
The flue gas contains a large amount of fly ash particles. The dust
laden flue gas entering at a htgh velocity erodes the tubes in the hot
end block thus reducing the life of the tubes. On the other handfthe
velocity of the flue gas at the cold end block being very low,does not
sufficiently carry the fly ash particles along with It. Thus, settlement
of ash occurs in the cold end block resulting in poor thermal
performance. As the cold end block is prone to the low temperature
corrosion, the tubes in the cold end block, are generaIly made of low
alloy steel.
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OBJECTS OF THE INVENTION:
Accordingly, It is an object of the present invention to provide an
Improved stepped block Tubular Air Preheater adaptable to thermal
power boilers which eliminates the disadvantages of the prior art.
Another object of the present Invention is to provide an improved
stepped block Tubular Air Preheater adaptable to thermal power
bollers which maintains uniform and optimum gas velocity at the
inlet of each of the stepped blocks of the preheater,
A further object of the present invention is to provide an improved
stepped block Tubular Air Preheater adaptable to thermal power
boilers which increases the tube life of each of the stepped blocks by
reducing the tube erosion at the gas Inlet.
A still further object of the present invention is to provide an
improved stepped block Tubular Air Preheater adaptable to thermal
power boilers which minimizes ash settlement in the tubes of the
lower block of the preheater.
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An yet another object of the present invention is to provide an
improved stepped block Tubular Air Preheater adaptable to thermal
power boilers which entails less initial cost and reduced maintenance
cost.
SUMMARY OF THE INVENTION:
According to the invention, there is provided an improved stepped
block tubular air preheater adaptable to thermal power boilers, the
preheater comprises atleast three blocks each block being comprised
of a plurality of hollow metal tubes extending at both ends so as to
terminate into a plurality of tube plates thereby forming atleast three
blocks. The atleast three blocks are arranged one after the another
and enclosed in a steel casing which forms a passage for gas flow via
the internals of the tubes. The air flow is conducted over the tubes in
a cross-flow arrangement. The atleast three blocks are configured to
have a height, a width, and a depth. The device features of the
atleast three blocks are selected to maintain a gas inlet velocity £12
M/Sec and a gas leaving velocity ³ B M/Sec, The relationship of
height, width, and depth of the three blocks which has a dimensional
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ratio of 2;3:1.5 result in a depth of the hot-end block ,a depth of the
middle block, and a depth of the cold end block at a ratio-
relationship of 1:0.9:0.8
The stepped block design reduces the tube erosion at hot end block
and minimizes the ash settlement in the cold end block by
maintaining uniform and optimum gas velocity at the inlet of all the
blocks. The reduced tube erosion at the hot end block gives higher
life of the tubes. The minimized ash settlement in the cold end block
helps to maintain cleaner tubes resulting in better performance.
The improved Stepped block Tubular Air preheater has different block
sizes to maintain uniform and optimum gas velocity at the inlet of
each block. A bigger block size for the hot end , a medium size for
the middle block and a smaller size for the cold end block are
provided. The arrangement of the Tubular Air preheater blocks
instepped block configuration ensures a uniform and optimum gas
velocity at the inlet of each of the blocks by providing a selected free
area for the gas flow in each of the blocks. As the free flow area for
the flue gas in the hot end block is more, the flue gas velocity is
reduced in the hot end block to an optimum level. This reduces the
erosion of the hot end tubes resulting in higher life of the tubes. The
cold end block is designed at a smaller size compared to the hot end
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block such that the gas inlet velocity is increased to an optimum level
to improve the performance of the cold end block which minimizes
the ash settlement in the cold end block tubes.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1- Shows a general assembly of a Tubular Preheater block,
Figure 2(a)-Shows a prior art configuration of a Tubular Preheater
block.
Figure 2(b)- Shows a configuration of a Tubular Preheater block
according to the present invention,
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF
THE INVENTION
As shown in figure-1, a Tubular Air Preheater comprises a plurality of
tubes (1) extending at each end so as to terminate into a plurality of
tube plates(2). Such a configuration forms a plurality of tube banks.
The tube banks are disposed in a steel casin (3) which forms a
device comprising a passage for air flow. The device is supported by
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atleast two support beams (4). The gas flow is allowed internally via
the tubes (1), wherein the air flows over the tubes (1) in a cross-flow
arrangement.
As shown in figure 2(b), according to the invention, the size of each
block (8,9,10) is selected to maintain an uniform and optimum gas
velocity (£ 12 M/Sec) at the inlet of each block. To reduce the gas
inlet velocity to the optimum level in the hot end, the hot end block
(8) is configured as the largest compared to the other two blocks
(9,10)To increase the gas inlet velocity in the cold end block (10) to
the optimum level (³ 8 M /Sec),the cold end block (10) is made
smallest of the three blocks (8,9,10). This change in size of the
blocks (8,9,10) from top (hot end) to bottom (cold end) constitutes a
step line arrangement and is named as 'Stepped block design' for
Tubular Air Preheaters.
TAPH Blocks size can be varied by changing either height (H) or
width (W) or depth (D) or a combination of each parameter. A
change in depth / width changes gas velocity. A change In height /
width changes air velocity. The flue gas temperature is higher at the
inlet of the hot end block (8) and hence the volume of the flue gas
leads to have more impact in higher inlet gas velocity in the hot end
block (8). In order to bring down the gas velocity to the optimum
level, according to the invention,the free flow area for gas has been
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increased by increasing the depth (D) of the block. This facilitates
maintaining optimum gas velocity at the inlet of the hot end block
(8),which minimizes the tube (1) erosion and increases the life of the
tubes (1). The flue gas temperature is low at the inlet of the cold end
block (10) and hence the volume of the flue gas is having less impact
in lower inlet gas velocity in the cold end btock (10). In order to
Increase the gas velocity to the optimum level, the free flow area for
gas has been reduced by decreasing the depth of the block (10). This
ensures maintenance of an optimum gas velocity at the inlet of the
cold end block (10), which minimizes the settlement of ash in the
tubes (1) of the cold end block (10) and improves the performance.
As shown in figure 2(a), the height (h), width (w) and depth (d) of
the blocks (5,6,7) according to the prior art generally ranges as
under:
Height (h): 2 to 8 Metres
Width (w): 3 to 8 Metres
Depth (d): 1.5 to 4 Metres
According to the invention, the depth (D) of the blocks (8,9,10) is
varied. The depth(D2) of the Middle block (9) Is around 10% less
than the depth(D1) of the Hot end block (8),and the depth(D3) of the
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cold end block (10) is around 10% less than the depth (D2) of the
Middle end block (9),
Normally the sizing of the blocks depends on the Gas & air mass
velocities, Heat duty, type of fuel, and allowable pressure drop
values. In stepped block design,the sizing of the blocks depends on
optimum gas velocities at the inlet and outlet of the blocks instead of
mass velocity factor normally used. All other factors remain
applicable for stepped block design also.
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WE CLAIM;
1. An improved stepped block tubular air preheater adaptable to
thermal power boilers, the preheater comprises three blocks(8,9,10),
each block being comprised of a plurality of hollow inetal tubes(1)
extending at both ends so as to terminate into a plurality of tube
plates (2) thereby forming atleast three blocks, the three blocks
being arranged one aftar the another and each block having a steel
casing(3) which forms a passage for air flow over the
tubes(1),whereas the gas flow is conducted through the tubes (1) in
a cross-flow arrangement, the atteast three blocks (8,9,10)
configured to have a height(H),a width(W), and a
depth(D),characterized in that the device features of the atleast three
blocks(8,9,10) are selected to establish the following relationship:-
(a) Gas inlet velocity to gas outlet velocity at 3:2;
(b) Height(H), width(W), and depth (D) of atleast the three
blocks(8,9,10) having a typical dimensional ratio of 2:3:1.5,
and resulting in a depth (D1) of the hot-end block(8) ,a
depth (D2) of the middle block (9), and a depth (D3) of the
cold end block (10) at a ratio-relationship of 1:0.9:0.8.
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2. The preheater as claimed in claim 1, wherein the atleast three
blocks (8,9,10) disposed in the casing (3) Is provided with support
beams (4).
3. An improved stepped block Tubular Air Preheater adaptable to
thermal power boilers as substantially described and illustrated herein
with reference to the accompanying drawings.
Dated This 24th Day of March, 2006