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 Tubular
Air preheater named Stepped Block Tubular Air preheater 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 boiler
to the entering combustion air through the tube wall separating air and gas. The
Tubular Air Preheater 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 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 middle block and the last block where the gas exits is called the cold end
block. All the three blocks of the Tubular Air Preheater 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 high velocity erodes the
tubes in the hot end block thus reducing the life of the tubes. On the other hand,
the 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 generally made of low alloy steel. Hence there was a need to invent a
new design to eliminate all the above disadvantages.
OBJECTS OF THE INVENTION:
Accordingly, it is an object of the present invention to provide an improved
Tubular Air Preheater named 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 Tubular Air
Preheater named Stepped Block Tubular Air preheater adaptable to thermal
power boilers 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 Tubular Air
Preheater named 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 Tubular
Air Preheater named Stepped Block Tubular Air preheater adaptable to thermal
power boilers which minimizes ash settlement in the tubes of the lower block of
the preheater.
An yet another object of the present invention is to provide an improved Tubular
Air Preheater named 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 tubular air preheater
named stepped block tubular air preheater adaptable to thermal power boilers,
the preheater comprises alteast 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 provide a relationship of height, width, and
depth of the atleast three blocks which has a dimensional ratio of 2:3:1.5. And a
depth of the hot-end block ,a depth of the middle block, and a depth of the cold
end block has a ratio of 1:0.9:0.8
The stepped block Tubular Air preheater 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 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 in stepped 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 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 Normal Tubular Preheater block.
Figure 2(b)- Shows a configuration of a Stepped block 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 casing (3) which forms a device comprising a passage for air flow. The
device is supported by 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 (=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 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 block (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 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.
WE CLAIM:
1. The stepped block tubular air preheater adaptable to thermal power boilers,
the preheater comprises three blocks(8,9,10), in step line configurations of a
bigger size hot end block, a middle size at middle block, a small size at cold
end block, the three blocks being arranged one after another starting big
block at top and a small block at the bottom and each block having a steel
casing (3) in order to minimize ash settlement and maintain uniform and
optimum gas velocity, each block being comprising of a plurality of hollow
metal tubes (1) extending at both ends so as to terminate in to a plurality of
tube plate (2) thereby forming a passage for air flow over the tubes (1)
where as the gas flow is conducted through the tubes (1) in a cross-flow
arrangement; the at least 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 at least three blocks(8,9,10) are selected to establish the
following relationship: -
(a) Height (H), width(W), and depth (D) of the at least three
blocks(8,9,10) has a dimensional ratio of 2:3:1.5, and
(b) a depth (Di) of the hot-end block(8) ,a depth (D2) of the middle block
(9), and a depth (D3) of the cold end block (10) has a ratio-
relationship of 1:0.9:0.8 as illustrated in the accompanying drawings.
2. The preheater as claimed in claim 1, wherein the at least three blocks
(8,9,10) disposed in the casing (3) is provided with support beams (4).

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 plate thereby forming atleast three blocks. The atleast
three blocks are arranged on 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 as gas inlet velocity ≤12 M/Sec and a gas leaving velocity ≥ 8 M/Sec. The relationship of height, width, and depth of the three blocks which has a dimension ration 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.