FIELD OF THE INVENTION
The present invention relates to an improved arrangement for supporting
backpass economiser enclosure duct of Circulating fluidized bed combustion
boiler at various load conditions.
BACKGROUND OF THE INVENTION
Circulating fluidized bed combustion (CFBC) is an efficient and known method of
burning various grades of fossil fuels in particular low grade coal, lignite, pet
coke etc. Fuel feeding is done by means of a drag link chain feeder which
transports the lignite from a storage bunker to the combustor through a self
cleaning rotary valve. Fluidization of particles take place in the combustor by
preheated primary air from a primary air (PA) fan through air distribution
nozzles. For complete combustion of the fuel, secondary air from a SA fan is fed.
The flue gases with hot soild particles from the products of combustion flows
from a grate of the combustor to cyclone elevation by imparting heat to water
walls of the combustor. In the combustor, the burning coal raises from the grate
level to the top where an entry for the cyclone is provided. During the
transportation, the coal particles burn and liberate heat then the flue gas to
enters the cyclone, and separate the solid particles gets separated from the flue
gases, which is then re-directed to the combustor via seal pot. The remaining
flue gases after separating the solid particles, flows to the backpass which is a
gas tight enclosure, with vertical four sided water/steam cooled walls. The
backpass consists of various horizontal heat transfer sections of super heaters
and reheaters in the form of coils. Economiser coils are arranged below the low
temperature super heater section in the gas tight four sided duct enclosure

in which the feed water is heated up by utilizing the residual heat available in the
flue gases, after transferring the heat to various stages of the super heater and
re heater sections in the water/steam cooled walls of the back pass.
Economizer enclosure duct consists of a plurality of banks of economizer coils
placed horizontally to pick up the heat from the flue gases and increase the
temperature of the feed water to a near saturation temperature, and enhancing
the thermal efficiency of the boiler. The temperature of the flue gases ranges
from 506°C (At inlet of a top bank of the economizer coils) to 300°C (At outlet of
bottom bank economizer coils).The construction materials of the enclosure duct
depends on temperature of the flue gas. Nevertheless, up to 420°C, alloy steel is
to be used and below 420°C carbon steel material is used.
Expansion joint is used to isolate the economizer enclosure duct both
mechanically and thermally (in respect of thermal expansion) from water/Steam
cooled walls of the back pass.
Supporting the economizer enclosure duct constitutes one of the critical
parameter in the mechanical design of a CFBC boiler which includes stiffening of
the duct against the dead load, live load, ash load and wind load in combination
with the flue gas design pressure. The Economizer enclosure duct is stiffened
externally with buck stays at various levels to prevent distortion of the four sides
of the enclosure duct from said load combination. Vertical levellers are provided
between two buck stay levels as a lateral support to an outboard buck stay
flange and to keep the buck stay perpendicular to the duct wall. Further support
members to transfer all the dead load of duct components are provided which
include duct plates, duct stiffening components, insulation and insulation fixing
components to the adjacent structural members.

Prior art provides a arrangement for supporting an economizer enclosure duct of
size 10972.8x6502.4mm. In the present case, the size of the economizer
enclosure duct is 15342x12395mm, which is quite larger when compared to prior
art, the duct becomes susceptible to large load conditions and needs a support
device which further requires catering for large thermal expansion movements
(flexible supporting arrangement) in addition to transferring the dead load to the
adjacent structural members.
Although support arrangement for larger size economizer duct is known, which
includes four supports at one elevation two at each side to take care of the
entire load transfer to the structural members. However, this known method fails
to take care against bending of the duct plate at support elevation.
OBJECT OF THE INVENTION
It is therefore an object of the invention to propose an improved support
arrangement for a large-size economizer enclosure duct of circulating fluidized
bed combustion (CFBC) boilers to eliminate failure due to bending moment,
which eliminates the disadvantages of the prior art.
SUMMARY OF THE INVENTION
Accordingly, an improved support arrangement is provided for large-size
economizer enclosure duct of circulating fluidized bed combustion (CFBC) boilers
to eliminate failure due to bending moment, the improvement is characterized
by comprising :a plurality of additional trusses (2) forming structural frames
provided at four planes along the width of the enclosure duct; a plurality of buck
stays for structural strengthening of the duct enclosure provided externally;

An expansion joint to mechanically and thermally isolate the enclosure duct from
the back pass which attaches a top portion of the duct to a bottom ring header
of the backpass; a plurality of horizontal heat transfer coil of the economizer
interposed between the top portion of the enclosure and the bottom portion of
the duct acting as a hopper to collect the fly ash from the flue gases, the
collected fly ash from flue gas being periodically discharged from the boiler; a
regenerative air pre heater receiving the residual heat of the flue gases directed
via a third pass of the boiler from the bottom portion of the economizer
enclosure duct; and a plurality of support members provided at the top of the
hopper to transfer the total load of the duct to adjacent structural members.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1- is a general arrangement of a CFBC boiler of prior art indicating the
typical location of an economizer enclosure duct.
Figure 2- is an improved arrangement for supporting at various load conditions
of a back pass economizer enclosure duct according to the invention.
Other objects and advantages of this invention will become apparent from the
following description taken in conjunction with the accompanying drawings
wherein are set forth, by way of illustration and example, preferred embodiments
of this invention.
DETAILED DESCRIPTION OF THE INVENTION
The economizer enclosure duct shown in Figure 1, includes a tight seal
rectangular duct enclosure (1), to which buck stays at various elevations with

corresponding vertical stiffeners are attached externally for stiffening of the duct
enclosure against the dead load, live load, ash load and wind load, in
combination and further considering the flue gas design pressure. Top of the
enclosure duct is attached to the bottom ring header of the backpass with a
metallic expansion joint to isolate the duct enclosure from the back pass both
mechanically and thermally. The top portion of the enclosure duct
accommodates a plurality of horizontal heat transfer coils of the economizer and
a bottom portion of the enclosure duct which act as a hopper to collect the flyash
from flue gases. The collected fly ash in the bottom hopper portion is discharged
from the boiler at frequent intervals. From the bottom portion of the economizer
enclosure duct the flue gases are directed to a third pass of the boiler to transfer
residual heat of the flue gases to Regenerative Air Pre heaters.
At top of the hopper portion of the enclosure duct supports have been provided
to transfer the total load of enclosure duct to the adjacent structural members.
As the duct size is very large, providing supports at one elevation is not sufficient
as it leads to induce bending moment on the enclosure duct which causes
distortion of the enclosure duct, and jeopardizing its functionality. After rigorous
structural analysis a plurality of truss members (2) at four planes along the width
of the enclosure duct are provided.
The structural members shown in Figure 2 act trusses (2) and form frames to
accommodate bending moments induced in the operating condition.
Accordingly, the shape of the enclosure duct remains under formed throughout
the life of the boiler for provision of the inventive support arrangement.

WE CLAIM
1. An improved support arrangement for large-size economizer enclosure
duct of circulating fluidized bed combustion (CFBC) boilers to eliminate
anticipated failure due to bending moment, the improvement is
characterized by comprising :
a plurality of additional trusses (2) forming structural frames provided
at four planes along the width of the enclosure duct;
a plurality of buckstays for structural strengthening of the duct
enclosure provided externally;
a metallic expansion joint to mechanically and thermally isolate the
enclosure duct from the back pass is provided which attaches a top
portion of the duct to a bottom ring header of the back pass;
a plurality of horizontal heat transfer coil of the economizer interposed
between the top portion of the enclosure and the bottom portion of
the duct which acting as a hopper to collect the fly ash from the flue
gases, the collected fly ash from flue gas being periodically discharged
from the boiler;
a regenerative air pre heater receiving the residual heat of the flue
gases directed via a third pass of the boiler from the bottom portion of
the economizer enclosure duct; and

a plurality of support members provided at the top of the hopper to
transfer the total load of the duct to adjacent structural members.

An improved support arrangement for large-size economizer enclosure duct of
circulating fluidized bed combustion (CFBC) boilers to eliminate anticipated
failure due to bending moment, the improvement is characterized by
comprising:a plurality of additional trusses (2) forming structural frames provided
at four planes along the width of the enclosure duct; a plurality of buckstays for
structural strengthening of the duct enclosure provided externally; a metallic
expansion joint to mechanically and thermally isolate the enclosure duct from the
back pass is provided which attaches a top portion of the duct to a bottom ring
header of the back pass; a plurality of horizontal heat transfer coil of the
economizer interposed between the top portion of the enclosure and the bottom
portion of the duct which acting as a hopper to collect the fly ash from the flue
gases, the collected fly ash from flue gas being periodically discharged from the
boiler; a regenerative air pre heater receiving the residual heat of the flue gases
directed via a third pass of the boiler from the bottom portion of the economizer
enclosure duct; and a plurality of support members provided at the top of the
hopper to transfer the total load of the duct to adjacent structural members.