FIELD OF THE INVENTION
The present invention generally relates to a method of supporting heat transfer
coil assembly used in fluidized bed heat exchangers (FBHE) in circulating
fluidized bed boilers (CFBC) to avoid welding related failures and reduce the
manufacturing time.
BACKGROUND OF THE INVENTION
Shrink fit between two concentric hollow cylinder are commonly used throughout
wide variety of industrial applications to transmit either mechanical or thermo
mechanical loads in the form of axial, radial forces and torque. For example, in
the heat transfer coil assembly in accordance with the present invention, the
coupling is required to support the horizontal heat transfer sections of super
heater, Reheater and evaporator heat transfer coils. The intended heat transfer
coils are disposed in a highly agitated fluidized bed of ash particles with high
temperature in the order of 850°C. At times, the supporting components of such
heat transfer coils in the FBHE requires stringent vertical load carrying capacity
along with a flexible support arrangement, which is challenging. Even an
adequately provided vertical load application, lacks flexibility between the heat
transfer coils and support components, and causes damage to the supporting
components along with the heat transfer surfaces, resulting in heavy loss to
the productivity. As such, present invention may be advantageously put in
fluidized bed heat exchangers of Circulating fluidized bed ash in about 850°.

Prior art of conventionally supporting horizontal heat transfer coil assembly in
such environment is welding the sleeve with coil tube to transfer the load, which
requires cumbersome preheating and post heating activities in turn cracks may
also may be developed in the weldment due to improper pre and post weld heat
treatment. Normally, the metal temperature of heat transfer coils is less when
compared to sleeve metal temperature due to flow of less temperature medium
in the heat transfer coils, which inter alia requires different material specification
for tube and sleeve. When welding two different materials at different
temperatures there is a possibility of restraining differential thermal expansion
between tube and sleeve, resulting in unduly induced thermal stresses which
eventually causes failure of support components and coils. In addition to the
above mentioned limitations of the prior art of welding, thermal contact may not
be proper between coil and sleeve, which causes high metal temperature of the
sleeve.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method of weld-less
coupling of a thermo mechanical load bearing assembly between two heat
transfer coil tubes and a load carrying sleeve operating under high temperature
and erosion prone environment in a circulating fluidized bed combustion boilers.

SUMMARY OF THE INVENTION
A shrink-fit supporting device of the fluidized bed heat transfer coils of the
present invention allows to overcome the problems associated with conventional
supporting arrangement. Shrink-fitting is a technique in which an interference
between a coil tube and a support sleeve is achieved by a relative size change
after assembly. This is usually achieved by heating or cooling one component
before assembly and allowing it to return to the ambient temperature after
assembly, employing the phenomenon of thermal expansion to make a load
bearing joint. According to the present invention, the required shrink fit between
the coil tube and sleeve has been achieved due to the difference in the outer
radius of the coil tube and the inner radius of the sleeve which is known as a
radial interference because of which a contact pressure is created on the
contacting surface of two bodies. The amount of interference is selected such
that the contact pressure on either the sleeve or the tube remains within the
yield limit of both the materials and also a shrink fit is maintained in both the
cold and intended operating conditions. Insertion of the sleeve inside the tube is
not possible in cold conditions due to the radial interference. To make the
required fit, sleeve is heated to a specific calculated temperature for achieving
the required clearance to make the insertion easy.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Typical location of the device of the invention in a fluidized bed heat
exchanger of circulating Fluidized bed boilers.
Figure 2: Detailed and enlarged view of the device illustrating the components
that make up the preferred embodiments.
Figure 3: detailed and enlarged view of a floating sleeve with coil tube showing
a radial clearance.
DETAILED DESCRIPTION OF THE INVENTION
Figure 1 shows the typical application of the inventive device in the fluidized bed
heat exchanger system of CFBC boilers. The fluidized bed heat exchangers are
additional heat transfer surfaces which have been provided to increase the heat
transfer area of the super heater, Reheater and evaporative circuits of the
circulating Fluidized bed boilers. It also serves the purpose of maintaining a
uniform temperature of the combustor in all the operating load conditions of the
boiler. An improved sulphur capture efficiency over a wide operating load range,
a simplified plant operation and high boiler efficiency even at part load conditions
are the additional advantages by providing the fluidized bed exchanger in the
circulating fluidized bed boilers. In the fluidized bed heat exchanger, hot ash
from the combustor at a temperature of 850°C is fluidized to transfer the heat
from ash to the heating surfaces. Supporting the heat transfer coil tube in such
an environment is very critical, and also suspected to out of plane vibrations due
to excitation arising from fluidization of high temperature ash particles. Hence,

sufficient stiffness of geometry of the heat transfer surfaces along with the
support components is required to avoid near resonance condition. Number of
sleeves (1) at a predetermined locations of the heat transfer coils have been
shown in figure 1, to transfer the vertical loads of the coils to the intended
header and out of all, some of the sleeves(l) have been fixed with the coil tubes
and remaining sleeves have been made floating (free with the tube). The
process of fixing and floating the sleeves with the coil tube has been made so
that the entire support device should be able to carry the intended vertical load
and further allows the free thermal expansion of the coil tube, without
restraining the differential thermal expansion between the tube and sleeve.
Figure 2 is the enlarged view of the connection between the sleeve and heat
transfer coil tube showing the radial interference between the fixed sleeve (2)
and the heat transfer coil (3) which is the primary embodiment in the present
invention. Figure 3 depicts the detailed and enlarged view of the connection
between a floating sleeve (4) and the heat transfer coil tube (3) with the radial
gap.

WE CLAIM :
1. A method of weld-less coupling of a thermo mechanical load bearing
assembly between two heat transfer coil tubes and a load carrying
sleeve operating under high temperature and erosion prone
environment comprising the steps of:-
locating a plurality of sleeves at predetermined locations along the heat
transfer coil tubes, a first plurality of the plurality of sleeves being fixed
to the tubes in a floating manner to allow differential thermal expansion
between the tubes and the floating sleeves, and
rigidly fixing a second plurality of sleeves with the tube maintaining an
intra-radial interference.
2, The method as claimed in claim 1, wherein the sleeves and heat transfer
coil tubes are shrink-fitted.