FIELD OF THE INVENTION
The present invention relates to in-situ thermocouple replaceable heat flux
measurement device. More particularly, the invention relates to a device with
online replaceable thermocouples for measuring absorbed heat flux.
BACKGROUN D OF THE INVENTION
Data representing absorbed heat flux in fluid cooled furnaces and heat
exchangers is important for improving and optimizing heat transfer. Steam
generators are one such application where the heat absorption keeps varying
due to the nature and type of ash /dust deposition on heat exchange surfaces.
Capturing the information on variation in heat absorption requires an on-line
measuring instrument capable of withstanding robust working environment.
Instruments briefed in prior art below with varying physical constructions and
limitations are in use for this purpose.
Indian Patent Application No.493/DEL/2004 discloses a device for similar
application having two thermocouples brazed to a stainless steel sensor at known
axial distance in the direction of heat flow to measure the cold end and hot end
temperatures which inter-alia provides the differential temperature enabling
computation of heat flux. In this heat flux sensor assembly, if the thermocouple
fails to measure the temperature due to improper handling of the assembly or
damage during the maintenance period or loss of shelf life, etc., the entire
assembly has to be disassembled from the furnace and a new sensor assembly
is to be welded to bring back the system into operation at the desired location
and time. This replacement could not be done when the boiler is in operation
and requires shutting down of the boiler.
US 6848373 B2 teaches a method of measuring heat flux and corrosion in a
furnace, in which one thermocouple of a pair of thermocouples is attached to a
tube and the second thermocouple is attached to a web connected to that tube.
A temperature differential is determined for each pair of selected time intervals.
A decrease in the difference between the temperature differentials indicates
melting of slag on the furnace wall, thereby forecasting a possibility occurring
corrosion. However, according to this invention, the thermocouples are fixed at
different locations, one on tube and the other on the web in a gap between the
adjacent tubes, in the horizontal direction, on the backside of the water wall
panel, a true radial heat flow to water from hot furnace side is not possible in the
method of this prior art.
US 2005/02177841 teaches a device of measuring a heat flux. The measuring
device includes a heat exchanger pipe having a pipe wall with a circumference,
and an indentation extending over and deforming a portion of the circumference.
At least one thermocouple is disposed concentrically in the portion of the
circumference deformed by the indentation. Filling material fills the indentation.
The method of measuring a heat flux, are also provided. The size of indentation
can be decreased for a given size of the thermocouple, so that the heat flux is
obstructed to a comparatively small degree by the pipe wall while local
overheating of the pipe wall is prevented. This approach requires sophisticated
manufacturing process involving the high pressure tube to form a dent,
depositing metal in the dent and accurate machining of grooves in the deposited
metal.
US 7,077563 B2 discloses an apparatus and method for the monitoring and
measurement of chemical and / or biological deposition in heat exchangers and
other processing vessels. The sensing system includes at least two hollow fluid
vessels conductively mounted across a constant heat transfer path. Thin film
heat flux sensors are attached to a heat transfer surface of the vessels to
measure changes in differential heat flux that occur when deposition begins to
accumulate in the vessel. In this way, it is shown that differential heat flux
measurements can be used to detect and measure the early onset of chemical
and / or biological deposition. This method is applicable for assessing the impact
of internal deposits in vessels and requires a heat generating source. This
concept could not be applied for pressure pipes.
In all the above prior patents, method or device for the heat flux measurement
described and method of replacement of thermocouples in case of failure of the
component are not given. Thus the prior art only envisages knowledge of
measuring the flux in a furnace or heat exchanger for monitoring the deposition
and corrosion.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a device to measure
the heat flux inside the water wall of boiler and other heat exchanger tubes to
monitor impact of dust /ash deposition on heat transfer.
Another object of the invention is to propose a method for replacement of the
thermocouples used in the heat flux measuring devices while the boiler / heat
exchanger component is in operation avoiding boiler shut down for repair and
increasing the availability of the measuring device.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 - shows a device with online replaceable thermocouples for measuring
absorbed heat flux.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
As shown in figure - 1, the device for measuring absorbed heat flux with online
replaceable thermocouple essentially consists of a stainless steel sensor stud (2)
machined to allow push fit of a plurality of thermocouples (5) and brazed to a
boiier/heat exchanger tube (1). This sensor stud (2) is constructed with a narrow
annular gap and a semi-spherical shaped thermal guard ensuring unidirectional
heat flow through the central cylinder in it. At least two thermocouple (±5)
passing through a straight carbon steel guard tube (6) and stainless bend tubes
(4) so as to perfectly fit in the grooves on both sides of the stud ending at the
same central axis with a predetermined axial spacing. The stainless steel bend
tubes (4) having bend on one end matching perfectly with the radius of
curvature of the boiler / heat exchanger tube (1) including the groove in the
sensor (2). The bend tubes (4) are welded to both the boiler / heat exchanger
tube (1) and the groove in the sensor (2). The straight end of the tubes (4) pass
through the straight carbon steel guard tubes (6) which are clamped to a
supporting clamp system (3). A flat portion of the supporting clamp system (3) is
welded to the boiler/heat exchanger tube (1). The thermocouples (5) have
stepped lengths of varying diameter and a quick connecting fitting means (7) on
the outer end with two leads ending in a terminal box (8). The terminal box (8)
is fixed to the supporting clamp assembly (3).
In accordance with the additional feature of the invention, the two
thermocouples (5) can be replaced during the operation of the boiler/heat
exchanger component. The present invention enables immediate replacement of
failed thermocouples while the boiler/heat exchanger is in operation and
improves the availability of the measuring device, Also, the invention drastically
reduces the time and money spent in repairing the failed sensor.
The measuring device of the invention having following advantages features:
a. Enables online replacement of failed thermocouples improving availability
of the sensor.
b. Avoids material and erection cost involved in replacement of failed
sensor.
c. Use of higher size thermocouples increase the life of thermocouple.
d. Combining core sensor and the annular dome in one piece ensures
concentricity and uniform radial gap around the sensor. This ensures one
directional heat flow close to theoretical condition.
e. Accommodating thermocouples on either side reduces complications in
replacement of thermocouples and selection of stainless steel bend tube.
f. The new type of the thermocouples selected enables accommodating
higher thermal cyclic loading leading to longer life.
WE CLAIM:
1. A device for measuring absorbed heat flux in a Boiler furnace/ heat
exchanger, comprising a boiler / heat exchanger pressure pipe brazed
with a stainless steel sensor stud facing the heat source; at least two
online replaceable thermocouples disposed on either side of the pressure
pipe within configured grooves in the stud spaced at different radii from a
center of a pipe for measuring absorbed heat flux, wherein said stud
having annular narrow gap and a semispherical shaped thermal guard
brazed perpendicular to the axis of said pressure pipe.
2. The measuring device as claimed in claiml, comprising stainless steel
bend pipes to provide thermal protection for the thermocouples.
3. The measuring device as claimed in claim 1, wherein said thermocouples
pass through a protective pipe attached to said pressure pipe on the
backside.
4. The measuring device as claimed in claim 1, comprising an electric
terminal box fixed to the protective tube.
5. The measuring device as claimed in claim 1, comprising the core sensor
and annular thermal guard machined in single piece ensuring concentricity
and uniform gap all around the core sensor
6. The measuring device as claimed in claim 1, comprising thermocouples on
both sides enable accommodating higher thermocouple diameter and
protective pipe alternatives
7. The measuring device as claimed in claim 1, comprising a new type of
thermocouple having higher thermal cycle withstanding capability leading
to longer life
8. The measuring device as claimed in claim 1, wherein the thermocouples
are replaceable when the boiler / heat exchanger is working.

ABSTRACT

The invention relates to a device for measuring absorbed heat flux in a Boiler
furnace / heat exchanger, comprising a boiler / heat exchanger pressure pipe
brazed with a stainless steel sensor stud facing the heat source; at least two
online replaceable thermocouples disposed on either side of the pressure pipe
within grooves configured in the stud spaced at different radii from a center of a
pipe for measuring absorbed heat flux, wherein said stud having annular narrow
gap and a semispherical shaped thermal guard brazed perpendicular to the axis
of said pressure pipe.