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1
Technical University of CzestochowaThe Faculty
of Process Material Engineering and Applied
PhysicsThe Department of Industrial Furnaces and
Environmental Protection
Marian Kieloch, Agnieszka Klos, Jaroslaw Boryca,
Edyta Warwas
Possibilities of measuring the surface
temperature of charge in heating furnaces
Abstract The knowledge of the precise value of
temperature is necessary for the proper control
of the heating process. Based on laboratory tests
carried out so far, a proposal has been made to
utilize digital photopyrometry as a method that
could be used for the measurement of temperature
in the future. The paper has also compared
temperature measurement results obtained using
thermovision and digital pyrometry, which could
be an alternative to the former.
2.The influence of capacity on the temperature
distribution in the furnace heating chamber The
process of heating charge in industrial heating
furnaces relies on the complex phenomena of heat
and mass transfer, both within the furnace
chamber and in the bulk (cross-section) of the
charge being heated. The full understanding of
those phenomena is extremely difficult, both in
model studies and the industrial operation of
furnaces. The performance of heating furnaces is
determined by energy intensity (unit consumption
of heat), steel loss for scale, CO2 emission,
scale adhesion to the substrate, and decarburized
layer thickness. Very often, the process of
heating charge in rolling mills and forges is
also decisive to the quality of finished product.
The thesis could be put forward that the basis
for the determination of the performance a
furnace (charge heating) should be the
information on the course of thermal phenomena in
the furnace chamber, as well as in the charge
itself. It is the most favourable course of these
phenomena that should govern the process
technology, and thus the thermal conditions in
the furnace heating chamber. On the basis of
theoretical derivations and model study results
it can be stated that the performance of heating
furnaces is determined by heating technology, and
for a given technology by the capacity of those
furnaces. Requirements imposed on the processes
of heating charge before plastic working are
extremely simple and come down to obtaining the
following at the end of the process the assumed
value of charge surface temperature, and the
specific temperature difference on the charge
cross-section. In industrial furnaces, the
continuous measurement of charge surface
temperature is very difficult, or even
impossible. The furnace temperature is the output
parameter in the control system of any fuel-fired
furnace. It must, however, precisely inform about
the value of the heat flux taken up by the
charge, so a strict relationship must exist
between the furnace temperature and the charge
surface temperature. This relationship is
extremely complex, and the furnace temperature
depends on the temperature of combustion gas in
the chamber, the temperature of the inner furnace
wall surface, the charge surface temperature, and
on the heat exchange conditions, including
furnace capacity, chamber geometry, combustion
gas radiation properties, the radiation
properties of the walls and the charge, and
combustion gas motion in the furnace chamber.
Despite the fact that so measured furnace
temperature is little objective and does not
directly indicate the temperature of the charge
surface, it remains invariably the parameter
defining heating process technologies. The
surface temperature of charge at the exit from
the furnace chamber is equal to the preset
temperature and amounts to 1250?C, with the
assumed temperature difference in the
cross-section of ?t50 K. After reducing the
capacity, for the same technology, the surface
temperature values increase. Further reducing the
capacity results in a continued increase in the
surface temperature. This temperature increase
causes directly an unnecessary increase in usable
heat and a significant increase in the loss of
steel for scale. At the same time, over more than
35 of the furnace length, the charge surface
temperature increases to over 1300?C. Reducing
the furnace capacity results in a slight increase
in wall temperature. As a consequence, the
thermal balance items associated with the above
temperatures also increase or remain unchanged.
The combustion gas temperatures, apart from the
equalizing zone, are slightly lower for reduced
furnace capacities. The temperature of combustion
gas existing the furnace decreases with reducing
capacity. The computation results indicate that,
for a given heating technology (T1), the heat
consumption is determined by furnace capacity
(Fig. 1). This is entirely consistent with the
results of industrial tests. From the theoretical
derivations, computation results and the results
of model studies and industrial tests it can be
concluded that it is impossible to obtain low
heat consumption indices for small furnace
capacities. The results of the performed
computations show also that the lowest heat
consumption will be reached for any heating
conditions by adapting the heating technology to
the conditions of the ideal process.
7. The effect of temperature on the greyscale
level of the digital photo The effect of
temperature on the greyscale level of the digital
photo is illustrated in Figure 5. This
dependence was determined for three exposure time
settings, namely S1/60s,S1/125s and S1/250s.
It has been established that the effect of
temperature on the greyscale level can be
described with the following equation
where a, b, c, d constant value, r degree
greyss, t temperature, C. On the basis of
measurement results, e.g. for S1/60s, the
following relationship is obtained The
calculation of the value of temperature from the
developed relationship is difficult. Therefore,
relationship (1) is represented in the following
form
The effect of the greyscale level of the object
examined on the value of temperature is
represented graphically in Fig. 6.
Fig.5. Influence of temperature on the degree
greyss of digital photo for carbon steel , ISO
80, A4
In the identical manner, the relationship under
consideration can be developed for other
conditions of measurements carried out.
8. Comparison of measurement results Measurement
results obtained by three different methods are
shown in Fig. 7. The temperature values obtained
using the digital camera differ from the
reference temperatures to a considerably lesser
extent compared with the temperature values
measured with the thermovision camera. In the
case of the digital camera, a larger error is
observable at lower temperatures. By contrast,
for the thermovision camera, the error increases
with the increase in specimen temperature.
Fig.6. The effect of time of time exposure on the
course of dependence t f(r), ISO 80, A 4
3. Methodology of measurements taken using the
digital camera Digital photopyrometry is a
contact-less method. The measurement and
recording of temperature is done using a digital
camera 3. The method consists in making a
photograph of the radiating object under
examination. The core of this measurement is the
recording of the spectral emission density,
ec?. The digital camera has a CCD converter,
instead of the traditional photographic plate,
and a processor that processes the data and
enables them to be stored in the cameras memory.
Data are recorded in the RGB (Red, Green, Blue)
mode as a 24-bit colour image. Then, using
suitable software, this image can be transformed
into an 8-bit multi-grade bitmap to be red out in
greyscale. 256 greyscale levels (from 0-Black to
255-White) are recorded. Such transformation
enables the temperature of the examined object to
be represented as a function of the absolute
greyscale level. The greyscale level is
determined by the computerized analysis of
digital photos using a suitable graphical program
4.
Fig.7. Results of temperature measurements with
utilization of three method
Table 1. Comparison of the result of temperature
measurement
9. Assessment of measurement accuracy It can be
found from the performed assessment of accuracy
that the results obtained using digital
photopyrometry exhibit relatively small
deviations from the reference temperatures.
Differences between the reference temperatures
and the measured temperatures for photopyrometry
are even smaller than for the thermovision
camera. Digital photopyrometry is distinguished
by higher measuring accuracy, particularly in
temperature ranges above 750?C. Comparison of
temperature measurement results obtained using
the thermovision camera and the digital camera
against the reference temperature values is given
in Table 1.
The greyscale level determination is a basis for
developing a temperature characteristic. Such a
characteristic will define the dependence of the
surface temperature of the heat
radiation-emitting object being photographed on
the average greyscale level of the photograph
showing the object under examination. The Fig.2
is presented the digital pictures example.
Fig. 1. The influence of heating technology and
furnace capacity on the consumption of heat
Fig. 2. The digital photography of sample for two
temperature a) 700ºC and b) 1100ºC
4. The effect of camera settings on the form of
the temperature characteristic The effect of the
ISO sensitivity of the CCD matrix on the
behaviour of the relationship t f(r) is
illustrated in Fig. 3. This relationship is
rectilinear in a certain section of the
temperature range. A large increase in ISO
sensitivity produces an effect in the form of a
shift of the rectilinear section of the
characteristic curve toward lower temperatures.
Fig. 3. Influence of affection ISO on course of
dependence t f(r), S 1/60 s, A 4 12
5. Methodology of measurements performed using a
thermovision camera A thermovision camera
converts the temperature radiation coming from
the object being observed into an electronic
signal. Thermovision apparatus makes it possible
to conduct the scientific observation and
examination of thermostatic and thermokinetic
variations and enables the contact-less
measurement of temperature. Thermovision finds
application in the assessment of the construction
of furnaces and heaters, the monitoring of charge
heating and cooling processes, the determination
of heat losses and the examination of heating
element operation 13. However, in order to make
a correct temperature measurement with a
thermovision camera, the emissivity, e, of the
material being examined will have to be
determined. This method is expensive, which
substantially limits its application.
Nevertheless, it will be irreplaceable in the
observation of both static and dynamic surface
temperature distributions.The termographs example
is illustrated in Fig. 4.
Conclusions The analysis of the test and
calculation results has shown that the obtained
results of temperature determination using the
digital camera little differ from the reference
temperatures. The average measuring error within
the entire temperature range for the use of
digital photopyrometry is larger only by approx.
0.2 compared to thermovision. Above 800?C, on
the other hand, the error obtained with the use
of photopyrometry does not exceed 1. Moreover, a
prerequisite for the use of thermovision is the
correct determination of the emissivity of the
material examined. The incorrectly selected value
of emissivity causes measuring errors reaching up
to several hundred degrees. Thus, digital
photopyrometry is an accurate method of measuring
steel charge temperature, being, at the same
time, much cheaper than thermovision. Digital
photopyrometry has a potential to be a major
competitor for thermovision among the methods
used for industrial temperature measurements.
Fig.4. The termographs of sample in two
temperatures a) 700ºC and b) 1100ºC
6. The measuring stand Tests were carried out on
a round steel specimen. The specimen was heated
up directly in the chamber of an electric-gas
oven. The reference specimen temperature was
measured using an Ni-CrNi thermocouple and red
out in stationary heat flow conditions, with the
simultaneous photographic recording of the image
of the specimen being examined. The distance of
the camera from the test specimen was approx.
1.5m. A reflex camera by Olympus was used for
the tests. The technical specification of the
camera is as follows ISO 80, 160, 320 S 1/60
s, 1/125 s, 1/250 s A4 12. For comparison,
the recording of test specimen temperature was
also done using a thermovision camera. The
distance of the thermovision camera from the test
specimen was approx. 1.5m, similarly as for the
photographic camera. The emissivity, on the other
hand, was measured by adjusting the temperature
measured with the camera to the temperature value
indicated by the thermocouple. The measured e
value was e0.9.