Faculty of Process & Materials Engineering and AppliedPhysics

of The Czestochowa University of Technology

The Department of Industrial Furnaces and Environmental Protection

Influence of changes objects temperature on the factor emissivity value

Mgr inż. Krystian Nowak*Dr hab. inż .Lech Szecówka, Prof. PCz

Dr inż. Henryk Radomiak

*Author is a participant of a program called „ Plan Rozwoju Politechniki Częstochowskiej” which is sponsored by European Social Found, European Union

Emissivity is one of basic bright property of matter. With temperature it characterizes radiation heat

transfer, and it plays very important role in radiation-convectional, radiation-conductional and radiation-

convection-conductional heat transfer.

Thermo visions technique

Thermo visions technique

Each body which temperature is superior than absolute zero emits heat energy in the form of thermal radiation energy in seen as well as invisible ( infrared ) range. It strictly depends on objects temperature. Thermo

vision camera taking advantage of this dependence and registering this radiation

appoints researched surface temperature.

Thermo visions technique To main adwantages of thermo vision belong:

-very wide temperature range,

-capability appointment temperature of flame and gasses,

-big accuracy of measurement

-received results give information about holistic temperature arrangement on researched surface.

Emissivity

Perfectly black body is an ideal source of thermal radiation ( temperature ). Quantity defining defection of radiation between perfectly black

body( theoretical ) and real body we call emissivity. This quantity depends (among other) from temperature.

It is possible to appoint emissivity with the aid of ration equivalent bright quantities of real and perfectly black bodies

dA

Ede

..

E.

- stream of emissivityA - field of emitting surface.

100

4.TCce

C - constans of radiation which value is 5,6693 W/(m2K4)

T - temperature

ce

e.

.

Emissivity:

Exemplary results of industrial thermo vision measurements

300.0

1076.7 °C

400

600

800

1000

FLIR Systems

Object Parameter Value

Emissivity 0.85

Reflected Temperature 1100.0 °C

Label Value

Ar1: Max 1045.3 °C

Ar1: Min 1013.2 °C

Ar1: Max - Min 32.1 °C

Ar1: Average 1030.7 °C

Ar2: Max 1029.9 °C

Ar2: Min 977.3 °C

Ar2: Max - Min 52.6 °C

Ar2: Average 996.9 °C

The presented exemplary thermogram comes from second heating zone of unclogging furnaces. Temperature measured in this zone with the aid of thermocouples was 1100oC. Exactly this temperature is accepted as parameter which defining closest surroundings. On thermogram there are singled out two fields: „ Ar1” and „ Ar2 ”, which are subject to detailed analysis.

Measuring position and software used to analyses

A sample (dimension: 30x30x50 mm) has been placed in heating chamber of electric furnace (dimension 295x105x120). It has been put on isolating pads about 10 mm high, what was suppose to minimize uneven heating by participation of heat transfer on conductive way. Sample was preliminary oxidized, however, thin coat of oxidized material strictly adhere to the remainings.

Measuring position and software used to analyses

Possible range of measuring temperature is between - 40oC +2000oC, at accuracy of measurement to 2%. Basic

element of this camera is uncooled mikrobolometric detectors matrix. Analysis has been executed in the form

of tables basing on standard software -ThermaCAM Reporter 7.0.

Heating chamber was fullfiled by nitrogen, to obtain indifferent atmosphere. It has allowed to eliminate oxidizing of farthest materials. Such phenomenon could cause to

rise of considerable unsimilarity on researched surface , and what goes behind it, differences in emissivity and registered temperatures.

Under the surface of the researched sample two thermocouples connected to measuring card DaqLab2000 have been placed, which were a comparative fix-up for thermovision camera. To determinate surrounding temperature in heating chamber of

furnace thermocouples from furnace measuring systems were taken.

Experiment

The purpose of the experiment was to determine a change emissivity coefficient influenced by temperature, for sample with

oxidized surface. Additional element is the verification of how erroneous introduction of emissivity coefficient effect accuracy on

measurement executable at utilization of thermovision camera. For the purpose of the realization comparative method was done. Measurement of temperature was executed with pin method and

lecture from thermovision camera with alternate emissivity coefficient, introduce in period of processing and analyses

thermogram.

Result and analyses

Ar1

-10.0

481.8 °C

0

100

200

300

400

FLIR SystemsObject Parameter

Value

Object Distance 0.5 m

Reflected Temperature

370.0 °C

Label Value

Ar1: Average 245.6 °C

Ar1: Emissivity 0.82

472.4

862.5 °C

500

600

700

800

FLIR Systems Object Parameter

Value

Reflected Temperature

852.0 °C

Label Value

Ar1: Average 800.1 °C

Ar1: Emissivity 0.88

Lp.T1 T2 Tśr

1

1

2

2Tot.

[oC] [oC] [oC]

1 238,63 251,07 244,85 0,81 243,90 0,82 245,60 370,00

2 300,92 302,56 301,74 0,82 301,89 0,83 302,70 370,00

3 347,59 349,86 348,73 0,84 348,10 0,85 349,00 421,00

4 399,74 399,07 399,41 0,86 399,70 0,87 400,10 435,00

5 451,77 452,64 452,21 0,86 452,00 0,87 452,60 500,00

6 500,40 499,06 499,73 0,87 499,70 0,88 500,20 537,00

7 550,83 550,66 550,75 0,87 550,40 0,88 550,80 585,00

8 599,77 600,64 600,21 0,87 600,20 0,88 600,60 638,00

9 650,71 650,13 650,42 0,90 650,40 0,91 650,90 689,00

10 700,86 701,24 701,05 0,89 700,90 0,90 701,40 739,00

11 749,89 750,01 749,95 0,90 749,60 0,91 750,00 786,00

12 800,09 801,24 800,67 0,88 800,10 0,89 800,70 852,00

T T

Result and analyses

Checked off field “Ar1” has been chosen as part subjected analysis, where for presentation from this area was chosen average value of temperature. Conditionality

was accommodation and depth that thermocouples have been fixed. Reflected temperature was accepted on base of lecture executed from regulating match of

furnace. For each measurement two emissivity coefficient was adopted, such values of temperatures was closest from average pin measurements.

0,8

0,82

0,84

0,86

0,88

0,9

0,92

200 250 300 350 400 450 500 550 600 650 700 750 800 850

Temperature on surface of sample

Em

issi

vity

val

ue Changes of emissivity

value for e1

Changes of emissivityvalue for e2

Result and analyses

200,00250,00300,00350,00400,00450,00500,00550,00600,00650,00700,00750,00800,00850,00900,00

1 2 3 4 5 6 7 8 9 10 11 12

Successive measurements

Tem

per

atu

re [

oC

]

Averege fromthermocouple number 1and 2

Souranding tempreture

Temperature for loweremissivity coeficient

Temperature for higheremissivity coeficient

300.0

992.4 °C

400

600

800

FLIR Systems

1,7

0,810,9

0,4

0,60,5

0,4 0,40,5 0,5

0,4

0,6

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

1 2 3 4 5 6 7 8 9 10 11 12

Successive measurements

Tem

per

atu

re d

iffe

ren

ce [

oC

]Result and analyses

Error in designation emissivity at the level of 0,01 can cause inaccuracy in lecture of temperature (in showed case even 1,7oC).

It was executable during farthest attempt of charge heating. Distinct, visible differences in colors on surface of sample are an effect of progressing from 800oC temperature of separate oxidize coat from the rest of researched material. Reason for this phenomenon was thermal expansibility differences between external oxidized coat and internal not oxidized.

Resume

Usefulness of thermo vision measurement in steel industry in big degree depends on measuring techniques. In order to the result was to be reliable, it is required from the person carrying measurement elementary knowledge from range of heat transfer, acquaintances of objects, researched material and certain dependences. Often indispensable condition is the execution of preliminary research allowing the assignment of basic factor effecting the measurement. It is important to pay attention to such elements:

- emissivity

- surroundings temperature

- state (condition) researched surface.

Despite certain troubles in practice of thermo vision measurements, continuous perfecting of measurement technique and obtained new practical experiences can cause generalization of these investigative methods, giving full information about temperature and schedule on researched surface of the material ( object ).

Thank you for your attention