Infrared thermography 230 pages ebook – electrical and industrial applications PDF

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This is a limited preview of the book. You can buy the whole 226 pages book at: http://www.saige.it/Details.aspx?id=44 SUMMARY 1. THE PHYSICAL LAWS OF REFERENCE _____________________________________________ 4 1.1 Heat and temperature __________________________________________________________ 4 1.2 C...

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Infrared thermography 230 pages ebook – electrical and industrial applications Davide Lanzoni

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Building t hermography - including blower door and heat flux met er Davide Lanzoni TABLE OF CONT ENT S I N M Sect ion E A S U R E M E N T A N D C O N VOLUME 1—NON-CONTACT T EMPE… José Luis Roque COMMON SENSE APPROACH T O T HERMAL IMAGING BBlf ISM LIBRAW ~OCUMENT SUPPLY CENT RE , 2 … Cihan Köseoğlu

This is a limited preview of the book. You can buy the whole 226 pages book at:

http://www.saige.it/Details.aspx?id=44

SUMMARY

1.

2.

3.

THE PHYSICAL LAWS OF REFERENCE _____________________________________________ 4 1.1

Heat and temperature __________________________________________________________ 4

1.2

Conduction: Fourier's law ________________________________________________________ 4

1.3

Convection: Newton's law _______________________________________________________ 6

1.4

Specific heat, latent heat, thermal capacity, thermal diffusivity _________________________ 8

1.5

Radiation and Stefan-Boltzmanns' law ____________________________________________ 10

1.6

The photon and the electromagnetic spectrum _____________________________________ 11

1.7

Transmissivity into the atmosphere ______________________________________________ 13

1.8

Planck's law _________________________________________________________________ 15

1.9

Wien's law___________________________________________________________________ 16

1.10

Stefan-Boltzman's law _________________________________________________________ 17

1.11

The black-body, the grey-body and the real-body ___________________________________ 18

1.12

Kirchoff's law: absorbance, emittance, reflectance and transmittance ___________________ 19

1.13

The radiance measured ________________________________________________________ 22

1.14

Emissivity ___________________________________________________________________ 24

1.15

Absorptivity _________________________________________________________________ 32

1.16

Transmissivity ________________________________________________________________ 33

1.17

Lambert's cosine law __________________________________________________________ 35

1.18

Materials with specular and diffuse reflectivity and measurements _____________________ 38

TECHNOLOGY AND SPECIFICATIONS OF THERMAL IMAGING CAMERAS _______________ 39 2.1

Operation of infrared cameras __________________________________________________ 39

2.2

How to choose a thermal imaging camera _________________________________________ 48

2.3

Subjective and complementary characteristics: example of a thermal imaging camera. _____ 50

2.4

Possibilities offered by the image processing software _______________________________ 53

2.5

Spectral band ________________________________________________________________ 58

2.6

Temperature measurement range (RANGE) ________________________________________ 58

2.7

Geometric and optical parameters (FOV, AFOV, IFOV) ________________________________ 59

2.8

Slit response function (SRF) _____________________________________________________ 63

2.9

Choice of the FOV and of the IFOV ________________________________________________ 67

2.10

Thermal sensitivity (NETD) ______________________________________________________ 67

2.11

Spatial frequency, MRTD, MDTD _________________________________________________ 70

2.12

Influence of ambient temperature on the measurement ______________________________ 73

2.13

Saving and data capture: what can't be changed____________________________________ 74

2.14

How to obtain a good thermographic image _______________________________________ 74

THERMOGRAPHIC MEASUREMENT ____________________________________________ 77

1

4.

5.

3.1

Measurement of emissivity _____________________________________________________ 77

3.2

Techniques for measurement of reflected temperature _______________________________ 84

3.3

Quantifying the uncertainty of measurement_______________________________________ 91

3.4

Measurement of transmissivity __________________________________________________ 92

3.5

Problems relating to measurements through infrared windows ________________________ 98

3.6

Reliability of the measurement _________________________________________________ 104

3.7

Common errors and prejudices _________________________________________________ 106

3.8

The importance of the magnitudes of influence ____________________________________ 108

THERMOGRAPHY APPLIED TO ELECTRICAL YSTEMS ______________________________ 111 4.1

Definitions __________________________________________________________________ 111

4.2

Conditions for thermographic surveys on electrical systems __________________________ 112

4.3

Typical thermal anomalies in electrical installations ________________________________ 117

4.4

Transformers________________________________________________________________ 125

4.5

Thermographic surveys on transformers __________________________________________ 130

4.6

Thermographic surveys in transformation stations _________________________________ 133

4.7

Thermographic surveys on capacitors ____________________________________________ 144

4.8

Criteria for assessing the seriousness of anomalies _________________________________ 149

4.9

Electric motors ______________________________________________________________ 159

4.10

Thermographic check of photovoltaic systems _____________________________________ 171

4.11

Thermographic surveys on batteries and accumulators ______________________________ 179

4.12

The conduct of the thermographer in terms of electrical safety _______________________ 181

4.13

The calculation of savings achieved as a result of thermographic surveys _______________ 183

THERMOGRAPHY APPLIED TO INDUSTRIAL SYSTEMS _____________________________ 187 5.1

Measurement of the temperature of heat exchanger pipes in furnaces _________________ 187

5.2

Gas leak detection ___________________________________________________________ 195

5.3

The use of filters to measure thin plastic __________________________________________ 196

5.4

Moisture control in paper mills _________________________________________________ 199

5.5

Refractory control____________________________________________________________ 200

5.6

Controlling the levels in silos and tanks __________________________________________ 202

5.7

Checking of steam traps _______________________________________________________ 206

5.8

Research of corrosion under insulation ___________________________________________ 212

5.9

Temperature control on moulds ________________________________________________ 215

5.10

Control of fibreglass boats _____________________________________________________ 216

5.11

Various applications __________________________________________________________ 222

2

1. THE PHYSICAL LAWS OF REFERENCE This is a limited preview of the book. You can buy the whole 226 pages book at:

http://www.saige.it/Details.aspx?id=44 1.1

Heat and temperature

Heat is a form of energy that is transferred between two bodies, or between two parts of the same body, that are found in different thermal conditions. Heat is therefore energy in transit then: it always flows from the points at higher temperature to those at a lower temperature, until a thermal equilibrium is reached, that is, until the two bodies reach the same temperature. Heat is measured in joules (J). Temperature, however, is an index of molecular agitation.

1.2

Conduction: Fourier's law

Heat conduction occurs between bodies at different temperatures, in direct contact with each other (fig. 1.1). The ability of a means to enable heat to flow is determined by its thermal conductivity k. The heat flows from high to low temperature The thermal resistance (R Thermal) is given by the unit of distance (L) divided by the thermal conductivity (K) (R Thermal = (T1 – T2)A/Q = L/K) (R Electric = V1 – V2/I)

Fig.1.1: heat conduction in a solid

Thermal conduction follows Fourier's Law: Φ=

Q k ∗ ( T i − T e) = A L

where: • Q/A is the heat flow [W/m2]; • k (or λ) Is the thermal conductivity [W/(m °C)]; • L is the thickness of the material relative to the direction of heat [m]; • Ti and Te are the inside and outside surface temperatures [°C or K], for example, if the body conducts heat from the inside warm surface to the outside cold surface. 4

A body is considered a good conductor when the thermal conductivity k is high. Air is a good insulator while water is not. Therefore if an insulation material that covers a wall absorbs water, its conductivity increases, increasing heat dispersions. The best thermal conductors are metals. The table in fig. 1.2 shows a number 2

Material

λ at 20°C (W/m Material °C) Steel with 5% Ni 29 Cast iron Steel with 30% Ni 105 Graphite Water (still liquid at 20°C) 0.63 Granite Heavy water 10÷100°C 0.56÷0,65 Lime plaster and gypsum Aluminum 206 Glass wool Still air 0.03 Spruce and pine wood Silver 420 Oak wood Asphalt 0.64 Linoleum Dry concrete 0.81 Marble Wet concrete 1.39 Solid dry bricks Cardboard 0.14÷0,23 Hollow dry bricks Plasterboard panels 0.21 Stone masonry Natural rubber 0.13÷0,23 Sandstone Celluloid 0.35 Compact limestone Compressed cellulose 0.24 Polystyrene Powdered cement 0.070 Dry sand Electrolytic iron 87 Wet sand Gypsum 0.39 Expanded cork Ice 2.20÷2.50 Common glass Fig. 1.2 - thermal conductivity (k) values of common materials

λ at 20°C (W/m °C) 50 4.9 3.13÷4,06 0.81 0.04÷0,05 0.13÷0,16 0.18 0.19 2.1÷3,5 0.46÷0,7 0.35÷0,81 1.39÷2,9 1.28÷1,74 0.7 0.03 0.32 1.16÷1,74 0.04 1÷2

2

Length L of the path of heat through the material influences the flow of heat: good thermal insulation is not therefore simply achieved with a material with a low k but also with the consistent thicknesses: the lower the heat dispersion the higher the thickness of the material. In summary: • the ratio k/L is defined thermal conductance and is measured in W·°C−1·m−2; •

the ratio L/k is defined thermal resistance R of the material, measured in W-1·°C*m 2;

•

total transmittance U, measured in W·°C-1·m−2, is the sum of the inverse of thermal resistance R with the superficial conductance resistances αi and αe (see paragraph 1.2.2).

EXAMPLE: 5 kW of heat are conducted by a wall which is 515° C through a surface of area 10 m², thickness 10 cm and thermal conductivity 0,3 W/mK. What is the temperature on the other side of the wall? Q = deltaT/R ⇒ delta T = Q*R = Q (L/kS) = 5000 Watt (0,1 m/(0,3 Watt*m*-1°C-1*10 m2)) = 166,7 °C Tcold = Twarm - deltaT = (515-166,7)°C = 348,3°C If a wall is composed of several layers of materials with different thermal conductivities, k1, k2, ..., kn having respective thicknesses s1, s2, ... sn, its total transmittance is: U = 1/Rtot = 1/(1/αi + s1/k1 + s2/k2 + …. sn/kn + 1/αe)sn/kn + 1/αe)

5

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Fig. 1.25 – polar diagram of emissivity for non-metallic materials

For metals, the angular variation of emissivity can show the opposite behaviour, but with more limited variations and contained in the low values of these materials: note that in fig. 1.26 the diagram scale is different from the previous one. For example the emissivity of polished chrome (used for radiation screens) increases with the angle of emission, passing from 0.04 for an angle perpendicular to 0.14 to 80° from the normal to the surface.

Nickel - shiny Nickel - opaque

Fig. 1.26 – polar diagram of emissivity for polished metals

This may mean that within the same thermographic image, the points, shot with a lower angle, appear at temperatures other than those taken with a wider angle. In the example of the image in fig. 1.27, the surface of photovoltaic panels is warmer than the environment and the emissivity of their glass surface• falls with an increase in the angle of shooting (as for building and insulating materials), lower panels appears hotter than upper panels because their emissivity is greater (the angle between the shooting direction B of upper panels is greater than shooting of lower panels A). This creates the diversity of apparent temperature in photovoltaic modules of the image in fig. 1.27, with upper modules which seem colder than lower ones.

27

Figure 1.27 – points A and B are in the same image with different angle with respect to the perpendicular

Another factor that influences emissivity is the shape of the material: holes and concave forms cause multiple reflections of the radiation inside them and therefore an increase of emissivity. See images 1.28 in the image left, the body is warmer than the environment, in the image to the right it is cooler, but because of its low emissivity, the surface of the body appears in both cases to be at ambient temperature. The holes instead, because of their greater emissivity, better "reveal" the real body temperature. Note that the deeper holes have greater emissivity than those that are less deep.

Figure 1.28: influence of shape on emissivity – Images courtesy of Reidar Gustafsson

28

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3. THERMOGRAPHIC MEASUREMENT As the thermal imaging camera measures a radiance Em which is the sum of the energy emitted (Ee), reflected (Er ) and eventually transmitted (Et) by the object (if it is not opaque), to determine the temperature of the object it is necessary (paragraph 2.1.4): •

to subtract from the total energy measured (Em) parts Er and Et to obtain the sole energy emitted, which is the only one able to provide information on the actual temperature of the object: Ee = Em – Er – Et

•

to divide the energy emitted for the emissivity ε of the surface of the object in order to understand which energy a black-body emits (Ecn) that is at the same temperature of the object (and within the same range of wavelength of the thermal imaging camera, where the real object is assumed to be grey):

Ecn = Ee / ε Only once these steps have been addressed is the thermal imaging camera able, by using the stored calibration curve it contains, to derive the temperature of the object. The calibration curve, as it is obtained in the laboratory, refers to a black-body, which does not reflect or transmit energy and has ε = 1: for this reason, it is necessary to determine the energy emitted by a black-body at the same temperature of the object, eliminating the reflected and transmitted components.

3.1

Measurement of emissivity

There are 5 methods to evaluate emissivity: 1. using standard values (the simplest but least reliable method): - organic materials have values of between 0.85 and 0.95 approximately - metals have very variable emissivity: generally low for shiny metals and high for those that are oxidised and painted 2. using emissivity tables with values reported depending on the wavelength 3. estimating the emissivity from similar objects and searching for a representative sample of them 4. locally modifying the emissivity of the object with known emissivity finishes (good procedure), for example, insulating tape, paint or sprays (all methods that bring the emissivity to approximately ε = 0.96≈0.98) 5. measuring the emissivity using standard procedures (eg. ASTM E-1933). 77

In the following, we will examine the methods for measuring the emissivity of an object. All the methods require contact with the object and involve modification of the object's temperature to at least 10°C above ambient temperature to be more reliable (the measurement of a body that is colder than the environment is most affected by reflected radiation and is therefore more uncertain). As the emissivity also depends on the temperature (see section 1.15.3), the methods determine the emissivity of the object at the temperature at which it is brought for measurement: if in the situation of actual measurement the object is found to be at a different temperature, its emissivity may also be different. In addition, the emissivity found experimentally also depends on the make and model of the thermal imaging camera used in the test (due to the different wavelength at which several thermal imaging cameras work even if for example it involves 2 models of thermal imaging cameras operating in the LW), and also for several measurements made with the same thermal imaging camera: in fact each measurement consists of 2 measurements (of the reflected temperature and of the object), each affected by random uncertainties. In the procedures defined by the ASTM E-1933 American Standard, the material required for the test is as follows: 1. a controlled environment, with uniform radiant temperature (walls at the same temperature, for example, air-conditioned room) and without air currents that might produce convective effects 2. calibrated thermal imaging camera with the possibility of setting emissivity and reflected temperature 3. a diffuse reflector of infrared radiation (see 3.2.2) 4. a system of heating the object to at least 20°C above ambient temperature 5. a system to bring locally the emissivity of the object to a value that is higher and known, for the procedure defined in paragraph 3.1.1 6. a calibrated contact thermometer (e.g. thermocouple), for the procedure defined in paragraph 3.1.2. In the following paragraphs 3.1.1 and 3.1.2, for performing steps 3, 4 and 5 it is recommended to read paragraph 3.2.2.

3.1.1

Technique using marker of known emissivity

The method of measurement of the emissivity with the marker with known emissivity required by ASTM E-1933 includes the following steps (fig. 3.1): 1. uniformly heat the object up to a temperature of at least 20°C higher than the ambient temperature and maintain it at a constant temperature 2. position the thermal imaging camera at the desired distance from the object, focussing on the area where the emissivity of the object to be measured 3. ensure the diffuse reflector is parallel to the object (see 3.2.2) 4. set the emissivity = 1 in the thermal imaging came...