Preface xi

1.1 The Importance of Temperature Measurement/1

1.2 Why Use Radiation Thermometry?/3

2.1 Introduction/5

2.2 Heat Transfer/5

2.2.1 Conduction/6

2.2.2 Convection/6

2.2.3 Radiation/7

2.2.4 Combined effects/8

2.3 Blackbody Radiation/9

2.3.1 Planck's law/10

2.3.2 Wien's displacement law/12

2.3.3 Approximations to Planck's law/12

2.3.4 Total radiation and the Stefan-Boltzmann law/14

2.4 Emissivity/15

2.5 Reflected Radiation/16

2.5.1 Bi-directional reflectance/17

2.6 Absorption, Emission, and Scattering of Radiation/20

3.1 Introduction/23

3.2 Spectral-Band Thermometers/24

3.2.1 Spectral responsivity/24

3.2.2 Output signal/25

3.2.3 Monochromatic approximation/26

3.2.4 Radiance temperature/28

3.2.5 Instrumental emissivity/28

3.3 The Gold-Cup Pyrometer/29

3.4 Thermal Imagers/33

3.5 The Laser Pyrometer/34

3.6 Ratio Thermometers/35

3.7 Multi-Wavelength Thermometers/36

4.1 Introduction/39

4.2 Emissivity Errors/39

4.2.1 Emissivity versus surface finish/40

4.2.2 Emissivity versus wavelength/41

4.2.3 Emissivity versus angle/42

4.2.4 Measuring emissivity/43

4.2.5 Temperature error due to emissivity error/46

4.3 Reflection Errors/47

4.3.1 The measurement equation/48

4.3.2 Strategy 1 (Tw << Ts)/50

4.3.3 Strategy 2 (Tw ? Ts)/51

4.3.4 Strategy 3 (Tw >> Ts)/52

4.3.5 Reflection error nomograms/55

4.3.6 Multiple tubes/57

4.3.7 Effective background temperature/59

4.3.8 Calculating the geometric view factors/61

4.4 Absorption and Emission Errors/62

4.4.1 Effective absorption coefficient/63

4.4.2 Analogy with reflection errors/64

4.4.3 Correcting absorption and emission errors/65

4.4.4 Multiple corrections/67

4.5 Errors Due to Flames/70

4.6 Scattering Errors/71

4.7 Viewing through Windows/72

4.8 Size-of-Source Effect/74

4.8.1 Scattering/75

4.8.2 Poor focus and diffraction/76

4.8.3 Misalignment/77

4.9 Vignetting/77

4.10 Ambient Temperature Dependence/78

4.11 Signal Linearization/79

5.1 Introduction/81

5.2 The Meaning of Uncertainty/82

5.3 Combining and Propagating Uncertainties/84

5.3.1 Combining uncertainties/84

5.3.2 Propagating uncertainties/85

5.4 Uncertainty in the Corrected Temperature/88

5.4.1 Target temperature uncertainty/88

5.4.2 Emissivity uncertainty/92

5.4.3 Effective background temperature uncertainty/94

5.4.4 Combined uncertainty/96

5.4.5 Optimum wavelength/99

6.1 Introduction/101

6.2 Blackbody Cavities/102

6.2.1 Effective emissivity/102

6.2.2 Temperature uniformity/104

6.3 Calibration Methods/105

6.3.1 Calibration procedure/106

6.3.2 Calibration example/109

7.1 Introduction/113

7.1.1 Measurement procedure/113

7.1.2 Data analysis procedure/116

7.2 Top-Fired Multi-Row Box Furnaces/117

7.2.1 Measurements/118

7.2.2 Analysis/121

7.3 Side-Fired Single-Row Furnaces/126

7.3.1 Measurements/126

7.3.2 Analysis/129

7.4 Floor-Fired Cubic Furnaces/131

7.4.1 Measurements/133

7.4.2 Analysis/134

7.5 Floor-Fired Cylindrical Furnaces/135

7.5.1 Measurements/137

7.5.2 Analysis/139

7.6 Furnaces with Horizontal Tubes/141

7.6.1 Measurements/142

7.6.2 Analysis/143

References/149

1 um thermometer, 750 °C to 1050 °C/151

1 um thermometer, 800 °C to 1000 °C/152

1 um thermometer, 750 °C to 950 °C/153

3.9 um thermometer, 750 °C to 1050 °C/154

3.9 um thermometer, 400 °C to 800 °C/155

3.9 um thermometer, 500 °C to 700 °C/156

Index/157

Preface

More than a decade ago, I embarked on a research program in the field of radiation thermometry. As a recent Ph.D. graduate, I was (and still am) employed by the Measurement Standards Laboratory of New Zealand, New Zealand's national measurement institute. But rather than just being confined to the relative safety of a calibration laboratory, I was given the task of seeking solutions to industrial radiation thermometry problems 'in the real world,' particularly in the petrochemical industry. After visiting several plants, it soon became apparent that there was a lack of practical knowledge in the industry on how radiation thermometers should be used, and that most of the advice being followed was of an anecdotal nature. I was frequently met with the most fundamental of questions: 'What emissivity setting should we use?' Over the years, the questions have become more difficult and my research activities more interesting and in-depth. The knowledge I have accumulated through a combination of theoretical modeling and practical experience forms the basis of this book.

The aim of this book is simple: to provide the means for petrochemical engineers to confidently assess the temperatures of their process tubes, whether they are in the methanol industry, the ammonia industry, the refining industry, or any industry utilizing fired furnaces for the heating of process tubes. This book is an introduction to the use of radiation thermometers for achieving this aim.

Radiation thermometry has been applied in the petrochemical industry for many decades, and while recognized as being the only viable method to measure temperatures in the harsh environment of flame exhausts, its credibility has suffered due to its many, sometimes puzzling, sources of error. In fact, some past researchers have stated that some of the problems associated with these errors are insoluble. I beg to differ. By understanding the nature and origin of the errors it is possible to unravel them from the radiation thermometer measurements to yield an estimate of both the true temperature and its all-important uncertainty.

The book is organized as follows. A brief introduction outlining the need for temperature measurement in industrial furnaces is given in Chapter 1. Chapter 2 then introduces the physical principles that are important to gaining an understanding of how radiation thermometers work, including the essential concept of blackbody radiation. Chapter 3 discusses a number of different types of radiation thermometer that are used in industrial furnaces. This chapter largely focuses on spectral-band thermometers as these are the most commonly used thermometers. The errors associated with the use of spectral-band thermometers are analyzed in Chapter 4 along with techniques or measurement strategies that can be used to minimize or eliminate the errors. Anyone serious about their temperature measurements should be equally serious about estimating the size of the uncertainty associated with their measurements, as the uncertainty provides a measure of the quality of the measurement. Methods for assessing uncertainties are given in Chapter 5. Calibration of radiation thermometers is discussed in Chapter 6, and a procedure is provided for those who wish to carry out their own in-house calibrations. Finally, in Chapter 7, measurement and analysis procedures are presented for carrying out a full furnace tube temperature survey. These procedures are demonstrated through a series of detailed examples for a number of different furnace geometries and operating conditions. These examples are derived from real-life plants, with the details changed somewhat to protect the confidentiality of the original results.

I would like to express thanks to my colleague and long-time mentor, Rod White, for his constant encouragement and support for this book, for his insightful contributions to the development of many of the ideas that this book encompasses, and for his critical reviewing of the manuscript.

Peter Saunders
December, 2006