1、Advanced TemperatureMeasurement and ControlSecond EditionAdvanced TemperatureMeasurement and ControlSecond EditionGregory K. McMillanNoticeThe information presented in this publication is for the general education of the reader. Because neither the author(s) nor the publisher has any control over th
2、e use of the information by the reader, both the author(s) and the publisher disclaim any and all liability of any kind arising out of such use. The reader is expected to exercise sound professional judgment in using any of the information presented in a particular application.Additionally, neither
3、the author(s) nor the publisher has investigated or considered the effect of any patents on the ability of the reader to use any of the information in a particular application. The reader is responsible for reviewing any possible patents that may affect any particular use of the information presente
4、d.Any references to commercial products in the work are cited as examples only. Neither the author(s) nor the publisher endorses any referenced commercial product. Any trademarks or tradenames referenced belong to the respective owner of the mark or name. Neither the author(s) nor the publisher make
5、s any representation regarding the availability of any referenced commercial product at any time. The manufacturers instructions on use of any commercial product must be followed at all times, even if in conflict with the information in this publication.Copyright 2011 International Society of Automa
6、tionAll rights reserved. 67 Alexander DriveP.O. Box 12277Research Triangle Park, NC 27709Printed in the United States of America. 10 9 8 7 6 5 4 3 2ISBN 978-1-936007-38-7No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form orby any means, electronic, mecha
7、nical, photocopying, recording or otherwise, without the prior writ-ten permission of the publisher.Library of Congress Cataloging-in-Publication Data in processvContentsPreface ixChapter 1 Temperature Measurement 11-1. Introduction and Overview 11-2. Thermocouples and RTDs 121-3. Selection of a Tem
8、perature Sensor 201-4. Specifications 211-5. Setup and Calibration 231-6. Installation 231-7. Maintenance 33Exercises 35References 35Chapter 2 Measurement Error 372-1. Heat Conduction Error 372-2. Radiation Error 462-3. Dynamic Error 482-4. Velocity Error 512-5. Electronic Error 522-6. Sensor Error
9、562-7. Nonlinearity Error 612-8. Decalibration Error 632-9. Insulation Error 652-10. Leadwire Error 662-11. Error Accumulation 682-12. New Sensors 70Exercises 74References 75Chapter 3 Basic Feedback Control 793-1. Introduction 793-2. PID Modes, Structure, and Form 82vi Advanced Temperature Measureme
10、nt and Control3-3. PID Tuning 943-4. Adaptive Control 1093-5. Set-Point Response Optimization 113Exercises 119References 119Chapter 4 Process Dynamics 1214-1. Introduction 1214-2. Performance Limits 1224-3. Self-Regulating Processes 1324-4. Integrating Processes 137Exercises 141References 141Chapter
11、 5 Exchangers 1435-1. Process and Equipment Design Considerations 1435-2. Disturbances and Difficulties 1475-3. Effect of Loop Performance 1485-4 Controller Tuning 1495-5. Control Errors 1525-6. Control Strategies 156Exercises 157References 158Chapter 6 Reactors 1596-1. Process and Equipment Design
12、Considerations 1596-2. Disturbances and Difficulties 1606-3. Loop Performance 1636-4. Controller Tuning 1646-5. Control Errors 1706-6. Control Strategies 171Exercises 177References 178Chapter 7 Columns 1797-1. Process and Equipment Design Considerations 1797-2. Disturbances and Difficulties 1817-3.
13、Effect of Loop Performance 1837-4. Controller Tuning 183Contents vii7-5. Control Errors 1847-6. Control Strategies 185Exercises 191References 192Chapter 8 Vessels, Desuperheaters, Dryers, Kilns, Calciners, Crystallizers, Extruders, Chambers, and Rooms 1938-1. Process and Equipment Design Considerati
14、ons 1938-2. Disturbances and Difficulties 1958-3. Effect of Loop Performance 1968-4. Controller Tuning 1978-5. Control Strategies 197Exercises 206References 206Chapter 9 Wireless 2079-1. Introduction and Overview 2079-2. Principles 2169-3. Security and Reliability 2209-4. Communication Rules 2249-5.
15、 Process Control 2389-6. Installation of a Wireless Network 251Exercises 253References 254Appendix A Suggested Readings and Study Materials 257Appendix B Solutions to Exercises 259Appendix C Unification of Controller Tuning Relationships 271Appendix D Physical Property Data 281Appendix E Emissivitie
16、s 285Appendix F First Principle Process Gains, Dead Times, and Time Constants 287Appendix G FORTRAN Subroutine for Dynamic Simulation ofExtruders 301viii Advanced Temperature Measurement and ControlAppendix H Convective Heat Transfer Coefficients 307Appendix I Implementation Checklist for Best Perfo
17、rmance 311Index 315ixPrefaceTemperature is one of the four most common types of loops. While the other com-mon loops (flow, level, pressure) occur more often, temperature loops are gener-ally more difficult and important. It is the single most frequently stated type of loop of interest to users, and
18、 the concern for better control extends to the widest variety of industries.Temperature is a critical condition for reaction, fermentation, combustion, drying, calcination, crystallization, extrusion, or degradation rate and is an inference of a column tray concentration in the process industries. T
19、ight temperature control translates to lower defects and greater yields during seeding, crystal pulling, and rapid thermal processing of silicon wafers for the semiconductor industry.For boilers, temperature is important for water and air preheat, fuel oil viscosity, and steam superheat control. For
20、 incinerators, an optimum temperature often exists in terms of ensured destruction of hazardous compounds and minimum energy cost. For heat transfer fluids such as cooling tower, chilled water, brine, or Therminol, good temperature control minimizes upsets to the users.Temperature control in cold ro
21、oms is used to reduce the contamination and deg-radation rate in pharmaceutical, biochemical, beverage, and food research and production.Temperature control in plant growth chambers is important for studying the effects of hybridization, genetic engineering, and plant growth regulators.The great imp
22、act of temperature control on conductivity and pH loops was not widely realized until recently. For aqueous solutions of acids, bases, and salts, the inferred concentration from conductivity and the pH of the solution often changes at the rate of about twenty percent and four tenths of a pH unit, re
23、spec-tively, per ten degrees centigrade. These changes, which reflect a fundamental alteration in the solution due to different ion dissociation and mobility, should not be confused with temperature effects on the signal developed by the elec-trode, which are corrected for by conventional temperatur
24、e compensators. Thus, x Advanced Temperature Measurement and Controltemperature control can be important for raw, cooling tower, deaerator, boiler, and wastewater treatment.Good temperature control is important during the research, reaction, separation, processing, and storage of products and feeds
25、and is thus a key to product qual-ity. It is also of importance for environmental control and energy conservation.Tight temperature control can extend the life of process equipment (e.g., reactor glass lining, scrubber fiberglass trays, or furnace firebrick) by prevention of excur-sions beyond the t
26、emperature rating. Abrupt changes in coolant or steam flow can shock equipment and upset other utility users. Thus, it is also important to monitor the controller output and use methods (e.g., set point velocity limits and split-range, criss-cross prevention logic) to prevent rapid changes or oscill
27、ations. Since temperature control is typically achieved by the direct or indirect manipula-tion of heat flow into or out of the system, a reduction in the overshoot and oscil-lation of temperature loops can also correspond to a decrease in energy consumption.Curiously, the slowness of the response o
28、f the temperature process is the biggest source of problems and opportunities for tight temperature control. The slowness makes it difficult to tune the controller because the persistence and patience required to obtain a good open- or closed-loop test exceeds the capability of most humans. At the s
29、ame time, this slowness, in terms of a large major process time constant, enables gain settings larger than those permissible in other types of loop except for level.The nonlinearity of the process further aggravates the tuning problem. The dependence of the process gain on operating conditions and
30、load has been discussed but not simplified and quantified in enough detail to facilitate online compensation.The slowness of the response of the thermocouple or resistance temperature detector (RTD) in a thermowell slows down the ability of the controller to identify and react to upsets and consider
31、ably affects all of the tuning settings (i.e., gain, reset, and rate) of many temperature loops.Once a properly implemented temperature loop is correctly tuned, the control error is often less than the tolerance (error limits) of the sensor. If one considers that the accumulated error of an installe
32、d thermocouple or RTD system can be about five times larger than the error limits of the sensor, one realizes that system measurement error seriously limits temperature loop performance.Preface xiThe major contributors to the measurement system error for thermocouples and RTDs will be detailed and n
33、ew sensor technologies will be identified in Chapter 2. The user can in many cases reduce the error significantly by modification of the installation or by the use of a breakthrough in technology.The temperature control error can be less than the system measurement error for large back mixed volumes
34、 (e.g., vessels and columns) if the controller gain is maximized to take advantage of the large process time constant. For volumes without appreciable back mixing (e.g., desuperheaters, exchangers, and extruders), the process dead time exceeds the process time constant. The controller gain is smalle
35、r and consequentially the control errors are larger for these dead time dominant loops.11Temperature MeasurementLearning ObjectivesA. Appreciate the importance of temperature measurements.B. Understand the relative performance of thermocouples and resistance temperature detectors.C. Gain an overview
36、 of optical pyrometer performance.D. Learn about the latest technological advances in transmitters and sensors.E. Find out how to select specific sensor and thermowell designs.F. Understand important aspects of extension wire effects. G. Gain some insight into best transmitter and communication opti
37、ons.H. Learn the basic thermowell location and installation requirements. I. Learn terminology and issues to intelligently discuss industrial applications.1-1. Introduction and OverviewTemperature is a measure of a materials internal molecular activity. As the level of molecular activity rises, the
38、temperature of the material increases. Hot and cold are subjective, qualitative descriptions of a change in molecu-lar activity. Temperature is often the most important of the common measurements because it is an indicator of process stream composition and product qual-ity. It would be nice if we ha
39、d online analyzers throughout the process but 2 Advanced Temperature Measurement and Controlthe fact of the matter is that most plants have infrequent, offline lab analy-sis at best. In the chemical industry, nearly all loops that are controlling the composition of a unit operation use temperature a
40、s the primary con-trolled variable. Even when online or at-line analyzers exist, these are usu-ally relegated to monitoring and the manual trim or optimization of temperature set points by supervisory or model predictive control. Tem-perature measurements are also essential for equipment protection
41、and performance monitoring. Some examples of common unit operations that have a critical dependence upon tight (minimum variability) temperature control are: Bioreactors and fermentors Calciners and kilnsChemical reactors Columns (e.g., absorber, distillation, stripper)Condensers CrystallizersDryers
42、EvaporatorsExtruders Furnaces Superheaters and desuperheatersVaporizersOver the years, the need in the process industry for more consistent and accurate ways to describe temperature led to the invention of tempera-ture-measuring devices, or sensors. Sensors use standard, universally rec-ognized temp
43、erature scales. Because these scales rely on fixed points in nature (e.g., freezing point of water), they provide a way to describe tem-perature that is both objective and quantitative. The four temperature scales in use today are Fahrenheit, Celsius (also called Centigrade), Kelvin, and Rankine. In
44、 commercial applications, Fahrenheit and Celsius are the most commonly used scales. In industrial environments, high process temperatures, pressures, and vibration make it necessary to have a robust temperature sensor. Fast response time, accuracy, and stability are also needed. While several types
45、of temperature sensors are available, such as thermistors, infrared pyrom-eters, fiber optic, and others, the two most commonly used in the process measurement industry are resistance temperature detectors (RTDs) and thermocouples (TCs).1 Temperature Measurement 3Comparison of Thermocouples and Resi
46、stance Temperature DetectorsIn the process industry as a whole, 99% or more of the temperature loops use thermocouples (TCs) or resistance temperature detectors (RTD). The RTD provides sensitivity (minimum detectable change in temperature), repeatability, and drift that are an order of magnitude bet
47、ter than the ther-mocouple, as shown in Table 1-1 1, 2. Sensitivity and repeatability are 2 of the 3 most important components of accuracy. The other most impor-tant component, resolution, is set by the transmitter. Drift is important for extending the time between calibrations. The data in this tab
48、le dates back to the 1970s and consequently doesnt include the improvements made in thermocouple sensing element technology and premium versus standard grades. However, the differences are so dramatic that the message is still the same. A Resistance Temperature Detector (RTD) has a much better sensi
49、tivity and repeatability, a lower and more predictable drift, and a higher signal level than a thermocouple (TC).Table 1-1 includes data on thermistors, which have seen limited use in the process industry despite their extreme sensitivity and fast (millisecond) response, primarily because of their lack of chemical and electrical stabil-ity. Thermistors are also highly nonlinear but this can be addressed by smart instrumentation.For bare sensing elements, thermistors are much faster-responding than thermocouples, which are slightly faster than RTDs. Thi
