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Chapter Eleven

Temperature Control

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Introduction

• Controlling temperature is one of the most

common processes in industrial electronics

and manufacturing.

• Manufacturing processes that are affected

 by temperature are referred to as thermal 

 systems. 

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Fundamentals of Temperature

• Molecular motion creates heat known as

thermal energy

• Thermal movement from hot to cold is

called thermodynamics

• Absolute zero (no molecular motion) means

no heat is produced

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Thermal Control Systems

• Thermal systems supply heat energy from a

source, provide a path for distribution, and convertthe energy into some form of work.

• Temperature in a Thermal Control System is

controlled either manually or automatically.

• Two types of control systems used are:

 –  ON/OFF

 –  Proportional

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Thermodynamic Transfer 

• Transfer of heat through a

solid is calledconductance. 

• Transfer of heat through

fluids (liquids and gases)

is called convection.• Heat transfer may also

occur by radiation, as is

the case in a vacuum.

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Thermodynamic Transfer Examples

• Convection•

Radiation

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Thermal Energy Sources

• Industrial Furnaces

 –  Combustion Furnace• Electric Furnaces

 –  Arc Furnaces

 –  Resistance Furnaces

 –  Induction Furnaces

• Cooling Systems

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Temperature Measurements• Reasons to monitor temperature:

 –  Control chemical reactions

 –  Prevent over-temperature conditions

 –  Maximize fuel efficiency

• Temperature Scales –  Fahrenheit Scale

 –  Celsius Scale

• Thermal Energy Work Units –  BTU - British Thermal Unit

 –  Calorie

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Temperature Indicating Devices

• Crayons - melt at specific temperatures

• Paints - turn glossy at predetermined

temperature

• Pellets - same principle as crayons; used

when temperature is observed from a distance

• Labels - change color based upon temperature

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Temperature Indicating Devices

•Liquid Crystal Indicator 

• Liquid Filled Thermometers

 – Glass thermometer 

 – Filled-bulb thermometer • Bimetallic Thermometers

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Temperature Indicating Devices

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Electronic Sensors

•

Two main types:  – Thermoresistive - change resistance as the

temperature varies

 – Thermocouples - produce a voltage

 proportional to the change in temperature

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Thermistors

• Thermistor - thermal resistor 

• Exhibits large change in electrical resistancewhen subjected to small amount of 

temperature change

• Exhibits negative temperature coefficient   – Resistance is inversely proportional to

temperature

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How Thermistors Are Used• Temperature measurement

• Temperature compensation

• Surge Suppression

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Resistance TemperatureDetectors

• Metals have a positive temperature

coefficient 

• RTD’s use: 

 –  Nickel - more sensitive to temperature changes

 – Platinum - wider range of temperatures

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RTD Applications

• Overcurrent protection

• Motor Starting

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Thermocouples

• Dissimilar wires joined at both ends.

• EMF is developed based upon the Seebeck effect.• Converts heat into voltage.

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Temperature Sensor Comparison

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Radiation Pyrometry•  Noninvasive temperature detection

• Uses a pyrometer to detect temperature• Useful where conventional sensors may not be

employed

 –  Moving objects

 –  Extremely hot temperatures –  Non-contact measurement required

 –  Corrosive or hazardous environments

 –  Where temperatures must be taken from a distance

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Categories of Pyrometers

• Broadband

• Optical Detector 

• Ratio Pyrometer 

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Pyrometer Illustrations

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Pyrometer Illustrations