7/31/2019 Ch11 - Temperature Control
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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
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