Transcript Temperature Controls

Temperature Controls
Andrew Blackwell
For a copy of this presentation please E-mail me at
[email protected]
This presentation mainly covers sections 9.1, 9.2, 9.3,
9.5, and 9.6
Importance of Temperature Control
• Safety – Used to prevent overheating that
can cause an unsafe work environment.
• Troubleshooting – Overheating is often
due to loose connections, improper
lubrication, or defective bearings.
• Means of processing material – The
Correct temperature must be maintained
in order to make a good product.
Makeup of Temperature Controls
1. Temperature controls are made up of two parts
a. A temperature sensing element –
Discussed in future slides.
b. A switching element – Consisting of
normally open and/or normally closed contacts.
2. The main difference among temperature
elements is the way by which the temperature
information is transferred from the sensing
element to the switching element.
Selection of Temperature
Controllers
• Temperature range available – Not all
controllers cover the entire temperature
range used in industrial control.
• Type of sensing element – Electronic,
differential expansion of metal, and
expansion of fluid, gas, or vapor.
• Response time – Rapid response
controllers are faster, but generally more
expensive.
Selection of Temperature
Controllers Continued
• Sensitivity – A relatively sensitive controller
is usually desired, however a controller
can be too sensitive for an operation.
• Operating Differential – Difference in
temperature between the make and break
point of a controllers contacts.
Electronic Temperature Controllers
(Pyrometer)
• Thermocouple (200-5000 degrees F)– Joins two
dissimilar metals at their extremities (see figure
9-2A on page 170). A temperature difference
between points T1 and T2 will create a voltage.
• Thermistor (100-600 degrees F)– A
semiconductor with decreasing resistance as
temperature increases. These are used in
combination with a voltage source and relay.
Electronic Temperature Controllers
Continued
• Resistance Temperature Detector (3001200 degrees F) – Uses resistance of a
coil to make a voltage, which is then
compared to a reference voltage.
Two Types of Pyrometer
• Millivoltmeter – Uses the small voltage
produced by the sensor and an amplifier.
When the potential reaches a certain point
the control action begins. This type of
controller is adversely effected by shock
and vibrations.
Two Types of Pyrometer Continued
• The Potentiometric Controller – The signal
from the temperature sensor is compared
to a reference temperature. It has the
disadvantage of more circuitry, but it has
these advantages:
– A. These have no moving parts
– B. Does not have to be calibrated for external resistance
– C. Not affected by shock and vibration
- For more information read section 9.3 in the textbook
Temperature Switches
(Thermostats)
• Mechanical -link (100-1500 degrees F) –
Consists of one piece of metal. The metal
contracts or expands producing
mechanical action that operates a switch.
• Fused Bimetal (40-800 degrees F) – Two
different metals fused together will deform
differently due to temperature change.
This deformation will cause mechanical
action that operates a switch.
Other Temperature Switches
a. Liquid filled (150-2200 degrees F)
b. Gas filled (100-1000 degrees F)
c. Vapor filled (50-700 degrees F)
These operate on the principle that as
temperature increases the medium expands and
creates more pressure. This pressure then
operates a switch.
These units have a fast response time, and
the liquid filled type is entirely self contained.
Important Terms
• Band Width – Proportion of the
temperature range which will be tolerable.
It consists of a mean temperature and a
percentage which it can go above or
below.
• Offset – difference between the desired
mean and the actual mean.
• Automatic Reset – Electronically shifts the
band up or down to eliminate offset.
Important Terms Continued
• Rate – Valuable in applications where
rapid temperature changes may occur. It
controls how hard the heating or cooling
element will work.
• Mode – The operational functions that a
controller has.
• Analog and Digital Set Point – Analog puts
the temperature on a scale. Digital puts
displays the temperature with numbers.
Temperature Conversions
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Fahrenheit degrees
200 – 5000
100 – 600
300 - 1200
100 – 1500
40 – 800
150 – 2200
100 – 1000
50 – 700
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Celsius degrees
93 – 2760
38 – 316
149 – 649
38 – 816
4 – 427
66 – 1204
38 – 538
10 – 371
Refrences
• Electronic Control for Machines by
Kenneth Rexford and Peter R. Giuliani
copyright 2004