Temperature Controls

Andrew Blackwell

For a copy of this presentation please E-mail me at adb016@latech.edu

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 (300-1200 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

• Fahrenheit degrees• 200 – 5000• 100 – 600• 300 - 1200• 100 – 1500• 40 – 800• 150 – 2200• 100 – 1000• 50 – 700

• 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