Chapter 2 Temperature

Process Control & Instrumentation by Moaz Waqar

Chapter 1: Temperature Measurement

Temperature: Temperature is a measure of the thermal energy in a body, which is the relative hotness or coldness of a medium and is normally measured in degrees using one of the following scales; Fahrenheit (F), Celsius or Centigrade (C), Rankine (R), or Kelvin (K).

Heat: Heat is a form of energy; as energy is supplied to a system the vibration amplitude of its molecules and its temperature increases. The temperature increase is directly proportional to the heat energy in the system.

Specific heat: Specific heat is the quantity of heat energy required to raise the temperature of a given weight of a material by 1.

Thermal Conductivity: Thermal conductivity is the flow or transfer of heat from a high temperature region to a low temperature region.



Temperature Measuring devices:

There are several methods of measuring temperature that can be categorized as follows: 1. Expansion of a material to give visual indication, pressure, or dimensional change 2. Electrical resistance change 3. Semiconductor characteristic change 4. Voltage generated by dissimilar metals 5. Radiated energy

Q 1: What temperature in Kelvin corresponds to 115F?

Q 2: What is the heat required to raise the temperature of a 1.5 kg mass 120C if the specific heat of the mass is 0.37 cal/gC?

Q 3: A furnace wall 12 ft2 in area and 6-in thick has a thermal conductivity of 0.14 BTU/h ft F. What is the heat loss if the furnace temperature is 1100F and the outside of the wall is 102F?

Q 4: How much heat is transferred from a 25 ft 24 ft surface by convection if the temperature difference between the front and back surfaces is 40F and the surface has a heat transfer rate of 0.22 BTU/h ft2F?

Q 5: The radiation constant for a furnace is 0.23 108 BTU/h ft2F4, the radiating surface area is 25 ft2. If the radiating surface temperature is 750F and the room temperature is 75F, how much heat is radiated?

Q 6: Calculate the length and volume for a 200 cm on a side copper cube at 20C, if the temperature is increased to 150C.



Temperature Measuring devices:

There are several methods of measuring temperature that can be categorized as follows: 1. Expansion of a material to give visual indication, pressure, or dimensional change 2. Electrical resistance change 3. Semiconductor characteristic change 4. Voltage generated by dissimilar metals 5. Radiated energy



1. Liquid Expansion Thermometers

i) Mercury in Glass Principle: Mercury expands linearly with temperature Construction: The device consisted of a small bore graduated glass tube with a small bulb containing a reservoir of mercury. Working: As temperature increases, mercury expands and the level of mercury indicates the temperature. Range: The operating range of the mercury thermometer is from 30 to 800F (35 to 450C). Advantages: Relatively low cost, highly accurate method, linear. Disadvantages: Toxicity of mercury, ease of breakage, high response time, non-digital output.

ii) Liquid in glass: Range: -300 to 600F (170 to 330C).



2. Bimetallic Strips

Principle: metals are pliable and different metals have different coefficients of expansion Construction: Bimetallic strips are usually configured as a spiral or helix for compactness and can then be used with a pointer and a scale. Its one end is fixed and other end is free to move. Working: As temperature increases, spiral tries to open itself up and the free end moves the pointer to indicate the temperature Range: 180 to 430C Advantages: Relatively low cost, highly rugged Disadvantages: Relatively inaccurate, slow to respond, hysteresis, non-digital output.



3. Pressure-Spring Thermometers

Principle: Fluid expands with increasing temperature and exerts pressure if constricted Construction: It has a metal bulb made with a low coefficient of expansion material with a long metal tube, both contain material with a high coefficient of expansion; the bulb is at the monitoring point. The metal tube is terminated with a spiral Bourdon tube pressure gage (scale in degrees). Working: As the temperature in the bulb increases, the pressure in the system rises, the pressure rise being proportional to the temperature change. The change in pressure is sensed by the Bourdon tube and converted to a temperature scale Advantages: Relatively low cost, highly rugged, low maintenance cost, remote indication Disadvantages: slow to respond, non-digital output

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3. Pressure-Spring Thermometers

i) Liquid Filled Range: 130 to 315 oC Iii) Gas filled Range: 270 to 760 oC



4. Resistance Temperature Devices Principle: The electrical resistance of pure metals is positive, increasing linearly with temperature Construction: Resistance temperature devices (RTD) are wire-wound resistors. Working: In a resistance thermometer the variation of resistance with temperature is given by RT2 = RT1 (1 + Coeff. [T2 T1]) where RT2 is the resistance at temperature T2 and RT1 is the resistance at temperature T1 Range: 170 to 780C for platinum, -180 to 300 oC for nickel Sensitivity: 0.004/oC for platinum. 0.005/oC Response Time: 0.5 to 5 seconds Advantages: Relatively low cost, highly rugged, low maintenance cost, linear Disadvantages: slow to respond, relatively low sensitivity



5. Thermistors Principle: The electrical resistance of semiconductors decrease with increase in temperature. Construction: Thermistors are a class of metal oxide (semiconductor material). They can be formed as discs, beads, rods and discs. Range: -50 to 300 oC Sensitivity: 0.1/oC Response Time: 0.5 to 5 seconds Advantages: Low cost, highly rugged, low maintenance cost, avaialble in different size, shapes and values, quite high sensitivity Disadvantages: slow to respond, non-linear



5. Themocouples Principle: An emf is produced proportional to the temperature. Construction: Two wires of different metals/alloys are joined/twisted together at one end, whereas other two ends of wires are used to measure the emf/voltage. Working: Seebeck Effect is responsible for the generation of emf. = (T2-T1) = Emf produced = Seebeck Constant (V/K) T2,T1 = junction temperatures in K. emf produced can be then calibrated for temperature output using standard devices. Response Time: 10 ms 20s Advantages: Highly accurate, higher temperatures can be measured, linear Limitations: Compensation is required, wire resistance, decalibration, thermal shunting.

Type Composition Temperature Range

(oC)

Seebeck Coefficient

(mV/K)

Properties

K Nickel Chromium / Nickel

Alumel

-270 to 1260 0.04 Inexpensive, accurate and

reliable

J Iron - Constantan -210 to 1200 0.051 Inexpensive, low life at

high temperatures

E Chromel - Constantan -270 to 1000 0.06 Strong signal, higher

accuracy

T Copper - Constantan -270 to 400 0.04 Stable at low

temeratures, used in

ultra-low temperature

freezers

N Nichrosil - Nisil -270 to 1300 0.038 Same as K type

S Pt (10% Rh) Pt -50 to 1760 0.011 Stable at high

temperature, accurate

B Pt (30% Rh) Pt (6% Rh) 0 to 1700 0.008 Used for very high

temperature

measurements

R Pt (13% Rh) - Pt -50 to 1760 0.012 Expensive, stable at high

as well as low

temperatures



5. Themocouples Seebeck effect: As mentioned earlier, this effect describes that when two junctions Jh and JC of two different conductors are placed at higher and lower temperatures T2 ( ) and T1 () respectively, electromotive force is generated in the loop, consequently a current (conventional) is generated from cold junction to hot junction. Peltier Effect: This effect describes that when a current is passed through two junctions Jh and JC of two different conductors, heat is generated at Jh and heat is absorbed at JC. It is actually a reverse state of Seebeck effect. Thomson Effect (Lord Kelvin effect): This effect describes that a current carrying conductor having a temperature gradient or a temperature change at two different points either gives out heat or absorbs heat which depends on the type of material.



5. Themocouples Law of intermediate metals: If a third wire is inserted in to the junction which has a homogenous temperature along its length, then it does not pose any difference on the emf produced i.e. the results remain unaltered by the insertion of this new wire. Law of homogeneous metals: If a thermocouple consists of junctions made up of conductor of one material only, there would be no emf produced no matter how large is the temperature difference between the junctions. It also describes that there would be no emf produced if the temperature at the two junction is same even if the junctions are made up of two different type of conductors. Law of intermediate temperature: A thermocouple A with one junction at T1 and other at T2 gives an emf E1, and another thermocouple with one junction at T2 and other at T3 gives a value E2. This law states that if a thermocouple with one junction at T1 and other at T3 gives emf E3 which would always be equal to E1 + E2.



6. Pyrometers Principle: All bodies above 0 K emit radiation Construction: Range: 650 to 1800C Advantages: Temperature can be measured from a distance, no upper limit Disadvantages: Not suitable for low temperatures, cannot be used in the presence of light absorbing or scattering medium.



6. Pyrometers Total Radiation Pyrometer Radiation from the target falls on the concave mirror, which can be moved back and forth to focus radiation on the radiation receiver. Thermocouple is attached to the receiver, emf is calibrated to a temperature scale -Use of concave mirror -non-linear, not suitable for lower than 650 C.



6. Pyrometers Selective Radiation Pyrometer Disappearing filament optical pyrometer utilizes the photometric principle of comparison of the intensity of incoming radiation at a particular wave band of that of a lamp. An image of the target is superimposed on a heating tungsten filament. Brightness of lamp is calibrated to corresponding temperature. -Not lower than 650 C. -More accurate -Red filter

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Chapter 2 Temperature