Download Nirma University Chemical Engineering Department Handouts for AI

Nirma University
Chemical Engineering Department
Handouts for AI 2014
Lecture 1
Heat & Temperature
Temperature Scales
Different types of temperature scales and their comparison
Temperature Measuring Instruments:
Expansion Thermometers: Solid, Liquid, Gas
Bimetallic Thermometers and Thermostats
Bimetallic thermometers are contact temperature sensors found in several forms if
you know where to look, e.g. inside simple home heating system thermostats.
They're the coil of metal that has some electrical contacts affixed to it. They are
more familiar to many people in industry and commerce as miniature pocket dial
thermometers that many people use to check the temperature of fat in a deep frier
or a vat on a small process line.
Perhaps the best part is that the pocket thermometers are sealed and do not
require batteries! The major uses are where a quick check of the temperature of
an object is desired, or, in the case of the thermostats, a simple, reliable, low cost
sensor that provides the input and sometimes a good part of the control action in a
home heating system.(Nowadays electronic thermostats are more commonly seen
and since they need a battery to power the clock inside them, they usually have a
different type of temperature sensor, one that also runs with a battery.)
Lecture 2
Filled System Thermometers
Different types of Gas filled, Liquid filled, Vapour filled thermometers and their
principle, construction, working, advantages, disadvantages etc.
Nirma University
Chemical Engineering Department
Handouts for AI 2014
Lecture 3-4
Resistance Temperature Detectors (RTDs)
What are RTDs?
Resistance Temperature Detectors or RTDs for short, are wire wound and thin film
devices that measure temperature because of the physical principle of the positive
temperature coefficient of electrical resistance of metals. The hotter they become,
the larger or higher the value of their electrical resistance.
The Advantages of RTDs
The advantages of RTDs include stable output for long period of time, ease of
recalibration and accurate readings over relatively narrow temperature spans.
Their disadvantages, compared to the thermocouples, are: smaller overall
temperature range, higher initial cost and less rugged in high vibration
environments.
They are active devices requiring an electrical current to produce a voltage drop
across the sensor that can be then measured by a calibrated read-out device.
RTDs Other Than Platinum
RTDs can be made cheaply in Copper and Nickel, but the latter have restricted
ranges because of non-linearities and wire oxidation problems in the case of
Copper.
Standard Platinum RTDs(SPRTs)
The ITS-90 (International Temperature Scale of 1990- used as a worldwide
practical temperature scale in national metrology labs like NIST, NPL et al) is
made up of a number of fixed reference points with various interpolating devices
used to define the scale between points. A special set of PRTs, called SPRTs, are
used to perform the interpolation in such labs over the ranges 13.8033 K (Triple
point of Equilibrium Hydrogen) to the Freezing point of Silver, 971.78 °C.
The German DIN standard DIN 751 for RTDs recognizes only platinum material
with a temperature coefficient of resistance of 0.00385
Nirma University
Chemical Engineering Department
Handouts for AI 2014
Lecture 5-6
Thermistors
(THERMal resistors)
Thermistors are special solid temperature sensors that behave like temperaturesensitive electrical resistors. No surprise then that their name is a contraction of
"thermal" and "resistor". There are basically two broad types, NTC-Negative
Temperature Coefficient, used mostly in temperature sensing and PTC-Positive
Temperature Coefficient, used mostly in electric current control.
Thermistor Terminology
A glossary slightly modified from that given in a US government publication: MILPRF-23648D. Note that the term being described is in bold typeface.
A thermistor is a thermally sensitive resistor that exhibits a change in electrical
resistance with a change in its temperature. The resistance is measured by
passing a small, measured direct current (dc) through it and measuring the voltage
drop produced.
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standard reference temperature
zero-power resistance
resistance ratio
The zero-power temperature coefficient of resistance
A NTC thermistor
A PTC thermistor
The maximum operating temperature
The maximum power rating
The dissipation constant.
The thermal time constant
The resistance-temperature characteristic
The temperature-wattage characteristic
The current-time characteristic
The stability
Nirma University
Chemical Engineering Department
Handouts for AI 2014
Lecture 7-8
Thermocouples (TCs)
Thermocouples are among the easiest temperature sensors to use and obtain and
are widely used in science and industry. They are based on the Seebeck effect
that occurs in electrical conductors that experience a temperature gradient along
their length. They are "simple", rugged, need no batteries, measure over very wide
temperature ranges and more. They have their quirks, too, like everything else.
The solution to thermocouple uses and problems lies in the details of a given
application.
What is a Thermocouple?
Thermocouples are pairs of dissimilar metal wires joined at least at one end, which
generate a net thermoelectric voltage between the the open pair according to the
size of the temperature difference between the ends, the relative Seebeck
coefficient of the wire pair and the uniformity of the wire-pair relative Seebeck
coefficient.
Thermocouple Types and More
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Thermocouple Types, Letter Designations, Construction The ISA in the
USA started the standards on Thermocouple nomenclature that includes
the letter conventions used to describe certain alloy wire pairs. This is now
carried on around the World and ASTM Committee E20 on Temperature
provides the standard (ASTM E 230) that is now the American National
Standard for the letter designation, the calibration tables, the color coding
and recommended use limits of the most common types of thermocouple
wire pairs
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Thermocouples are used in many places with many things like indicators
and controllers to do something useful, such as control a heating system to
heat a product through a temperature-time profile that causes it to soften or
cook or set or transform from a stressed condition to an annealed one or
any number of physio-chemical changes that produce a desired end result.
Nirma University
Chemical Engineering Department
Handouts for AI 2014
This page has a growing collection of Web sites offers a look at some of
those things and places.
Nirma University
Chemical Engineering Department
Handouts for AI 2014
Lecture 9-12
Pyrometers
A Pyrometer, or radiation thermometer, is a non-contact instrument that detects an
object's surface temperature by measuring the temperature of the electromagnetic
radiation (infrared or visible) emitted from the object.
The wavelength of thermal radiation ranges from 0.1 to 100 µm (4 ~ 4,000 µin),
i.e., from the deep ultraviolet (UV) across the visible spectrum to the middle of the
infrared region (IR).
Pyrometers are essentially photodetectors which are capable of absorbing energy,
or measuring the EM wave intensity, at a particular wavelength or within a certain
range of wavelengths.
Common pyrometers include:
•Optical Pyrometer (a.k.a. Brightness Pyrometer or Disappearing Filament
Pyrometer)
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Designed for thermal radiation in the visible spectrum.
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Utilizes a visual comparison between a calibrated light source and the
targeted surface. When the filament and the target have the same temperature,
their thermal radiation intensity will match causing the filament to disappear as it
blends into the targeted surface in the background.
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When the filament disappears, the current passing through the filament can
be converted into a temperature reading.
•Infrared Pyrometer
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Designed for thermal radiation in the infrared region (0.75 ~ 1000 µm;
30 µin ~ 0.04 in) usually 2 ~ 14 µm (80 ~ 550 µin)
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Constructed from pyroelectric materials, e.g., triglisine sulfate (TGS), lithium
tantalate (LiTaO3), or polyvinylidene fluoride (PVDF).
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Similar to the charge generated by stressed piezoelectric materials, a
pyroelectric charge dissipates in time. Hence, a rotating shutter is required to
interrupt the incoming radiation to obtain a stable output.
Nirma University
Chemical Engineering Department
Handouts for AI 2014
Pros and Cons
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Pros:
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Non-contact measurement
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Fast response time
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Good stability
Cons:
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Expensive
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Accuracy maybe affected by suspended dust, smoke, and thermal
background radiation
Comparison of Temperature measuring sensors and their selection.
Lecture 13-14
Direct Level Measurement
Direct methods employ physical properties such as fluid motion and buoyancy, as
well as optical, thermal, and electrical properties. Direct level measurement does
not require compensation for changes in level caused by changes in temperature.
Direct level measurements show the actual level of the interface.
· Indirect Level Measurement
Indirect level measurement involves converting measurements of some other
quantity, such as pressure to level by determining how much pressure is exerted
over a given area at a specific measuring point, the height of the substance above
that measuring point can also be determined.
Continuous Level Measurement
In many processes, continuous level measurement is required because it is
necessary to know at all times the exact position of the interface in relation to one
or more specific reference points. A gage or sight glass, can be used to
continuously observe the position of the interface.
· Point-to-Point Level Measurement
· Selecting Measurement Devices
Visual sensors.
Nirma University
Chemical Engineering Department
Handouts for AI 2014
Dip Sticks and Lead Lines, Sight Glasses and Gage Glasses, Float Devices,
Magnetic-Type Float Devices, Variable displacement sensors.
· Archimedes' Principle
· Principles of Variable Displacement
· Liquid-Liquid Interface Measurement
· Variable Displacement Level Measuring Devices
Nirma University
Chemical Engineering Department
Handouts for AI 2014
Lecture 15
Pressure/Density Level Instrumentation
Bubbler Tubes
Floats & Displacers
Float Level Switches
The buoyant force available to operate a float level switch (that is, its net buoyancy) is the
difference between the weight of the displaced fluid (gross buoyancy) and the weight of the float.
Floats are available in spherical, cylindrical and a variety of other shapes.
Applications & Installations
Displacer Switches
Continuous Level Displacers
Displacers are popular as level transmitters and as local level controllers, particularly in the oil and
petrochemical industries. However, they are not suited for slurry or sludge service because coating
of the displacer changes its volume and therefore its buoyant force. They are most accurate and
reliable for services involving clean liquids of constant density. They should be temperaturecompensated, particularly if variations in process temperature cause significant changes in the
density of the process fluid.
Interface Applications
When measuring the interface between a heavy liquid and a light liquid (such as oil on water), the
top connection of the displacer is placed into the light and the bottom connection into the heavy
liquid layer
Nirma University
Chemical Engineering Department
Handouts for AI 2014
Lecture 16
Continuous Level Floats
Of the various float sensor designs used for continuous level measurement, the oldest and
arguably most accurate is the tape level gage (Figure 7-12A). In this design, a tape or cable
connects the float inside the tank to a gage board or an indicating take-up reel mounted on the
outside of the tank. The float is guided up and down the tank by guide wires or travels inside a
stilling well. These level indicators are used in remote, unattended, stand-alone applications, or
they can be provided with data transmission electronics for integration into plant-wide control
systems.
Float Control Valves
Float-operated control valves combine level measurement and level control functions into a single
level regulator. While simple and inexpensive, they are limited to applications involving small flows
and small pressure drops across the valve.
Radiation-Based Level Gages
An entire class of level instrumentation devices is based on a material's tendency to reflect or
absorb radiation. For continuous level gages, the most common types of radiation used are
radar/microwave, ultrasonic, and nuclear.
Ultrasonic Level Gages
The origin of ultrasonic level instrumentation goes back to the echometers used in measuring the
depth of wells by firing a blank shell and timing the return of the echo. SONAR detectors used in
naval navigation also predate industrial applications of this principle.
Ultrasonic Transducers
Level Switches