PC Unit 3 PDF

# Unit 3 ## Types of Measurement - Flow measurement (liquid, gases) - Level measurement (liquid) - Temperature measurement (solid, liquid, gases) - Pressure measurement (solid, liquid, gases) ## Uses instrument - To measure pressure, flow, level, temperature. ## Control the parameters - To con...

# Unit 3 ## Types of Measurement - Flow measurement (liquid, gases) - Level measurement (liquid) - Temperature measurement (solid, liquid, gases) - Pressure measurement (solid, liquid, gases) ## Uses instrument - To measure pressure, flow, level, temperature. ## Control the parameters - To control process. - Optimized product (max.yield) ## Process - Raw material - Process - Finished product - Series of unit operations - Single operation performed on raw material. ## Unit operation - Crashing - Grinding - Filtration - Represented by flow diagram. ## Process - Integration of Unit Operation - **Vacuum** - negative pressure or absence of pressure. ## Consider Distillation - Atmospheric distillation - Vacuum distillation - High temperature distillation ## Process Instrumentation Diagram - **Temperature difference** - Temperature gradient (across distillation column) - Reflux - Control panel - **Feed point plate** - Reboiler coil - Remove bottom product - **Indicators**: - Level indicator - Pressure indicator - Temperature indicator - Composition indicator ## Control Panel - Control these parameters - Control product/process - Optimized product (max.yield) ## Redwood Viscometer - Determine viscosity of oil - Uniform temperature: single unit input ## Fermentor - **Drive PPM** - PH sensor (controller) - PH1 - O2 supply - Aeration device - Temperature sensor. - Impeller shaft / RPM - Impeller - Head ## Temperature Measuring Devices - The temperature can be measured using different sensors for measurement of temperature. - **There are four basic types of temperature measuring devices.** Each of these uses a different principle. - **Mechanical temperature measuring devices:** - Liquid in gases, glass thermometers, bimetallic strips, bulb and capillary, pressure type devices, etc. - **Thermo Conjuctive Devices (thermocouples)**. - **Thermo resistive devices** (RTD's and thermistors). - **Radiative** (infrared and optical pyrometers). ## Mechanical Temperature Measuring Devices - **Thermometer** - Graduations of 10 units - Thermometer Bulb - Mercury - **Principle of Operation (Working)** - Thermal measurement. - Expansion of mercury in thermometer. - Mercury thermometer works on the principal of expansion of mercury due to the heat applied to the mercury bulb. - Now-a-days, we get mercury thermometer which measure the temperature digitally. - In such thermometer, there is no such graduation or marking on the thermometer. - The thermometer reading is directly digitally displayed. - We have other types of thermometers also like a thermometer with a probe and the temperature can be measured on different scales & the scales are inter convertible i.e the temperature from one scale can be converted to another scale. - **Bimetallic Thermometer** - A bimetallic stripe works on the principal of thermal expansion of the metal on obligation of heat. - **Metal A** - Bimetal - **Metal B** - Expanded - **Different metals have different coefficients of thermal expansion & they expand differently on application of heat.** - Generally, the metals having a greater conductance or good conducting metals expand more than bad conducting or less conducting metals. - **A bimetal consist of two different metals, one of them is a good conductor whereas the second one is less conducting. When heat is applied the good conductor bends first and then the less conducting will bend along with the good conducting metal.** - **The principle of bimetallic expansion is used for making & breaking of the contact in the thermostat devices. ** - **Thermostat devices:** - The thermostat devices are those devices which require controlled heating for some purpose. - The bimetal incorporated in the thermostatic device makes and breaks the circuit on achieving the set point. - The setpoint in a device & is the temperature. or pressure which is setup in the device and which is to be achieved by the device... On reaching the set-point, the device automatically breaks the circuit. - **Electrical contact** - **Adjustment** - High expansion metal - Low expansion metal - Contacts - **Bulb and Capillary Sensor:** - **Capillary:** - To remote sensing bulb - **Bellows** - **Contacts** - **Adjustment** - **Sealed Bellows - Seed Silloid temperature sensor:** - This is filled with a gas, vapour or liquid. - It responds to change in temperature by variation in volume and pressure causing expansion or contraction - **Bulb and Capillary Sensor Thermometer:** - **Bulb and capillary elements are used where temperatures are to be measured in ducts, pipes, tanks, or similar locations remote from the controller.** - The bulb is filled with liquid from a gas or refrigerant depending on the temperature range required. - Expansion of fluid in the heated bulb exerts a pressure which is transmitted by the capillary to the diagram & there it is transmitted into the movement. - **Pressure Thermometer:** - **Bulb** - **Tube** - **Pressure Gauge** - **Pressure Thermometer** - The pressure thermometer while still considered mechanical operates by the expansion of a gas instead of liquid or solid. - Suppose the gas inside from the thermometer is an ideal gas then the ideal gas will follow ideal gas law that is $PV=nRT$ Where P is pressure, V is volume, n is mass of gas, R is gas constant for specific gas, T is the absolute temperature. - The bulb and tube are of constant volume so V is constant. Also the mass (n) of the gas in the sealed bulb is also constant. - Hence the above equations reduces to $P=KT$ where K is a constant. - **A pressure thermometer therefore measures temperature indirectly by measuring pressure. The gauge pressure is calibrated in terms of temperature.** - Hence the device can be used for measurement of temperature. ## Sample Questions - What are the different parameters that are measured in a chemical industry? How these parameters affect the processing in a chemical industry. Explain in brief. - What are the different devices for measurement of temperature? Discuss them in brief. - What is a bimetallic strip? What are the different applications of a bimetallic strip in industry? - With a neat sketch explain the temperature measurement with a bourdon tube. ## Characteristics of Instruments - The characteristics of instruments are divided into two types: - **The static characteristics and the dynamic characteristics.** - **The static characteristics do not have any relation with the real time.** - **The static characteristic of an instrument are as follows:** - **Desired Characteristic** - Accuracy - Reproducibility - Sensitivity - **Opposite Characteristic** - Static error - Drift - Dead zone - **Accuracy:** - The accuracy of an instrument is defined in percent as ± x% accurate. - Generally, the accuracy of the instrument varies in between 5-10%. - The accuracy of an instrument is an important characteristic of an instrument. - **Static error:** - The Static error may be defined as the difference between the true value & the actual reading of the instrument. - The true value + static error = actual reading of the instrument. - **Reproducibility:** - The reproducibility of an instrument may be defined as the accuracy with which the instrument produces the same reading. -**Drift:** - The drift in an instrument is defined as the shift in the instrument calibration over period of time. - In some instruments the calibration may not shift for days and months, whereas in some cases the calibration may shift immediately within a day. - There are different types of drifts in instruments. - **Sensitivity:** - The sensitivity of an instrument may be defined as the immediate response given by the instrument to the input. - A Sensitive instrument reacts immediately to the smallest input given toit. - A non-sensitive instrument will not react to the input given to it. - **Dead Zone:** - It may be defined as the zone in which no response is observed through the input. - i.e. the dead zone is a zone in which the instrument fails to respond. ## Dynamic Characteristics - These are those characteristics which vary with the real time. - **Desired Characteristics** - Speed of Response - Fidelity - **Undesired Characteristics** - Lag. - Dynamic Error - **Speed of Response:** - It may be defined as the immediate response or the rate of response given by an instrument to the input provided to the instrument. - **Lag:** - It may be defined as the time taken by the instrument to respond to the given input. - The lag in an instrument may be calculated in seconds. - **Fidelity:** - The fidelity of an instrument may be defined as the degree to which an instrument indicates the changes in measured variable without dynamic error. - Dynamic error is defined as the difference between the true value and the quantity changing with time. - And the value indicated by the instrument if no static error is assumed. ## Thermocouples - These are the instruments used in the industry for high-temperature measurement. - Thermocouples measure the temperature between 2000°F to 3000°F. - Initially, the thermocouples were used to measure the temperature between the range 2000°F to 3000°F but nowadays the thermocouples are also able to measure lower temperature also. - Thermocouples can measure temperature as low as 200°F. - They are generally used in industries like metal industry, glass, molten salt industry & with the reactors & furnaces. - **There are different combinations of thermocouples along with the thermocouple lead wires.** - The different combinations of thermocouples are given in the **table below:** | Sr.no | Metal 1 | Metal 2 | Leadwire Metal 1 | Leadwire Metal 2 | |---|---|---|---|---| | 1 | Copper | Constantan | Copper | Constantan | | 2 | Iron | Constantan | Iron | Constantan | | 3 | Chromel | Alumel | Chromel | Alumel | | 4 | Iron | Constantan | Iron | Copper-Nickel Alloy | | 5 | Chrome | Alumel | Copper | Constantan | | 6 | Platinum - Rhodium | Platinum | Copper | Copper - Nickel Alloy | ## Desirable Properties in a Thermocouple for Industrial Use 1. Thermocouples should have a large thermal EMF for a given temperature range so that a simple and durable measuring instrument can be coupled with the thermocouple. Generally, instruments measuring voltage are connected with the thermocouple. 2. The thermocouple must have a precise calibration. - The thermocouples are used in high-temperature operation hence the thermocouple should have precise measurement so that they do not have any kind of drift in their output or reading. 3. **Resistance to corrosion and oxidation** - The thermocouple is used for measurement of high temperature. - A metal during operation at high temperature is subjected to different thermal stresses. - These stresses are responsible for thermal corrosion in the metal. - Hence while selecting the material for thermocouple proper material which can handle the operating conditions should be selected. - Such a material will resist both corrosion and oxidation, both points should be considered.. 4. **Linear Relationship with Temperature:** The thermocouple should have a linear relationship with the temperature, that means as the temperature increases, the thermocouple output in whatever units) it is should increase linearly i.e. if we plot temperature against output (millivoltas) we must get a straight line in the graph. - Only then the relationship will be a linear relationship. ## The Principle and Working of a Thermocouple - The principle of working of a thermocouple can be understood from the following: - (i) When two dissimilar metals are connected together, a small voltage called as the thermojunction voltage is generated at the junction, this is called as peltier effect. - (ii) If the temperature of the junction changes, it causes the voltage to change. This voltage can be measured by means of an electric circuit connected to a voltmeter or millivoltmeter. The output voltmeter voltage is proportional to the temperature difference between the free ends. This is called as thomson effect. - (iii) Both these effects can be combined to measure temperature by holding one junction at a known temperature (reference junction & measuring the voltage). The temperature at the sensing junction can be calculated or known. The voltage generated is directly proportional to the temperature difference. The combined effect at the thermojunction is known as thermojunction effect or the Seebeck effect. ## Thermocouple can be represented by a diagram given below - **Metal A** - Measuring junction - Voltmeter - **Metal B** - Reference junction - **Figure representing a thermocouple (okt) along with thermocouple probe, and a measuring device. The thermocouple probe, and a measuring device produces temperature difference which is directly proportional to the EMF generated.** - Hence we can use the thermocouple for measurement of temperature after calibration of temperature in either volts or millivolts. - Based on the sensing position of the thermocouple measuring junction, the thermocouple are divided into three types. 1. **The grounded junction thermocouple - The measuring junction directly touches the probe wall.** - Such type of arrangement gives good heat transfer from the outside through the probe wall to the thermocouple junction. - Probe wall - Metal A - Voltmeter - Grounded junction thermocouple - Underground junction thermocouple - Exposed junction 2. **Underground junction thermocouple - The thermocouple junction is not touching the wall of the probe instead it is far away-om the wall of the probe. Such a thermocouple offers electrical isolation.** - Hence the underground thermocouple is recommended for corrosive environments where electrical isolation is required. 3. **The exposed junction thermocouple - The electrical circuit i.e. the measuring junction coming out of the probe, such type of thermocouple offers best response time but it is limited to the use in non-pressurized application.** - Robust can work in any condition - different (corrosive, reactive) are present. ## Advantages of Thermocouple - The advantages of a thermocouple are: - (i) It has high accuracy. - (ii) It is a robust instrument and can be used in harsh environments and high vibrations. - (iii) The thermal reaction is fast (heat transfer). - (iv) The operating range of thermocouple is very wide. - (v) The cost of thermocouple is not very high ## Disadvantages of Thermocouple - (i) Non-linearity. - (ii) Requires low voltage. - (iii) Reference instrument is required. - (iv) The thermocouple recalibration is required. ## Applications of Thermocouple - The applications of thermocouples include the following: - (i) The thermocouples are used in the temperature sensors in thermostats, offices, laboratories, research institutions, etc. - (ii) These are used in industries for monitoring temperatures of metals, metal furnaces, reactors, etc. ## Difference between a Thermocouple and a Thermostat | Sr.no | Feature | Thermocouple | Thermostat | |---|---|---|---| | 1 | Range of temperature | 500 - 2000 °F | -112 - 302 °F (negative to positive) | | 2 | Stability | Provides less stability | Provides medium stability | | 3 | Price range | Less | High | | 4 | Sensitivity | Less | More | | 5 | Linearity | Moderate | Poor | | 6 | System cost | High | Medium |