PC Unit 3 HW 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 ### Process - Integration of Uni...

## 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 ### Process - Integration of Unit Operation - Vacuum - negative pressure or absence of pressure ### Consider Distillation - Atmospheric Distillation - Vacuum Distillation - High Temperature Distillation ### Process Instrumentation Diagram - The image shows a diagram illustrating the process instrumentation of a distillation process. - It includes different components and their functions: - Temperature difference - Gradient (across dist column) - Feed point plate - Reboiler coil - Condenser - Product - Control panel - Level indicator (Li) - Pressure indicator (Pi) - Temperature indicator (Ti) - Composition indicator (Ci) ### Control Panel - Control these parameters: - Control product /process - Optimized product (Max Yield) ### Redwood Viscometer - Determine viscosity of oil - Uniform temperature: single unit input - The image shows a diagram of a Redwood Viscometer, a device used to determine the viscosity of oil. - It includes: - A stick with a wire - Oil bath at 50°C - A temperature controller - A measuring cylinder ### Fermentor - The image shows a diagram of a fermentor, a bioreactor used to cultivate microorganisms. - It includes: - A drive with a PPM (parts per million) unit - A pH sensor with a controller - An impeller shaft with an RPM (revolutions per minute) unit - An impeller - An aeration device - An oxygen supply - A temperature sensor ### Temperature Measuring Devices - Temperature can be measured using different sensors for measurement of temperature. - There are four basic types of temperature measuring devices, each of which 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 - The image shows a diagram of a thermometer, explaining the main components and working principle. - **Principle of operation (Working): Thermal measurement (use)** - Expansion of mercury in thermometer. - Mercury thermometers work on the principle of expansion of mercury due to the heat applied to the mercury bulb. Nowadays, we get mercury thermometers which measure the temperature digitally. In such thermometers, 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 and 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 principle of thermal expansion of the metal on obligation of heat. The image shows a diagram of a bimetallic stripe and explains its working principle. - Different metals have different coefficients of thermal expansion, and they expand differently on application of heat. Generally, metals having greater conductance or good conducting metals expand more than bad conducting or less conducting metals. - A bimetal consists of two different metals: one of them is a good conductor, and the other one is less conducting. When heat is applied, the good conductor bends first, and then the less conducting metal will bend along with good conducting metal. - The principle of bimetallic expansion is used for making and 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 thermostat devices makes and breaks the circuit on achieving the set point. The set point is a temperature or pressure which is set up in the device, and it is to be achieved by the device. On reaching the set point, the device automatically breaks the circuit. The image shows a diagram, explaining the operation of a thermostat device. - **Bulb & Capillary Sensor:** - The diagram shows a bulb and capillary sensor. - This is filled with a gas, vapor, or liquid. It responds to change in temperature by variation in volume and pressure, causing expansion or contraction. - **Bulb & 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 diaphragm, and there it is transmitted into the movement. - **Pressure Thermometer:** - The diagram shows a 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 no. of mass of gas,R is gas constant for the gas, T is the absolute temperature. - The bulb and tube are of constant volumes, so V is constant. Also, the mass (n) of the gas in the sealed bulb is also constant. Hence, the above equation reduces to P = KT, where K is 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 Question - **1. What are the different parameters that are measured in a chemical industry? How these parameters affect the processing in chemical industry. Explain in brief.** - **2. What are the different devices for measurement of temperature? Discuss them in brief.** - **3. What is a bimetallic strip? What are the different applications of a bimetallic strip in industry?** - **4. With a neat sketch explain the temperature measurement with a bourdon tube.** ### Characteristics of Instruments - The characteristics of instruments are divided into two types: - Static characteristics and - Dynamic characteristics. - **Static characteristics:** - Static characteristics do not have any relation with the real time. - The static characteristics of an instrument are as follows: - Accuracy - Reproducibility - Sensitivity - 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 and the actual reading of the instrument. 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 a 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 to it. - 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 the measured variable without dynamic error. Dynamic error is defined as the difference between the true value and the quantity changing with time. The value indicated by the instrument if no static error is assumed. ### Thermocouple - **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 2000°F and 3000°F, but nowadays the thermocouples are also able to measure lower temperature. - Thermocouple can measure temperature as low as 200°F. - They are generally used in industries like the metal industry, glass, molten salt industry, and with the reactors and 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 | Headwire Metal 1 | Headwire 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| Chromel | Alumel | Copper | Constantan | |6| Platinum-Rhodium | Platinum | Copper| Copper - Nickel Alloy - **Desirable properties in a thermocouple for Industrial use:** - 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. - The thermocouple must have a precise calibration. The thermocouples are used in high temperature operation, and hence the thermocouple should have precise measurement so that they do not have any kind of drift in their output or reading. - 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 a thermocouple, proper material which can handle the operating conditions should be selected. Such a material will resist both corrosion and oxidation, both points are important. - Linear relationship with the temperature. The thermocouple should have a linear relationship with the temperature. That means as the temperature increases, the thermocouple output in whatever unit it is should increase linearly, i.e., if we plot temperature against output (millivolts), 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: - **When two dissimilar metals are connected together, a small voltage called as the thermojunction voltage is generated at the junction (this is called as the Peltier effect).** - **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 the Thomson effect.** - **Both these effects can be combined to measure temperature by holding one junction at a known temperature (reference junction), and 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 the thermojunction effect or the Seebeck effect.** - **The thermocouple can be represented by a diagram given below:** - **Figure representing a thermocouple, probe, and a measuring device:** - The thermocouple probe and a measuring device produce 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, thermocouples are divided into three types:** - **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. - **Underground junction thermocouple:** The thermocouple junction is not touching the wall of the probe. It is far away from 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. - **Exposed junction thermocouple:** The measuring junction is coming out of the probe. This type of thermocouple offers best response time, but it is limited to the use in non-pressurized applications.