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Temperature Control System

Added on - 17 Jun 2021

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INTRODUCTION:The purpose of this paper is to design a control system for the annealing furnace. Annealingfurnace is nothing but a furnace that heats up to a maximum temperature and then cools down toobtain their properties. The alloys are heated up to 860 degree Celsius and then it is cooled forabout 10 hours. The cooling of the furnace is carried out under an argon atmosphere with thewater circulation of about 20 degree Celsius. The on/off control valve provides the supply of thewater. In this paper we have discussed about the temperature measuring device that can be usedin this furnace. This paper also presents the PLC logic diagram for the temperature controlsystem.a) TEMPERATURE MEASUREMENT:Requirement: The temperature in the annealing furnace of about 860 °C with an accuracy ofcertain plus or minus of the degree temperature should be measured.Measuring device: There are several temperature measurement devices that include RTD,thermistors, thermocouple etc. Out of this thermocouple can be used for this process.Thermocouple is formed with the two dissimilar metals joined together causes a potentialdifference across the junction [1]. This process purely depends on the type of the metal used.When the temperature across both the junction is same then the potential difference is zero. Inorder to get an e.m.f, the difference in the temperature should be produced. Due to this process,one junction should be kept at 0 °C and the other at a nominal temperature. The potentialproduced is given by the formula,e.m.f= at + bt2....................................................equation (1)Where, a and b are constants. There are several types in the thermocouple. Thermocouples withthe base-metal types include E, J, K and T are of lesser cost but they become worse with theirlifespan. Their accuracy varies between ± 1 to 3%. Thermocouple of R type has noble-metal thatare very high in their cost. They have a long lifespan with the accuracy of about ± 1% even betterthan that. Generally, thermocouple is mounted in a sheath to provide the mechanical andchemical resistance [2]. If the thermocouple has a wide sheath then the response could be slowand with thin sheath the response will be much faster with the time. The chromel-alumelthermocouple is shown in the figure given below.
Figure 1: chromel-alumel thermocoupleTable 1: Description of thermocouple types with its range and sensitivityTypeMaterialsRange °CSensitivity mV/°CEChromel-constantan0 to 98063JIron-constantan-180 to 76053KChromel-alumel-180 to 126041RPlatinum-platinum/rhodium 13%0 to 17808TCopper-constantan-180 to 37043b) TEMPERATURE CONTROL SYSTEM:Figure 2, shows the block diagram of the temperature control system. Data acquisition is done bymeasuring the temperature of the furnace with the thermocouple and this temperature signal isconverted into electrical signal in a standard value of 4-20 mA and transmits the signal throughthe transmitter [3]. This serves as an analog input to the EM235 module which is a part of thePLC. Data is received through EM235 and the output is send through the temperature displayand the control element.Figure 2: Block diagram of the automatic temperature control system.
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