Designing A Control System For Annealing Furnace - Temperature Control And Monitoring With Thermocouple And PLC

Requirement and Measuring Device

Discuss about the Control Of Reheat Furnace Based On Distributed Model.

The purpose of this paper is to design a control system for the annealing furnace. Annealing furnace is nothing but a furnace that heats up to a maximum temperature and then cools down to obtain their properties. The alloys are heated up to 860 degree Celsius and then it is cooled for about 10 hours. The cooling of the furnace is carried out under an argon atmosphere with the water circulation of about 20 degree Celsius. The on/off control valve provides the supply of the water. In this paper we have discussed about the temperature measuring device that can be used in this furnace. This paper also presents the PLC logic diagram for the temperature control system.

Requirement: The temperature in the annealing furnace of about 860 °C with an accuracy of certain 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 potential difference 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. In order 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 potential produced 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 with the base-metal types include E, J, K and T are of lesser cost but they become worse with their lifespan. Their accuracy varies between ± 1 to 3%. Thermocouple of R type has noble-metal that are very high in their cost. They have a long lifespan with the accuracy of about ± 1% even better than that. Generally, thermocouple is mounted in a sheath to provide the mechanical and chemical resistance [2]. If the thermocouple has a wide sheath then the response could be slow and with thin sheath the response will be much faster with the time. The chromel-alumel thermocouple is shown in the figure given below.

Figure 1: chromel-alumel thermocouple

Thermocouple Types and Characteristics

Table 1: Description of thermocouple types with its range and sensitivity

Figure 2, shows the block diagram of the temperature control system. Data acquisition is done by measuring the temperature of the furnace with the thermocouple and this temperature signal is converted into electrical signal in a standard value of 4-20 mA and transmits the signal through the transmitter [3]. This serves as an analog input to the EM235 module which is a part of the PLC. Data is received through EM235 and the output is send through the temperature display and the control element.

Figure 2: Block diagram of the automatic temperature control system.

In controlling the temperature of the furnace, the on/off controller plays a significant role that controls the heating and cooling of the furnace. A particular set point should be given in the operation of the control system [4]. Lower the value than the set point, turns the heater on that heats the alloy and higher the temperature than the set point turns the heater off.

For coolant we circulate the water at a flow rate of about 280 litres / min. This is carried out at an argon atmosphere. This argon gas should be provided from the argon containers at a pressure of about 180 kPa. To provide the gas at a sufficient pressure we require control valves. There are certain control valves like pressure control valves, flow control valve, Directional control valves etc. the best use for this control system is the pressure control valves [5]. This control valves protect the system from the sudden surge in pressure due to the opening of the valve. The system can be affected due to the pressure surge that happens due to the fluid demand.  These are often known as the pressure regulator valve that turns the gas on/off depending on the pressure. They can control the force produced by the actuator with the varying pressure.                     

Actuator force= Pressure × Area…………………………Equation (2)

Figure 3: Schematic diagram of the pressure control valve

A programmable logic controller (PLC) plays a vital role in the field of microprocessor. It’s very simple and easy for the users to handle since it could consist of hardware and software components which could handle all the industrial components, and the work of the engineer is to program the plc using ladder logic which could do certain type of automation and control in the industrial equipments.

Block Diagram of the Automatic Temperature Control System

One of the commonly used controllers in the automatic temperature control system is the On/Off controller. The input value should be compared with the lesser than or equal logic as well as with the greater than or equal logic. This input value is compared with the set point. The first line of the ladder logic is connected with the S1 and the last line is connected with heater output which is responsible for the control of the whole circuit [6]. The MOV instruction addresses the thermocouple that measures and displays the output of the device. The set point value that is in the source B is compared with the thermocouple output that is present in the LEQ and GEQ of source A. These are all present in the same integer file. The set point of LEQ will be a lesser value and the set point of GEQ will be greater. The ladder diagram works in the following logic:

1) when source A< B in the LEQ logic then the heater turns ON until the output of the low temperature is true and the output of the high temperature is false [7]. The heater is on till the high temperature becomes false although low temperature is false.

2) When the high temperature value is reached then Source A will be equal to source B in the GEQ logic. This will turn off the heater, since the high temperature is true. This is continued till the low temperature is reached.

This cycle is repeated that helps in maintaining the set point of the furnace [8]. The PLC ladder diagram is seen in the figure given below.

Figure 4: PLC Ladder Logic for the annealing furnace.

There are certain methods to tune the controller. Here we have used simple form of the temperature control device which is said to be an on/off controller. The output from this controller is either on or off where it does not support any middle states. As mention, the output of the controller turns on the heater when the temperature is below the set point and turns off when it reached the set point. On-off differential or hysteresis is added to the controller In order to avoid any damage that may happens to the system. When a precise control is not required then we can use on-off controller. PID controllers can also be used in this case where we have to tune the parameters of the proportional (kp), Integral (ki) and derivative (kd) values. For this case on/off controller is more sufficient.

Given:

RC = 29 min

Rq₁ = 1280 k

T₀ = 350 k

Equation:

RC dT/dt + T = Rq₁ + T₀

Hence, 29 dT/dt + T = 1280 + 350

dT/dt + T = 56.207

From the equation, we know dT/dt is the temperature varying with the time and T is the room temperature.

Conclusion:

This paper deals with the automatic control system to control the temperature with the control valve. To optimize the furnace for a better performance the following measures should be carried out: 1) Tapered return air drop with the proper ductwork setting and the joints should be properly sealed 2) reduction of turbulence and maintain low pressure drop the throat radius elbow and turning vanes should be fitted perfectly 3) Choosing suitable filter cabinet with proper quality 4) dual pipe combustion 5) Proper selection of thermocouple. These following measures could lower the cost of maintaining the furnace.

References:

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[2] M. Jelali S. Zareba, A. Wolff. Mathematical modelling and parameter identi- fication of a stainless steel annealing furnace. Simulation Modelling Practice and Theory, 60, 2016.

[3] F. Lawayeb D. Dumur A. Mouchette X.M. Nguyen, P. Rodriguez-Ayerbe. Temperature control of reheating furnace based on distributed model predictive control*. 2014. Cited on page 17.

[4] J. Löfberg. Yalmip : A toolbox for modeling and optimization in matlab. In Proceedings of the CACSD Conference, 2014.

[5] T. Kiefer A. Kugi A. Steinboeck, D. Wild. A mathematical model of a slab reheating furnace with radiative heat transfer and non-participating gaseous media. International Journal of Heat and Mass Transfer, 53, 2010.

[6] Curtis D Johnson, “Process Control Instrumentation Technology,” Pearson Education, 2009.

[7] Zhuzhen wang, Xiaodong Zhao, and Haiyan Wang, Design of series leading correction PID controller,” In the proc. of IEEE International Conference, 2009

[8] Wang Zhenglin, Guo Yangkuan,. Process control engineering and simulation. Beijing: Electronic Industry Press, 2009.