Physical Process Modeling

    . . .            

ELECTRO-RESISTANCE FURNACE
part II


Created by Physical Process Modeling



    APPLICATION
    The furnace with movable hearth is designed for heat-treatment of metal pans at temperatures of up to 1000oC. The furnace is used for temper hardening normalization, heating before plastic deformation, etc. In addition, the furnace finds application, in the drying and baking of moulds for precision founding of steel as well as in drying and baking of ceramic glaze.

    CONSTRUCTION
    The electric resistance box furnace is composed of two parts: a stationary body and a movable hearth. The body of the furnace is built up of shaped and sheet steel and the heating chamber is built up with interlocked bricks. The equipment is insulated with special type heal-resistant ceramic tiles and asbestos sheets. The parts to be heat-treated are arranged on the car-bottom when it is drawn out of the furnace in the extreme front position. The car-bottom is operated by an electromechanical device. The car-bottom seals the opening to the furnace. A pull-out lead supplies power to the healing chamber. A limiting controller checks the extreme front and back positions of the car-bottom. A contactor unit supplies power to the healers in the movable hearth. An automatic blocking device locks the heaters when the car-bottom is brought out. The heating chamber is divided into two thermal zones, and each one of these is equipped with a separate heat-regulating unit composed of a thermocouple and a thermoregulator.
    The furnace is fitted out with an electric panel where the switching and controlling equipment is installed. Loading capacity-max 2000kg.




    APPLICATION
    These furnaces are widely applied in thermal processing at maximum temperatures of up to 1000oC. They are designed for heating metal of metals and more specifically for heating metal parts before age hardening and temper hardening. They also find extensive application in the banking of non-metal materials as ceramic glaze and others. The furnace is not suited for heating materials whose vapours would react with the heaters or the heat-resistant coating of the furnace.

    CONSTRUCTION
    The furnace has a welded construction of sheet and shaped steel. The heating chamber is built up with interlocked bricks and is insulated with thermal insulation ceramic tiles and asbestos sheets. The heating coils made of high quality resistance wire are installed on the bottom, vault and waits of the heating chamber. An automatic blocking device locks the heaters when the furnace door is opened. The door is operated electromechanically or manually. A thermocouple and a thermoregulator control the temperature. The furnace is fitted out with an electrical panel where the switching equipment is installed.

    INSTALLATION AND OPERATION
1. The user installs the furnace on a suitable site. The first two types of furnaces as indicated m the specification table, i. e. the small size and middle size furnaces, are delivered in a furnace state. The third type of furnaces, i.e. the large size furnaces, require a foundation specially laid for the purpose; therefore the user is required to construct a suitable foundation to install the furnace and the electric panel. The third type of furnace is built up by the manufacturer on the site.
2. The furnace should be accurately dried up before initial use, after repair work or after a longer stand-still period. The drying should be carried out in agreement with the operating instructions.
3. The equipment should be operated as indicated in the operating instructions.
The normal and trouble-free operation of the furnace and the equipment is guaranteed: under the following conditions: - ambient temperature +5oC - +40oC - humidity up to 80% - altitude up to 1000 m


furnace heat field



The video material (simulation) illustrates some of the transitional processes discussed in this chapter. For more information, contact us.



MOVIE 1: Heat Transfer in Electro-Resistance Furnace (ERF)
MOVIE 2: Heat Transfer in ERF
MOVIE 3: Heat Transfer in ERF
MOVIE 4: Heat Transfer in ERF
MOVIE 5: Heat Transfer in ERF
MOVIE 6: Heat Transfer in ERF
MOVIE 7: Heat Transfer in ERF

MOVIE 8: Heaters 2D;     MOVIE 13: Heaters 3D
MOVIE 9: Heaters 2D;     MOVIE 14: Heaters 3D
MOVIE 10: Heaters 2D;   MOVIE 15: Heaters 3D
MOVIE 11: Heaters 2D;   MOVIE 16: Heaters 3D
MOVIE 12: Heaters 3D;   MOVIE 17: Heaters 3D

MOVIE 18: Dual chamber 1;
MOVIE 19: Dual chamber 2
MOVIE 20: chamber-loading     +3D
MOVIE 21: cylindrical-loading
MOVIE 22: Cooling 1     MOVIE 23: Cooling 2
MOVIE 24: Cooling 3     MOVIE 25: Cooling 4


Theory Background

Heat equation:
Learn more ...
Radiation:

Conduction and convection:


Furnace-3D field - Temperature
Furnace-3D field - Total heat flux
Furnace-3D field - Temperature, Total heat flux
Furnace-3D field - Temperature, Total heat flux



Finite element method (FEM)- Heat transfer modeling
Furnace 1



high furnace heat field


Regulation Of The Process Of Heating
(simulation)

model-experiment numerical method


Regulation Of The Process Of Heating
(simulation and experiment)


model-experiment numerical method

model-experiment numerical method





EXPERIMENT
Furnace 1



thermal processing experiment-simulation

thermal processing experiment-simulation

electric resistance furnace - experiment

electric resistance furnace - experiment

electric resistance furnace - experiment

electric resistance furnace - experiment

electric resistance furnace - experiment







    APPLICATION
    The furnaces find wide application in thermal processing at temperatures up to 1000oC. They are mainly used for heat-treatment of metals and more specifically for heating metal parts before age hardening and temper hardening. They also find extensive application in the baking of non-metals materials as ceramic glaze and others. The furnaces are not suited for heating materials whose vapors would reach with the heaters or heat-resistant coating of the furnace.

    CONSTRUCTION
    The furnaces have a welded construction of sheet and shaped steel. The heating chamber is built up with interlocked bricks and is insulated with thermal insulation ceramic tiles and asbestos sheets. The coil heaters made of high quality resistance wire are installed on the bottom, vault and walls of the heating chamber. Automatic breaking is provided on opening the furnace door. The temperature is controlled and regulated automatically by means of a thermocouple and a thermoregulator. The furnace is fitted out with an electric pane where the switching and control equipment is installed.





MODEL
Furnace 2



furnace simulation - field


Regulation Of The Process Of Heating
(simulation)


temperature regulation - matlab








EXPERIMENT
Furnace 2




electric resistance furnace - experiment

electric resistance furnace - experiment

electric resistance furnace - experiment

electric resistance furnace - experiment

electric resistance furnace - experiment






MODEL
Furnace 3



Regulation Of The Process Of Heating
(simulation)


temperature regulation - matlab

temperature regulation - matlab

Heaters
(simulation)


furnace heater simulation

furnace heater simulation







EXPERIMENT
Furnace 3




electrical furnace experiment

electrical furnace experiment


electrical furnace experiment

electrical furnace experiment

Heaters Modeling

heaters modelling - mesh




Transient Processes In High Temperature
Vacuum Electrical Resistance Furnace With MoSi2 Heaters




Abstract

    In this publication is analyzed the work of high temperature vacuum ERF with MoSi2 heaters, loaded with details with different thermal parameters and to implement of processes with given requirements by controlling different parameters of the power system. For analysis of transient heat processes in high temperature vacuum ERF with water cooled walls, a system of differential equations is composed corresponding to the thermal schematic and is visually modeled in MATLAB/SIMULINK.

Introduction

    Thermal processing in high temperature vacuum electrical resistance furnace (ERF) is characterized by predominant radiative heat transfer. The coefficient of radiative exchange is very high because of its proportional temperature dependence. That causes body surface to heat very rapidly. The rate of temperature change inside the body is determined by its thermodynamic properties and may be evaluated through the coefficient of thermal diffusity, a=/(c.). Thermal inertness of the body is defined by its interaction with the environment and is characterized by the Bio's criterion: Bi=(l.) /. Great value of the Bio's criterion (Bi>0,5) is obtained for massive details with bad thermal conductivity that may cause dangerous thermo-mechanical strains to occur.
    Certain publications exist numerous publications but they are for ERF with metal heaters. In high temperature ERF metal-ceramic heaters are used, wich are known by the great value of the thermal coefficient of electrical resistance. That's why the process of heating up the furnace is always achieved by controlling electrical power, resp. voltage. The information in firm literature gives certain recommendations but they are not conformable to the specific technological process.     The aim of the following publication is to analyse the work of high temperature vacuum ERF with MoSi2 heaters, loaded with details with different thermal parameters and to implement of processes with given requirements by controlling different parameters of the power system.

Research

    For analysys of transient heat processes in high temperature vacuum ERF with water cooled walls, a system of differential equations is composed corresponding to the thermal schematic on figure 1. All thermodynamic properties - specific heat, coefficient of thermal conductivity are temperature dependent. The system is visually modeled in MATLAB/SIMULINK and solved using Runge-Kuta's numerical method with variable step ode45.

    The analysis is carryed out at the following conditions:
-     Vacuum electrical resistance furnace with cylindrical chamber and dimensions : diameter d=0,3m, height h=1,1m;
-     MoSi2 heating element with power P=2000W and dimensions: diameter d=0,006m, lenght l=0,6m;
- Load parameters:
        metal body with dimensions 0.2X0.2X0.5 m, coefficient of thermal conductivity =25 W/m.oC, specific heat c=500 J/kg.oC, density =7720 kg/m3, coefficient of thermal diffusity a = 6.4.10-6 m2/s;
        body with dimensions 0.2X0.2X0.5 m, coefficient of thermal conductivity =0,98 W/m.oC, specific heat c=890 J/kg.oC, density =1800 kg/m3, coefficient of thermal diffusity a = 6.11.10-7 m2/s.

heat transfer equation


where: Tn, TstSEP, Tst, Tc - are respectively temperatures of: heater, furnace wall (equal to 50oC), detail's surface (which is the temperature at the center of 1mm thick layer), detail's center; Cn, Cs, Cd are respectively thermal capacities of: heater, surface layer of, inside the detail; Rr1, Rr2 thermal resistance of radiation respectively to the furnace wall an to the detail's wall; Rs, Rd thermal resistance of conductivity respectively for the surface layer and for the inside layer.

high temperature furnace - substitution scheme

Fig. 1


    Theoretic analysis is carryed out by connecting the heating element to a system with:
    - constant current;
    - constant voltage;
    - constant power;
    - power step regulator.

    Calculations for the appropriate values of the current and voltage are made based on the temperature dependence of the electrical resistance which is approximated by third degree polynomials.
R(T) = A +BT + CT2 + DT3
where:
A = 7.489.10-3 ;
B = 4.351.10-5 /oC;
C = 1.944.10-8 /(oC)2;
D = -7.285.10-12 /(oC)3;

Work at constant current

    The value of the current is chosen equal to the maximum value for the specified type of heater - 150A.
    Graphically the results are presented on fig. 2 5, where:
    - fig. 2 and fig. 4 shows the beginning of the heat up process for furnace loaded with chamot resp. metal detail;
    - fig. 3 and fig. 5 shows the whole process.

furnace transient processes

Fig. 2

furnace transient processes

Fig. 3

furnace transient processes

Fig. 4

furnace transient processes

Fig. 5

    While heating a chamot detail, due to its bad thermal conductivity a great temperature difference occurs between surface and inner layers compared to the metal body (fig. 3, fig. 5). Temperature at the surface reaches rapidly temperature of the heater and inside the heating rate is much slower because of the great thickness of the body.
    The thermophysical properties of the materials determine the difference in time constant of the process. Transient process ends earlier for the metal body, but in both cases the established values of the temperature and the power are equal, although temperature rising is considerably greater at the beginning for the chamot body.

Work at constant voltage.

    The value of the voltage is chosen on condition that the power dissipated by the heating element will not be above the specified value: furnace equations, where Rmin is the resistance at room temperature. For the given type of heater element that voltage is 4V. In vue of the positive temperature coefficient the power drops down.
    Graphically the results are presented on fig. 6-9, where:
    - fig. 6 and fig. 8 shows the beginning of the heat up process for furnace loaded with chamot resp. metal detail;
    - fig. 7 and fig. 9 shows the whole process.

    Temperature difference between surface and center of the body is smaller than at constant current (fig. 7). The values of temperature and power at the end of the process are equal for chamot (fig. 7) and metal body (fig. 9) but they are attained for a different amount of time.

vacuum furnace - ode simulation

Fig. 6

vacuum furnace - ode simulation

Fig. 7

vacuum furnace - ode simulation

Fig. 8

vacuum furnace - ode simulation

Fig. 9

Work at constant power

    In this case the heater is loaded at nominal power-2000W. The value of the established temperature is higher than the one at constant current.

    Graphically the results are presented on fig. 10 13, where:
    - fig. 10 and fig. 12 shows the beginning of the heat up process for furnace loaded with chamot resp. metal detail;
    - fig. 11 and fig. 13 shows the whole process.
    The temperature difference between surface and center of the metal body at the beginning of the process is twice the one at constant current (fig. 12). Temperature of the heating element rises sharply and then changes with much slower rate while heating chamot details (fig. 10) and in case of the metal body the heater temperature rises more fluently.

furnace constant power

Fig. 10

furnace constant power

Fig. 11

furnace constant power

Fig. 12

furnace constant power

Fig. 13

Work at step power regulator

    Graphically the results are presented on fig. 14-17, where:
    - fig. 14 and fig. 16 shows the beginning of the heat up process for furnace loaded with chamot resp. metal detail;
    - fig. 15 and fig. 17 shows the whole process.
    This regime is achieved by step change of the electrical power at given temperature according to table 1

Table 1

ToC 20 - 100 200 - 500 600 - 800 900 - 1000 1100 - 1300
P, W 200 600 1000 1300 1900


    By carefully selection of the switching steps, the temperature difference between surface and center of the body may be reduced to a safer value, but the commutation of the power supply causes a step change in heater's temperature. That is more significant in case of chamot detail (fig. 15). Because of chamot's bad thermal conductivity surface layer heat up rapidly and almost reach heater's temperature. Due to great value of the thermal resistance, heat flow to the body is small and temperature of the heater is higher than in case of metal body (fig. 17).

vacuum furnace regulator

Fig. 14

vacuum furnace regulator

Fig. 15

vacuum furnace regulator

Fig. 16

vacuum furnace regulator

Fig. 17

Conclusion

    In viru of the analysis of the work of high temperature vacuum electrical resistance furnace with MoSi2 heaters the following conclusions may be made:
    - Thermophysical properties of heated details have influence on process time constant. The established values of temperature and power are equal for chamot and metal body.
    - Thermal processes at constant current and constant voltage are characterised by sharp change of the power at the beginning (drop down at constant voltage and rise at constant current) and then it fix at certain level till the end of transient process.
    - Increase of temperature is much higher in case of chamot body, witch along with its bad thermal conductivity cause a great temperature difference between surface and center of the body. That imposes chamot details for thermal treatment to be much thinner.
    - By carefully selection of the switching steps, the temperature difference between surface and center of the body may be reduced to a safer value.







MoSi2 Heaters
Simulation

MOVIE

heaters - heat field

heaters - heat field

Vacuum Electrical Furnace

vacuum furnace - experiment

vacuum furnace - experiment


Heat Transfer Module. Modelling, Analysis and Design.



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