The Basic Principle of Resistance Wire Furnace

It consists two parts,the resistance wire and the insulation material, which are used to convert electrical energy into heat energy,and The furnace can reach the required temperature and the suitable temperature distribution . Besides, the furnace body also includes the furnace tube, the furnace rack,the shell,the terminal and so on.The furnace tube is used to place the test material, the furnace shell contains insulation material, the furnace supports the furnace body, and the terminal rod is connected with the power supply and resistance wire.

The Resistance furnace is a device that can convert the electrical energy into heating energy,when the electric currentI go through the conductor of resistance R,it can creat heat energy Q after period t.

The purpose of control heating body is achieved by controlling I,R and t, It is necessary to choose the resisitance wire reasonably ,and on the other hand, even if the resisitance wire can produce enough heat, the furnace can reach enough high temperature ,which is largely determined by the heat dissipation of the electric furnace. In fact, the temperature of the electric furnace depends on the heating and cooling of the furnace. Therefore,the heat preservation ability of the furnace is very important.It is necessary to use thermal insulation materials to reduce heat loss.

Resistance wire is generally divided into metal and non-metal.In the metal resisitance wire the iron chromium alloy the nickle chromium alloy,platinum-rhodium, molybdenum,tungsten and tantalum resistance wire is widely used. Non-metallic resistance wire includes graphite resistance wire and silicon carbide resistance wire.

2.selection of resisitance wire

nichrome resisitance wire

Nickel-chromium alloy resistance wire normally use temperature should not exceed 1345℃,

or it will happens phase change,in use process,we do not wish the phase change happen ,because it will change the volume of the resistance wire and shorten its service life.Whereas, we also hope the using temperature of resistance wire can be higher .According to the phase diagram of Cr-Ni, control the percentage of the nichrome alloy in the zone between yellow lines can avoid the happening of phase change,and reach the related higher using temperature of resistance wire.

This kind of alloy resistance wire is suitable for the temperature under 1000℃，its model for Cr20Ni80,Cr15Ni60,its performance is shown in table 1. This kind of material is easy to process, has high resistivity and oxidation resistance, can produce Cr203 or NiCr4 oxidation film at high temperature, but should not be used in reducing atmosphere.The Ni-Cr alloy is very soft as long as it is burnt after high temperature use.

Table1 Ni-Cr Alloy Performance

In lab, According to various requirements of the design and production of the electric furnace are mostly small tubular furnace ,the power is generally around 10KW.The design mentioned here mainly includes the determination of power,the choice of electric heating body ,the selection of refractory materials and thermal insulation materials.

The determination of power

The electric furnace power is a index to measure the size of the furnace from energy area. In fact,due to the complicating of the circuit cooling condition,it is very difficult to make sure the loss of the furnace power and the temperature point under a sure power.So it usually rely on some basic concept of empirical or semi-empirical methods to help hence the balance of the energy

For a circular furnace the surface area of the heating tube is first calculated.Assuming the stove for moderate degree of heat preservation,can experience data detected by table 2 100cm2 wattage heating surface area within the required power, then multiplied by the heated surface area in the hearts obtains the need power.

           Table 2 power required for each 100cm2 tube surface at different temperatures.

For example: the inner diameter of the tube is 10cm,heating part length is 80cm,it is expected to be heated to 1100,what’s the power that the furnace needed under the medium heat preservation situation.

First Step: to calculate the inside surface area of hearth: S=∏*D*L=3.14*10*80=2513(cm2)

According to form 1-4,we can know that every 100cm2 surface area of hearth nees 220w, therefore, the total power of furnace tube is:

Although the calculation method is not rigorous ,it is still very useful in reality work.

The Calculation of thermal insulation

According to the maximum temperature and the working atmosphere, make a selection of the kinds of thermal materials.For example to make a maximum temperature of 1100 in airfunace, can choose Cr25Al5 heating wire as the heating element ,but it must be clear that the highest use temperature electric heating element refers to the electric heating body working in the highest surface tempreature in the dry air, not only refers to the heaven temperature .Because of different cooling conditions,general requirements furnace top temperature low than the highest use electric heating body temperature about100℃.In addition , it must be clear that the surface load of the electric thermal body refers to the number of power of the furnace charged by the electric heating body is large, and the amount of electric heating body is large, and the amount of electric heating body is low ,but the higher the surface load of the electric heating body in the unit surface area.Under the condition of certain furnace power,the surface load of the electric heating body in the unit surface area,the surface load of heating body is large ,and the amount of electric heating body is low,but the higher the surface load of the electric heating body is,the shorter the life expectancy,so the choice should be appropriate.Table 3 is the surface load valve of the Ni-Cr electric thermal body ,which is an important parameter in the calculation of electirc heat body.In the calculation of electric heating body allows the surface load to be removed normalloy in order to use safety, so that the voltage can be adjustable.The working voltage is usuallly calculated in 200v .

Below,the calculation procedure is illustrated with the example of resisitance wire furnace: Furnace tube size is 100*110*1000mm,furnace temperature 1100℃， voltage220v,oxidizing atmosphere,furnace medium heat preservation, heating zone length is 800mm,strives for the resistance wire length and diameter.

1)calculation of tropical area The surface area of the tropical zone

S=∏*D*L=3.14*10*800=2513cm2

The calculation of the power From table3,1100,100 per cm2 furnace area of the required power p=220w,so the electric power required:

2) The selection of electric heating body.

According to the requirement and the table 1-1,can choose Cr25Al5 iron chromium aluminum wire for the electric heating body ,according to table 1-5,Cr25Al5 electric hating at 1100 to allow body surface area load for 10w/cm2.Detected by the table 1-1 Cr25Al5 at 20℃ than the resistance of Po=Ω * was 1.45 / m, temperature coefficient = (3-4) * 10-5 / ℃, so than resistance at 1100 ℃

the power of furnace

w/cm2 Electric heating surface load

V volatage

mm diameter

put actual coefficences into formula (1-7),we can get the diameter of the Electric Heating Wire

The calculation of the length of the electric heating wire:

Sectional area of Electric Heating Wire

Heater resistance

Heater length

R- the total resistance of heater

f- the sectional area of resisitance wire

L- the length of heater

put actual figures into the formula

                                          The Surface load value of the Ni-Cr electric heater

Checking Computations

The above calculation is correct and can be checked according to the surface load formula

Put related data into the formula(1-12) and get the result that lower than the surface load value chosed during designing , therefore,safe use is guaranteed