Industrial resistance furnace design principle
Resistance furnace is a device and integrated device that converts electrical energy into thermal energy. According to joule's law, when the current I flows through the conductor of resistance R, the heat will be generated after time t Q: Q=0.24I2Rt (CAL) (1-1)
From the above formula, it can be seen that we control the three parameters I, R and t, which is equal to controlling the value of Q. The purpose of controlling the heating element can be achieved.
Before designing the technology, it is important to keep in mind that increasing the heat value does not mean increasing the resistance value of heating elements indefinitely, which often increases the manufacturing cost of the furnace. If the current of heating elements is appropriately increased, the calorific value will be multiplied. This is the essence of resistance furnace design principle to master.
Therefore, reasonable selection of heating elements is more important. According to the design principle, the necessary and reasonable value of R and sufficient value of I should be calculated first, so as to ensure that sufficient heat Q can be generated within the specified time t to reach the required temperature of the equipment. What needs to be emphasized here is that although it is important to choose the value of R, it is even more important to choose the value of I.
On the other hand, even if the heating element can send out enough heat, the equipment may not be able to reach the temperature we need to use. Less or very little heat loss from the stove is even more important. As virtually all experienced engineers know, the correct operating temperature of an electric furnace depends not only on how much heat is added to the furnace, but also on how much heat is lost. Therefore, it is one of the first and necessary measures for industrial furnace design engineers to design the insulation capacity of the furnace, how to reasonably choose the thickness of the insulation material and minimize the heat loss.
Electric heating elements are generally divided into two categories: metallic and non-metallic. Commonly used in metal heating elements are ferrochrome aluminum alloy and nickel chromium alloy, platinum-rhodium, molybdenum, tungsten, tantalum heating body and so on, non-metallic graphite heating body, silicon carbide heating body and so on.
(1) ferrochrome aluminum alloy heating element
At present, there are three kinds of domestic and common brands: Cr25Al5, Cr17Al5, Cr13Al4, etc. The alloy properties are shown in table 1-1. With the progress of technology, there are more and more brands of ferrochrome aluminum alloy heating elements including Mo, etc. They are applicable to work in the temperature range of 1000-1300℃, or even higher. They have good oxidation resistance, easy to process, high resistance, low temperature coefficient of resistance and low price. The dense oxide film of Cr2O3 can be generated at high temperature to prevent the further oxidation of the alloy by air, but it is not suitable to be used in the reducing atmosphere. In addition, contact with carbon, acidic medium, sodium silicate, asbestos and non-ferrous metals should be avoided as far as possible to avoid damaging the protective film. The main disadvantage of this heating element is that it has low strength at high temperature.
(2) nickel-chromium alloy electrothermal body
This kind of alloy electrothermal body is applicable to the temperature below 1000℃. Its models are Cr20Ni80, Cr15Ni60, etc. Its performance is shown in table 1-1. This kind of material is easy to process and has high resistivity and oxidation resistance. It can form Cr2O3 or NiCr4 oxide film at high temperature, but it is not suitable to be used in reducing atmosphere. Ni-cr alloy after high temperature use, as long as there is no overburning is still very soft.
(3) refractory materials and thermal insulation materials
In order to obtain a stable high temperature, two conditions must be met: first, there must be an electric heating body, that is, a heat source; second, there must be an insulator surrounding the electric heating body to prevent heat loss outward.
Refractory: in high temperature resistance furnace, the heating element itself temperature is higher, generally up to more than 1400℃, furnace structure needs to be arranged appropriate refractory. Qualified refractories must have the following conditions: high refractoriness, dense structure, good strength at high temperature, no obvious volatilization, and no reaction with the operating gas in the furnace.
Insulation materials: in order to reduce heat loss and increase the stability of the furnace temperature, often fill in the furnace shell insulation materials, they must have a small temperature coefficient, porosity, with a certain degree of fire resistance.
According to the temperature, it can be divided into three categories: A high-temperature insulation material greater than 1200℃, b900-1000 ℃, C less than 900℃.
High temperature insulation materials commonly used are light clay brick (1150-1400℃), light silicon brick not more than (1500℃), light high aluminum conversion not more than (1350℃), aluminum silicate fiberboard (1200-1400℃) and so on.
Medium temperature insulation materials commonly used are: light perlite and vermiculite.
Low temperature insulation materials are asbestos, mineral wool, asbestos is a very common heat insulation material, its chemical composition is water containing magnesium silicate, mineral wool is metallurgical slag with high pressure steam blowing fiber in the air quickly cooling and obtained from the artificial mineral fiber.
Now with the progress of technology, the best insulation material has been developed is a kind of high aluminum fiber cotton, it is light and soft like cotton, insulation performance is very good, the use temperature up to 1600℃
Of course, aluminum silicate fiber felt is also used in large quantities with better insulation performance.
So now for us to choose a spate of new thermal insulation material, enabled us to develop the design vision, abandon the past use of refractory brick and the concept of perlite, as far as possible the choice of the latest materials, heat preservation performance will be greatly improved, so that our industrial furnace manufacturing besides will become more and more light, and more and more to save energy.
