Furnaces & Types of Furnaces

Introduction:

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Furnace is a device that is used to heat a material in order to exchange their shape as like in shaping and forging or the properties of the material can be changed. These furnaces are mostly used for melting the metals that are used for casting. Furnace can be termed as a type of oven.

It is an enclosed structure inside, which the material is heated to very high temperatures. It is a device that is designed to perform the water or air heating and is circulated throughout a building in a heating system.

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Features:

The design of the furnaces is in such a way to consume fuel and heat as less as possible in a given time that requires low labor cost.

Efficient features:

The amount of heat that is to be given is prior determined to the material. There exists a sufficiency in the liberation within the furnace to heat the required stock. A proper heat transfer from the gases to the surface of the heating stock takes pace.

Temperature should be uniform within the stock. The heat loss from the furnace is minimized to the least possible. In the furnace the product of the flue gases directly comes in contact with the provided stock.

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Components of an industrial furnace:

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In industrial furnaces it serves the purpose of heating i.e. it serves as a reactor to provide heats to the reaction. The design of the furnaces varies, however the basic features are most common among all of them.

The basic components of a furnace are:

  • Radiant section
  • Convection section
  • Radiant coil
  • Burner
  • Soot blower
  • Stack
  • Insulation
  • Foundations
  • Access doors

Process:

The fuel flows in the burner is burnt from an air blower. The furnaces can have more than one burner. The furnaces can be arranged in cells with a particular set of tubes, or the burners can be flooring mount, on wall or roof mounted depending on the design demand. The flame of the burner heats up the tube those in-turn heats up the fluid that is present inside.

The first part of the furnace i.e. radiant section or fire box, where the combustion takes place by the action of radiation of the tubes around the fire inside the chamber. The heat fluid heats upto the desired temperature level. The gas that is formed from the combustion is known as flue gas. After the fire box the flue gases enters convection section where the heat is recovered before their vent into the atmosphere by convection. In industries commonly secondary fluids are heated in these furnaces with special additives like anti rust and with high heat transfer efficiency. Thus heated fluid is circulated in the whole plant where heat is needed instead of direct heating of the stock material in the product line. The material can be volatile or may be prone to cracking with the furnace temperature.

Types:

  • Blast furnace:

Blast furnace is a special type of furnace that is used to make iron from its ore. The dimensions of the blast furnace can be upto 60 meters tall and 15 meters in diameter. It is a biggest chemical reactors and also high ovens. The blast furnace is built of steel case and bricks that are made of magnesia that is very much resistant to heat and it does not melt. The furnace is cooled with water that runs inside the case and bricks. The process of making is simple, from the iron ore that is basically iron oxide. Iron is made from it on removal of oxygen. A crude iron oxide called pig iron is left over. Reduction is the process of removing oxygen where this process used carbon as this can take the ore to high temperatures.

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Ore, lime stone and carbon in the form of coke is put at the top of blast furnace in the form of layers. Special types of nozzles are used which are called tuyers, these put up the air in the furnace that are at the bottom of the furnace and the process is termed as blasting. The bottom portion is filled with liquid pig iron and slag that is called hearth. This content is removed from the bottom and the process is known as skimming. Slag pot is the collector of the liquid slag.  Iron can be taken from the hearth by a process called tapping. Pig iron is used in railway wagon applications.

Chemical reactions:

3 Fe_2O_3+C\to \: 2 Fe_3O_4+CO<br /><br /><br /><br /><br /><br /><br />

Fe_3O_4+CO\to \: 3 FeO+CO

FeO+CO\to \: Fe+CO

  • Open hearth furnace:

Open hearth process is also called as Siemens-martin process i.e. steel making technique for most of the applications in 20th century. This process seeks a means to increase the temperature in a metallurgical furnace that uses the waste heat given out from the furnace that directs the fumes from the furnace through a brick checker work. A brick is heated to a high temperature, and then it is introduced into the furnace. The device is first used to produce steel that is charged with pig iron and wrought iron along with their scrap. In US and Britain are best suited to the open hearth process.

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Natural gas or atomized heavy oils are used as fuel. The air and fuel are heated before combustion. The furnace gets charged with blast furnace iron in liquid form, steel crap that is combined with ore, lime stone, dolomite and fluxes. The roofs and body of the durance is made of highly refractory materials as magnetic bricks. 600 tons is the capacity of the open hearth furnaces that are installed in groups for massive auxiliary equipment, that is needed to charge up the furnaces and to handle the liquid steel that can be employed efficiently. It is most commonly used in industrial applications as electric arc furnace and oxygen process based furnaces.

  • Induction furnace:

Induction heating of metal will provide heat required for the electrical furnace. Induction furnace will provide clean, energy efficient and well controlled process of melting when it is compared with other processes. In most of the modern foundries use this type of furnaces instead of cupolas to melt cast iron as the former emits lots of duct along with other pollutants. The capacity ranges from less than one kilogram to hundred tones capacity are used in melting iron, steel, copper and aluminum and other precious metals. As the process does not use combustion, material temperature is not higher than the required to melt it. This processing prevents the loss of valuable alloying elements. The induction furnace lacks in the refining capacity where the charge materials clean the oxidation products and the known composition. The alloying elements may have a chance to lose alloying elements due to oxidation.

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The frequency of operation is high and is dependent on the material that is to be melted, furnace capacity required. If the volume of the melts is small then it requires higher frequency. A pre-heated furnace of the melting iron can melt the cold charge by tapping of the readiness within an hour. The power supply range is from 10 KW to 42 KW that melts in size ranges of 20 kg to 65 tonnes of the respective metals. The operating induction furnace can be used by the operators to identify the correct operation and its corresponding power level.

  • Electric arc furnace:

These furnaces are used to produce special quality steels that are lighter products as reinforcing concretes. Electric Arc Furnace does not use any hot metals as basic oxygen routes. The equipment is charged with a cold material that will generally be steel scrap. Other raw materials include the products from iron ore. Direct reduced irons, iron carbide, pig iron (iron product from blast furnace) that is casted and is allowed to get cooled are the included products. This process will not be charged directly.

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The steel scrap is induced in to the furnace by an overhead crane. Then a lid is used that is swung into position over a furnace. An electric current is passed through the electrodes that are positioned in the lowered that lowers the furnace. Other metals also get added to the steel that provides the required chemical composition. Oxygen is blown to purify the steel and lime in addition to the combined impurities. After sample collection the furnace is tilted to pour the floating slag on the molten steel. The modern furnace can melt up to 150 tonnes in around 90 minutes.

  • Reverberatory furnace:

In reverboratory furnace that is used in copper, tin and production where a furnace is used to smelting or in the refinement where the fuel in not in direct contact with the ore but the flame that is blown will heats out from the other chamber. The steel making is an open hearth process. The roof gets arched with the highest point overt the radiation section. This slopes in downward direction towards a flue bridge that detects flames in the reverberates. This hearth is made to be dense and impervious to meet heavy matte or else the molten metal that cannot penetrate through it. The walls are made of chemical resistive material. The process takes place continuously in the reverboratory furnace. The roof is charged with ore concentration, slag that flows from top to the end of the device. The matte is tapped at regular intervals from the deepest parts of the ore and then gets transferred to a converter where it gets further refined.

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Several technical innovations are used in the increment of the production capacity of the furnace. Its basic construction remained as earlier where the permitted higher temperatures that is refined at a faster rate. The smelting in the reverboratory gave a way to newer processes and the use of electric as well as flash furnaces.

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