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Understanding Furnaces and Numerous of It's Types and Their Uses

Updated on March 12, 2013
Furnace
Furnace | Source

A furnace can be considered as an equipment that is especially used in the melting of metals so that they could be cast into different shapes as required, or they are used just to heat the materials to various temperatures so that their shapes can be altered with external force; forging is a good example of this process, or to change the properties of a metal, as in the case of heat treatment.

Selecting a fuel for combustion, as in the case of furnaces that generate the heat for the process through combustion, needs ample thought. The components of a burning fuel, like flue gases, can cause great changes in the material that is being processed within the furnace. For instance, certain materials are least tolerant to the sulphur that may be present in flue gases. Likewise, solid fuels on burning releases particulates that can change the properties of materials placed within the furnace for processing.

These are many reasons as to why furnaces often use liquid fuels for its operation. Otherwise gaseous fuels are employed, and in certain instances electricity is employed. Electricity becomes the fuel in the cases of induction and electric arc furnaces. These furnaces are specifically employed in the steel industry in the melting of cast iron. When non-ferrous materials are being melted furnaces are usually fired by fuel oil. Likewise, furnaces employed in the processes of heat treatment and reheating also employ fuel oil for furnace firing. When sulphur needs to be totally ruled out, LDO or light diesel oil is employed.

Furnaces can be considered to be economically run only when the furnaces are successful in heating up the material that is being processed to the desired heat by consuming as little as fuel as possible and without the use of extensive labor. Only the complete combustion of the fuel with right amount of excess air can lead to efficient operation of the furnace. Furnaces operate at very low efficiency, and the very high temperatures they operate at are believed to be the reason for it. A furnace that is required to heat a material above 1000 degree Celsius will be generating exhaust gases which too are heated to temperature above 1000 degree Celsius, and this leads to loss of heat in the chimneys.

Parts of a furnace

Most of the furnaces will more or less be having similar components as discussed below:

  1. Furnaces will have a refractory chamber that is usually constructed of materials that provide insulation, and their primary purpose is to retain the heat that is produced within the furnace.

  2. Hearth is the platform that is usually made of refractory material and has the function of supporting the load that needs to be processed within the furnace. Certain parts of the hearth may be water-cooled.

  3. Burners are used to burn the fuel, either liquid or gaseous, to provide the necessary heat to raise the temperature of the furnace chamber to the desired levels and to maintain it at those levels.

  4. The chimneys are the source through which exhaust gases escape from the furnace chamber.

  5. Doors are through which the load is either brought into the furnace chamber or removed from it, and they are referred to as the charging and discharging door. Numerous other equipment like pushers, roller table, and conveyors may be employed in moving the load into or out of a furnace.

Furnace Types

Furnaces are classified into two types based on the method employed to generate the heat, combustion type or electricity powered. Further the combustion type furnaces are further classified into various types based on the exact type of the fuel that is being used, the way the materials are charged within the furnace, the way the heat is transferred to the material, and ways through which heat is recovered. However this classification is merely for reference and is not practical, because a particular furnace may employ various types of fuel for its operation, and may also employ various means to charge its load, and so forth.

Thus based on what was discussed, furnaces may be classified based on the type of fuel that is used and accordingly there are furnaces that are either oil, gas, or coal fired.

Based on the way the load is charged furnaces could be classified as intermittent or batch, periodical, or continuous. Examples of periodical furnaces are furnaces like forging, re-rolling, and so forth. Continuous furnaces walking beam, rotary hearth furnaces, and so forth.

Based on the way the heat is transferred furnaces are classified as radiation type furnace or convection type furnace.

Based on the heat recovery technique employed furnaces may be classified as recuperative and regenerative furnace.

What are forging furnaces?

Forging furnaces are employed to help certain materials attain a temperature after which they can be forged. These furnaces are expected to generate heat in the range of 1200 degree Celsius to 1250 degree Celsius. These furnaces transmit heat by radiation and use an open fireplace kind of a system. The furnaces are capable of processing a load of about 6 tons in a day if they were kept operational for about 18 hours a day. The operational cycle of this furnace includes a time to heat up the furnace, a time to bring the load to the required heat, and a time to forge the materials. The fuel that is consumed by the furnace will be directly dependent on the type of material being processed and the number of times the furnace is reheated, and so forth.

What is the kind of furnace employed in a re-rolling mill?

A re-rolling mill may employ a batch type furnace or a continuous furnace as per their convenience and the quantity of output they intend to churn out.

Batch type furnace

The box type furnace is a good example of a batch type furnace that is employed in a re-rolling mill. The furnace is used to heat up ingots, scrap, billets, and so forth for the re-rolling process. The materials are taken into the furnace for charging and are discharged once the process is complete. The material gets converted to rods, or strips, or as desired. The furnace may be operating in the temperature range of 1200 degree Celsius. The operational cycle of this furnace includes the time taken to heat up the material to the required temperature and the time taken to re-roll the material to the desired product. These furnaces are capable of providing an output of about 15 tons in a day and may consume fuel in the range of 180 kg per tonne of material heated.

Continuous type furnace

These furnaces too have similar operational cycle as of the batch type furnace but they are capable of producing an output of around 25 tons per day after operating for about 10 hours. The advantage of these furnaces are that the load within the furnace manages to recover the heat from the flue gases as it traverses the length of the furnace. In comparison to the batch furnace the load is heated up rather slowly but in a more uniform and steady manner.

Furnace that provides Continuous reheating

In a continuous reheating furnace the load is continuously on the move and as it does so it gets heated up to anywhere in the temperature range of 1000 to 1250 degree Celsius. The load temperature is raised to a level which enables pressing or rolling of the load into the desirable shape or size. The doors of these furnaces are kept small to prevent heat losses and prevent air from entering. These furnaces are further categorized into two distinct types based on the way the load is transported into and out of the furnace.

  1. When the entire load is kept in one bulk and pushed through the furnace it is called as a pusher type furnace.

  2. When the load is placed on a hearth that moves, because the hearth is supported by structures that allows the load to move forward, it is referred to as the moving hearth furnace. Examples of moving hearth furnaces are rotary hearth furnaces, bogie furnaces, walking beam furnace, and so forth.

Every furnace has certain advantages and disadvantages and only a good furnace manufacturer will be able to fabricate a furnace that suits your requirement best. For instance if you consider a walking hearth furnace, it is a continuous moving hearth furnace where the load is transported into the furnace and out of it in a walking the hearth kind of a manner. The design of such a furnace is simple, and easy to construct, and this furnace can cater to a variety of load requirements, the losses of this furnace due to water cooling is negligible, the furnace can be emptied, and there will be very minimal markings and scratches on the load. But, the disadvantages of this furnace is that the temperature across the load may not be uniform since the bottom part of the stock has no way of being heated, and moreover the spaces between the loads tends to skip the heating of the sides of the loads. This in turn requires the stock to stay put within the furnace for substantially longer hours thereby affecting the yield of the furnace and thereby limiting it's flexibility.

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