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Kent Kelin asked:


A Cupola is a type of furnace that is very much alike to blast furnace. The Cupola furnace is refractory lined, stack of steel that is around 20 to 35 feet high. It rests on a base plate that is made of cast iron and has four legs. The casting that is obtained from the cupola furnace is known as a cupola casting.

Cupola Furnace: The Cupola furnace is one of the oldest forms of furnace that have been used by the iron and metal foundries. The Cupola furnace is the crudest and the simplest furnace that has been used by the industries. However, the cupola’s use is declining and the furnace is fast being extinct.

Merits of the Cupola Furnace: Though the electric or the blast furnace has started replacing the Cupola Furnace, it has some special benefits of its own. The Cupola furnace can always remain continuously in action. The furnace also offers a very high melting rate. The most important merit of the furnace is that it has relatively very low melting cost. The furnace has also permits a lot of ease of operation.

However, due to the invention and development of the electric furnace, the use of Cupola furnaces has substantially declined. The electric furnace melts a much larger amount of metal. The operation cost of the electric furnace is also very low. The electric furnace also emits a much smaller level of smoke heat and also pollutants.

Construction of the Cupola furnace: The Cupola furnace is constructed in a crude and simple manner. The Cupola furnace is a vertical structure like a steel shell. The steel is lined with refractory bricks from the inside. The furnace has an opening half way in the vertical shaft. The charge is introduced through the opening. The charge is divided into different layers. It contains the metal that is to be melted. The metal is mixed with coke, fuel, and lime stone flux. The fuel is burnt directly into the air and is introduced in to the furnace through the tuyeres which are positioned above the hearth. The hot gases that have been produced ascend and pre heat the charge.

The cupolas in majority of the cases have drop down doors. The bottoms can be dropped down to facilitate the cleaning and the repairs. At bottom in front of the furnace is a tap hole. The molten metal is removed from the tap hole. At the rear there is also another slag hole. The top of the stag is covered with a spark or fume arrester.

Usually the cupola has a diameter of 405 to 2000 mm. It is operated on varying fuels for different metal ratios. The molten metal that can be produced in the copula of this size at a speed of 1 to 30 tones per hour.

Continuity of process: After the process of melting has ended, the charging is halted. However, the blast of hot air is always maintained till all the metal has been melted and has been taken out from the tap hole. When the air cools down, the doors at the bottom are opened and the residue that has been left behind is removed

DIY Metal Casting

Kent Klein asked:


In Metal Casting, the type of furnace you have at your foundry, or workshop determines a lot of the work you can or can not do. Many metal casters enjoy making their own furnaces and have done so quite successfully. The plans for home made furnaces are eagerly shared among the enthusiasts so finding blueprints and instructions should not be difficult. But before you rush off to find the blueprints you may want to take time to learn about the types of casting furnaces available to help determine which one will suit your needs. This is not saying that you should only have one furnace at any given time, but this way you will know what project will work best for which furnace.

Cupola

Cupola furnaces are among the most popular with backyard foundries. Many casters construct their own cupola which proves to be cost effective and highly efficient if done properly. Since cupola furnaces can achieve high melting temperatures they are primarily used for the melting of iron and bronze, though aluminum can also be melted when attention is paid to keeping the temperature low. The primary fuel source for cupola furnaces are coke using limestone for flux.

By not using a crucible, the Cupola furnace is able to melt the metal at a faster rate. While individual designs of these furnaces differ due to varying resources and design ideas, the basic component of a cupola furnace is the same. The cupola furnace must be elevated above the ground and installed with a drop bottom usually hinged, which will allow the excess metal, fuel source, and other waste to drop from the furnace. The tap hole and shaft which will allow the molten metal to flow from the furnace and into the ladle and air shafts are also required. A slag tap hole can be placed on the furnace usually in the back, higher then the tap hole, which will allow the slag to flow out easily.

With most small home foundries, the cupola furnaces are preferred over that of other furnaces for its high melting rates and for how easily the construction of such a furnace can be.

Reverberatory

The Reverberatory Furnaces are commonly found in industrial plants but there have been quite a number of home grown foundries that have used this furnace type with much success. The basic idea of a Reverbeatory Furnace is to use the heat reflecting off a surface, usually brick, to heat the metal, which is aluminum in most cases. This way, the metal does not come into contact with the fuel or the flame. This process is also used in Puddling Furnaces. The name Reverberatory and Puddling are often used interchangeably. By placing the metal in a shallow depression and then directing an intense flame over that depression and to the wall, the heat rebounds to melt the metal. Many casters will adjust the length of the flame since a longer path will mean that the heat will be more intense.

There are many designs for the reverberatory furnace, all of which will include exhaust ports and the rear of the furnace must be able to withstand the intense heat and be able to bounce it back into the chamber.

Bronze, aluminum, tin, and many other ores can be melted in a reverberatory furnace all depending on the construction of the actual furnace and the heat of the flame. This type of furnace has been around since at least the Middle Ages and was used primarily for bronze work.

Electric Arc

Electric Arc furnaces are normally used in Industrial foundries. Most hobby casters do not use these furnaces due to cost, space issues, and a general inconvenience. There are two forms of the electric arc furnace, the direct and indirect.

The direct arc has three electrodes which are used to heat metal by way of the arc. This creates an incredible amount of heat and so water jackets and other cooling devices will be needed for operation.

The indirect arc just uses one electrode and works in much the same way as the direct arc furnace.

Electric Induction

Electric Induction furnaces are becoming all the more popular with large industrial size foundries that enjoy the clean burning and efficiency of an electrical furnace. Copper coils surround the crucible with the metal inside which can reach extremely high temperature suitable for melting most metals.

Rotary

The Rotary style furnaces are hailed for their ease of use being fully or at the very least partially automatic. These furnaces tend to be harder to construct though there are resources and companies online that sell rotary furnace.

The different furnace types do affect the quality of the work being produced, so it is important to keep that in mind as you cast. With a little trial and error, you should find the perfect furnace to fit your needs so you can start building one of your very own.

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