Cupola Furnace working or constructionConstruction and working of Cupola Furnace   It is the constructed in the form of a hollow cylindrical vertical steel shell.It is lined from inside with a refractory material  .It is generally supported on four cast iron lags mounted on a concrete base .The bottom of the furnace is closed by two cast iron doors hinged to the bed plate of the furnace .A wind box cast iron encircles to the outside of the furnace bottom .This box is connected to the furnace blower by pipe known as the blast pipe .Air which supplies the oxygen necessary to burn the fuel ,is forced through the cupola by a blower .The top of the furnace is shielded by a mesh screen and topped with a cone shaped spark arrester ,which permits the free vent of the waste gas and deflects spark and dust back into furnace.working  

First of all ,the waste material dumped on the floor under the furnace after the previous heat must be cleared away.the cupola repair man then enters the cooled furnace and cleans out the slag and refuse on the lining and around the tuyeres from the previous run.Any bad spots or broken bricks are repaired with a refractory material to return the lining to its original shape .Then the bottom doors are swung into the position and the prop placed under them .all cracks are closed with fire clay and a layer of black moulding sand is placed on bottom .This sand is rammed down and given a slope towards the spout .The kindling material usually soft,dry pieces of wood is placed on the sand bed and a charge of coke is put in the furnace .The kinding and bed charge of coke are ignited through the tap-hole opening .As soon as the coke bed is thoroughly ignited ,the first charge of metal ,pig iron ,scrap and flux is placed on top of the bed charge of glowing coke .A weighed charge of coke is placed on top of the metal charge .After the cupola is fully charged upto the charging door ,the iron should soak in the heat for 45 mins or longer .No forced draft is used during this period .The only draft comes from tuyere peep hole and spout openings. As the charge descends in the furnace it is preheated untill it reaches the area where the melting takes place .The metal melts in small drops .Intermittently ,the tap hole is oppened ,allowing the metal to flow through the spout into the receving ladder.It is then closed again with a conical clay plug.This procedure is repeated ,until all the metal is melted and poured.

The Cupola Furnace

For many years, the cupola was the primary method of melting used in iron foundries. The cupola furnace has several unique characteristics which are responsible for its widespread use as a melting unit for cast iron.

1. The cupolas is one of the only methods of melting which is continuous in its operation2. High melt rates3. Relatively low operating costs4. Ease of operation

In more recent times, the use of the cupola has declined in favour of electric induction melting, which offers more precise control of melt chemistry and temperature, and much lower levels of emissions.

The construction of a conventional cupola consists of a vertical steel shell which is lined with a refractory brick. The charge is introduced into the furnace body by means of an opening approximately half way up the vertical shaft. The charge consists of alternate layers of the metal to be melted, coke fuel and limestone flux. The fuel is burnt in air which is introduced through tuyeres positioned above the hearth. The hot gases generated in the lower part of the shaft ascend and preheat the descending charge.

Most cupolas are of the drop bottom type with hinged doors under the hearth, which allows the bottom to drop away at the end of melting to aid cleaning and repairds. At the bottom front is a taphole for the molten iron at the rear, positioned above the taphole is a slaghole. The top of the stack is capped with a spark/fume arrester hood.

Typical internal diameters of cupolas are 450 mm to 2000 mm diameter wich can be operated on different fuel to metal ratios, giving melt rates of approximately 1 to 30 tonnes per hour.

A typical operation cycle for a cupola would consist of closing and propping the bottom hinged doors and preparing a hearth bottom. The bottom is usually made from low strength moulding sand and slopes towards a tapping hole. A fire is started in the hearth using light weight timber, coke is charged on top of the fire and is burnt by increasing the air draught from the tuyeres. Once the coke bed is ignited and of the required height, alternate layers of metal, flux and coke are added until the level reaches the charged doors. The metal charge would typically consist of pig iron, scrap steel and domestic returns.

An air blast is introduced through the wind box and tuyeres located near the bottom of the cupola. The air reacts chemically with the carbonaceous fuel thus producing heat of combustion. Soon after the blast is turned on, molten metal collects on the hearth bottom where it is eventually tapped out into a waiting ladle or receiver. As the metal is melted and fuel consumed, additional charges are added to maintain a level at the charging door and provide a continuous supply of molten iron.

At the end of the melting campaign, charging is stopped but the air blast is maintained until all of the metal is melted and tapped off. The air is then turned off and the bottom doors opened allowing the residual charge material to be dumped.

Cupola furnace,  in steelmaking, a vertical cylindrical furnace used for melting iron

either for casting or for charging in other furnaces.

René-Antoine Ferchault de Réaumur built the first cupola furnace on record, in

France, about 1720. Cupola melting is still recognized as the most economical

melting process; most gray iron is melted by this method.

Similar to the blast furnace, the cupola is a refractory-lined steel stack 20 to 35 feet

(6 to 11 metres) high, resting on a cast-iron base plate with four steel legs. The

bottom of the cupola furnace has two hinged doors supported in the closed position

by a centre prop. Molding sand is rammed over the closed bottom doors to support

the coke bed, molten metal, and succeeding charges. Forced air for combustion

enters the cupola through the openings (tuyeres) spaced around the rim of the lower

portion of the cupola.

Iron, coke, and limestone flux are placed on a bed of coke high enough to hold the

iron above the tuyere openings, where the temperature is the highest. The melting is

continuous, and molten metal may be allowed to flow continuously through an open

tapping spout at the base of the cupola, or it may be tapped intermittently.

Intermittent tapping is accomplished by piercing a clay bott, or plug, in the tapping

spout with a pointed steel rod to create a passageway called the breast of the

cupola. The tapping spout is stopped by plugging with a fresh clay bott. Wastes flow

out in the form of slag when the slag spout is tapped. At the end of the operation, the

prop is knocked from under the bottom doors and the remaining contents

discharged.