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Cupola coke bed requires care.

The most important factor in successful cupola operation is the proper preparation of the coke bed-getting the fire going. A big part of doing that is knowing how to concentrate heat uniformly and quickly.

To help the weekend barbecue chef who has trouble starting a reluctant grill while under pressure to "hurry up, the corn is done,' a simple, highly efficient backyard charcoal starter is available. it consists of a short, vented sheet metal stack that one fills with briquettes. The bottom is fitted with a charcoal bed screen under which is placed a lighted ball of newspaper. Presto! A great bed of white hot coals every time. The starter is really a mini-cupola. it uses side vents (like tuyeres) for air to support combustion and concentrates intense heat in a confined space just as a cupola furnace does.

A close friend tells of having to fire up a 96 in. cupola early every Monday morning so that a good coke bed was ready for his foundry's first shift. He talks about collecting kindling, piling in coke and lighting the pile with gas-fired lances. He acknowledges that the charcoal fire for cooking hamburgers and the coke fire to melt iron may be different in magnitude, but getting the 'right' fire bed is the key for both. Patience and preparation are absolutes for a good coke bed, but he adds that a successful cupola campaign begins with a sound furnace bottom.

Starting at the Bottom

Safe, efficient cupola operation requires a furnace bottom that is able to support the weight of a towering mass of coke and iron and that is sloped to allow the efficient tapping of melted metal that collects in the well. An improperly designed and installed bottom can cause tapping problems at minimum or a dangerous run out in the extreme.

Bottoms in short run cupolas are generally made of compacted sand or a sand/refractory mix. Bottoms for longer campaigns use a use a high-quaslity refractory material, usually a mixture of sand and aluminum silicate, fireclay, pitch and water.

The Coke Bed

Proper coke bed ignition is important to the quality of the metal produced. Melting rate and metal temperature are functions of the initial height of the coke bed above the tuyeres and the degree to which the bed is burned in. The mechanism by which melting is accomplished in the cupola is heat released by combustion of carbon through the reaction of air oxygen) and coke (carbon). Sudden changes in metal temperature during a campaign would indicate that the coke bed is too low. Facilitating the proper air/coke mix provides more uniform, hotter coke burning. Given the opportunity, selecting uniformly large chunks of coke provides greater void areas in the bed and improves the penetration of the air blast and the volume of molten metal in the well.

Initially, the bed is laid up to the tuyere level, and gas or oil igniters are inserted through ignition ports or tuyeres just above the sand bottom, using care to reach the bed's center. Once the bed is uniformly ignited, additional coke is added.

The two steps to bed preparation, ignition and burn-in, are mainly a function of materials at hand and foundry preference. Other bed considerations include additions, consolidation and measurement.

Some methods of ignition involve:

burners or torches; hot blast; external ignition; and kindling. Antipollution regulations, for the most part, preclude the use of kindling, leaving oil and gas torches the most used methods of lighting cupola coke beds. Hot blast systems don't need burners. Preheated air (8001000 F) directed into the cupola at a normal blast rate is sufficient to begin combustion and is less time-consuming than the alternatives.

Burning-in

After the first portion of the coke bed is thoroughly ignited, additional batches of coke are added until the top of the bed is just below the required height. The tuyeres are closed and the blowers turned on briefly to complete the ignition. The bed should be closely observed to ensure that it has been properly burned in and the coke bed prodded to consolidate the bed and eliminate voids. Consolidation reduces the bed height somewhat, and it should be gauged from above using a bed measuring-rod or chain. More coke is added to bring the bed up to the desired operating height.
COPYRIGHT 1991 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Bex, Tom
Publication:Modern Casting
Date:Sep 1, 1991
Words:721
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