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Basic cupola rules still apply after 200 years.

Time-tested truths, combined with modern technology, help maximize the efficiency of the venerable coke-fired melters.

On June 2, 1794, John Wilkerson of France patented the modern cupola. There is evidence that the Chinese had been melting iron to make cooking utensils in a cupola-like device centuries earlier. For 150 years after Wilkerson's invention, his patented cupola design remained almost intact.

Only during the last 50 years have modifications been made, including front slagging, protruding and water-cooled tuyeres, hot blast, recuperative hot blast, shell melting, oxygen enrichment and compensating computers. With all these changes, it helps to reflect on selections from an early writer of foundry lore, AFS member Edward Kirk. In his trade column, titled "Cupola Scraps" he made a number of observations on cupola operation as valid today as they were in 1910. For instance:

"Often, a melter 'don't know' why the cupola is working badly, because if he knew, he would be discharged for carelessness."

No operator would think of changing his charge materials 25-30% at one time, and a lot of thought and attention is given to properly weighing raw materials. Today's compensating computers can adjust from charge to charge for small variations of under- or overweight raw materials. However, more pounds of air are put into the cupola than the total combined weight of all metallics, coke and limestone.

Yet after such careful consideration of charge materials, blast rates can change 20-50% at one time. For example, a cupola blasting at 10,000 CFM has been known to jump to 15,000 CFM within a few minutes. This is like adding 50% to a coke or metallic charge, which causes an uneven melt rate, uneven analysis and, in short, a lot of problems.

One of the secrets to good, even cupola melting with a given melt rate and consistent temperature and analysis is a constant blast rate. Any blast,rate change should be given as much, if not more consideration as a change of metallics.

"A bad light-up makes a bad heat. The bed must be burned evenly or it will not melt evenly."

Many cupola operators have poor start-ups because, rather than measuring the bed, they rely on a weighed amount of coke. A weighed bed can be off by several inches due to variations in coke size, lining thickness, or buildup on and between the tuyeres. With the advent of pollution control and modern charging equipment, some cupolas have designs that make it physically difficult, if not impossible, to measure the bed. But a method must be found to do so, since start-up is the heart of the melting operation.

There are no shortcuts to a properly lit, burned-in and measured coke bed. It takes two people to make the bed one to do the actual measuring, and the other to observe. In all cases, the observer is held responsible for start-up.

"Don't burn up the bed before charging the iron."

An over-burned bed is caused by adding too much bed coke at charging. Rather than taking off the extra bed during the first charge, many operators turn on the blower (sometimes using oxygen) and burn the coke bed down to the proper height. This over-burning causes high metal carbons at the start, followed by low carbons in 1530 min, and results in off-analysis melting that can take as long as 5-6 hr to stabilize.

The proficient melter is always sure of on-analysis and temperature at tap. He is also sure of the bed because he has observed and measured it.

"A new cupola always effects a great saving in fuel when purchased, but it is often hard to find the fuel (saved) at the end of the year. A little more practical knowledge in managing the old cupola will often enable the foundrymen to find the fuel saved and the price of the new cupola besides."

Since it costs about the same to put in a larger melting unit when replacing a cupola, in most shops the cupola is too large to be efficient for the amount of metal required, and often must be slowed down. The only two ways to slow down a cupola are to reduce the blast rate, which causes temperature and melt rate inconsistencies, or to add more coke. Adding coke is impractical: a cupola at a 6:1 coke ratio would melt about 1 ton of metal per hr, while one at 12:1 would melt 2 tons per hr. Therein lie the economics of cupola melting - the more the coke split is reduced, the more fuel is saved.

Often, existing cupolas can be updated to meet new demands by adding new shells, water coolings and hot blasts, and the temptation of increasing its size can thereby be avoided.

"If you go into the foundry when the heat is being melted and find the tap hole almost closed, the spout all banged, and the melter picking at the spout with a tap bar and running a rod into the tap hole a yard or so in his efforts to get the iron out, and remark to him: 'You are having some trouble with your cupola today,' he will say: 'Yes, we have some very bad coke today!' or 'We are melting some dirty pig iron today, sir!' He never thinks: 'We have a very poor melter today, sir!'"

Lack of training among cupola operators and tenders is a major concern to the modern-day iron foundry. A few years ago, most people started melting with small 32 or 36 in. cupolas and progressed up to the giant 100-plus in. models we have today. Along the line, they learned the practical basics of cupola operations.

The modern melter doesn't have this advantage - he has inherited these giant cupolas with little or no practical experience. Education in the basic principles of cupola operation is the way to realize the cost savings and metallurgical advantages of a modern, up-to-date design. High employee turnover also causes a disruption in cupola melting.

It takes many years of practical experience for a melter to become efficient and understand the daily problems of a cupola. The same holds true for both supervisors and labor.

And last but not least -

"Less fuel is generally required to melt iron in the foundry office than is required to melt in a cupola."

Before they are installed, most cupolas will melt at least a 12:1 or 14:1 ratio. It is imprudent for an engineer to design a very efficient cupola, then calculate a heat balance and say that by following these steps, a great amount of fuel will be saved. Careful consideration needs to be given to any modifications, and experienced people should be involved in decision making. Practical knowledge, combined with today's technical advantages, can result in a very efficient furnace that will surpass the expectations of even the most discriminating supervisors.

A cupola will digest everything its fed - dirty motor blocks, old bathtubs, radiators and used soil pipes. Large volumes of steel scrap and pig iron, even with minimum controls, will produce quality iron in volume or batches that meet today's demanding metallurgical requirements.

A cupola is still one of the lowest cost, most efficient iron melters available today. Update its design, install modern controls, put it in capable hands, and feed it a reasonable semblance of clean metallics, and it's as good as any melter on earth.
COPYRIGHT 1995 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1995, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Hopkins, Edgar
Publication:Modern Casting
Date:Oct 1, 1995
Words:1232
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