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Vertical shaft melter nets savings.

Foundry, furnace builder and utility unite to cut operating costs.

The selection of aluminum melting furnaces is heavily biased by the cost and availability of gas, oil or electric energy. Two other considerations also are important.

One is metal cost. Metal loss in a melting process with aluminum at $1.20 lb showed that it probably was a larger consideration than the cost of fuel. Even with the cost of aluminum nearly halved, energy and metal loss remain significant considerations.

The second factor is the economic effect furnaces have on molten metal production and casting quality. Examples of some furnace types and energy used per pound of aluminum melted are listed in Table 1. Balancing equipment selection and energy costs is a decision foundries must weigh.

This was a dilemma for the management at Meloon Foundries, Inc., Syracuse, New York, an aluminum foundry recently beset by rising operating costs.

Staying with the known is a big factor in resisting change; the unknown can be expensive. But with melt costs edging up to unacceptable levels, some change was necessary. A decade previously, the foundry had updated its melting facilities, opting for electric melting and holding furnaces. Since then, the cost of electricity tripled, prompting a reevaluation of melting methods.

The foundry had been approached by the local utility, Nisagara Mohawk Power, to consider the use of natural gas in place of electricity because gas was in better supply and less expensive. As part of a continuing effort to reduce energy costs for small businesses, the utility and the foundry eventually formed a joint research venture to evaluate a gas-fired aluminum melter. The foundry had been investigating a vertical shaft furnace of the type successful in Europe, so the joint effort was timely.

To compete with electric resistance melting furnaces, a gas-fired system would have to be cost effective, melt rapidly and produce metal equal to electrically melted aluminum. A vertical shaft Stack Furnace|TM~, manufactured by Denton Corp. of Syracuse, New York, was chosen for its ability to recover waste heat. A sloped hearth was included to facilitate the immediate discharge of metal into electric holding furnaces.

Melting efficiency was set at 1000 Btu/lb and capacity at a ton/hr. A theoretical model showed that by adjusting shaft height, burner size and charge makeup, melting costs would be less than $0.01/lb of aluminum charged, including preheating costs.

The melter's final design has met or exceeded design parameters and, theoretical assumptions. Vacuum tests show little hydrogen pickup and, unless application specifications are critical, the foundry has been able to bring the metal to temperature in holding furnaces and pour without degassing. Once the melting process is established and programmed, the furnace is automatic and requires no skilled operator.

As pictured in Fig. 1, the foundry mounted the furnace, combustion equipment, electric controls, loading skip equipment and access platform onto a movable rail platform. Thus, the furnace is capable of feeding any of three holding furnaces positioned parallel to the 36-ft track.

Vertical Shaft Melter

The vertical shaft melter is just now showing up in this country as a thermally and operationally efficient aluminum melter. The effect of melting rate, Btu/lb melted and metal loss during melting is demonstrated in Table 1. It charts the results for a small shop melting 7500 lb/day in a stack melter and shipping 5000 lb of finished castings.

The furnaces, where applicable, offer cost and metal quality improvement benefits.

Initially offered to North American foundries from offshore under licenses, the furnaces were of large capacity (6000 lb/hr and up). Special charging devices and elaborate controls and holding capacity made them major capital investments. The company now offers mechanically charged furnaces in the range of 2000-4000 lb/hr with fuel rates less than 1000 Btu/lb.

The stack furnace discharges metal at a low temperature (1225-1250F) with low gas and impurity levels. Aluminum needs to be delivered directly or via short launder to a holding furnace where the correct temperature and metal quality are set for foundry use. Charging is safe as the furnace provides ample time for drying to take place in the furnace's cycle. This lowers melting costs and raises molten metal quality.

Table 1. Typical Gas-Fueled Vertical Shaft Furnace Operating Costs Based on Tests. (Source: Meloon Foundry, Inc. and Niagara Mohawk Power Corp.)

Foundry shipping 5000 lb/day - Melting 7500 lb/day Natural gas at $5.50/mcf Aluminum ingot at 70|cents~/lb Melt cost at 3500 Btu/lb - $144.38 Melt cost at 1000 Btu/lb - $41.25

Benefit/day - $103/day Saving 20 day month - $2062/mo Melt loss @ 3% = 225 lb @ 70|cents~ = $157.50 M elt loss @ 1% = 75 lb @ 70|cents~ = $52.50 Benefit/day $105/day Saving - 20 day month = $2100/mo
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Copyright 1992, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Author:Bex, Tom
Publication:Modern Casting
Date:Nov 1, 1992
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