WHAT ARE GAS FIRED ALUMINUM REVERBERATORY MELTING FURNACES AND HOW DO THEY WORK?

INTRODUCTION

The original basic design concept of the reverberatory aluminum melting furnace has not changed. It remains to be a rectangular, refractory lined, steel box with a burner, or burners, mounted in the roof, sidewalls, or end wall of the furnace. They have a charging (wet) well, located on one end, or on the side of the furnace, separated by an archway, or a skimmer door.

The burners heat the refractory lining walls, and roof, which in turn reverberate, or re-radiate the heat to the molten aluminum in the hearth, or bath. The surface emissivity of molten aluminum is between 0.004 to 0.055. However, in actual fact the molten bath forms an oxide layer which has an emissivity of 0.11 to 0.19, resulting in poor heat transfer qualities. These factors dictate that a reverberatory furnace be designed with a large bath surface area vs. melt rate ratio to achieve the required heat transfer to melt the hourly melting rate. Commonly, and proven bath to melt ratios should be 10:1, with a minimum of 8:1.

There are three modes of heat transfer:
1) CONVECTION
2) CONDUCTION
3) RADIATION

This article will cover the two modes of heat transfer and are based upon the “real world “ experience pertaining to the most commonly used by reverberatory manufacturers today, being, Radiation, and Convection.

WET HEARTH ( WELL ) REVERBERATORY FURNACES

With a wet well reverberatory furnace the charge, (ingot/sow, or returns) is introduced into the molten heel of a charging well, separated from the main bath / holding zone. This well can be physically separated by a submerged refractory archway or by a powered, vertical rising skimmer type door. The heat transfer is sub-surface via conduction through a refractory archway, or a vertical rising, powered door with refractory nose blocks/blades.

The proper charging rate to achieve 100% utilization, and efficiency, should be 25% of the furnace melting rate, introduced every 15 minutes, at a ratio of 60% returns, and 40% ingot.

The molten metal is dispensed through one of the following methods for delivery to the holding/ladling furnaces at the casting cells:

Manual tap-out block & plug assembly, into a transfer bull ladle.
Molten metal pump, into a transfer bull ladle.
Pneumatic pressurized dispensing pump well, into a transfer bull ladle.
Direct connected electric resistance heated, gravity launder system to holding furnaces.
Hydraulic tilting the furnace and pouring into a transfer bull ladle.

There are basically two trains of thought when you look at the various manufacturers of wet hearth gas fired reverberatory furnaces:

1) LOW HEADROOM RADIANT ROOF BURNER TECHNOLOGY

2) HIGH HEADROOM CONVECTIVE SIDEWALL BURNER TECHNOLOGY

1) LOW HEADROOM RADIANT ROOF BURNER TECHNOLOGY

A low headroom radiant roof reverb furnace design (FIGURE # 1) is based on three principles:

The distance between the burners (heat source), and the aluminum (load)
The hold to melt ratio (bath to melt)
Evenly timed charging of the melting stock (ingot / sow / returns)

Design principle #1 is based on the “ STEPHAN – BOLTZMANN LAW “ of radiant heat, which states that the closer the object is to the heat source, and the greater the temperature differential, the faster the transfer of heat (BTU’s). This is the design criteria for radiant roof type reverb melting furnaces. Through the utilization of low velocity, flat flame, high thermal release burners that heat the burner block up to 2000 F, you realize a huge temperature differential between the aluminum, and the burner, The transfer of BTU’s into the aluminum is then at it’s greatest.

With this type of furnace, the closer the radiant heat source is to the receiving medium, the more efficient it is. The closer the heat source, the more sources (burners), you need to achieve maximum coverage of the medium (aluminum).

Design principle #2 is based on that the hold to melt ratio is critical as a dilution factor for the solid aluminum being charged into the furnace. The higher the ratio, the less of a bath temperature fluctuation you get. The ideal ratio is 10:1 ( 10 lbs. capacity for every 1 lb. melted, and in any case NEVER less than 8:1.

Design principle #3 is based on the charging rate of the melting stock, This should be 25% of the melting rate, every 15 minutes. This will ensure that you will maintain a metal temperature of plus or minus 15 degrees F, during the melting process. Overcharging the furnace will create a sludge buildup in the bottom of the bath (temperature drops below the sludge point of the alloy), resulting in poor metal quality, and increasing dross losses.

2) HIGH HEADROOM CONVECTIVE BURNER TECHNOLOGY

A high headroom convective burner technology type reverb furnace design (FIGURE # 2) is also based on three design principles:

The use of side, or end-wall mounted, high velocity (momentum) burners with adjustable flame lengths, and a high forward velocity to provide convective heat transfer.
The hold to melt ratio.
Do not overcharge.

Design principle #1 is based upon the high velocity, high momentum burner technologies. The burners are side-wall, or end-wall mounted, toed in, and angled down from the furnace wall and directed towards the aluminum bath surface. This is based on providing convective heat transfer with the products of combustion exiting the burner tile at velocities of 15,000 ft / min. to 35,000 ft. / min. The philosophy is that these high velocity gases entrain the furnace atmospheres gases, and circulates them, vigorously, throughout the heating chamber, thereby maximizing “convective” heat transfer to the aluminum bath.

Simply put, as the velocity of hot air passing over a surface, it increases the amount, and rate of heat transfer proportionally, to the medium (aluminum).

The upside to this is that the high velocity products of combustion circulating through the heating chamber will cause a ripple effect on the bath surface, creating a large bath surface area to increase the rate of heat transfer.

The downside to this theory is that by rippling the bath surface and increasing the surface area, you also increase the oxidation rate and melt losses because there is a larger surface area exposed to the oxidizing atmosphere in the furnace.

High headroom furnaces also have more interior refractory surface area to heat up and store BTU’s, robbing the BTU’s needed to melt. Additionally, these furnaces have more square feet of steel casing, and since all efficiencies are also measured in lost BTU’s per sq. ft. of casing, per hour, logic say’s, you will have greater total heat losses.

The design principles #2 and #3, both pertain to the convective burner technology reverbs, and the radiant roof burner technology reverbs;

Bath to melt ratio ideally should be 10:1. Never less than 8:1
Charge rate should be 25% of the hourly melt rate, every 15 minutes
Do not overcharge.

EFFICIENCY : RADIANT vs. CONVECTIVE

When determining furnace efficiencies, it is important to include melt losses along with the fuel efficiencies. Depending on the design / burner technology, and placement, different percentages of melt loss will be encountered. The term “melt loss” defines the percentage of metal loss per pound melted through either oxidation, drossing-off procedures, flux usage, and vapor/flue gas losses.

These losses result in having to melt additional metal to make up for the losses. This results in additional energy input/costs, plus the cost of the additional metal required.

Experience, has shown that the fuel usage, and melt losses, that are “real world”, are :

RADIANT ROOF TECHNOLOGY

BTU per lb. melted————— 1500 -1650 BTU

% Melt Loss————————- 2 – 3 %

CONVECTIVE SIDEWALL TECHNOLOGY

BTU per ib. melted—————- 1800 – 2000 BTU

% Melt Loss————————– 4 – 5 %

NOTE: These figures are based on gas fired reverberatory (wet well) furnaces, with 100%, room ambient temperature returns, scrap, or ingot, charged into an exterior charge well.

AVAILABLE OPTIONAL ENERGY SAVING TECHNOLOGIES

The following technologies, when incorporated, can greatly reduce the energy cost when using gas fired reverberatory furnaces.

RECUPERATORS

A radiation tube type recuperator through which the hot furnace exhaust gases pass, and preheat the combustion air supplied to the burners at a temperature of 700 degrees F, can reduce fuel consumption by 20%.

INGOT / SOW PREHEAT HEARTH

An ingot, or sow preheat hearth, where the charge is placed so that the furnaces hot exhaust gases pass over, through, and around the charge prior to being introduced to the aluminum bath, will reduce fuel consumption by 25 – 30%.

INSULATING WELL COVERS

All open, or uncovered wells should have powered, with large wells, or, manual, with small wells, removable insulating covers. The surface radiation heat loss of a molten aluminum bath surface, at 1250 deg. F, is 7,147 BTU / sq. ft. / hour. (FIGURE: # 3)

SUMMARY

As stated at the beginning of this article, the original basic concept of the reverberatory gas fired aluminum melting furnace, has not changed. What has changed is, burner technology, refractory technology, control technology, recuperation, regeneration, etc.

There are a multitude of variables available today to save energy and reduce costs in the melting department. Be sure to check out the various sizes of Dynamo Reverb Furnaces here https://dynamofurnaces.com/cat/aluminum-gas-melting-furnaces/aluminum-gas-melting-tilting-reverb-furnaces/

AUTHOR / DISCLAIMER

This article was authored by Ed Lange, who is an Aluminum Furnace Consultant, elange2@cogeco.ca. The information presented in this article is based upon the authors 40 plus years of experience in the development, design, and sales of non-ferrous melting, holding, and handling equipment. The author does not in any way, endorse any particular furnace manufacturer, or their respective technologies.