Boost the efficiency of your gas-fired aluminum reverberatory furnace.

By adjusting your furnace's components and performing regular maintenance, you can improve the energy efficiency of your gas-fired reverb.

The growing demand for aluminum castings has placed the spotlight on efficiency in the aluminum foundry. The mistakes and losses that were hidden in small production runs (because they didn't represent a large dollar amount) are now magnified exponentially as the number and size of orders increases. At the heart of these inefficiencies for gas-fired aluminum reverberatory furnace users is melting costs. Gas-fired reverbs are designed to melt a large volume of metal fast, but not in the most energy efficient way.

The gas-fired reverb furnace consists simply of three parts: a fireplace at one end; a stack or chimney at the other end; and a bed between both on which the ingot or scrap is heated. In practice, the flame from the fireplace heats the ingot or scrap and then is reflected downward or "reverberated" by the refractory onto the melt below to maintain a melt temperature. This furnace simplicity is the hurdle foundries must leap, as the design is what reduces its energy efficiency.

Compared to other furnaces, the large open chamber of the reverberatory furnace requires a vast amount of energy to heat the refractory, the chamber space, and then "reverb" the neat to the ingot/scrap and the melt. To increase its efficiency, manufacturers have developed various configurations for aluminum melting to optimize the main benefit of the gas-fired reverb. Three examples of reverb furnace design are:

Wet Bath - This furnace design [ILLUSTRATION FOR FIGURE 1 OMITTED] works by charging all metal directly into the heel of the furnace's melt, which assists in the melting by conducting heat into the cold charge. For larger charges, preheat hearths are provided. A popular method of charging scrap is to convey it through an opening in the side wall of the furnace. In addition, most wet bath reverbs charge from one end and have a dip out well for pouring at the other side.

Dry Hearth - This furnace's melting method [ILLUSTRATION FOR FIGURE 2 OMITTED] is to charge all cold metal directly onto a dry refractory hearth, therefore avoiding the possibility of moisture entering the molten bath through the charge material. Dry hearth furnaces are manufactured as both medium size units (1000 lb of melt) for melting and tapping and large units (greater than 15,000 lb) for only primary melting.

Stack/Tower - Similar to a cupola furnace, the stack/tower furnace [ILLUSTRATION FOR FIGURE 3 OMITTED] uses a conveyor to charge all the cold metal (ingot or scrap) in the exhaust stack. In this stack, heated combustion products and direct flame are employed for preheating and softening charge material before they settle on the dry hearth. Because the heated combustion gases are used to preheat the charge, this furnace melts quickly and is considered the most efficient of the three groups.

Because a new furnace doesn't fit all operations or budgets, this article focuses on the components of a gas-fired reverb - burners, combustion controls, refractories, re-circulation, fluxing, charging methods, covers, transfer means and maintenance - and how they can be adjusted to improve energy efficiency.

Burners

Within a gas-fired reverb, the location and number of burners is significant. Furnaces designed for end or side firing have two burners that are arranged to fire in a "W" pattern, enabling the combustion products to be forced away from the burners before returning across the bath to exit through the flue [ILLUSTRATION FOR FIGURE 4 OMITTED]. Two types of these burners - regenerative or hot-air - can be used for considerable fuel savings (as much as 50%), depending upon the metal temperature requirements. Often, the additional cost of these burners makes them viable only for furnaces running 10,000 lb/hr.

Reverbs with roof-mounted burners (one or more depending upon melting requirements) have a lower roof design for energy savings benefits, when compared to end or side wall-mounted burners. The bath of the furnace is more uniformly covered with the direct heat release and radiation of these burners. The hot-air option is available, however, due to the burner location, the recuperative option isn't. For stack melting furnaces, a built-in heat recuperator that utilizes combustion products to preheat the charged material gradually as it enters the stack is available.

Another option is an external recuperator or heat exchanger, which can save considerable fuel but is subject to possible deterioration resulting from the normal requirements of fluxing.

Combustion Controls

Most reverbs are provided with a basic proportioning control consisting of an automatic air control valve working in conjunction with a burner of at least four to one turndown. If the furnace is running at full output to achieve maximum melting capabilities, the existence of proportioning control is minimal. An improved air-gas control system would be a fuel-air ratio control (micro-ratio valve) to maintain the exact ratio and conserve fuel.

An automatic flue damper [ILLUSTRATION FOR FIGURE 5 OMITTED] is an excellent feature of a wet bath reverb, but it is not practical for a dry hearth or stack melting furnace. The damper works in conjunction with the air valve, opening and closing as required. A popular type is a dual damper system with one damper (the low-fire damper) always open while the larger or high-fire damper is automatically controlled. Other types of dampers include air-jet (fuel savings of 15% are claimed) or venturi, both of which will conserve fuel.

Large melting installations might benefit by incorporating a complete combustion control system including a PLC-based controller, transmitters, oxygen sensors, air and fuel actuator drives, and pressure controls.

Refractories

The first step in reverberatory refractory design is the material that contacts the molten aluminum. It must have high thermal conductivity and thickness to provide a freeze plane prior to the next segment of refractory material. In terms of the temperature of the outer shell, it must remain "cool" at 200F (93C) or less. A reduction of the outer shell temperature results in energy savings, but it also means sacrificing the safe freeze plane area and/or increasing the overall refractory thickness to an impractical degree.

A better target for energy savings is the upper walls and roof of the reverb. Refractories of lower thermal conductivity can be used to minimize heat loss, with attention still being paid to refractory strength. Upper walls comprised completely of ceramic fiber are installed to substantially reduce heat losses, but the inherent splashing of aluminum, lack of resistance to oxide buildup and total lack of strength make fiber less than desirable, despite the savings.

Ceramic fiber is a viable lining for charge doors, where a reasonable amount of fuel savings can be realized. It is possible to reduce the amount of heat lost through the door by 50-90% depending upon the composition of the existing refractory.

Recirculation

Many reverberatory furnace users with capacities of greater than 50,000 lb have installed or are considering the addition of re-circulating molten metal devices [ILLUSTRATION FOR FIGURE 6 OMITTED]. These devices, which are available in electromagnetic and motor-driven designs, boost energy savings by increasing melting rates and productivity while improving metal homogeneity. In addition, the re-circulating devices are recommended for the melting of fines, such as chips and turnings, when the same benefits are desired. Manufacturers of re-circulating devices claim fuel savings as high as 20%.

Fluxing

Observing proper fluxing techniques is another way to save energy. Removal of oxides from the metal increases fluidity and thus fills the mold more effectively, reducing scrap. The flux application should never be less than the recommended volume and should never be left in place longer than required. If possible, the flux injection that is used should distribute the flux evenly and aid in the circulation of heat within the metal.

Charging Methods

The melting of solids (ingot and scrap) can be performed in an outside well, inside the furnace or with a combination of both. In a dry hearth furnace, all the metal is deposited on the hearth where it receives direct flame impingement (not a means of conserving energy). To receive maximum fuel efficiency, the dry hearth melter would be batch-charged and, if possible, the ingot would be charged over the scrap.

Wet bath furnaces are most efficient if the combustion products as well as heat transfer can be utilized through the bath. The furnace should be designed to route the combustion products across the main bath and over a preheat hearth or scrap charging area.

Stack/tower melters make full use of direct flame and combustion products by charging all ingot and scrap through the stack or flue. The furnace operates most efficiently when the stack is continuously charged.

Considerable fuel savings also can be realized if all melting is performed at the metal supplier's plant and the user receives only hot metal.

Covers

Use highly insulated furnace covers on open wells whenever possible and/or practical. At 1300F (704C), the radiation heat loss from a reverb is 12,000 btu/sq ft. Since the furnace must provide another 12,000 btu/sq ft to make up for the heat loss, the total fuel saved can be estimated at 24,000 btu/sq ft.

Transfer Means

The most common method of transferring molten metal from the central melter to other holding furnaces is via a transfer ladle from a furnace tap-out hole or pump. However, this is an energy efficiency nightmare because the molten metal loses temperature as it's tapped or pumped as well as during the transit from the melter to the holding stations. To compensate for the loss, the temperature in the melter is raised or the melt's time in the holding furnace is increased. Therefore, the user is faced with higher fuel costs, an increase in metal loss, further maintenance, shorter furnace life, ladle upkeep costs and the hazards of transporting molten metal. Some of these problems can be overcome with the use of high-quality insulating materials in ladles, electrically heated ladles or covered ladles.

When a single alloy is used in a plant or melting cell, the transfer problems can be eliminated with the use of a covered molten metal launder system, which connects the melter with the holding furnaces. Whether it is gas-fired or the more energy efficient electrically-heated system, the launder system conserves fuel by maintaining metal temperature through transport, in addition to providing safer, less hazardous working conditions and a reduction in scrap and labor content. Launder systems are an option for new plants and any existing facilities where the furnaces are in a straight line.

Maintenance

The last, but easiest way to lower energy costs with a gas-fired reverb is maintenance. A minimum daily cleaning of the furnace walls and molten metal surface should be performed without exception as excessive oxide dross and sludge result in higher fuel costs.

The basic plant rules for reverb maintenance include:

* remove any oxide build-up from the furnace side walls at least once a day;

* flux the furnace and remove dross daily;

* check thermocouple protection tubes, remove build-up and replace broken tubes daily;

* check temperature controllers for proper calibration at least twice a year;

* repair door seals when necessary;

* check combustion equipment for proper operations and have it serviced once a year by a qualified furnace or combustion service engineer.

Further Energy Conservation

Besides focusing on the specific components as mentioned above, other opportunities exist to conserve energy with a gas-fired reverb. They include:

* charging on a continuous basis rather than by the batch in a wet bath furnace;

* operating the furnace at full-rated capacity if it is not equipped with an automatic flue damper. With a stack/tower melter, it is also important to keep the stack charged with metal;

* keeping furnace doors closed except when necessary;

* maintaining as low a metal bath temperature as possible.

The reverberatory furnace user has many available energy savings options. Although some may be too costly or impractical, foundries always have the option of good furnace and burner maintenance habits, which conserve energy and cash by keeping the furnace working at full duty for the longest possible time. This article was adapted from an article that appeared in the November/December 1997 issue of Die Casting Engineer.

RELATED ARTICLE: Measuring Furnace Efficiency and Melt Loss

The cost to melt 1 lb of aluminum is an often overlooked part of a foundry's operating costs. Although foundrymen must use the melting equipment in their foundry - equipment that may be quite old and inefficient - they still can exercise significant control over melting costs.

Table 1 lists approximate efficiencies and melt loss for various aluminum melting furnaces. It is based on published literature and actual melting trials by various furnace manufacturers. The btu per lb and efficiency ranges are purposely wide because this is the area where the foundry can make a big difference by increased melting controls and proper maintenance of melting equipment.

The gas reverbs in this table include both wet bath and dry hearth reverbs, thus the efficiency ranges listed are exceptionally wide. The individual foundry melt controls can place a foundry at the top or bottom efficiency for the particular furnace type. In addition, electric furnaces are rated by KWh per lb. This has been converted to btu per lb for easy comparisons.

Table 2 examines another often overlooked melting cost - the cost of metal lost as the molten aluminum is oxidized through contact with the atmosphere and furnace combustion gases. This table shows the dramatic difference that reducing the melt loss can make on overall costs.

Table 1. Aluminum Melting Furnace Efficiency

Furnace           Melt Loss     btu/b     Efficiency

Gas Reverb          2-12%     1500-5000     12-28%
Electric Reverb        1%           820     70-75%
Stack Melter         1-3%      850-1250     40-60%
Table 2. High Cost of Melt Loss (Daily Melt = 7143 lb)

% Melt       Metal       Metal     Cost     Daily     Annual
Loss     Melted (lb)   Loss (lb)    ($)   Loss ($)   Loss ($)

1           7215           72      0.90       65       16,900
2           7289          146      0.90      131       34,060
4           7441          298      0.90      268       69,680
6           7599          456      0.90      410      106,600
8           7764          621      0.90      559      145,340

This table assumes that the foundry requires 7143 lb of molten aluminum for its daily production output. At 2% metal loss during melting, the foundry must melt 7289 lb of metal to meet this daily production need (an additional 146 lb to compensate for the loss). At $0.90/lb for aluminum ingot, that 2% melt loss becomes $34,060/year in lost metal. Add this cost to the cost per lb of melting the additional 146 lb of metal daily and that yearly cost escalates even more. A simple reduction in melt loss, through better furnace controls, improved burner efficiency and charge material controls, can make a substantial bottom line profit improvement for the foundry.

- Steve Robison, American Foundrymen's Society