Furnace and Regenerative Combustion Design in Open-Die Forging
Furnace and regenerative combustion designs are used for the production of large open-die forgings such as those produced at North American Forgemasters in New Castle, Pa. Regenerative combustion systems integrated with specialized large-furnace designs offer efficiency, emissions control, temperature uniformity and economic performance.
No matter the application, high-temperature furnaces are an integral part of the open-die forging process, not to mention a sizable portion of the capital expense for a new press project. Often, however, conflicting desirable furnace attributes – including but not limited to energy efficiency, production, emissions control, temperature uniformity and (often most importantly) economic performance – make finding the ideal furnace difficult. That said, synergies between these attributes can be achieved with the right design expertise and the right teams.
Finding the Right Design Balance
Recently, three regenerative furnaces were installed at North American Forgemasters in New Castle, Pa., to support their new 10,120-ton/90-MN open-die forging press. Two furnaces were box-style with front doors, and the other was a car-bottom-type furnace. Our team at Fives North American Combustion (FNAC) in Cleveland, Ohio, designed the furnaces to promote the synergies among the previously mentioned desirable furnace attributes.
The two box furnaces have dimensions of 28 feet wide x 31 feet deep x 15 feet high. They are capable of heating 500 tons of steel product. Based on the pieces being processed, it was determined that approximately 20 MMBtu/hour of heating capacity be installed using one pair of TwinBed II burners mounted on the rear wall. The furnace must be capable of +/-50°F inside the work zone per AMS 2750E. Two flues, one in each rear corner, were installed to manage and direct flue gases within the furnace volume.
The car-bottom furnace has dimensions of 15 feet wide x 52 feet deep x 18.25 feet high and is capable of heating 600 tons of steel product. Based on the pieces being processed, it was determined that approximately 20 MMBtu/hour of heating capacity be installed using four pairs of TwinBed II burners mounted on one of the side walls. Two flues were placed on the burner wall in the front and back corners. Similar logic was used on this furnace to control flue gases.
The furnaces and accompanying combustion technology were tailored for the large ingots North American Forgemasters produces. The combustion technology, coupled with full furnace integration, allows for significant fuel savings, improved temperature uniformity, reduced emissions and enhanced production. The result is minimized CO2 production, thereby reducing the forge’s total carbon footprint and impact on the environment while also maximizing the facility’s production capability.
Consumer sensitivity to energy costs and performance issues has increased during the past decade. Both considerations are now a regular part of the purchasing-decision process for consumers, especially industrial manufacturers. For instance, at forging temperatures for steel (1650-2300°F), efficiencies of traditional gas combustion are typically low, and performance issues – both environmental and process-related – must be considered. Forging and heat treatment have market-driven performance standards that must be followed to achieve certain quality, as well as regional environmental standards such as NOx or CO2 emission regulations, that further complicate matters for industrial forgers.
Fives is an industrial engineering group with a long heritage of engineering expertise. Fives designs and supplies machines, process equipment and production lines for the world’s largest industrial groups in sectors such as aluminum, steel, glass, automotive, logistics, aerospace, cement and energy, in both developing and developed countries.
In all these sectors, Fives designs and manufactures equipment and solutions that anticipate and meet the needs of its customers in terms of performance, quality, safety and respect for the environment.
In 2016, Fives achieved a turnover of 1.8 billion Euros and employed close to 8,400 people in over 30 countries.
Fives North American Combustion offers ultimate solutions to its customers, from design, manufacture, installation, commissioning and customer technical support throughout the equipment lifecycle. With a strong legacy based on 100 years of expertise, our company is a leader in outstanding combustion solutions, delivering performance at the heart of wide-ranging applications throughout the world.
Regenerative Combustion Systems
The seamless pairing of combustion system and furnace has been rare in the past, but our design team is in the strategic position to design around the total furnace product for the customer. In the past decade, FNAC has installed over 40 forge furnaces in the field, of which more than 25 use regenerative combustion systems. For example, we offer a regenerative burner that has been in the combustion industry for over 30 years, with installations on over 200 forge applications. The TwinBed II design, which has been improved in recent years, offers high energy efficiency paired with lower emissions levels compared to conventional combustion technologies.
Regenerative systems allow for up to 50% fuel savings when compared to cold-air systems on these types of furnaces. The regenerative burner is more compact, allowing for more media inside the heat exchanger to increase efficiency at higher temperatures. To comply with environmental standards, the combustion system has an outboard fuel injector for extremely low NOx operation above autoignition. This allows for NOx levels comparable to cold-air technology on a volume basis but with a 50% reduction on a mass basis.
Other advances in the flow-control system have added value to the design of the furnace. Accurate flow metering allows the combustion system to remain on ratio setpoint at lower firing rates, where large portions of these large-load heating cycles will operate. Mass flow control is used on both combustion air and fuel to ensure flow rates.
Overcoming Obstacles for Large Furnaces
Large furnaces capable of heating and supporting 200- to 500-ton loads have their unique set of difficulties. Refractory and steel selection become more difficult at such substantial furnace sizes. Consequently, certain special insulating fibers capable of enduring high temperatures must be chosen. Also, thought must be dedicated to the design of the hearth – specifically the cooling of the hearth’s steel plate.
On smaller furnaces, natural convection is sufficiently capable of cooling the hearth plate, ensuring structural stability. On large-footprint furnaces, however, the cooling effect of natural convection will not sufficiently lower the temperature of the hearth’s structural plate. To combat this effect, the North American Forgemasters’ forge furnace employs blowers exhausting the hot air under the hearth plate. A series of passages are engineered as part of the hearth support to allow for the exhaust to route through all areas of the plate. This has been found to significantly lower the temperature in certain areas, thus improving the structural integrity and life of the furnace.
Controlling furnace pressure is also of great concern because it affects energy efficiency and performance, especially temperature uniformity. On regenerative furnaces, ingress of ambient air can raise furnace oxygen levels and yield unacceptable NOx and scale formation. We overcome this obstacle with proprietary control logic that maximizes flue-gas extraction while maintaining positive furnace pressure over the complete range of operating temperatures and fuel turndown. One of our designs, for instance, has multiple doors to suit the customer’s needs while maintaining a tight seal.
If the customer has longer pieces that extend through the door opening, the door can be designed with air cylinders to clamp it in any position. These clamps are also used to provide a tight seal when the door is fully closed. A removable bulkhead is needed to seal the opening around the piece to block furnace radiation. Another popular design employed on large doors uses a cam/roller that leverages the weight of the door to create the seal.
Temperature uniformity can prove difficult on large furnaces, especially those with few regenerative burners. This can be remedied by placing flues in troublesome cold spots to direct the flue gas. FNAC has recently patented a control technology that employs multiple flues on large furnaces. This control technology will automatically bias the furnace gas to a specific flue based on the temperature feedback inside the furnace volume. This way, the furnace is actively controlling uniformity in the furnace volume, ensuring uniform temperatures with varying loads.
Both furnace designs achieve tight NOx emissions levels, with cycle averages just below 0.086 pound/MMBtu. No official uniformity tests have been conducted to date, but all indications show the furnace being uniform to the standard requested based on resident furnace control thermocouples. Fuel consumption tests were conducted and met contract efficiencies at 70%.
Rely on Regenerative Technology
Since the introduction of regenerative burners to forge furnaces, the industry has come to recognize the quality, performance and fuel-consumption benefits of the technology. The design described herein demonstrates an even more holistic view of the benefits of a regenerative forge furnace, specifically the benefits of the two systems where the furnace and combustion system function as a single machine.
For additional information visit www.fivesgroup.com.