Thin-Slab Reheating with Tunnel Furnaces
June 14, 2006
The advent of tunnel furnaces, as we know them today, began in 1989 when the first thin-slab casting facility was commissioned at Nucor Steel's Indiana facility. As shown in Figure 1, the thin-slab casting process utilizes a tunnel furnace to link a continuous slab caster to a rolling mill. The tunnel furnace heats the slab to rolling temperature and provides storage space or "buffer" during the casting/heating/rolling process. Since 1989, there have been over 50 tunnel furnaces installed in 30 different facilities in 13 different countries.
The tunnel furnace system consists of a refractory-lined steel enclosure with burners mounted in the sidewalls. Furnace sections are designed using a modular concept and include removable roof sections for effective overhead access into the furnace interior for maintenance. Cast slabs are supported and transported through the furnace by individually driven in-furnace rolls. See Figure 2 for a cross-sectional view of a typical tunnel furnace.
Waste gases are evacuated through a flue system equipped with recuperators and natural draft stacks. The recuperators utilize the heat in the waste gas stream to preheat the combustion air to improve combustion efficiency.
Furnace RollsTunnel furnaces typically utilize water-cooled rolls (Fig. 3), which provide support for the product as it is transported through the furnace. Water-cooled roll construction consists of a tubular carbon steel shaft, heat resistant alloy "tires," insulating refractory and anchors, and a tubular "corebuster" which engages a rotary union, directing water to and from the roll.
Scale RemovalTunnel furnaces employ scale removal "chutes" in the bottom of the furnace hearth sections. The furnace hearth is sloped to direct scale to the bottom of the chutes for selective dumping to the basement floor below the furnace. Thus, at the mill floor level, general cleanliness in the furnace area is much easier to maintain.
Many tunnel furnaces utilize a "slidegate" concept, which consists of a sliding plate fitted to the bottom of the chute opening. Figure 4 shows an open slidegate as viewed from the furnace basement. More recent designs incorporate a layer of insulating castable (monolithic) refractory for thermal protection and wheels for smooth and reliable slidegate movement.
RefractoryIn general, tunnel furnaces utilize a refractory system, which is designed to provide adequate insulation effectiveness while being cost effective and easily maintainable.
Roof, Sidewalls and Endwalls - Tunnel furnaces employ 2600°F, 9-in. thick ceramic fiber modules in the roof, sidewalls, and end walls of the furnace. This provides the insulation required and characteristically has a low thermal inertia, which allows for rapid cool down of the furnace. The lightweight ceramic fiber lining allows for easy removal of roof sections. The refractory on the sidewalls extends from the roof to the bottom of the square openings in the furnace sidewalls for the rolls. The ceramic fiber modules are installed using a stud gun, which welds the stud, assembled to the internal hardware of the fiber module, to the steel panel to which the hardware is attached. This approach has proven to be very effective and easy to maintain. In the event of module failure (e.g., stud weld failure), a replacement module can be easily installed using the same procedure utilized during the original installation.
Furnace Hearth - As mentioned previously, the hearth sections contain sloped chutes in the bottom for the purpose of scale removal. Each hearth module typically contains four openings. The hearth is lined with a 2-in. layer of 1900°F block insulation at the cold face, a 4-in. intermediate layer of 2400°F insulating castable and a 4-in. thickness of 2600°F high-strength castable at the hot face. This refractory configuration extends up the sidewalls to the bottom of the roll openings and is installed with appropriately located ceramic fiber joints to accommodate thermal expansion. This design is relatively simple and easy to maintain. Cracks that develop in the hot face must be cleaned of scale and stuffed with ceramic fiber.
Flues and Stack - Typically, two hearth sections of a single line tunnel furnace are equipped with large rectangular openings to allow the flow of waste gases to the flue system. The circular flues are lined with a 2-in. layer of 1900°F block insulation, which is covered with a 4-in. layer of 2400°F gunned insulating castable. This insulation system has also proven to be very effective and easily maintainable.
Control SystemMaterial Handling - The material-handing system for the tunnel furnace utilizes individually AC motor-driven furnace rolls for transporting slabs from the caster to the finishing mill. The roll speeds are varied to meet the variety of system requirements, with pulse width modulated variable frequency drives (VFD) controlled through a programmable logic controller (PLC). Multiple VFD's and a power switching system allow numerous slabs to be manipulated at various speeds within the furnace. For example, the charge end furnace rolls must be able to match caster speed while the discharge end rolls must run at mill entry speed. Speed feedback from the roll motors is accomplished using encoders mounted to several rolls in the system. The encoders, in conjunction with a series of photocells, allow the PLC to track multiple slabs and display their respective positions on the Human Machine Interface (HMI).
Combustion - Early tunnel furnace installations used distributed control systems for combustion control. Recent design utilizes a PLC-based system with the following benefits:
- Simpler Programming - Today's PLC's utilize simple ladder logic as opposed to proprietary programming languages employed by distributed control systems.
- PC Based Access - PLC systems use single PC access to all control screens versus a dedicated management station.
- Networking Capability - Distributed control systems are connected to a proprietary network whereas a PLC can be networked to other system hardware, which improves data access and information sharing.
Operator Interface - A PC-based HMI is used, which utilizes commercially available software for the development of control screens. Material handling and combustion control screens are developed with the same software, and access to all control screens for the system can be accomplished at any network PC. Figure 5 represents a typical HMI screen for zone combustion control.
Level 2 - Supervisory control using real-time thermal modeling is another feature of some tunnel furnace systems. Automatic remote set-point determination and automatic selection of the transfer point (i.e., the roll location at which a slab is accelerated from casting speed to in-furnace transfer speed) are capabilities of the Level 2 system. Level 2 also provides a means of archiving the thermal history of each slab through its cross-section, as well as determining the heating quality of each piece processed. Level 2 optimizes furnace operation during delays, saving fuel while minimizing overheated slabs with the associated scale formation. Finally, Level 2 is the means through which the tunnel furnace system is integrated into the scheduling and product tracking systems of modern plants.
Tunnel Furnace Analytical ModelsTunnel furnace design is accomplished by the utilization of analytical models designed to meet the unique operational and process requirements of each customer. Four analytical models are used to accomplish this objective: (1) transient heat transfer models; (2) dynamic simulation models; (3) heat balance models; and (4) flow distribution models.
Transient Heat Transfer Models - One-dimensional, finite-difference, transient heat transfer models are used to calculate the slab discharge temperature along its length and temperature uniformity through the slab thickness. Tunnel furnaces, unlike conventional reheat furnaces, realize significant differences in the heating times for the leading and trailing edges of the slabs being heated. This head-to-tail time differential is caused by a slow casting speed (typically 5 m/min) and a faster rolling mill takeaway speed (typically 15 m/min). Under these conditions, the slab head can experience as much as 10 to 20 minutes of extra heating time as compared with the slab tail. Figure 6 shows a comparison of the heating transients for the slab head and the slab tail.
Dynamic Simulation Models - From an operational standpoint, the furnace must be able to handle mill upsets and delays. The measure of a furnace's ability to handle such incidents is called Caster Abort Buffer, or CAB. This measures the amount of time available to the operators to cast at the present rate before the furnace has no more room for additional steel slabs. CAB has a direct impact on the furnace design and is dependent on caster speed, mill takeaway speed, slab length, and transfer point.
An example of a scheduled mill delay, roll changes typically require 8 to 12 minutes to complete. Unscheduled delays, such as mill upsets, could take much longer. During a mill upset or roll change, the rolling mill does not take any slabs from the furnace while the caster continues to cast new slabs. This causes the CAB to decrease linearly over the duration of the delay. Once the delay is over, a slab is taken by the rolling mill and the CAB begins its recovery. If the delay is long enough, however, the operator will have to resort to cutting slabs or even aborting the caster.
Heat Balance Models - A furnace system heat balance takes into account the iterations of process heating and furnace dynamics to properly size the combustion system of burners, recuperators, and exhaust stacks. This model takes into account the heat-to-steel and heat-to-system losses such as refractories and water-cooled in-furnace rolls. The Sankey diagram (Fig. 7) demonstrates the heat flows in the tunnel furnace system.
Additionally, there is another challenge to this model. That is the determination of the flow patterns of the combustion waste gases that carry heat throughout the furnace. In a conventional reheat furnace, the gas flows move in a direction opposite the steel movement, from the discharge end to the charge end and out through the uptake or downtake. In a tunnel furnace with multiple flues positioned over lengths of over 200m, the gas flows are no longer unidirectional.
Flow Distribution Models - Flow distribution models are developed to analyze the flow characteristics of the tunnel furnace design. The gas flow model utilizes Computational Fluid Dynamics (CFD) techniques to determine the precise flow directions along the length of the furnace. As a result of this flow model, flue positions can be selected to optimize the furnace pressure distribution, thus minimizing air infiltration and hot gas stingout.
As with conventional reheat furnaces, pressure control is important to tunnel furnace operations. Both types of furnaces operate best with a slightly positive pressure to prevent air infiltration, which affects fuel usage and scale formation. With tunnel furnaces, the more complex pressure distribution along a much longer length poses the challenge of collecting as much of the waste gases as possible in the stacks. Some operations have been observed to have large stingouts through the charge door and roll openings, which result in a large amount of hot gases bypassing the recuperators. This causes the recuperator performance to decrease and therefore affects the fuel consumption. Besides affecting the recuperator performance, the stingouts affect refractory and peripheral equipment life, not to mention posing a hazardous condition for work around the furnace.
It is not until all four of these models are solved that a tunnel furnace design is completely satisfactory. The intensive modeling effort ensures that the furnace will be properly designed for optimized process heating and operational stability. Additionally, the design should be economical from a fuel consumption standpoint and result in a safe work area.
Additional related information may be found by searching for these (and other) key words/terms via BNP Media LINX at www.industrialheating.com: refractory, ceramic fiber, castable, heat transfer, monolithic, thin slab casting, tunnel furnace, recuperators, Sankey diagram