Types of Reheating Furnaces
Types of Reheating Furnaces
Reheating furnaces are divided into two general classes:
Continuous type, including pusher type, rotary-hearth type, walking-beam type, walking-hearth type and roller-hearth type.
Batch furnaces are those in which the charged material remains in a fixed position on the hearth until heated to rolling temperature. Continuous furnaces are those in which the charged material moves through the furnace and is heated to rolling temperature as it progresses through the furnace.
Batch furnaces are the older type; though they are capable of heating all grades and sizes of steel, in practice, they are primarily used to heat relatively large billets, leaving small billets for continuous furnaces.
Batch furnaces are fired with either gaseous or liquid fuel, with preheated or cold air for combustion. The air may be preheated by either regenerators or recuperators. When the air is preheated with regenerators, the furnace fires reversed from one end to the other. If the air is preheated by recuperators, they are not reversed and firing is continuous from one or both ends, depending upon the location of the gas outlet port. The steel to be heated in a batch furnace commonly is charged and drawn through front doors by a charging machine. Batch furnaces vary in size from those with hearths of less than a square metre (only a few square feet), with a single access door, to those about 6 metres (20 feet) in depth by 15 metres (50 feet) long, with five or six doors.
Batch furnaces provide means for heating steels of various types and sizes. They can be operated to heat steel to temperatures above 1315C (2400F) more satisfactorily than a continuous furnace. If needed, they are suitable as a reservoir for holding hot steel directly from the primary mill for later rolling in the finishing mills. Primary disadvantages exist in the high capital investment per unit of production, low hearth area efficiency, high man-hours per ton of heated steel, lack of flexibility, and limitation on length of pieces to be heated, etc.
Continuous pusher-type furnaces were designed initially for heating billets and small bloom sections. The hearths were relatively short in length and were sloped downward longitudinally towards the discharge end to permit an easy movement of billets through the furnace. Pushers were used to push forward the charge of cold billets.
Longer furnaces generally are constructed now. Some have hearths about 24.5 to 32 metres (80 to 105 feet) long, with top and bottom firing, and contain preheating, heating and soaking zones. Recuperators are utilized to provide waste-heat recovery. Multiple-zone furnace (e.g. five-zone slab heating furnace) have been evolved from one-zone furnace in the early designs. to the modern five-zone slab heating furnace.
The steel to be heated in a continuous furnace can be charged either from the end or through a side door. In either case, the steel is moved through the furnace by pushing the last piece charged with a pusher at the charging end. As each cold piece is pushed into the furnace against the continuous line of material, a heated piece is removed. The heated piece is discharged by several methods, such as through an end door by gravity upon a roller table which feeds the mill, or pushed through a side door to the mill table by suitable manual or mechanical means or withdrawn through the end door by a mechanical extractor.
Advantages of the pusher-type furnaces are collected as follows:
High production per dollar investment, high hearth area efficiency, and high production per square foot of ground space occupied.
Low maintenance, ease in charging and drawing steel, less trouble from temperature inequalities between each succeeding piece drawn.
Better means for controlling the rate of heating at all temperature levels. Gradual rise in temperature permits charging all grades of cold steel without cooling furnace.
Can be built for any reasonable length of piece to be heated, resulting in higher mill yield.
Some of the important disadvantages of pusher-type continuous furnaces are:
Limited stock cross-section: face of contacting surface of stock must be square to prevent piling
Lack of flexibility for heating efficiently small orders of different lots of steel or thicknesses
Trouble from water-cooled skid maintenance: possible colder "stripes" on the hot steel; limited thickness of product (300 to 350 mm, or 12 to 14 inches) if water-cooled skids are used
Trouble from building up of scale on hearth, expensive to empty furnace at end of schedule.
Difficulty in pushing mixed sizes through furnace.
A distinctly different type of continuous reheating furnace is the rotary-hearth type, shown schematically in Fig. 1. It is used frequently for heating rounds in tube mills and for heating short lengths of blooms or billets for forging. The rotary-hearth type permits the external walls and roof to remain stationary while the hearth section of the furnace revolves.
Fig. 1: Rotary hearth furnace [?240?]
Rotary-Hearth Furnaces-Some of the important advantages of rotary-hearth furnaces are:
Rotary-hearth furnaces eliminate either the manual labor required for rolling rounds forward on horizontal or moderately sloped hearths, or the disadvantages of excessively sloped hearth in continuous furnaces. They have better means for controlling the rate of heating at all temperature levels than batch-type furnaces.
However, high capital cost per unit of production, high space per unit ratio, and low hearth area efficiency are expected with the otary-hearth furnaces. In addition, seals and wall refractories at the hearth level need to be well maintained.
The early design of walking beam furnaces used alloy steel walking beams that were exposed directly to the heat of the furnace and were subject to heat corrosion, so it operated at maximum temperatures of about 1065C (1950F), compared with reheating furnaces that must heat steel to temperatures up to 1315C (2400F).
Today the walking beam may consist of water-cooled steel members topped with refractories in such a manner that only the refractories are exposed directly to the heat of the furnace. Alternatively, the beams and supports may be constructed of water-cooled tubular sections (with "buttons" on the top surfaces to keep the hot steel from direct contact with the water-cooled tubes). Walking beam furnaces are now used to reheat slabs, billets and blooms, etc.
Walking-beam furnaces can be designed for side or end charging and discharging. Either hydraulic or mechanical methods can be used to actuate the beams. Cross firing with side-wall burners above and below the stock being heated have been employed. In some furnaces the stocks are heated with radiant-type burners in the furnace roof or in both the roof and below the stock.
Some of the important advantages of walking-beam furnaces are:
Pieces can be separated from one another on the hearth, so stickers are avoided.
Pile-ups and furnace retention time are reduced.
Furnace can be emptied easily from either end by activating the beam mechanisms.
Skid marks are eliminated since there is no line contact with water-cooled skids.
Hearth wear and stock damage are minimized since there is no rubbing or friction between the stock and the hearth.
By selecting the proper number of walking beams, better hearth utilization can be obtained when charging mixed sizes.
The potential for the extension of overall furnace length to improve the utilization of furnace waste gases and reduce fuel consumption. A similar advantage is not available with other furnace types because of limitations on overall furnace length.
Disadvantages of walking-beam furnaces exist in the system complexity, high cost, maintenance of hearth seals and hearth refractory, and possible problems from scale that drops off the stock being heated, etc.
In a walkinghearth furnace, travel of the work through the heating chamber follows the same general path as in the walking-beam furnace. The main difference in method of conveyance in these two furnace types is that, in the walking-hearth furnace, the work rests on fixed refractory piers. These piers extend through openings in the hearth and their tops are above the hearth surface during the time when the work is stationary in the furnace. The furnace gases can thus circulate between most of the bottom surface of the work and the hearth.
To advance the work toward the discharge end of the furnace, the hearth is raised vertically to first contact the work and then raise it a short distance above the piers. The hearth then moves forward a preset distance, stops, lowers the work onto its new position on the piers, continues to descend to its lowest position and then moves backward to its starting position toward the charging end of the furnace to await the next stroke.
In general, the same advantages and disadvantages of the walking-beam furnace apply to walking-hearth furnaces.
Roller-Hearth Reheating Furnaces
Roller-hearth furnaces, although used in many types of heat-treating operations, have not been used extensively for reheating steel to hot-working temperatures in conventional steelmaking practices. However, the advent of continuous casting has prompted studies of how this type of furnace might be used to advantage in heating much longer slabs than would be practical in a pusher-type or walking-beam-type furnace.
Some advantages of the roller-hearth design for reheating furnaces are:
Ability to handle very long pieces.
Zone control is simpler when cross-firing is employed.
Stock suffers little mechanical damage.
Skid marks are avoided.
Furnace is self-emptying.
Some disadvantages of the roller-hearth furnace are:
High initial cost per unit of capacity.
Water cooling of the rollers results in high heat losses, unless adequately insulated.
Although relatively narrow, roller-hearth furnaces must be considerably longer than pusher-type or walking-beam furnaces of the same capacity.
General notes on reheating furnaces
For continuous furnaces
Single-zone firing come with higher scale losses
Single-zone firing furnaces have greater tendency to cause decarburization of high-carbon steel than the top-and bottom fired furnaces, since the steel is in the furnace longer and is exposed to furnace gases with hydrogen and water vapor combinations. The scaling of steel is practiced sometimes deliberately to remove the decarburized surface layer.
Those furnaces provided only with top firing require longer hearths for equal production than those with top and bottom firing, but, in the case of pusher-type furnaces, do not require a special soaking zone to eliminate cold spots on the work caused by contact with water-cooled skids.
Side-discharge furnaces have less air infiltration at the hot end than end-door discharge furnaces. End-door discharge of the usual gravity type induces cold air into the furnace by the stack effect at the discharge section of the furnace. End-door discharge, however, is mechanically simpler for removing the heated stock; particularly slabs and heavy blooms.
A level hearth eliminates the stack effect of hearths sloping upwards towards the charging end. This stack effect draws cold air into the furnaces at the hot end and therefore causes higher fuel consumption and scale losses.
For a batch-type furnace, it is preferred to preheat certain grades of alloy and high-carbon steels in a supplementary furnace before the stock is transferred into the hotter furnaces. The preheating zone of a continuous furnace makes this unnecessary.
Finally, regenerators or recuperators act as a reservoir of heat supply, which is especially valuable for efficient soaking of steel.
 W.T. Lankford, Jr. et al (ed.): The Making, Shaping and Treating of Steel. United State Steel. 1985. ISBN 0-930767-00-4.