Side Firing Reheat Furnaces

Emissivity and conductivity at low product temperatures can have major effects on heat transfer and therefore furnace capacity. Higher gas temperatures in the furnace can increase heat transfer, which is why recuperation, oxygen enrichment, or regenerative burners can increase furnace capacity by as much as 15% and reduce fuel rates from 20 to 45%. 

Another problem that limits furnace capacity is bowing in top-fired-only furnaces wider than 25 ft (7.6 m). Excessive bowing in the charge zone is due to large temperature differentials between billet top and bottom. If the billet bows more than its thickness, pileups are sure to result. Pileups result in huge mill delays. Therefore, the furnace throughput must be reduced to a production rate that avoids serious bowing. 

To increase furnace productivity in wide furnaces, underfired enhanced heating burners should be used at the charge end of the furnace to reduce top-to-bottom temperature differentials within the load pieces. 

A second crosswise AT error is Iht variable temperature profile of the combustion gases across the furnace depending on the firing rate. With only one temperature measurement in a zone, the zone setpoint must be conservative to prevent rapid scale melting in any part of the zone hence, productivity is sacrificed. Modern burners 

Pusher Hearth Furnaces Are Limited by Buckling/Piling 

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Side-fired reheat furnaces can be troublesome in two ways (1) When conventional burners are installed directly opposite one another, the center of the furnace becomes very hot because the velocity pressures of the poc from the opposing burners negate each other and because the completion of the fuel burning is concentrated in the furnace center and (2) with staggered long-flame burners, a wide furnace s center gets hotter than the sides when on high fire, but at low fuel inputs the sidewalls get hotter than the centers. Both troubles can be prevented with controlled temperature profile burners and added T-sensors/controls. (See chap. 6.)... 

Fig. 1.3. Five-zone steel reheat furnace. Many short zones are better for recovery from effects of mill delays. Using end-fired burners upstream (gas-flow-wise), as shown here, might disrupt flame coverage of side or roof burners. End firing, or longitudinal firing, is most common in one-zone (smaller) furnaces, but can be accomplished with sawtooth roof and bottom zones, as shown. 

Hg. 5.10. Continuous steel pusher reheat furnace side fired with regenerative burners in the top and bottom heat and preheat zones, and root tired in the soak zone. Preheat zones often have been designed as unfired preheat zones, which are good for fuel economy. However, also firing the preheat zones with regenerative burners would add capacity while retaining high fuel efficiency. (For a discussion of controls for this furnace, see sec. 6.11.1.)... 

Fig. 5.22. Eight-zone reheat furnace, side sectional view with an aerial perspective view inset at top right. This furnace has longitudinal firing in all but zones 5 and 6, which are roof fired. Billets or slabs move from left to right, and poc move from right to left. An unfired preheat zone is left of zones 1 and 2.

Fig. 5.27. Continuous steel reheat furnace with nine pairs of regenerative burners in three top control zones and four pairs in a bottom zone. The sweep of hot poc from side burners can alternately proceed all the way across the furnace width, avoiding the former uneven heating when opposed burners created a hot spot pileup of heat in the center when on high fire, and a cool stripe down the middle on low fire.

Longitudinal firing of steel reheat furnaces in top and bottom heat and soak zones, including sawtooth-roof rotary furnaces, is used to reduce the number of burners and to develop a uniform temperature across the hearth. Otherwise, most of these furnaces would be side fired to hold the heat transfer temperature higher and longer (many times for as long as 40 ft, perhaps 25 ft, for longitudinally fired zones). 

S.4. Continuous Reheat Furnaces. Continuous reheat furnaces may be rotary or linear. Either can be side fired or top fired. Top firing may be done with conventional type A, F, or G forward thmst flames (fig 6.2) in a sawtooth roof or with type E flat flames in a flat roof. End firing alone can be used only in small linear reheat furnaces, but it is sometimes used in combination with roof- or side-firing in all sizes. (See also sec. 3.8.5.) For donut rotary hearth furnaces, much detail is discussed in section 6.4.1. 

Preliminary Decisions "Walking hearth foiu-zone reheat furnace, with all zones longitudinally or side fired. Zone 1 (charge end) is to be unfired. Zones 2 and 3 are to be side fired, and zone 4 (soak) is to be fired longitudinally, using ambiet air in all burners. Fuel = natural gas. Hearth width should include 2 ft clearance on each end of 30 ft long billets = 34 ft. 

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