Roof firing

Roof firing can provide uniform temperature across a hearth, especially in soaking zones. An almost-standard practice for soaking zones has been to use roof burners in three zones across the width of the furnace. Attempts to cut costs with only two zones have given very poor results. 

Roof firing can be accomplished either with type E ( flat flames) in a flat roof or with conventional (type A) flames or long, luminous (type F or type G) flames in a sawtooth roof. (See fig. 6.2.) 

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A continuous furnace may be heated so that the temperature of its zones is practically the same across the furnace. This temperature uniformity can be obtained by lengthwise firing in several zones (as illustrated by fig. 4.2), or by roof firing or side firing in several zones (as shown in fig. 4.3). In such furnaces, the heating capacity of a continuous furnace will equal or exceed the capacity of a same-size batch furnace. 

Fuel economy calculations are more complex for multizone furnaces, including rotary furnaces—side fired, roof fired, or longitudinally fired—with or without baffles between zones. (See sec. 2.6, 3.4, 3.5.) With thick loads, load placement is more critical. (See sec. 3.5, 6.9, 6.10.)... 

Rotary furnaces cannot be end fired, but they can be roof fired with type E flat flame burners or with a sawtooth roof. They may be side fired on the outside only, or inside and outside with a donut design. 

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.

The sawtoothed roof furnaces sometimes had several zones practically im-fired, but they at least had some firing even with reversed gas flow. Furnaces side fired, or roof fired with flat-flame (type E) burners had burners all along the walls or roof. Sawtoothed roof furnaces may have cost less, but with large loads and one fixed baffle, control was difficult. Regardless, a move to sawtooth roofs proceeded because of less cost. 

With the authors recommended six top heating zones and six bottom heating zones, the temperature measurement would control each small zone as the heating curve directs and would not get out of step as has been the case with very large zones. A furnace with the many zones recommended would probably be a roof-fired or side-fired furnace. Side firing would need ATP technology to control the loads temperatures evenly from end to end across the furnace width. 

Procedure—phase D3—Preheat Zone. If longitudinally fired, this zone would have a peak temperature of about 2250 F (1252 C) at a point 5 to 10 ft (1.5 m to 3.0 m) from the burner wall. The burner wall temperature would probably not be more than 2200 F (1204 C). The entry end of this zone is cooler because the product at the entry is generally at ambient temperature therefore, the temperature difference is greatest at that instant. The load temperature then rises rapidly because of the 4th power effect of radiant heat transfer. If roof fired or side fired, the slope of the temperature curve... 

Figure 7.11 (a) A roof-fired box-type reforming furnace, (b) Detail of top part of a... 

Engineering and Administrative Controls Sign Requirements Permit Requirements Tank Roofs Fire Watoh Atmospherio Testing LookoutATagout... 

Underwriters Laboratories 333 Pfingsten Road Northbrook, Id. 60062 Standards for Safety is ahst of more than 200 standards that provide specifications and requirements for constmetion and performance under test and in actual use of a broad range of electrical apparatus and equipment, including household appHances, fire-extinguishing and fire protection devices and equipment, and many other nongenerady classifiable items, eg, ladders, sweeping compounds, waste cans, and roof jacks for trader coaches. 

At the roofing plant, coating asphalts ate blended with mineral stabiHzet such as finely ground limestone, slate, flyash, or traprock. The stabiHzet increases the coating asphalt s resistance to fire and foot-traffic and adds durabiHty. 

Wood Shingles and Shakes. Early roofs in the United States were primarily hand-spHt hickory or cypress shakes. The natural beauty and style of these materials make them popular. Fire-retardant treatment and underlays may be needed to meet local fire codes. Wood shingles are sawn cedar having a uniform thickness. Wood shakes are usually hand-spHt and resawn. 

Fire and Wind Hazards. Weather resistance of roof covetings is not necessarily correlated to fire and wiad resistance. Underwriters Laboratory and the Factory Mutual System test and rate fire and wiad hazard resistance, and some durabiUty tests. Organic felt or fiber glass mat base shingles are commonly manufactured to meet minimum UL requirements, which, ia addition to minimum mass, require wiad and fire resistance properties. 

Metal deck assembhes are tested by UL for under-deck fire hazard by usiag their steiaer tunnel (ASTM E84). The assembly, exposed to an under-deck gas flame, must not allow rapid propagation of the fire down the length of the tuimel. FM uses a calorimeter fire-test chamber to evaluate the hazard of an under-deck fire. The deck is exposed to a gas flame and the rate of heat release is measured and correlated to the rate of flame propagation. A different FM test assesses the damage to roof iasulations exposed to radiant heat. 

Another problem occurs when some fire retardant formulations ate exposed to elevated temperatures (eg, when used as roof tmsses or as roof sheathing) thermal-induced strength reductions can occur in-service. The thermo-chemical factors were discussed by LeVan and Winandy (26), and a kinetic degrade model was developed (27). The treater should be consulted to obtain appropriate in-service modifications for specific fire retardant treatments. 

Wood in its untreated form has good resistance or endurance to fire penetration when used in thick sections for walls, doors, floors, ceilings, beams, and roofs. This endurance is due to low thermal conductivity, which reduces the rate at which heat is transmitted to the interior. Typically, when the fire temperature at the surface of softwood is 870—980°C, the inner char 2one temperature is - 290° C, and 6 mm further inward, the temperature is 180°C or less. The penetration rate of this char line is mm/min, depending on the species, moisture content, and density (45,46). Owing to this slow... 

Polyurethane foams for building purposes are normally of the rigid kind used for roof and wall insulation. Polyols containing halogen or phosphoms are used to increase the fire resistance of the foam. 

Combustible Dusts Dusts are particularly hazardous they have a very high surface area-to-volume ratio. When finely divided as powders or (dusts, solids burn quite differently from the original material in the bulk. Dust and fiber deposits can spread fire across a room or along a ledge or roof beam very quickly. On the other hand, accumulations of dust can smolder slowly for long periods, giving little indication that combustion has started until the fire suddenly flares up, possibly when no one suspects a problem. 

Honzontal-tube cabin heaters position the tubes of the radiant-section-coil horizontally along the walls and the slanting roof for the length of the cabin-shaped enclosure. The convection tube bank is placed horizontally above the combustion chamber. It may be fired From the floor, the side walls, or the end walls. As in the case of its vertical cylindrical counterpart, its economical design and high efficiency make it the most popular horizontal-tube heater. Duties are 11 to 105 GJ/h (10 to 100 10 Btu). 

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