Donut rotary hearth furnace

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. 

Sealing the sides of a car hearth or of disc or donut hearths of rotary hearth furnaces is usually accomplished with sand-seals or water-trough seals. 

Fig. 1.8. Rotary hearth furnace, donut type, sectioned plan view. (Disk type has no hole in the middle.) Short-flame burners fire from its outer periphery. Burners also are sometimes fired from the inner wall outward. Long-flame burners are sometimes fired through a sawtooth roof, but not through the sidewalls because they tend to overheat the opposite wall and ends of load pieces. R, regenerative burner E, enhanced heating high-velocity burner. (See also fig. 6.7.)...

Air/Fuel Ratio Control. Air flows may differ to burners in parallel in the same zone on the inside and outside of a rotary hearth furnace donut because of the long runs of air duct and the large number of tees and elbows. High design air velocity creates very different air flows to burners in a zone. One such furnace was designed for an air flow of 70 ft/sec (21 m/s) with three elbows and four tees to each burner. The fan s discharge pressure was 14"wc (3.5 kPa), but the pressure delivered to one burner... 

Example 6.1 This is a case study of a 45 ft (13.7 m) diameter donut (see glossary) rotary hearth furnace, similar to figure 1.8, that was having problems with low production capacity. The inside cross-section dimensions of the donut-shaped, circular gas and load passageway (a circular tunnel furnace) are 4.5 ft (1.37 m) high x 12 ft (2.66 m) wide. Most of the furnace gas flow is counter to the load movement. 

Alternatives to straight-line horizontal continuous furnaces are rotary hearth (disc or donut) furnaces (fig. 1.8 and secs. 4.6 and 6.4), inclined rotary drum furnaces (fig. 1.10), tower furnaces, shaft furnaces (fig. 1.11), and fluidized bed furnaces (fig. 1.12), and liquid heaters and boilers (sec. 4.7.1 and 4.7.2). 

Rotary hearth or rotating table furnaces (fig. 1.8) are very useful for many purposes. Loads are placed on the merry-go-round-like hearth, and later removed after they have completed almost a whole revolution. The rotary hearth, disc or donut (with a hole in the middle), travels on a circular track. The rotary hearth or rotating table... 

Fig. 6.6. Calculated time-temperature heating curves for a rotary hearth donut furnace showing the effects of delays before addition of enhanced heating burners. (Directions for calculating time-temperature curves are given in chap. 8.) The top two curves show what happens upon restart at normal tph after a delay. The bottom curve shows that loads charged after resumption will be too cold to roll, forcing a fall back to half the normal tph.

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