Radiant-roof furnace

The advantages of radiant-roof furnaces are no crucible required very low energy costs close temperature control clean, cool, silent working conditions. 

Although most low-energy holding furnaces serve individual casting machines, some larger ones are used as buffers between bulk melters and machine furnaces. This latter use allows better use of the melter, which is seldom an efficient holder. 

The rotary fiimace consists of a horizontal cylindrical vessel, in which the metallic charge is heated by a burner located at one side of the furnace. The flue-gases leave the oven through the opposite side. To generate the required heat, fuel or natural gas is used combined with air or pure oxygen. 

Once the metal is melted, and after a composition check and adjustment, a tap-hole in front of the furnace is opened and the melt in the furnace is discharged into ladles. Because of its lower density, the slag floats on the metal bath in the furnace and is finally collected through the tap-hole into slag pots. 

A melting cycle spans PA to several hours. For continuous molten metal production, foimdries install 2 to 4 rotary furnaces, which are operated consecutively. The thermal efficiency of the rotary furnace is very high, i.e. at 50 to 65 depending on the capacity. This high 5deld is achieved by using pure oxygen instead of air as the combustion medium. 

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Crucible, hearth type and radiant roof furnaces... 

Example 5 Radiation in a Furnace Chamber A furnace chamber of rectangular paraUelepipedal form is heated hy the combustion of gas inside vertical radiant tubes hningthe sidewalls. The tubes are of 0.127-m (5-in) outside diameter on 0.305-m (12-in) centers. The stock forms a continuous plane on the hearth. Roof and end walls are refractory. Dimensions are shown in Fig. 5-20. The radiant tubes and stock are gray bodies having emissivities of 0.8 and 0.9 respectively. What is the net rate of heat transmission to the stock by radiation when the mean temperature of the tube surface is SIG C (1500 F) and that of the stock is 649 C (1200 F) ... 

ASTM E 84 Standard Test Method for Surface Burning Characteristics of Building Materials ASTM E 108 Standard Test Methods for Fire Tests of Roof Coverings ASTM E 119 Standard Test Methods for Fire Tests of Building Construction and Materials ASTM E 136 Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750°C ASTM E 162 Standard Test Method for Surface Flammability of Materials Using a Radiant Heat Energy Source... 

A furnace firebox is 20 ft (6.1 m) long, 10 ft (3.05 m) wide, and 5 ft (1.5 m) high. Because of a rich fuel-air mixture, all surfaces have become coated with lampblack, so that they all act as black-body surfaces with virtually complete absorption and emittance of radiant energy emittance e is 0.97. The furnace is overfired i.e., its cold surface is the floor, composed of closely spaced tubes flowing water at 250°F (394 K). When the furnace is operating, its roof is at 1150°F (894 K), the sidewalls are at 920°F (766 K), and the end walls at around 800°F (700 K). A plant emergency suddenly shuts the furnace down. Determine the initial rate of heat transfer from each interior surface if the water in the tubes remains at 250°F. Assume that the tube surface is at the water temperature. 

Example 2.3 A reverberatory batch melting furnace, fired with natural gas, has a 36" high gas blanket between the molten bath surface and the furnace roof. The absorptivity of the 1500 F molten bath surface is estimated to be O.3. When the poc are at 2000 F, calculate the radiant heat flux from the poc gases to the load. 

Fig. 3.14. Heat treating furnace with radiant U-tubes on the roof and back wall. The return legs (2nd and 4th from the hearth) are less radiant than the burner legs (1 st and 3rd from the hearth). Tumbling around the bends completes gas-air mixing so the renewed delayed-mixing flame (type F, fig. 6.2) causes a glow in the second leg. Courtesy of Rolled Alloys, Temperance, Ml.

Fig. 3.16. A heat treating car-hearth (batch) furnace. Both sides of the furnace are heated by four W-radiant-tubes wifh a tofal of eight pairs of regenerative burners. Plug fans through the roof drive recirculation down between the load pieces.

Fig. 4.6. Continuous roller hearth furnace, side-elevation sectional view. Through-the-roof plug fans drive circuiation across radiant tubes above and beiow ioads on roiiers.

In the temperature range usually used for this process, the furnace walls, roof, and hearth provide excellent radiant heat transfer. The furnace height necessary to avoid flame impingement on the strip from lower burners also assures a good average beam for gas radiation to both top and bottom surfaces of the load. 

It should be understood that there are many variables to this desCTipiion. Burners may be located In the side walls or roof of the radiant section. Insulation may be reftactory brick, ceramic fibers, or a mineral wool blanket. The produa may flow into the convection section tubes, exit that seaion through crossover piping, and flow through the radiant seaion. Multiple furnaces may be tied to one common sack by breeching. 

Reformer In a reformer furnace, shown in Exhibit 7-5, preheated process fluid flows through catalyst-filled tubes, which are usually located in the center of the radiant section. This type of furnace may have single or multiple compartments burners may be mounted in the roof, wall, or floor. Heat recovery systems may also be employed through the use of waste heat boilers or the convection section s steam generation coils. 

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