Car-hearth furnaces

Fig. 3.5. Large car-hearth furnace such as used for stress-relieving large vessels. The fiber-lined 90° flues avoid black hole cold spots in the furnace roof preventing uneven load temperature. Courtesy of Hal Roach Construction Co.

Tunnel kilns, widely used in firing ceramics and carbon shapes, use a long train of cars as a conveyor Each car may be similar to, but often narrower than, the car of a batch-type car-hearth furnace. Much of what is discussed in this book can apply to ceramic kilns, but the ceramic industries have so many publications on kiln construction and operation that this text will not dwell on them specifically. 

Shuttle car-hearth furnaces and kilns are hybrids between batch and continuous furnaces and kilns, combining the compact lower cost of a batch operation with the productivity and fuel economy of a continuous furnace or kiln. A shuttle furnace has doors at both ends and with two rolling hearths, permitting quick unloading and reloading of the furnace with minimum cooling during the switch-around. (See fig. 4.8.) The capital cost is only about 65% of two furnaces, but the production rate is almost doubled. The fuel economy per year and per ton heated is better because the doors are closed and the burners are in use more often. 

Losses Through Open Doors, Cracks, Slots, and Dropouts, plus Gap Losses from Walking Hearth, Walking Beam, Rotary, and Car-Hearth Furnaces (see also sec. 4.6.9)... 

Fig. 6.8. Temperature patterns in a car-hearth furnace with three versus five zones, and modulated versus minimum firing rates. 3-zone T/s 5-zone T/s...

Of the previously mentioned tabulated five situations, situation 1 is probably most desirable for industrial heat-processing furnaces. If the hearth is tight so that there can be no inleakage from below, the pressure at hearth level should be controlled at -1-0.02 in. wc (0.51 mm H2O). For conveyor furnaces and car-hearth furnaces, there may be a chance of a leak below the hearth level (as at a water seal or sand seal), in which case the -1-0.02 in. wc (0.51 mm H2O) pressure should be the setpoint for that lowest leak level. The control sensor should be just high enough above the hearth to avoid blockage by accumulated scale or refractory crumbs, and the control setpoint biased upward per table 7.1 for the difference in elevation between the sensor and the lowest leak. This will achieve the three objectives listed previously. 

Side view of a batch car-hearth furnace with top-firing, bottom-fiuing. 

Fig. 7.8. Better heat treatment of railroad wheels with high-velocity burners and 50% higher exposure in a conventional furnace by stacking the wheels two-high on special piers.This sectional view could be of a rotary hearth or longitudinal continuous furnace, or a car-hearth furnace.

Car-bottom furnaces differ from standard types in that the charge is placed upon movable cars for running into the furnace enclosure. The top oi the car is refractory-lined and forms the furnace hearth. The top only is exposed to heat, the lower metal structure being pro-tec ted by the hearth brick, sand, and water seals at the sides and ends and by the circulation of cooling air around the car structure below the hearth. For use where floor space is hmited elevator furnaces serve similar purposes. 

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. 1.18 Car-hearth heat treat furnace with piers for better exposure of bottom side of loads. The spaces between the piers can be used for enhanced heating with small high-velocity burners. (See chap. 7.) Automatic furnace pressure control allows roof flues without nonuniformity problems and without high fuel cost.

Fig. 3.4. Car-hearth heat treat furnace with piers, ceramic fiber waiis, and high-veiocity burners top left and bottom right). Courtesy of Horsburgh and Scott Co.. Cieveiand, OH.

Figure 3.5 shows a 40 ft (12.2 m) long car-hearth in a 17.5 ft (5.3 m) high fiber-lined furnace with high-velocity burners at top and between the piers. Automatic furnace pressure control makes it possible to use top flues. Drilled square air manifolds shoot curtains of air across the flue exits as throttleable air curtain dampers for furnace pressure control. 

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. 6.1. Side-fired in-and-out furnace (with car-hearth), 18 wide x 12 deep x 8 high iD. Adjustabie fiame burners give uniform heating width-wise/depth-wise double-stacked piers help bottom uniformity. (See also figs. 3.26 and 6.23.)...

Rg. 6.5. Sectional view of a rotary hearth furnace (such as fig. 1.8) with enhanced heating. This also could be a car-hearth batch furnace or in-and-out batch-box furnace. In many cases, the higher velocity burners would be smaller (relative to the main burners above) than they appear in this drawing. In other than rotary hearth furnaces, the high-velocity burners should fire between piers and opposite the main burners—to further enhance circulation. 

Assume that the furnace has or will have cracks, and leaky seals around doors, peep sights, sensors, and car hearth or conveyor. Establish an ongoing inspection and repair program to minimize these possible sources of inleakage or outleakage. 

Doors should be checked often and repaired promptly because hot gas leaks can lead to mnaway ruin quickly. Seals around doors and car hearths need frequent repair or replacement. Doors should be checked for warpage and loss of refractory. Doors that are not used should be bricked up, but with addition of an observation port (with closure on a chain) and closure for monitoring furnace conditions during firing. If there are any gaps between doors and stationary furnace elements exceeding in. (3 mm), they should be adjusted for less leak. 

A natural-gas-fired car-bottom furnace is to be built for heating 175 000 pounds of steel ingots from 50 F to 2150 F in 16 hr. Using formulas and data from this book and References 51 and 52 as well as from refractory suppliers data, select hearth, sidewall, and roof construction. Then calculate heat loss, heat storage, and coldface temperatures for the selected hearth, wall, and roof. 

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