Roof flues

Of equal importance, there was no increase in refractory cost. The roof, flue, and curtain wall temperatures were closely monitored during the initial operation. No increase in the temperature of the monitoring thermocouples was noted. Inspection of the furnace refractory during a normally scheduled shutdown showed no signs of abnormal refractory wear. 

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.

Roof flues should generally be used only when there is bottom firing. Otherwise, hot gases will not flow to the bottom to maintain a hot gas blanket temperature, so bottom heat losses will take heat from the load(s) via solid radiation and conduction. The resultant nonuniformity in load temperature will be intolerable. 

Bottom flueing is preferred, but in-the-wall vertical flues have been found too costly, and they pull a harmful negative pressure at the hearth level. With top firing, the best arrangement is hearth-level flues with automatic furnace pressure (damper) control. If fired with top and bottom burners, use of a roof flue with automatic furnace pressure control is suggested. The flue location should be determined to enhance the design circulation pattern. (See chap. 7.)... 

Bottom firing (i.e., burners below the loads) delivers heat to the usually cooler hearth, making up for hearth losses that otherwise would be taken from the loads or from the gas blanket. (See fig. 7.3.) Bottom firing is sometimes used with roof vents, but roof flues can be undesirable because at low-firing rates, the gases may short-circuit direct to the roof flues (giving poor temperature uniformity and poor fuel economy). Roof vents also can cause negative or low furnace pressure therefore, oversize vents should be avoided, and furnace pressure should be controlled with a stack closure. Tall furnaces are especially susceptible to this problem. 

Roof flues can be used with top firing if the flames have sufficient momentum (even at low firing rates) that they will fly past the flues and not up the stack. (See fig. 7.12.)... 

The average American home is responsible for more annual pollution than the average car. This often comes as a surprise because the pollution attributed to homes is produced miles away at a power plant or out-of-sight from roof exhaust flues. However, eveiy time someone flips a switch, activates the air conditioning, or takes a shower, pollution is being produced. There will be growing appreciation for energy efficient homes that help prevent pollution. 

Two of these pane are usually worked together, and the hydrochloric acid gas brought into one main flue, as shown in the figure. The salt cake furnace is attached to the decomposing pan, as shown in Fig. 512. The sole of this furnace, which is flat, is constructed of good fire-brick or Welsh tiles, and the roof, which is only slightly arched, is of fire-brick. H ie the fireplace,... 

If we operate with a positive pressure in the firebox, although the burners may appear to operate normally, the hot flue gases will leak outward. This damages the roof arch supports and the steel structure so as to shorten the life of the heater. 

The sulphate pan A (see Fig. 109) was a shallow dish made of fire resistant white cast iron 2500 to 3000 mm in diameter and 400 to 700 mm deep. A firebrick arch was built above the dish with a flue in the middle for the escaping hydrogen chloride. Muffle E, connected to the pan by a narrow channel N provided with a slide was 4000 to 8000 mm long and 2000 to 3000 mm wide. The bottom of the muffle consisted of large firebrick blocks. Also the roof was built of the same material but was thicker (120 to 240 mm). Its heat resistance had to be greater than that of the bottom as the muffle was heated from above. 

Reverberatory furnaces3 are particularly suitable for fines and flue-dust from the blast-furnaces. Their walls and roof are lined with silica bricks, and the hearth with silica sand. A typical charge consists of2 ... 

Fig. 7.7. Photograph of catalyst bed converter, courtesy Outokumpu OYJ www.outokumpu.com Gas inlet and outlet flues are shown. Others are hidden behind. Fig. 7.6 s gas coolers are also hidden behind. Converter walls and roofs are designed to be strong enough to withstand their acid plant s main blower shutoff pressure without damage (Friedman and Friedman, 2004). Catalyst tray supports are also strong enough to withstand the downward force exerted by the descending feed gas (at the converter s operating temperature).

Cement sheet and boarding materials such as roofing sheets, cladding, flat sheeting, rainwater services, water tanks, moulded decorative panels or containers, pipework/flue pipes, non-slip external floor tiles etc. 

Fig. 1.4. Eight-zone steel reheat furnace. An unfired preheat zone was once used to lower flue gas exit temperature (using less fuel). Later, preheat zone roof burners were added to get more capacity, but fuel rate went up. Regenerative burners now have the same low flue temperatures as the original unfired preheat zone, reducing fuel and increasing capacity.

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.

To reduce fuel rates, the first fired zone should be controlled by temperature measurement in the roof about 6 ft from the uptake flue in the direction of load... 

Rotary furnaces once had flues in each fired zone, which reduced thermal efficiencies to 30 to 35%. Most such furnaces have been rebuilt with one flue in the roof of the charge area, except where they supply a waste heat boiler, and all the steam generated is used in the operation. 

Phase 2.1. As combustion gases (poc and excess air) flow from flames, they pass over load pieces, and may be directed across walls, roof, hearth, baffles, and piers in a circulation pattern, eventually finding their way to the flues. This flow phase delivers heat to loads and walls by convection and by gas radiation (largely from carbon dioxide and water vapor molecules). 

Change the location of the control measurement in the top preheat zone from the roof near the flue to 6 to 10 feet toward the furnace discharge. There, it can feel the gas temperature and see the product. 

With thick loads, the pieces should be on piers with high-velocity burners located in rows near the bottoms of both sidewalls, alternating on 4-ft (1.22 m) centers. With this arrangement, flues can be in the roof. One important point In batch operations, do not pass the poc gases of any zone through another zone because that will result in loss of temperature control for the second zone. 

Example 6.6 A pit fiirnace is being fired with natural gas and 10% excess air, and has a 2400 F (1589 C) flue gas exit temperature. The wall, hearth, and roof losses are calculated to be 1.55 kkBtu/hr. With cold air firing, there is a40°F (22°C) temperature difference from top to bottom of the ingots. Predict the corresponding temperature difference when using 1300 F (704 C) combustion air, and when using oxy-fuel firing. 

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