Poc gases

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. 

Flames from solid fuels may contain ash particles, which can glow, adding to the flame s luminosity. With liquid and gaseous fuels, flame luminosity usually comes from glowing carbon and soot particles. The effective flame emissivity, as measured by Trinks and Keller, is usually between that of the poc gases and a maximum value of 0.95, depending on the total surface area of solid particles. 

To have temperature uniformity within each load piece and among the pieces, furnace gases and solids must have low temperature differences. All heat supplied by the combustion reaction flows either (1) directly from the hot poc gases to the load or (2) from the poc gases to the refractory, and is then re-radiated to the load. Heat transfer is a form of potential flow, moving from high temperature to low temperature. Thus, the flame and poc gases must be hotter than the refractory, and the refractory must be hotter than the load. 

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. 

The heat transfer rate from the poc gases to the loads must be moderate because the load temperature will reflect the poc temperatures. Therefore,... 

Fig. 2.14 Triatomic gas radiation heat transfer coefficients for 36 to 72 in. (0.91-1.83 m) thick gas biankets with poc having 12% CO2 and 12% H2O. The data of figs. 2.13 and 2.14 are for gas blankets of 12% CO2 and 12% H2O, but most natural gases produce about 12 CO2 and 18% H2O, so the actual radiation will be somewhat higher. (Continued from fig. 2.13.)...

Rg. 2.21 The many concurrent modes of heat transfer within a fuel-fired furnace. Some refractory surfaces, r, and charged loads, c, are convection-heated by hot poc flowing over them. Triatomic molecules of the combustion gases, g, and soot particles, p, radiate in all directions to refractories, r and loads, c. The surfaces of r and c in turn radiate in all possible directions, such as r to r, r to c, c to c, and c to r. 

With an ATP-type burner, the heat release pattern of the flame can be automatically adjusted by the difference in temperatures sensed at two points in the furnace. One of those temperatures also can limit energy inputs so that both ends of the load(s) will be controlled to raise or lower their temperatures together. If ATP-type burners cannot be fitted to spaces that are too narrow, other means (discussed later) must be used to avoid load temperature nonuniformities. This is usually done by designing for no more than a 30°F (16°C) poc temperature drop as the gases pass from one end of the load to the other. 

Fig. 3.17. Batch recirculating oven passes gases through the loads many times, saving fuel. The circulating gases have burner poc, and thus help uniformity.

A6. Because the refractory at the exit could not have reached its temperature unless the passing furnace gases were hotter than the refractory itself. Those poc are the source for heat in the relf actory walls, and there must be a difference in temperature to drive the heat from the gases to the walls. 

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). 

In furnaces with top and bottom heat and preheat zones, there is greater resistance to poc gas flow below the loads and their conveyor. That resistance causes the bulk of the bottom gases to flow into the top zones, reducing the effective heat transfer exposure areas significantly. This movement of combustion gases into the top zones reduces productivity and lowers available heat, increasing fuel use per ton of product. 

Fossil fuel combustion transforms chemical energy into sensible heat, raising the temperature of the combustion gases. The resultant hot poc immediately transfer heat by convection and gas radiation to cooler solids and gasses, at rates proportional to their temperature differences. 

When the firing rate is lowered, the reverse phenomena take place Gases take longer to traverse the same path, and so each molecule of poc has more residence time during which to deposit its heat on the loads, but its coefficient of heat transfer is less (a function of velocity to only the 0.52 to 0.80 power). 

Sensible heat carried out of the furnace by the furnace gases (poc) is often the largest loss from high-temperature furnaces and kilns. It is evaluated by the available heat charts mentioned in section 5.1 100% — %available heat = %heat carried out through the flue. It can be reduced by careful air/fuel ratio control, use of oxy-fuel firing, and good furnace pressure control. 

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