Furnace gas exit temperature

Improving energy use in furnaces requires knowledge of the flue gas exit temperature. Many studies and articles oversimplify the measurement of furnace gas exit temperature or simply assume it to be the temperature of the furnace (refractory wall) at the flue entry—neither of which is correct. 

A shortcut method for estimating furnace gas exit temperature is offered by the graph of figures 2.20 and 5.3, adapted by coauthor Shannon from radiant tube data, and extrapolated above 1800 F (1255 C). Also refer to Estimating Furnace temperature profile for calculating heating curves in chapter 8. 

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. 2.20 Elevation of flue gas exit temperature atx>ve furnace temperature, for a variety of velocities (average across-the-furnace cross section in the vicinity of the flue). (Same as fig. 5.3.)...

Example 3.1 Find the rate of heat liberation needed to heat 0.4% carbon steel to 2200 F on a hearth. A loading rate of 80 Ib/ft hr is very good for a single zone batch furnace. From figure 2.2, interpolate the gain in steel heat content from 60 F to 2200 F as 365 Btu/lb, so 80 x 365 = 29 200 Btu/ft hr, which is 8.11 Btu/s for each square foot of hearth. From an available heat chart for natural gas (reference 51), the best possible efficiency for an estimated 2400 F flue gas exit temperature with 10% excess air would be 31.5%, so the rate of heat liberation required = 29 200 Btu/ft hr output divided by (31.5 useful output/100 gross input) = 92 700 gross Btu/ft hr. 

A2. Because fuel costs are much higher in high-temperature furnaces than in lower temperature furnaces as a result of the higher flue gas exit temperature causing higher stack loss. 

Q6. Why is it misleading to guess that a furnace zone s flue gas exit temperature is the same as the zone s inside refractory surface temperature ... 

Tat flue gas exit temperature will always be higher than the furnace temperature at the flue because otherwise heat would not flow from the furnace gases to the walls and loads. Accurate measurement of flue gas exit temperature can be difficult. A high-velocity thermocouple with several radiation shields is essential. Figure 5.3 helps estimate the temperature elevation of the exiting gases above the furnace temperature. The sum of the furnace temperature and this elevation is the temperature that should be used to enter the bottom scale of available heat charts 5.1 and 5.2 to determine the %available heat. 

Rg. 5.3. Elevation of flue gas exit temperature above furnace temperature, for a variety of stp velocities (average across-the-furnace cross section where the poc approach the flue). The stp velocity = stp volume divided by the cross-sectional area of the flowing stream. (Same as fig. 2.2.) NOTE The convention used in this txiok is to omit the degree mark (°) with a temperature ievei (e.g., water boiis at 212 For 100 C) and to use the degree mark only with a temperature difference or change (e.g., the difference, AT, across an insuiated oven waff was 100°F or 55.6°C, or the temperature changed20°F or II.VC in an hour). 

Rg. 5.4. Quick method for estimating flue gas exit temperature from the measured furnace temperature near the flue. 

To reduce fuel cost and improve productivity, an engineer must be able to adjust furnace gas temperatures to change the furnace temperature profile. In a longitudinally fired furnace, shortening the flame will raise the temperature near the burner wall. This can be accomplished by spinning the combustion air and/or fuel, which in turn spins the poc. The resultant increase in heat transfer near the burner wall will reduce the flue gas exit temperature, raising the % available heat. 

Continuous furnaces should be more fuel efficient than batch furnaces because they do not cool down during and after every load is removed, throwing away the heat stored in their walls. In addition, they are usually longer furnaces, and if fired only from one end, they give their hot gases more time and more surface contact with which to transfer heat to their loads, reducing the flue gas exit temperature. 

Flue gas exit temperature is affected by (a) flame length, (b) firing rate (furnace gas velocity), and (c) heat transfer from the furnace gases to the loads, and from furnace gases to the refractory and then to the loads. 

Step 2. The type E flames already selected are primarily radiation burners, so the flow of poc across the retort surfaces will be quite low, estimated at 15 fps. From figure 5.3, at 2000 F furnace temperature, read 60°F elevation of the flue gas exit temperature (fget) above furnace temperature, or fget = 2000 + 60 = 2060 F. 

If the furnace will have sophisticated automatic air/fuel ratio control, and is constructed with a steel outer shell so that tramp air will be minimal—say 5% excess air, then extrapolating at 5% XS air from figure 5.1 at 2060 F flue gas exit temperature and 400 F preheated air, read 49% available heat. 

Example 5.3 (Cost of fgr) A furnace burning natural gas has 1800 F (1255 C) flue gas exit temperature with 10% excess air. Use %available heat calculations to compare fuel costs for Cases a toe discussed next. 

Q4. Why is the flue gas exit temperature always higher than the furnace temperature ... 

Q5. If furnace temperature at the furnace entry (flue gas exit) is 1800 F (982 C), what will the flue gas exit temperature be ... 

Given A heat-treat furnace has a flue gas exit temperature of 1800 F (982 C) and is running with 10% excess air while burning 6 fuel oil. 

Example 7.1 Given A car furnace (batch) 10 x 20 x 9 high inside is to heat 40 tons of steel loads from 60 F to 2250 F at a rate of 250°F per hour. Specific heat of steel, fromp. 275 of reference 52 is 0.165 Btu/lb F. Average flue gas exit temperature will be 2200 F. The fuel will be natural gas with 10% excess air. Average losses, in Btu/ft hr are roof 900, walls 500, door 1100, and car 600. 

A13. A security factor of 1.3 is suggested, applied to the maximum burner firing rate and with flue gas exit temperatures 200°F (111°C) above the furnace running temperature at maximum rates. Some furnace designers may be irritated by these specifications, but they are needed to recover a furnace s normal temperature profile quickly. These specifications are more necessary for a mill with many delays to provide the versatility needed. It is important to be aware of different goals—furnace designers want to build an inexpensive furnace so that they can get the order, but operators want versatility to be able to heat and roll as many tons as possible. 

Too many engineers use furnace temperature as flue gas exit temperature when... 

When specifying a new furnace, input calculations should be based on the true flue gas exit temperature— NOT ON FURNACE TEMPERATURE Coauthor Shannon recommends adding a safety factor of 30% in general, but 40% in the charge zone to accommodate productivity expansion of the mill—the latter because inadequate charge-zone capacity can cause swings in input needs after delays. His experience... 

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