Flue gas exit temperature

A quicker approximate estimate of the temperature to use when entering the bottom scales on figures 5.1 and 5.2 is via fig. 5.4, from the empirical formula of equation 5.1. 

A higher temperature process must exhaust more heat to heat a load hotter. Similarly, there is a great difference between efficiencies of high-temperature industrial furnaces and lower temperature industrial ovens. 

TABLE 5.1. Fuel saved by use of various degrees of air preheat with 6 fuel oil with 10% excess air. For other fuels, send higher heating value and fuel analysis (volumetric for gas, gravimetric with liquid or solid fuel) to North American Mfg. Co. (Cleveland, OH 44105). Reproduced with permission from Ref. 49. 

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). 

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

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Condensing boilers are now available for both gas- and oil-fired plant, the advantage of these being that the flue gases are further cooled down to below 100°C so that the latent heat available in the flue gas water vapor is recovered. The condensate has to be removed and the boiler capital cost is higher than for conventional plant. However, the boiler plant efficiency is increased to the order of 90 per cent, based upon the fuel gross calorific value. Where the flue gas exit temperatures are in excess of 200° C a further economy can be obtained by the provision of a spray recuperator in the case of gas and flue gas economizers for oil and coal. 

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.

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. 

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. 

Total radiation heat transfer rate for eight W-tubes = 13 393 x 224 ft = 3 000 000 Btu/hr, or for one W-tube = 375 000 Btu/hr. The reader can estimate that the flue gas exit temperature with an average tube outside surface of 1600 F will be 1800 F. From an available heat chart for natural gas, at 1800 F and 10% excess air, read 48% available heat. Therefore, each of the sixteen regenerative burners should have a gross input capacity of 375 000 / 0.48 = 781 000 gross Btu/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 ... 

Reduction of flue gas exit temperatures by computer modeling... 

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. 

Fig. 5.1. Percents available heat for an average natural gas with cold air and with preheated air. (See fig. 5.3 for estimating flue gas exit temperature.) For other fuels, send fuel analysis and higher heating value to North American Mfg. Co., Cleveland, OH 44105-5600. Reprinted with permission from reference 52. (See also figs. 5.2, 5.3, and table 5.1.)...

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. 

Lowering the firing rate will lower flue gas exit temperature because of lower poc temperature, thus raising %available heat. However, if the firing rate is so low that... 

Lower flue gas exit temperature saves fuel Better heat transfer rate lowers gas exit temperature Lower firing rate lowers gas exit temperature Excess air can absorb heat intended for the load... 

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. 

Heat transfer lowers flue gas exit temperatures. Heat transfer rises if... 

Increasing flue gas recirculation (FGR) to reduce NOx emissions raises the concentration of inerts in a flame, thereby increasing the flame length. The longer flame raises the flue gas exit temperature and also lowers the reaction (flame) temperature, thereby raising the fuel rate. Using FGR to lower NOx can raise fuel costs considerably. (See sec. 5.12.)... 

Step 2. Predict the %available heat (which is 100% - %flue losses) by reading it from an available heat chart (figs. 5.1 or 5.2). Section 5.1 explains how to determine flue gas exit temperature. 

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. 

The loss caused by sensible heat in the flue gases (stack loss) can be evaluated as the %net heating value (90% for natural gas) minus the %available heat at the flue gas exit temperature, from Figure 5.1. At high temperature, the loss becomes excessive, especially with high excess air thus, such cases give payback by using heat recovery. 

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