Firebox temperature

Above temperatures of 900°F, the austenitic stainless steel and other high alloy materials demonstrate inereas-ingly superior creep and stress-rupture properties over the chromium-molybdenum steels. For furnace hangers, tube supports, and other hardware exposed to firebox temperatures, cast alloys of 25 Cr-20 Ni and 25 Cr-12 Ni are frequently used. These materials are also generally needed because of their resistanee to oxidation and other high temperature corrodents. 

Tfb = temperature, °F, of the radiant surface, which is essentially the firebox temperature. The reason for this latter value, 7, is that the flames heat not so much the tubes as the refractory, and the refractory then reradiates the heat to the tubes, so the main heat source becomes the refractory. 

A typical air preheater will reduce the fuel required to liberate a given amount of heat by 10 percent. The debit for this improved thermal efficiency is a higher flame temperature, and the possibility of overheating the radiant section. The only instance where there is a clear advantage to fit an air preheater to an existing furnace is when the firebox of that furnace is running below a desirable maximum firebox temperature. Three types of air preheaters are in common use ... 

The answer, of course, is to allow more air into the firebox, and thus generate more flue gas. The firebox oxygen was increased from 3 to 6 percent, which reduced the firebox temperature to its prerevamp state. This, in turn, increased the pounds of flue gas flowing through the convective section and increased the heat absorbed in the convective section. 

But without flow, there is no way to carry the heat away from the tubes. Therefore, the tubes overheat. The temperature of the tubes may approach the temperature of the refractory, at the point in time when flow is lost. The refractory temperature is indicated by the firebox temperature, or the temperature of the flue gas flowing from the firebox into the convective section. 

A typical firebox temperature is 1500°F. Thus, the heater tubes can reach 1300°F on loss of the process flow, even though the fuel flow has been immediately stopped. Tubes with a low chrome content may bend and distort as a result of such overheating. Even at 1000°F, residual liquid left in the tubes when flow is lost may thermally degrade to a carbonaceous solid or heavy polymer that fouls the interior of the tubes. 

In the simulation, feedrate, steam ratio, and inlet temperature were maintained constant. The pressure and feed conversion at the coil outlet were specified. The inlet pressure and firebox temperature profile were adjusted to meet the two specifications. A uniform temperature profile... 

Example Results. A demonstration of the fit of the model to commercial data is shown in Table II. The last column compares recent commercial test data for ethane-propane against model predictions. The close prediction of ethylene, ethane, and propylene yields provides confidence in the cocracking synergism mechanism built into the model. Figure 3 compares predicted and observed process-operating profiles for the case of the n-butane reactor. The firing level in the reactor simulation was adjusted to meet the observed conversion at the coil outlet. Agreement in the process, tubeskin, and firebox temperatures is well within the precision of the data. 

Firebox temperature high firebox temperatures result in higher NO, formation (Fig. 2). ... 

Boilers are higher NO , producers than fired heaters because the firebox temperatures are higher and they typically have air preheat systems. Firebox temperatures typically run in the 2100" F range, at full loads. 

Historically, there has been no other way to achieve low NO , levels because of the high firebox fluxes/firebox temperatures. 

The curves are for boilers only. Because of their high firebox temperatures, boilers tend to have high NO c and benefit more from FGR. Estimates are that fired heaters can achieve a maximum of about 30% NO c reduction from FGR. 

Perhaps the only way to achieve low NO on boilers, because of the high firebox temperatures and the potential impact from air preheat. 

Bussman, W., and Baukal, C. "The Effect of Firebox Temperature on NOx Emissions." Presented at 2004 Air Waste Management Association annual meeting, paper 04-A-664-AWMA, Indianapolis, IN, June 2004. 

Furnace cooling and temperature control are important for a number of reasons. A process burner test can be compromised by operating at temperatures significantly higher or lower than the design temperature specified by the client. A correct firebox outlet temperature is essential for an accurate prediction of the heat flux profile. Temperatures that are too low can negatively impact burner stability or overpredict CO production. Firebox temperatures that are too high could lead to excessive NO, production. 

Choose a center-to-center spacing for the radiant tubes which is compatible with the selected tube size. Wide tube-spacing permits high radiant absorption rates with relatively low firebox temperatures and gives good circumferential heat... 

When two points on opposite sides of the curve are obtained, join them by a straight line. The point of intersection of tiiis line with the absorption curve is the correct firebox temperature. For this temperature calculate tg2, read qg2 /q from Figure 1-10 and calculate Qr. This is the heat actually absorbed in the radiant section. 

Since there is no preheated air or fuel, the heat rate of the combustion air, qa, equals the heat rate of the fuel — which equals zero. Assume that the heat loss from the tubes, q l > divided by the rate of heat combustion, or Qi/g = 0.02. Estimate that the average firebox temperature equals 1,500°F. 

For this type of furnace, assume that the exit gas temperature is the same as the average firebox temperature. 

Figure 1-19 shows a small portion of Figure 1-11 with the absorption curve for 672° F tube wall. Plot the calculated qR/aA pF of 37,500 at the assumed firebox temperature of 1,500°F. This lies to the left of the absorption curve, so try another calculation at 1,700° F. 

Plot this point on Figure 1-19 and join the two calculated points by a straight line. It crosses the absorption curve at 1,610° F. This is the firebox temperature. 

Radiant heat flux is defined as heat intensity on a specific tube surface. Thus, heat flux represents the combustion intensity and is analogous to how hard a fired heater is run. More specifieally, keeping the firing rate within safe limits is equivalent to maintaining the peak heat flux at less than the design limit because high firebox temperatures could cause tubes, tube-sheet support, and refiractory failures. What is the peak flux and why is it so important to keep it within the limit These questions will be answered next. 

Ultralow NO c and the latest generation burners may have less turndown capability than conventional burners. High CO levels can occur when firebox temperatures are below 1240 °F. Flame instability and flameout can occur when firebox temperatures are below 1200 °F. Since ultralow NO and the latest generation burners are often designed at the limit of stability, a fuel composition change may cause a stability problem. 

The fuel gas to a fired heater is controlled by a BPCS control function (function TIC-1), which throttles a fuel control valve, CV-1, as shown in Figure F-3. A hazard analysis was performed to identify process hazards and to determine whether the safeguards were sufficient to mitigate the process hazards. The team determined that when the heater was firing hard, a low-pass flow through the tubes could result in a high firebox temperature with the potential for tube rupture, furnace fire and structural damage to the furnace. 

During the interval before feed flow is restored, however, the furnace tubes will continue to absorb heat from the refractory walls after the burners are shut off. If the firebox temperature had been running at 1,700 F, it is probable that the tubes will heat up to about 1,500°F shortly after the flow of process fluid is lost. 

To reduce the firebox temperatures to the pre-revamp level, the operators increased the excess oxygen from 3% to 6%. This adjustment reduced the burner flame temperature, increased the volume of flue gas, and consequently, increased the convective section heat absorbed. 

Heat liberation from the low-NOx burner is more uniform than with a conventional burner. This permits a higher average firebox temperature to be sustained, without promoting... 

Fired heaters rely on the flow of process fluids through the tubes to keep tube and firebox temperature down. If the temperature of the process fluid leaving the heater falls, the temperature indicator on the discharge stream calls for more fuel to be fed to the burners. 

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