Get the Most from Your Fired Heater
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Originally appeared in: March 2004 issue, Chemical Engineering Reprinted with publisher’s permission. Get the Most From Your Fired Heater Though the functioning of these widely used heat- ers it appears simple, there is more to efficient operation than meets the eye. A common stum- bling block is the control of draft. Ashutosh Garg Furnace Improvements Fired heaters are major consumers of en- savings of almost $500,000/yr. Automatic Combustion ergy in the chemical process industries draft control can improve the efficiency of Combustion, the exothermic reaction re- (CPI) especially at petroleum refineries fired heaters if it is designed and installed sulting from rapid combination of fuel with and petrochemical plants. Accounting for correctly. Before explaining how, we pro- oxygen, produces heat and flue gases. Fuel as much as 70% of total plant energy con- vide a brief refresher on the concepts in- and air must be mixed thoroughly for com- sumption in some instances. While most volved. plete combustion. In theory, it is possible plant engineers and operators are aware of to burn fuel completely with just the the importance of controlling excess oxy- Fired Heaters stoichiometric amount of combustion air. gen in fired heaters, they often overlook a In a fired heater, the thermal energy liber- However, under actual operating condi- key determinant of efficient heater opera- ated by the combustion of fuel is trans- tions, perfect mixing of fuel and air is not tion; the control of their draft, namely, the ferred to fluids contained in tubular coils possible within the short time that is in- negative pressure inside the vessel with within an internally insulated enclosure. volved in combustion. If only the theoreti- respect to the atmosphere. A typical fired heater consists of cal amount of combustion air were pro- A recent survey indicates two three major components; the radiant sec- vided, then some fuel would not burn com- extremes in draft management. In most tion, the convection section and the stack. pletely. So, excess air is needed, expressed fired heaters, the draft is maintained at Figure 1 shows a typical cross-sectional as a percentage of the theoretical quantity almost four times the value recommended. view of a vertical cylindrical fired heater. of air required for perfect combustion. This At the other end of the spectrum, some The fired heater is fired by oil or gaseous excess air shows up as excess oxygen in heaters run with no draft – in fact, with fuel. The process fluid, passing through the flue gas. Table 1 shows the effects of positive pressure at the radiant arch (the tubes in the heater, absorbs the heat mostly excess air and stack temperature on the transition zone between the radiant and by radiant heat transfer, and by convective thermal efficiency of the fired heater. As a convection sections). Neither situation is heat transfer from the flue gases. rule of thumb, every 10% increase in ex- desirable; they can cause considerable loss The flue gases are vented to the cess air reduces the heater efficiency by of energy, and can even be hazardous. atmosphere through the stack. Burners are almost 1%, whereas every 35oF reduction Plants can save substantial amounts of en- located on the floor (as stylized in Figure in stack flue gas temperature increases ergy by training operators in proper draft 1) or on the sidewalls of the heaters. Com- efficiency by 1%. control and making minor hardware modi- bustion air is drawn from the atmosphere. fications. For a 100,000-bbl/d (BPD) refin- Combustion is directly affected by the Burners ery in the U.S., even a 1% improvements draft. Burners start and maintain combustion, in in thermal efficiency translates into energy the firebox. They introduce fuel and air in the correct proportions and mix them, pro- Furnace Improvements Services www.heatflux.com 1 Get the most from your Fired Heater vide a source of ignition, and stabilize than (and thus at lower pressure than) the Natural Draft: As implied above, this is the flame. How the air is supplied to the colder ambient air outside. the most common system (Figure 2). Air burners is largely related to the concept In a given situation, the theo- is drawn into the burners by means of of draft, discussed in more detail now. retically available draft, in inches of wa- the draft created by the radiant section. In most fired heaters, the burn- ter column (inWC ) can be calculated as The taller this section, the greater the ers are natural draft, as explained below. follows: available draft. Typical draft gains are of These burners are the most dependent on the order of 0.1 inW.C. per 10 feet of the draft, as all natural draft burners are Draft = 0.53 HP [(1/ Tambient) – (1/ Tflue gas)] box height in the radiant section. sized for a specific draft loss across the Draft at the heater floor is the burner. Providing a higher draft than that Where H is stack height in feet, P is at- order of 0.3 to 0.7 in. for tall, vertical design value will induce more air, mospheric pressure in pounds per square cylindrical heaters. Natural draft is the whereas providing lower draft will lead inch absolute (psia). Tambient is the ambi- most simple and reliable type of heater, to insufficient air for combustion. ent temperature in degrees Rankine and as the air supply does not fail. System The other type of burners used Tflue gas is the flue gas temperature, in the performance is directly linked to the in fired heaters is forced-draft burners same units. draft available in the heater. In these which get their air supply from a fan. Combustion air is drawn into heaters, draft control is the most impor- These are not dependent on the heater the burners from the atmosphere, and hot tant operating parameter. draft. gas rises due to buoyancy and flows out There are also self-inspirating of the stack to the atmosphere. While Forced-Draft: In this type of heater, the pre-mix burners, used in special heaters passing through the heater’s convection air is supplied by means of a centrifugal such as those for steam methane reform- section and the stack, flue gases encoun- fan, commonly known as a forced-draft ing, or for ethane cracking. Most of these ter friction resistance, known collec- (FD) fan. A FD fan provides air at rela- burners are partially dependent on the tively as draft losses. Sufficient stack tively high pressure, in the range of 2 to draft available in the heater. height is given to provide the buoyancy 6 in.WC , leading to better air-fuel mix- effect needed to overcome these losses, ing and smaller burners. Draft and to ensure that pressure is always In this option, too, the stack is Draft is the pressure differential between negative inside the firebox. required to create a negative draft inside air or flue gas in the heater and ambient There are four types of draft the fired heater.Draft control is impor- air. It materializes because hot flue gases systems in the fired heaters: tant in these heaters, to minimize air inside the firebox and stack are lighter Furnace Improvements Services www.heatflux.com 2 Get the most from your Fired Heater leakage and to ensure negative pres- with a fired heater, the combination is known the radiant section. In the convection sure throughout the whole heater. as a balanced-draft system. Most air preheat- section, flue gases admittedly encounter ing installations are, in fact, balanced draft. resistance due to tubes, but gain some Induced-Draft: When the height of In a typical air preheating system, the draft due to the height of this section. If the stack is inadequate to compen- draft loss across the air preheater could be on the convection section becomes fouled, sate for the draft-loss requirements, the order of 2-6 inWC. The stack by itself the pressure drop across that section an induced-draft (ID) fan is provided cannot compensate for a loss of this magni- goes up and the draft at the arch can, in on top of the fired heater (Figure 3). tude. Instead, the FD fan supplies the combus- fact, becomes positive. The resulting negative pressure in- tion air, and the ID fan takes care of flue gas Similarly in the stack, the stack side the heater ensures adequate disposal. In these systems, draft control is re- controls the draft. If the damper is closed draft for the burners from the atmos- quired for efficient combustion. Figure 4 too far, the arch draft will become posi- phere. Most heaters in cracking and shows a typical balanced-draft heater with an tive; if it is instead opened too far, it will reforming units fall into this cate- air preheating system. lead a very high draft in the arch. The gory. The size of the convection sec- right stack height provides the draft need tion in these fired heaters is very Draft Profile to maintain negative pressure at the arch large, and the draft control is very Maintaining a negative pressure at all times and to take care of friction losses in the important. throughout the fired heater makes the device convections section and stack. inherently safe, and ensures that hot flue gases Balanced-Draft: When both forced- will at no time escape. By contrast, a positive Draft Control draft and induced-draft fans are used pressure inside the heater can be hazardous for In natural or forced-draft systems, the operating personnel, would draft in the fired heater is controlled by cause flue gas leakage, as well the means of a stack damper, as just dis- as damage to the fired-heater cussed.