Get the Most from Your Fired Heater

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 heaters 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-

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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

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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. In induced-draft and balanced- casing and overall structure. draft heaters, the draft is controlled by The typical draft profile ID fan. Because the arch of the heater for a balanced-draft heater ap- has the highest pressure, it is commonly pears in Figure 5. Other types of used as a point of control. heaters have similar profiles, A value of 0.1 in W.C. is typi- except for some minor variations cally maintained at the arch in all fired associated with the (lone) ID or heaters, except for some special, down- FD fan installation. fired reformer heaters. This value en- As can be seen from the sures safe operation and minimal air draft profile, the radiant arch of leakage. Excess air must be minimized the heater sees the highest abso- for efficiency improvement. On the other lute pressure throughout the hand, enough air must be provided to whole heater, except for the obtain the correct and desirable flame stack tip. If draft at the arch can shape and complete combustion. Closing be controlled to be negative, the air registers reduces air flow but in- engineer can be sure that the creases heater draft. Closing the stack entire heater will be at negative damper reduces the fired-heater draft. In pressure. order to regulate excess air effectively, The floor of the heater the damper and registers must be ad- or the hearth, where the burners justed jointly. are typically located, experience draft due to the stack effect in

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Air Leakage A fired heater is not a pressure-tight structure. Air can leak into the heater through all openings available to it. This air does not take part in combustion, instead showing up in the stack. It can lead to ineffi- cient combustion, to a waste of energy due to excess draft, and to the generation of NOx emissions*. Even with fuel prices at only $3 million Btu, one square inch of leakage area can lead to $32,000 in energy cost per 0.1 in W.C. of excess draft. These precautions can minimize air leakage a fired heater:

 Keep all peepholes closed.

 Make sure that the doors are tight on the header box, which houses fluid-tubing U bends in the convection section.  Keep the explosion door closed.

 Ensure there is only minimal air leakage via the penetrations of the tube guides (which hold the fluid tubes in place) into the NOx burners or raw –gas burners (in floor of the heater. which the fuel gas and air become One reliable indication of air mixed, externally, at the burner tip); leakage is the production of CO even at both versions, unlike the premix burn- high oxygen levels. Carbon monoxide ers, are draft-dependent. Therefore, will be generated at the burners if the air these heaters need to provide the re- to them is insufficient, but the leaked air quired draft. Old fired heaters that (which does not help the burners) raises have not been thus modified are the the oxygen content of flue gases and thus most-significant sources of fired- masks that insufficiency. heater energy loss today.

Typical heater configurations Heater with stack damper: Most fired Several heater and damper configura- heaters installed in the last 30 years tions can be found in chemical process fall in this category, having been de- plants: signed with a manually operated stack damper. Fired heater with no stack damper: The damper is typically oper- Heaters of this type were built in the ated from grade, by means of a cable 1950s and 1960s. The burners installed and a winch. The damper is provided in these heaters were typically of the with an external position indicator; premix version; in these burners; the also, the winch is calibrated. amount of air inspirited is automatically proportional to the fuel gas pressure. Overtime, however, the burners in such heaters became replaced by either low- *Conversely, if the draft in the heater is (unfortunately) positive, hot gases from the Furnace Improvements Services www.heatflux.com 4 Get the most from your Fired Heater

firebox can leak out through the openings, which that could change the flue gas flow pattern poses a safety hazard. in the furnace. Heaters with ID fan: The two types of heaters that use ID fans to maintain However, dampers of this type are of poor Multiple heaters with common stack: the draft in the heater are: induced- quality; they often get stuck, and sometime Similarly, in several installations, a num- draft fired heaters, as discussed earlier; remain fully open. Operators tend to be ber of heaters are connected to a common and balanced-draft fired heaters with reluctant to touch them so as to make ad- stack (Figure 6). This configuration is par- air-preheating systems. justment to drafts. These dampers should ticularly common in Europe, where the In both types, the draft is be replaced with more-reliable versions, local pollution laws may dictate using a controlled by the fan. Generally, the whether manually or pneumatic operated, stack as high as 200 to 300 ft. Such stacks fans are provided with an inlet-box from grade or at a control panel. are based upon grade, and the fired heaters damper to control the draft; in some are connected through the ductwork. cases, the fan is instead provided with Heaters with off-take dampers: A number In these installations, the draft a variable-sped drive (VFD) for that of cabin-type fired heaters with long con- control becomes tricky. Any change in the purpose. Furthermore, some installa- vection sections are equipped with single firing conditions of one heater can affect tions have a VFD on the ID fan, as or multiple off-take ducts, which connect the draft in all the other heaters and require well as a damper in the ID suction to the convection sections to the stack. In their readjustment. In such circumstances, control the draft. As a damper in the some such heaters, the dampers are in- it is common to have an automatic draft ID suction to control the draft. stalled in the off-takes instead of stack. control system for each heater. An alterna- Heaters with ID fans are gen- Multiple off-take dampers should be oper- tive consists of having a manual loading erally large, so it is especially impor- ated uniformly, as to avoid any imbalance station, along with pressure indicators, in tant that the correct draft be main- the control room. tained. Due to the large number of

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burners and peepholes in large heaters, is that most of the points are not easily move to the position specified by the high draft can readily affect the opera- accessible. Another common approach is purchaser in the event of control signal tion adversely. to bring all the (pneumatic) gauge con- failure or motive force failure. nections to ground level, and then install It is also important to check Draft Control separate draft gauges there. the stack damper at every shutdown, Controlling draft requires the following A third variation, rather com- make sure it is working properly, and instruments and hardware: monly employed, is to manifold all the make repairs or modifications as connections and install a single draft needed. Damper operation is especially Draft gauges: These gauges are simple gauge. This option is economical, but it critical if the heater has an air preheat- instruments designed to measure draft requires the operator to open and close ing system; in this case, a tight-shutoff, or differential pressure. Typical draft- valves every time the draft must be quick-acting damper should be em- gauge locations in a fired heater are as checked. ployed. shown in Figure 7. An advanced approach is to Many plants using air pre- install pressure transmitters at the arch heaters tend to keep the stack damper Heater floor: A minimum of two gauges and send the signal to the control room; slightly open, for the fear of it getting are recommended for the heater floor. in this case, the other draft gauges are stuck. usually field-installed. But as a result, either cold flue Heater arch: Having at least two gauges Regardless of the installation gas starts recirculating back into the at the heater arch or at the convection option chosen, it is important that the system or the hot flue gas leaks into the section inlet is likewise recommended. gauges have a correct range. Failure to atmosphere. Both of these scenarios meet this simple requirement is among cause a loss of efficiency. Instead, the Convection section exit: Gauges here the most common problems found with damper should be kept fully closed, and serve to check the total draft loss across fired heaters in the field. its motion should be tested every two the convection section. The minimum weeks. recommendation is one draft gauge, Stack Damper Reliability right above the stack damper. As an The American Petroleum Institute’s API Automatic Draft Control alternative, an arrangement with two 560 code specifies several requirements As noted above, draft in fired heaters gauges (above and below the damper) is for a good stack damper. For example, it can be controlled automatically. One useful monitoring the stack damper is requires one blade for every 13 ft2 of control scheme is shown in Figure 8. adjusted, the draft upstream and down- internal cross-section area. The blades Although automatic draft control often stream will change. should be of equal area, with their move- suffered from damper-quality, pressure- ments opposed. measurement and other problems in the Installation options: These are several The code also calls for the past, improvements in equipment qual- prevalent practices for installing draft damper controls to be provided with ex- ity have removed risks. gauges. For instance, each gauge con- ternal position indicators, and they nection can be made locally; a drawback should be designed so that the dampers

Edited by Nicholas P. Chopey

The Author Ashutosh Garg is a senior Thermal Engineer at Furnace Improvements (Sugar Land, Texas; Tel 281-980- 0325; Fax: 832-886-1665; email: [email protected]). He has almost 30 years of experience in design, engineering, and troubleshooting of fired heaters and combustion systems. He began his career as a gradu- ate engineer in an ammonia plant; this work was followed by six years in KTI India and eight years at EIL, New Delhi, in the latter firm’s heater group. For the past seven years, he has been with Furnace Improve- ments, where he provides services to the petroleum refining and petrochemical industries related to fired heaters and NOx emissions reduction. He has published several papers on those two topics in trade magazines. A registered professional engineer and a member of AIChE, he is a member of API subcommittee on heat transfer. He holds de- gree in chemical engineering from Indian Institute of Technology, Kanpur, India.

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