Document ymDo089Og7wwrqn7Kr2NQKoR2

490 CHAPTER 34 1959 Combustion Adjustments The present-day oil burner with mechanical oil and air supply, properly installed and equipped with an automatic draft regulator, is capable of maintaining efficient combus tion for a considerable period following the initial adjust ments of oil and air. Eventually, certain changes will occur, and may be such that the amount o! excess air will de crease below allowable limits. A decrease in air supply while the oil delivery remains constant, or an increase in oil delivery while the air supply remains constant, will make the mixture of oil and air too rich for clean combus tion. The more efficient the adjustment, the more critical it will be. The oil and air supply rates must remain constant. The following factors may influence the oil delivery rate: (1) changes in oil viscosity due to temperature change or variations in grade of oil delivered; (2) erosion of atomising nozzle; (3) fluctuations in bypass relief pressures; and (4) possible variations in methods of atomization. Any change due to partial stoppage of oil delivery will increase the pro portion of excess air. This will result in less heat, reduced economy, and possibly a complete interruption of service. The following factors may influence the air supply: (1) changes in combustion draft due to a' variety of causes (i.e., changes in chimney draft because of weather changes, sea sonal changes, back drafts, failure or inadequacy of auto matic draft regulator, use of chimney for other purposes, possible stoppage of the chimney, and changes in draft re sistance of boiler due to partial stoppage of the flues); and (2) changes in air inlet adjustments at the fan. There should be no air leakage into the boiler or furnace setting. The overfire draft should be reduced as low as is facturer's instructions. It b important that the chamber be as nearly airtight as b possible, except when the particular burner .requires a secondary supply of air for combustion. The atomizing burner b dependent upon the surrounding heated combustion chamber surfaces to vaporize the oil and support combustion. Unsatisfactory combustion may be due to inadequate atomization and mixing. A combustion cham ber can only compensate for these things to a limited extent. If liquid fuel continually reaches some part of the fire brick surface, a carbon deposit will result. The combustion chamber should enclose a space having a shape similar to the flame, but large enough to avoid flame contact. The nearest approach in practice b to have the bottom of the combustion chamber flat, but far enough below the nozzle to avoid flame contact, the sides tapering from the air tube at the gams angle as the nozzle spray, and the back wall rounded. A plan view of the combustion chamber resembles in shape the outline of the flame. This insures quick vapori zation, rapid combustion and better mixing by eliminating dead spaces in the combustion chamber. An overhanging arch at the back of the fire pot b sometimes used to increase the flamn travel and give more time for mixing and burning, and sometimes to prevent the gases from going too directly into the boiler flues. When good atomization and vigorous miring are achieved by the burner, combustion chamber de sign becomes a less critical matter. Where secondary air b used, combustion chamber design b quite important. When installing some of the vertical rotary burners, the manufac turer's instructions must be followed carefully when install ing the hearth upon which class successful performance de pends in this type of burner. consistent with the proper disposal of the products of com Boiler Settings bustion. When using mechanical draft burners with average Since the volume of space available for combustion b a conditions, tire combustion chamber draft should not be determining factor in oil consumption, it b general practice allowed to exceed 0.02-0.05 in. water. An automatic draft, to remove grates and extend the combustion chamber down regulator is necessary in order to maintain constant draft ward to include or even exceed the ashpit volume; in new conditions which, in turn, improve efficiency of operation. installations the boiler may be raised to make added volume The draft regulator should be adjusted in accordance with available. Approximately 1 cu ft of combustion volume the manufacturer's instructions. should be provided for every developed boiler horsepower, \ Even though a fan b generally used to supply the air for and in thb volume from 1.5 to 2.5 lb'of oil peT hour can combustion, in most oil burners, the importance of a proper property be burned. Thb corresponds to an average libera chimney should not be overlooked. The chimney should have tion of about 38,000 Btuh per cu ft. At times much higher sufficient height and size to insure a uniform draft within fuel rates may be satisfactory. For best .results, care should tiie limits given, if maximum efficiency throughout the heat be taken to keep the gas velocity below 40 fps. Where ing season b to be maintained. checkerwork of brick b used to provide secondary air, good Measurement of the Efficiency of Combustion practice calls for about 1 sq in. of opening for each pound of oil fired per hour. Such checkerwork b best adapted to Since efficient combustion b based upon a clean flame fiat flames, or to conical flames that can be spread over the and definite proportions of oil and air employed, it b pos floor of the combustion chamber. The proper bricking of sible to determine the results by analyzing the combustion a large or even medium sized boiler for oil firing b impor gases. It b customary to analyze only for carbon dioxide tant, and frequently it b advisable to consult an authority (CO,) and to obtain the temperature of the stack gas. A on thb subject. It b essential in combustion chamber design showing of 10 to 12 percent indicates the best adjustment, to prevent flame impingement upon either metallic or fire if the flame b clean. Most of the good installations operate brick surfaces. Manufacturers of oil burners usually have with 8 to 10 percent CO, Taking into account the potential available detailed plans lor adapting their burners to var hazard of. low excess air (high CO,), a setting to give 10 ious types of boilers, and such information should be uti percent OO, constitutes a reasonable standard for most oil burners. Commercial Standard CS-75 requires that oil burners labeled as complying with the standard shall ob lized. Controls tain smoke-free combustion at 10 percent CO,. In all cases smokeless combustion b a requirement for oil burners. The control of oil-fired equipment b discussed in the sec tion Controb for Automatic Fuel-Burning Equipment. Combustion Chamber Design COMMERCIAL AND INDUSTRIAL OIL BURNERS With burners requiring a refractory combustion chamber, the size and shape should be in accordance with the manu Oil-burning equipment for commercial and industrial ap plications b usually designed for burning the lower cost Automatic Fuel Burning Equipment . 491 heavy fuel oils such as U- S. Commercial Standard Grades Nos. 4, 5, and 6. The viscosity of these oib b much greater than that of the lighter domestic grades and, therefore, the equipment required for satisfactory storage, pumping; and combustion differs greatly from, that used in the typical domestic oil-burning system. Both the initial cost and the operating cost of an oilburning system may be affected materially by the charac teristics of the industrial fuel oil to be used. These charac teristics should, therefore, be given careful consideration when preparing the specifications of the fuel oil for which the system b to be designed. Gassrftcation of Burners Oil burners are usually classified according to the method used for atomizing the oil as (1) horizontal rotary-cup atomizing, (2) mechanical pressure-atomizing, (3) steamatomising, and (4) air-atomizing. Horizontal Rotary-Cup Burner In thb type of burner, oil b delivered into a horizontal cup that b rotated at high speed. As the thin film of oil b spun from the rim of the cup, it enters a cone of high ve locity primary air where very effective atomization b ob tained. In most applications secondary air for combustion b supplied by natural draft through checkered openings in the floor of the combustion chamber. In some applications, however, the secondary air b admitted through openings surrounding the burner nozzle and may be supped by forced draft. The rotary-cup burner has been the most popular type in the capacity range from approximately 25 to 500 boiler horsepower. Its principal advantages are that it b a selfcontained integral unit; it b readily adaptable to manual, semi-automatic, or fuUy-automatic control; and the firing rate may be easily modulated through a wide range while maintaining high combustion efficiencies. These burners are frequently used for firing packaged steam generator units. Mechanical Pressure-Atomizing Burner In thb burner oil b atomized by passing it. through a specially designed nozzle under pressures of approximately 100 to 250 psig. The air for combustion may be supplied by either natural or forced draft, and b usually admitted through an air register surrounding the burner nozzle or through checkered openings in the floor of the combustion chamber. Thb burner b popular for marine service and for large capacity multiple installations. It b not readily adapted to fully automatic operation. Steam-Atomizing Burner High-pressure steam b used in thb burner.for atomizing the oil. Oil b delivered to the nozzle at pressures ranging from approximately 5 to 50 psig, and steam at approxi mately 50 to 150 psig b admitted to the nozzle by various methods to assist in the atomization. Combustion air b usu ally supplied by natural draft through checkered openings in the floor of the combustion chamber or through an air register surrounding the nozzle. Thb burner b relatively low in first cost but b not adapted to fully automatic opera tion or wide variations in firing rate. Air-Atomizing Burner The air-atomizing burner b similar to the steam-atomiziag type, except that air under pressure instead of steam b used for atomization: The performance characteristics of the two burners are also similar but higher temperature oil b usually required when air for atomization. STORING AND HANDLING HEAVY FUa OILS Preheating of Heavy Fuel Oils Unlike the lighter grades of fuel oib that can be pumped, atomized, and burned efficiently without prior preheating, the heavier grades of residual fuel oib require conditioning by the application of heat before use. Preheating b employed to reduce the viscosity of the oil so that it can be pumped and atomized property by the burner. The function of the burner b to atomize the oil and mix it thoroughly with the proper amount of air for prompt ignition and efficient com bustion. The optimum viscosity will vary from 100 to 450 seconds Saybolt Universal viscosimeter, with the type of burner and b reached at different temperatures for different grades of oil. The range of atomizing viscosity b about as follows for normal grades of oil: Type of Burner Mechanical-A tom tying Steam-Atomizing Rotary-Cup-Atomizing Range of Atomizing Viscosities 100-300 SSU 200-250 SSU 250-450 SSU ` Table 1 lists temperatures needed to bring various oil vis cosities to the proper atomizing leveb. These ran be used with some variation to suit individual operations. The correct degree of preheat must be used for each grade of oil. Underheating or overheating affects the terminal vis- Table 1 .... Preheating Temperatures to Obtain Suitable Viscosities for Atomizing Vhcottfy of OiP OS Tanperatarw (Fofavnbarf) Required In SSF In SSU of 100 F D*riredSSU Vbcojify 100 150 200 300 450 100 100 80 70 61 50 150 120 100 90 77 66 200 135 110 100 87 75 2i 300 150 128 115 100 88 26 400 160 138 128* 108 96 30 500 170 145 133 115 102 39 750 185 160 147 126 114 50 1000 190 168 155 133 120 60 1200 200 175 160 138 125 75 1500 205 180 168 145 132 100 2000 215 190 175 150 137 115 2500 220 195 182 156 143 135 3000 225 200 187 160 146 145 3500 229 205 190 162 150 160 4000 235 208 194 165 152 170 4500 237 212 197 167 155 190 5000 - 240 215 200 168 157 200 5500 245 218 204 172 160 220 6000 248 220 208 175 162 260 7000 250 225 210 177 165 285 8000 254 227 2li 180 i67 320 9000 258 230 216 183 169 342 10,000 260 235 218 186 171 * SSF Meonda Saybott Furoi viscosimeter. SSU KWnnrfi Sayboit Uni* renal vocpidmetcr. From Fud OH Manual, by Paul F. Schmidt (The Industrial Pres*) 1951, p. 105. Column* ter 300and 430 viscosity from T>arii Fnriirrrini fnrr