Document DwZ1Vr05nK54mOMYV9mqvob4

502 CHAPTER 34 1960 Guide and may be such that the amount of 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, can 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) rhangas in oil viscosity due to temperature change or variations in grade of oil delivered; (2) erosion of atomizing 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., fthangBR in chimney draft because of weather changes, sea sonal changes, bade 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 consistent with the proper disposal of the products of com bustion. When using mechanical draft burners with average conditions, the combustion chamber draft should not be allowed to exceed 0.02-0.05 in. water. An automatic draft regulator is necessary in order to maintain constant draft conditions which, in turn, improve efficiency of operation. The draft regulator should be adjusted in accordance with the manufacturer's instructions. Even though a fan is generally used to supply the air for combustion, in most oil burners, the importance of a proper chimney should not be overlooked. The chimney should have sufficient height and size to insure a uniform draft within the limits given, if maximum efficiency throughout the heat ing season is to be maintamed. Measurement of the Efficiency of Combustion Since efficient combustion is based upon a clean flame and definite proportions of oil and air employed, it is pos sible to determine the results by analyzing the combustion gases. It is customary to analyse only for carbon dioxide (CO,) and to obtain the temperature of the stack gas. A showing of 10 to 12 percent indicates an excellent operation if the flame is clean. Many good installations operate with 8 to 10 percent CO, - Commercial Standard CS-75 requires that oil burners la beled as complying with the standard shall obtain smoke-free combustion at 10 percent CO,. In all cases smokeless com bustion is a requirement for oil burners. Combustion Chamber Design With burners requiring a refractory combustion chamber, the size and shape should be in accordance with the manu facturer's instructions. It is important that the chamber be as nearly airtight as is possible, except when the particular burner requires a secondary supply of air for combustion. The atomizing burner is dependent upon the surrounding heated combustion chamber surfaces to reflect heat and help vaporize the oil. 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. Tbe 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 is 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 same 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 is sometimes used to increase the flame 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 is used, combustion chamber design is quite important. When installing some of the vertical rotary burners, the manufac turer's instructions must be followed carefully with regard to the hearth upon which successful performance depends in this type of burner. Boiler Settings Since the volume of space available for combustion is a determining factor in oil consumption, it is general practice in converting coal-fired boilers to remove grates and extend the combustion chamber downward to include or even ex ceed the ashpit volume; in new installations the boiler may be raised to make added volume available. Approximately 1 cu ft of combustion volume should be provided for every developed boiler horsepower, and in this volume from 1.5 to 2.5 lb of oil per hour can properly be burned. This corre sponds to an average liberation of about 38,000 Btuh per cu ft. At times much higher fuel rates may be satisfactory. For best results, care should be taken to keep the gas velocity be low 40 fps. Where checkerwork of brick is used to provide secondary air, good practice calls for about 1 sq in. of opening for each pound of oil fired per hour. Such checkerwork is best adapted to flat flames, or to conical flames that can be spread over the floor of the combustion chamber. The proper brick ing of a large or even medium sized boiler for oil firing is im portant, and frequently it is advisable to consult an authority on this subject. It is essential in combustion chamber design to prevent flame impingement upon either metallic or fire brick surfaces. Manufacturers of oil burners usually have available detailed plans for adapting their burners to var ious types of boilers. Such information should be utilized. Controls The control of oil-fired equipment is discussed in the sec tion Controls for Automatic Fuel-Burning Equipment. COMMERCIAL AND INDUSTRIAL OIL BURNERS Oil-burning equipment for commercial and industrial ap plications is usually designed for burning the lower cost heavy fuel oils such as U. S. Commercial Standard Grades Nos. 4, 5, and 6. The viscosity of these oils is 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 oil burning system may be affected materially by the charac- Automatic Fuel Burning Equipment leristics 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 is to be designed. Classification 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) steamatomizing, and (4) air-atomizing. Horizontal Rotary-Cup Burner In this type of burner, oil is delivered into a horizontal cup that is rotated at high speed. As the thin film of oil is spun from the rim of the cup, it enters a cone of high ve locity primary air where very effective atomization is ob tained. In most applications secondary air for combustion is supplied by natural draft through checkered openings in the floor of the combustion chamber. In some applications, however, the secondary air is admitted through openings surrounding the burner nozzle and may be supplied 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 is a selfcontained integral unit; it is readily adaptable to manual, semi-automatic, or fully-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 this burner oil is atomized by passing it through a specially designed nozzle under pressures of approximately 100 to 300 psig. The air for combustion may be supplied by either natural or forced draft, and is usually admitted through an air register surrounding the burner nozzle or through checkered openings in the floor of the combustion chamber. This burner is popular for marine service and for large capacity multiple installations. It is not readily adapted to fully automatic operation. Steam-Atomizing Burner High-pressure steam is used in this burner for atomizing the oil. Oil is delivered to the nozzle at pressures ranging from approximately 5 to 50 psig, and steam at approxi mately 50 to 150 psig is admitted to the nozzle by various methods to assist in tbe atomization. Combustion air is usu ally supplied by natural draft through checkered openings in the floor of the combustion chamber or through an air register surrounding the nozzle. This burner is relatively low in first cost but is not adapted to fully automatic opera tion or wide variations in firing rate. Air-Atomizing Burner. The air-atomizing burner is similar to the steam-atomiz ing type, except that air under pressure instead of steam is used for atomization. High-pressure air-atomizing burners use air pressures from 30 psig upwards and can operate at oil pressures and preheat temperatures in the same range as steam-atomizing burners. Low-pressure air-atomizing burn ers require airpressures of Vt to 5 psig and oil pressures from 5 to 20 psig. This latter type of burner requires a, slightly lower oil viscosity at the nozzle for good atomization than either the high-pressure air or steam-atomizing burner. 503 STORING AND HANDLING HEAVY FUEL OILS Preheating of Heavy Fuel Oils Unlike the lighter grades of fuel oils that can be pumped, atomized, and burned efficiently without prior preheating, the heavier grades of residual fuel oils require conditioning by the application of heat before use. Preheating is employed to reduce the viscosity of the oil so that it can be pumped and atomized properly by the burner. The function of the burner is 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 80 to 450 seconds Saybolt Universal viscosity, with the type of burner, and is reached at different temperatures for different grades of oil. The range of recommended atomizing viscosities is about as follows for normal grades of oil: Range of Type of Burner Atomising Viscosities Mechanical-Atomizing 100-300 SSU Steam and High-Pressure Air-Atomizing 150-250 SSV Rotary-Cup-Atomizing 150-450 SSU Low-Pressure Air-Atomizing 80-90 SSU Table 1 lists temperatures needed to bring various oil vis cosities to the proper atomizing levels. These can 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- Toble 1 .... Preheating Temperatures to Obtain Suitable Viscosities for Atomizing Viscosity of Oft* OS Temperature (Fahrenheit} Acquired in SSF at 122 f In SSU at 100 F Desired SSU Viscosity 100 IJO 200 300 450 21 26 30 39 . 50 GO 75 100 115 135 145 160 170 190 200 220 260 285 320 342 100 150 200 300 400 500 750 1000 1200 1500 2000 2500 3000 3500 4000 - 4500 5000 5500 6000 7000 8000 9000 10,000 100 80 70 61 120 100 90 77 135 110 100 87 150 128 115 100 160 138 128 108 50 66 75 88 96 170 145 133 115 102 185 160 147 126 114 190 168 155 133 120 200 175 160 138 125 205 180 168 145 132 215 190 175 150 137 220 195 182 156 143 225 200 187 160 146 229 205 190 162 150 235 208 194 165 152 237 212 197 167 155 240 215 200 168 157 245 218 204 172 160 248 220 208 175 162 250 225 210 177 165 254 227 214 180 167 258 230 216 183 169 260 235 218 186 171 * SSF -- seconds Seybolt Faro! viscosimeter. SSU seconds Seybolt Uni verse! viscosimeter. From Fed Oil Manual, by Poi F. Schmidt (The Industrie! Frege) 1851, p. 105. Columns for 100 end 450 viscceity from Davis Eozmeerinj Ccrp.