Document 3eQEQoga0p20emg7gvZabJJVO

334 CHAPTER 17 1948 Guide with the liquid fuel particles, mixing, vaporization and burning: occur air at once, in the same space. This produces a luminous flame. A deficient amount of air is indicated by a dullred-ordark orange flame with smoky tips. An excessive supply of air may produce a brilliant white flame or a short ragged flame with incandescent sparks flashing- through the com bustion space. While extreme cases may be detected, it is not possible to distinguish, by eye, the effect of the finer adjustment which competent installation requires. 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 combustion for a considerable period following the initial adjustments of oil and air. Eventually certain changes may occur, however, that will cause the per cent of excess air to decrease 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 con stant, will make the mixture of oil and air,too rich for clean combustion. 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 atomizing nozzle,.(3) fluctuations in by-pass relief pressures, and (4) possible variations in methods of atomization. Any change due to partial stoppage of oil delivery will increase the proportion 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, seasonal changes, back drafts, failure or inadequacy of automatic draft regulator, use of chimney for other purposes, possible stoppage of the chimney, and changes in draft resis tance of boiler due to partial stoppage of the flues), and (2) changes in air inlet adjustments at the fan. - Air leakage into the boiler or furnace setting should be reduced to a minimum. The amount of air leakage will be determined-by the draft in the combustion chamber. It is important that this draft should be reduced as low as is consistent with the proper disposal of the gases of combustion. When using mechanical draft burners with average con ditions, the combustion chamber draft should not be allowed to exceed 0.02:0.05 in. water. An automatic draft regulator is very helpful in maintaining such values. 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 that the draft will be uniform within the limits given if maximum ef ficiency throughout the heating season is to be maintained. 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 possible to determine the results by analyzing the combustion gases. It is usually sufficient to analyze only for carbon dioxide (CO*). A showing of 10. to 12 pier cent indicates Automatic Fuel Burning Equipment.: 335 the best adjustment if the flame is clean. Most.ofthe good installations show, from 8 to 10 per cent CO*. Taking into account the potential hazard of low excess air (high COj), a setting to give 10 per cent CO* constitutes a reasonable standard for most oil burners. Combustion Chamber Design With burners requiring a refractory combustion chamber the size and shape should be in accordance with the manufacturer's instructions. It is important that the chamber shall be as nearly air tight 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 refractory or firebrick surfaces to vaporize the oil and support combustion. Unsatisfactory combustion may be due to inadequate atomization and mixing. A combustion chamber 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 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 atthe 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. In this way as much firebrick as possible is close to the flame so it may be kept hot. This insures quick vaporization, 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 mixing are achieved by the burner, combustion chamber design 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 manufacturer's instructions must be followed carefully when installing the hearth, as.iii this class successful performance depends upon this factor. Boiler Settings As the volume of space available for combustion is a determining factor'in oil consumption, it is general practice to remove grates and extend the combustion chamber-downward to include or even exceed the ash pit 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 corresponds to an average liberation of about 38,000 Btu per cubic foot per hour. At times much higher fuel rates may be satisfactory. For best results, care should be taken to keep the gas velocity below 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 bricking of a large or even medium sized boiler for oil firing is .important and frequently it is advisable to consult an authority on this subject. The essential in combustion chamber design is to. provide against flame impingement upon either metallic or firebrick surfaces. Manufacturers of