Document 4m99NgEgDz38dD4Gxqnk9n4G

Heating Ventilating Air Conditioning Guide .1938 and the blowers for air supply being mounted in sets adjacent to the boilers. In such cases, one pump set can serve several burner units, and common prudence dictates the installation of spare or reserve pump sets. Pre-heaters and other essential auxiliary equipment also should be in stalled in duplicate. Boiler Settings As the volume of space available for combustion is the determining factor in oil consumption, it is general practice to remove grates and extend the combustion chamber downward to include or-even exceed the ashpit volume; in new installations the boiler should 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 lb of oil can properly be burned. This cor responds to a maximum liberation of about 38,000 Btu per cubic foot per hour. There are indications that at times much higher fuel rates may be satisfactory. This in turn suggests that the value of 38,000 Btu per cubic foot per hour might be adjusted according to good engineering judgment. For best results, care should be taken to keep the gas velocity below 40 ft per second. 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 fire brick surfaces. Manufacturers of oil burners usually have available detailed plans for adapting their burners to various types of boilers, and such information should be utilized. The Combustion Process Efficient combustion, as previously indicated, must produce a clean flame and must use relatively small excess of air, i.e., between 25 and 50per cent. This can be done only by vaporizing the.oil quickly, completely, and mixing it vigorously with air in a combustion chamber hot enough to support the combustion. A vaporizing burner prepares the oil, for combustion, by transforming the liquid fuel to the gaseous, state through the application of heat. This is accomplished before the oil vapor mixes with air to any extent and if the air and oil vapor temperatures are high and the fire pot hot, a clear blue flame is produced. There may be a deficiency of air as shown by the presence of carbon monoxide {CO) or an excessive supply of air, depending upon burner adjustment, without altering the clean, blue appearance of the flame. An atomizing burner i.e., gun and rotary types is so named because the oil is mechanically separated into very fine particles so that the surface exposure of the liquid to the radiant heat of the combustion chamber is vastly increased and vaporization proceeds quickly. The result of such practice is the ability to burn more and heavier oil within a given com bustion space or furnace volume. Since the air enters the fire pot with the liquid fuel particles, it follows that mixing, vaporization and burning are 218 Chapter 11. Automatic Fuel Burning Equipment all occurring at once in the same space. This produces a luminous instead of a blue or non-luminous flame. In this case a deficient amount of air is indicated by a dull red or dark orange flame with smoky flame tips. An`excessive supply of air may produce a brilliant white flame in some cases or, in others, a short ragged flame with incandescent sparks flashing through the combustion space. While extreme cases may be easily detected, it is generally not possible to distinguish, by the eye alone, the finer adjustment which competent installation requires. Certain tests indicate that there is no difference in.economy between a blue flame and a luminous flame if the position, shape and the per cent of excess air of both flames are about the same. Furnace or Combustion Chamber Design The furnace or combustion chamber may be defined as that part of a boiler or conditioner in which combustion is established. 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. It is evident that the atomizing burner is dependent upon the surround ing heated refractory or fire brick surfaces to vaporize the oil and support combustion. While the importance of the combustion chamber is obvious, its design has been troublesome. 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. Fundamentally, 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 at the same angle as the nozzle spray and the back wall-rounded.- A plan view of the combustion chamber thus 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 quite hot. This insures quick vaporization, rapid combustion and better mixing -by eliminating dead dr inactive'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 pre-; vent;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. js used, combustion chamber design is quite, important. | With some of the vertical rotary burners- considerable care must be exercised in definitely following, the manufacturers instructions, when installing the hearth as in this class successful performance depends upon this factor. Combustion Adjustments . -- j Where adjustments of oil- and air have been made which give efficient combustion, the-problem of maintaining the adjustments constant be comes an important one. Particularly is this true when he change causes the per cent-of excess air to decrease below allowable limits of the