Document 4JQem5p7jbmmX5YzKmeMdZpEa

204_____________ Chapter 10_______________________________ 1945 Guide combustion--When using. mechanical draftburners-with average -eon--- .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 above if maximum efficiency 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 (C02). A showing of 10 to 12 per cent indicates the best adjustment if the flame is clean. Most of the good installations show from 8 to 10' per cent C02. Taking into account the potential hazard of low excess air (high C02), a setting to give 10 per cent C02 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 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. 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 in 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 Automatic Fuel Burning Equipment' ' . 205 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 oil burners usually have available detailed plans for adapting their burners to various types of boilers, and such information should be utilized. Controls Controls for oil burner operation, including devices for the safety and protection of a boiler or furnace, are fully described in Chapter 33. GAS-FIRED HEATING EQUIPMENT A gas burner is defined by the American Gas Association as "a device for the final conveyance of the gas, or a mixture of gas and air, to the combustion zone." Burners used for domestic heating are of the atmos pheric injection, yellow flame, or power burner types. The use of gas has resulted in the production of a number of types of domestic gas heating appliances, and systems. These may be classified in types designed for central heating plants, and those for unit applica tion. Gas-designed units and conversion burners are available for the several kinds of central systems. Unit heaters, space heaters and circu lators may be had for installation in the space being heated. Central Heating Systems Boilers and furnaces specially designed for gas-firing incorporate design features for obtaining maximum efficiency and performance. Small flue passes to secure good heat transfer, the use of materials resistant to. the corrosive effects of products of combustion, and draft hoods are notable features. Control equipment includes gas pressure regulators, thermostatic pilots and limit controls designed to protect the appliance and to insure safety of operation. A boiler designed for gas-burning is illustrated in Fig. 17. Conversion burners are usually complete burner and control units designed for installation in existing boilers and furnaces. Burner heads are of circular or rectangular shape in order to fit in space available. The control equipment is generally the same as for gas boilers and furnaces. Various baffles made of clay radiants or metal are used for the purpose of guiding the products of combustion along the heating surface in the-, firebox or flues. Automatic air. dampers are supplied on many models to prevent flow of air into the firebox when the burner is not operating. One form of central heating system is the warm air floor furnace7. The 'Gas Floor Furnaces, Gravity Circulating Type (U. S. Department of Commerce, National Bureau of standards. Commercial Standard No. CS90-42).