Document EJB2m8MDqnggezvr5zvX8aB0

HEATINC VENTILATING AIR CONDITIONING GUIDE 1942 design becomes a less critical matter. Where secondary air is used, com bustion chamber design is quite important. With some of the vertical rotary burners considerable care must be exercised in definitely following the manufacturer's instructions when installing the hearth as in this class successful performance depends upon this factor. Combustion Adjustments 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 the change causes the per cent of excess air to decrease below allowable limits of the burner. 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 combustion. The more efficient the adjustment (i.e., 25 per cent excess air) the more critical it will be of variations. The oil and air supply rates must remain constant. The following factors may influence the oil delivery rate: (a) changes in oil viscosity due to temperature change or variations in grade of oil delivered, (5) erosion of atomizing nozzle, (c) fluctuations in by-pass relief pressures and (d) possible variations in methods 2b (3) and 2b (4) listed in the previous classification table. Note that 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 inter ruption of service but usually no soot will form. The following factors may influence the air supply: (a) 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 (b) changes in air inlet adjustments to the fan. It is recognized that a secondary source of air. due to leakage in the boiler setting is present in many installations and it is highly desirable that this leakage be reduced to a minimum. Obviously 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 mechani cal 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 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 whole heating season is to be maintained. Measurement of. the Efficiency of Combustion Efficient combustion being based upon a clean flame and certain proportions of oil and air employed, it is possible to determine the results by analyzing the gases formed by the combustion process. Except in the case of a non-luminous flame it is usually sufficient to analyze only for 212 CHAPTER 10. AUTOMATIC FUEL BURNING EQUIPMENT 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 at the present time show from 8 to 10 per cent C02. Taking into account the potential hazard of oil or air fluctuations with low excess air (high COt) a setting to give 10 per cent C02 constitutes a reasonable standard for most oil burners. Controls Controls for oil burner operation, including devices for the safety and protection of a boiler or furnace, are. fully described in Chapter 34. GAS-FIRED APPLIANCES The increased use of gas for house heating purposes has resulted in die production of such a large number of different types of gas heating systems and appliances that today there is probably a greater variety of them than there is for any other kind of fuel. Gas-fired heating systems may be classified as follows: I. Gas-Designed Heating Systems. A. Central Heating Plants. 1. Steam, hot water, and vapor boilers. 2. Warm air furnaces. B. Unit Heating Systems. 1. Warm air floor furnaces. 2. Industrial unit heaters. 3. Space heaters. 4. Garage heaters. II. Conversion Heating Systems. A. Central Heating Plants. 1. Steam, hot water and vapor boilers. 2. Warm air basement furnaces. These systems are supplied with either automatic or manual control. Central heating plants, for example, whether gas designed or conversion systems, may be equipped with room temperature control,, push-buttoncontrol, or manual control. Gas-Fired Boilers and Furnaces Specially designed boilers are available for gas-firing such as shown in Fig. 16. Additional information on gas-fired boilers will be found in Chapter 12. Either snap action or throttling control is available for gas boiler operation. Throttling control is especially advantageous in steam systems because steam pressures can be maintained at desired points, while at the same time complete cut-off of gas is possible when- the thermostat calls for it. Warm air furnaces are variously constructed of cast-iron, sheet metal and combinations of the two materials. If sheet metal is used, it must be of such a character that it will have the maximum resistance to the corrosive effect of the products of-combustion. With some varieties of manufactured gases, this effect is quite pronounced. Warm air furnaces 213