Document jyaybN1YvoQagN5bomb7w12Z2

American Society of Heating and Ventilating Engineers Guide, 1937 ' Indirect heaters generally consist of steam boilers in connection with heat exchangers of the coil or tube types which transmit the heat from the steam to the water. This type of installation has the following advantages: 1. The boiler operates at low pressure. 2. The boiler is protected from scale and corrosion. 3. The scale is formed in the Heat exchanger in which the parts to which the scale is attached can be cleaned or replaced. . The accumulation of scale does not affect efficiency although it will affect the capacity of the heat exchanger. 4. Discoloration of water may be prevented if the water supply comes in contact with only non-ferrous metal. Where a steam heating system is installed, the domestic hot water usually is obtained from an indirect heater placed below the water line of the boiler. FURNACE DESIGN Good efficiency mid proper boiler performance are dependent on cor rect furnace design embodying sufficient volume for burning the par ticular fuel at hand, which requires thorough mixing of air and gases at: a high temperature with a velocity low enough to permit complete com bustion of all the volatiles. On account of the small amount of volatiles contained in coke, anthracite, and semi-bituminous coal, these fuels can be burned efficiently with less furnace volume than is required for bi tuminous coal, the combustion space being proportioned according to the amount of volatiles present. Combustion should take place before the gases are cooled by the boiler heating surface, and the volume of the furnace must be sufficient for this purpose. The furnace temperature must be maintained sufficiently high to produce complete combustion, thus resulting in a higher COt content and the absence of CO. Hydrocarbon gases ignite at temperatures varying from 1000 to 1500 F. The question of furnace proportions, particularly in regard to mechani cal stoker installations, has been given some consideration by various manufacturers' associations. Arbitrary values have been recommended for minimum dimensions. A customary rule-of-thumb method of figuring furnace volumes is to allow 1 cu ft of space for a maximum heat release of 50,000 Btu per hour. This value is equivalent to allowing approxi mately 1 cu ft for each developed horsepower, and it is approved by most smoke prevention organizations. The setting height will vary with the type of stoker. In an overfeed stoker, for instance, all the volatiles must be burned in the combustion chamber and, therefore, a greater distance should be allowed than for an underfeed stoker where a considerable portion of the gas is burned while passing through the incandescent fuel bed. The design of the boiler also may affect the setting height, since in certain types the gas enters the tubes immediately after leaving the combustion chamber, while in others it passes over a bridge wall and toward the rear, thus giving a better opportunity for combustion by obtaining a longer travel before entering the tubes. To secure suitable furnace volume, especially for mechanical stokers or oil burners, it often is necessary either to pit the stoker or oil burner, or 446 Chapter 25--Boilers where water line conditions and headroom permit, to raise the boiler on a brick foundation setting. Smokeless combustion of the more volatile bituminous coals is furthered . v the use of mechanical stokers. (See Chapter 28.) Smokeless com bustion in hand-fired boilers burning high volatile solid fuel is aided (1) by the use of double grates with down-draft through the upper grate, (2) by the use of a curtain section through which preheated auxiliary air is introduced over the fire toward the rear of the boiler, and (3) by the intro duction of preheated air through passages at the front of the boiler. All three methods depend largely on mixing secondary, air with the partially burned volatiles and causing this mixture to pass over an incandescent fuel bed, thus tending to secure more complete combustion than is pos sible in boilers without such provision. HEATING SURFACE Boiler heating surface is that portion of the surface of the heat transfer apparatus in contact with the fluid being heated on one side and the gas or refractory being cooled on the other side. Heating surface on which the fire shines is known as direct or radiant surface and that in contact with hot gases only, as indirect or convection surface. The amount' of heating surface, its distribution and the temperatures on either side thereof influence the capacity of any boiler. Direct heating surface is more valuable than indirect per square foot because it is subjected to a higher temperature and also, in the case of solid fuel, because it is in position to receive the full radiant energy of the fuel bed. The heat transfer capacity of a radiant heating surface may be as high as 6 to 8 times that of an indirect surface. This is one of the reasons why the water legs of some boilers have been extended, especially in the case of stoker firing where the extra amount of combustion chamber secured by an extension of' the water legs is important. For the same reason, care should be exercised in building a refractory combustion chamber in an oil-bumirig boiler so as not to screen any more of this valuable surface with refractories than is necessary for good combustion. The effectiveness of the heating surface depends on its cleanliness, its location in the boiler, and the shape of. the gas passages. Investigations1 by the U. S. Bureau of Mines show that: 1. A boiler in which the heating surface is arranged to give long gas. passages of small cross-section will be more efficient than a boiler in which the gas passages are short and of larger cross-section. 2. The efficiency of a water tube boiler increases as the free area between individual tubes decreases and as the length of the gas pass increases. 3. By inserting baffles so that the heating surface is arranged in series with respect to the gas flow, the boiler efficiency will be increased. The area of the gas passages must not be so small as to cause excessive resistance to the flow of gases where natural draft is employed. Heat Transfer Rates Practical rates of heat transfer in heating boilers will average about 3300 Btu per sq ft per hour for hand-fired boilers and 4000 Btu per sq ft 1See V. S. Bureau of Mines Bulletin No. 18. The Transmission of Heat into Steam Boilers. 447