Document EZvnnkmmY4rQ1J7q9YGmxReN

516 CHAPTER 35 1960 Guide Boilers for hot water supply are classified as direct, if the water heated passes through the boiler, and as indirect, if the water heated does not come in contact with the water or steam in the boiler. Direct heaters are built to operate at the pressures found in city supply mains, and are tested at pressures from 200 to 300 psig. The life of direct beaters depends almost entirely on the scale-forming properties of the water supplied and the temperatures maintained. If low wateT temperatures are maintained, the life of the heater will be much longer due to decreased scale formation and minimized corrosion. Direct water heaters in some cases are designed to bum refuse and garbage. Indirect heaters generally consist of steam boilers in con nection 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 sup ply comes in contact with only nonferroua metal. Where a steam or a forced circulation hot water heating system is installed, the domestic hot water may be heated by an indirect heater attached to the boiler. For most satisfac tory performance in the steam system, this heater is placed just below the water line of the boiler. In a forced circulation hot water system, it should be located as high as practicable with respect to the boiler. BOILER DESIGN CONSIDERATIONS Furnace Design Good efficiency and proper boiler performance are depend ent on correct furnace design. There must be sufficient vol ume for burning the particular fuel which is used, and means to obtain a thorough mixing of air and gases at a high tem perature and at a velocity low enough to permit complete combustion of all the volatiles. For hand-fired boilers, the furnace volume should be. large enough to hold sufficient fuel for reasonably long firing periods. (See Chapters 33 and 34.) 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 high** 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 effectiveness of the heating surface depends on its cleanliness, its location in the boiler, and the shape of the gas passages. 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. Inserting baffles so that the heat ing surface is arranged in series with respect to the gas flow, increases boiler efficiency and reduces stack temperature, but increases the draft, loss through the boiler. Heat-Transfer Rate Practical average overall heat transfer rates, expressed in Btu absorbed per square foot of surface per hour, will aver age about 3300 for hand-fired boilers, and 4300 for mechani cally-fired boilers when operating at design load. When me chanically-fired boilers are operating at maximum load, as defined in this chapter under heading Selection of Boilers, these values will run between 5000 and 6000. Boilers operating under favorable conditions at these heat transfer rates, will give exit gas temperatures that are considered consistent with good practice, although there are boilers which have high efficiencies and also operate at higher transmission rates. RATING AND TESTING CODES Heating boilers are usually rated according to codes de veloped by the Steel Boiler Institute, the Institute of Boiler and Radiator Manufacturers, the American Gas Association, the Mechanical Contractors Association of America, and the Packaged Firetube Branch of the American Boiler and Affili ated Industries. These test codes have been prepared specifi cally for the purpose of obtaining information required for es tablishing acceptable ratings. The various current rating and testing codes will be discussed in following paragraphs in which limitations, methods of application, and resulting rat ings will be described. The Steel Boiler Institute has adopted a rating code for boilers designated Table 1 Steel Boilers, Table 2 Steel Boilers, and Table S Steel Boilers, and a testing code for Oil-fired Table 2 and Table S Steel Boilers (SBI Rating Code, Eighth Edition, February 1958). Tables 1, 2, and 3 of this chapter show the SBI Gross Outputs and Net Ratings of these three classes of boilers. 'Die Table 1 boilers (defined as those having 129 to 3571 sq ft of heating surface and formerly designated as Com mercial boilers) are rated in square feet EDR design load (steam) on the baas of heating surface with limitations set for grate area, furnace volume, and furnace height. The Table boilers (defined as those having not more than 294 sq ft of heating surface and formerly designated as Residential boilers) are rated on the basis of hearing surface with ratings confirmed by test. The Table S boilers (defined as having no limits of minimum heating surface or furnace volume, and having Gross Outputs and Net Ratings listed in Table 3) are rated on the basis of performance under the provisions of Section III, Part 2, of the SBI Rating Code. The minimum stack area dimensions to be cataloged for Table S boilers are shown in Table 4. Required stack heights are found in the catalogs of the manufacturers. Stoker-fired and gas-fired Table boilers are rated (SBI Net Rating) not in excess of the oil-fired rating. Eland-fired Table boilers are rated (SBI Net Rating) not greater than fourteen times the beating surface. The Institute of Boiler and Radiator Manufacturers has adopted a code1 for rating cast-iron heating boilers, based upon performance obtained under controlled test conditions. This code applies to all sectional cast-iron heating boilers ex cept those of magazine-feed type. The Gross / = B = R Output is obtained by test, and is subject to certain limiting factors. For hand-fired boilers, the number of boilers of a series to be tested, the minimum over- Heating Boilers, Furnaces, Space Heaters to to 8| CO NNN W -* M f- O O 388 OO w ef to eo ill I I i 188 538 1 CT 3 88S CO .H< <E> a> -v eo o nn o f -- NOON V to QO 8SSS e >o o' w o o' e*" to 288 2 3S how to 2 55 ci of N f<aO.0 mnto g5 rt" V Sosoiso < - NO N oo Iss NOONOO 517 ! I 5 d I sI 1