Document byXdg0mKmV8GpO3NM5E30OpOo

: 340 CHAPTER 18 1946 Guide large push nipples at top to permit-the circulation of water between adjacent sections at both the water line and bottom of the boiler, which is necessary to enable the use of an indirect water heater with the boiler for summer-winter hot water supply. Round and sectional boilers may be increased in size by the addition of sections and corresponding plate work. ' Capacities of cast-iron boilers range generally from capacities required for small residences up to about 12,000 sq ft of steam radiation. There are a few boilers made withcapacities-up to 18,000 sq ft of steam radia tion. For larger loads, boilers must be installed in multiple. .Steel Boflers. Steel boilers may be of the fire-tube type, in which the gases of com bustion pass through the tubes and the boiler water circulates around them, or of the water-tube type, in which the gases circulate around the .tubes and the water passes through them. ' Either the fire-tube or water-tube type may be designed with integral water jacketed furnaces or arranged for refractory lined brick.or refractory lined jacketed furnaces. Those with integral water jacketed furnaces are called portable firebox boilers and are the most commonly used type. They are usually shipped in one piece, ready for piping. Refractory furnaces are usually installed in refractory lined furnace boilers after they are set in place. _: Capacities of steel boilers range from those required for small residences up to about 35,000 sq ft of steam radiation. : Boilers for Special Applications One of these is known as the magazine feed boiler developed for the bupiing of small .sizes of anthracite and coke and has a large fuel carrying capacity which results in longer firing periods than would be'the case with the standard types burning coal of buckwheat size. Special atten tion must be given to proper chimney sizes and connections when in stalling in order to insure adequate draft. . 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 hot 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 lb per square inch. The .life of direct heaters depends almost entirely on the scale-forming properties of the water supplied and the temperatures maintained. If low water 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 burn refuse and garbage. 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 tp 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. " Heating Boilers and Furnaces_______________ ____________ y;341 Where a steam or a forced circulation hot water heating system is installed, the domestic hot water may be heated by ah indirect heater attached to the boiler. For most satisfactory 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 possible with respect to the boiler. . BOILER DESIGN CONSIDERATIONS Furnace Design Good efficiency and proper boiler performance are dependent bn correct furnace design. There must be sufficient volume for burning the particular fuel which is used, and means to obtain a thorough mixing of air and gases at a high temperature 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 16 and 17.) 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 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 heating 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-heat transfer rates in heating boilers expressed in Btu absorbed per square foot of surface per hour will average about 3300 for hand fired boilers and 4000 for mechanically fired'boilers when operating at design loadJ When mechanically 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 trans mission rates. TESTING AND RATING CODES The Society has adopted four solid fuel testing codes, a solid fuel rating code and an oil fuel testing code. ASHVE Standard and Short Form Heat Balance Codes for Testing Low-Pressure Steam Heating Solid Fuel Boilers--Codes T and 2;-- /