Document DD75DE5GY8ygd6KYMa43a37e4

396 CHAPTER 16 1956 Guide Cast-iron boilers are usually shipped in sections, and assembled at the place of installation. However some small boilers are shipped factory assembled. In the majority of boilers the sections are assembled with push nipples and tie rods but external headers and screw nipples are also used successfully. Many sectional boilers are provided with 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. Small sectional type boilers are available with wei-base construction, wherein the ashpit or combustion chamber sides and bottom are sur rounded by extensions of the water legs of the boiler sections, and thus no separate base is required. This type of construction permits the boiler to be set directly on a wood or composition floor without danger of fire. The wei-base also provides some additional heating surface. Capacities of cast-iron boilers range generally from capacities required for small residences up to about 12,000 sq ft of steam radiation but boilers are made with capacities up to approximately 32,000 sq ft E.D.R. Steel Boilers Steel boilers may be of the fire-tube type, in which the gases of combus-. tion 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 connections. Refractory furnaces are usually1 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 50,000 sq ft E.D.R. (steam). Boilers for Special Applications One of these is known as the magazine feed boiler developed for the burning 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 attention must be given to proper chimney sizes and connections 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 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 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 Heating Boilers, Furnaces, Space Heaters 397 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. Tne accumulation of scale does not affect efficiency, although it will affect the capacity of heat exchanger. 4. Discoloration of water may be prevented if the water supply comes in contact with only non-ferrous metal. Where a steam or a forced circulation hot water heating system is in stalled, the domestic hot water may be heated by an indirect heater at tached 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 pos sible with respect to the boiler.. BOILER DESIGN CONSIDERATIONS Furnace Design Good efficiency and proper boiler performance are dependent on 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 14 and 15.) 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, in fluence 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, in creases 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 average about 3300 for hand-fired boilers, and 4000 for mechanically-fired boilers when operating at design load. 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 con ditions at these heat transfer rates, will give exit gas temperatures that