Document ykQGdkgbm1v0BXExbXxVyvdg2

958 CHAPTER 50 " `1949 Guide equipment, and-controls, or may consist'of1a: separate storage tank and external direct-fired water heater,1 which may be a so-called side-arm heater for small capacity or a gas-fired boiler for larger capacity. Gas boilers used for direct hot water supply must be able to withstand the city water operating pressure. While direct-fired gas heaters are used generally for residences and small installations of 100 gal storage capacity or less, indirect heaters are recommended for larger installations. Chimney connections for all direct-fired, fuel burning water heaters are an important consideration. Refer to Chapter 19. Electric water heaters for domestic, hot water supply are described in the section Heating Domestic Water by Electricity in Chapter 30. Ih.the indirect method either steam or hot water is used for heating the water. With steam the water to be Heated is preferably circulated around the1 outside of the steam tubes which are submerged within a tank. A Water Services 959 is in the form of a number of straight copper tubes with rear U bends or a floating head,-inserted through a special opening in the boiler. While the coil may be located in the steam space above the water line of a steam boiler, it operates more satisfactorily when below the water line since clogging of the water tubes may thereby be delayed. . This method is widely used with out storage tanks since the intimate contact and efficient circulation of the water in this arrangement permit the' utilization of the heat stored in the water of the boiler. A thermostatic three-way mixing valve is frequently used to maintain a uniform temperature of, the hot water supply to. the plumbing fixtures. In order to reduce clogging by precipitated; solids, water heating plants sometimes develop steam in a closed circuit, transferring the heat through 3 hot water to fixtures "?T Ii typical indirect heater using steam is shown in Fig. 8. The coils usually are of copper and are {/-shaped to permit expansion and contraction. The shell'may be of.steel, copper, or with a special inside protective lining.. Where straight heating tubes are used, one end of the tube is usually ex-; panded into a floating head to take care of expansion. The coils should be capable of easy withdrawal for inspection and for removal of.scafe.. Instead of steam, the heating medium may also be hot water inside the tubes. Another method of transferring heat from a heating boiler to the domestic water is illustrated in Fig. 9. The' water heater is generally; a cast-iron shell withiin which there is located a spiral copper coil. Hot water from the boiler circulates inside the shell and'around'the coil, and returns to the boiler, while domestic water from the storage tank circulates inside the coil. The storage tank should be installed with the bottom of the tank as far above the boiler Us possible. Horizontal storage tanks smaller than 18 'or ` 20 in. diameter are not recommended because of the difficulty of preventing the hot and cold water from mixing,' and especially is this an important consideration when large quantities of water are withdrawn. In Fig. 10 the heat transfer surface is placed inside the boiler instead of in a separate vessel, but otherwise the operation1 is similar to that .of Fig. 9. This arrangement with vertical tank is commonly used for small domestic installations. :... v . / : '' ' : Sometimes the heating element is located inside.of.the larger type fire tubeboilers and small residential boilers.' In this case the heat transfersurface Fig. 9. Indirect Water Heater Mounted on Side of Boiler a tubular heater to the domestic' water. The water in the primary heater, exposed to the high temperature of the fire, is repeatedly used and hence has no appreciable tendency to deposit'scale, while the domestic water, heated by steam at a much lower temperature than that of the fire, also exhibits a .much reduced tendency to precipitate dissolved salts. . . COMMUTING HEAT TRANSMITTING SURFACE The area of the inside surface of a heating coil inky be determined from EquationT. '' ' ' ; '' .'. ^ 11 -. . ' : . - , Q X 8.330,- h) A= .... - ' (1: where; . ., .- .... . A -- surface area of coil, square feet: Q = quantity of water heated, gallons per hour. 'f, = hot'water outlet temperature, degreeS'Fahrenheit;' (if'= cold water inlet temperature, degrees Fahrenheit: U = coefficient ofheat transmission, Btu per (hour)'(square foot) (degreeFahren heit logarithmic mean'temperature difference). : -/; ' -For,copper or brass coils U = 240 (steam) and 100 (hot water). p: : For iron coils U = 160 (steam) and 67 (hot water).. v \\ /