Document em91vbe4My0b1D4O2y4E0B1qG

1212, CHAPTER 49 1958 Guide easy withdrawal for inspection and for removal of scale. . 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 within 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 as possible. Horizontal storage tanks of less than 18 or 20 in. diameter are not recommended because of the difficulty of preventing the Hoi, and cold water from mixing, and especially is this an important consideration when large' quantities of water are withdrawn.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 operation is similar to that of Fig. 9. This arrangement with vertical tank is,commonly used for small domestic installations. - Sometimes the heating element, is,located inside of the larger type fire tube boilers and small residential boilers. Inthis case the heat transfer sur-. face isin 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 placed in the steam space,above,the water line of a, steam boiler; it is usually placed below the water line. Long coils of small diam eter tubing,:immersed in the water-,<are widely:used without storage tanks. The rate of flow through the coil is limited .by the friction loss in the coil; - tofhturcs Fio. 10. Indirect Water Heater Placed in Boiler Water Services 1213 and by fittings and restrictions, so that the water .attains, the; desired temperature, in one passage;through the coil, i: This arrangement is;fre quently found in oil burner :installations where the 'heating-boiler, either steam or hot water type, is used to supply hot water during the summer. 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 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. . Water characteristics, the effect of impurities and means of improving the quality of the water, axe important iteqis, as brought out in Chapiteri42. . COMPUTING HEAT TRANSFER SURFACE V.' ; The area of the inside surface of a heating coil iriay be'determined'from Equation 2. Q X 8.33(fa - U) A = uxtm (2) where A = surface area of coil, square feet. Q = quantity of , water heated, gallons per hour.. It = hot water outlet temperature, Fahrenheit, li = cold water inlet temperature, Fahrenheit. U = coefficient of heat transmission, Btu per (hour) (square foot) (Fahrenheit de gree logarithmic mean, temperature difference). For copper or brass coils U = 240 (steam) and 100.(hot water). ., For iron coils If = 160 (steam) and 67 (hot water). - v" ta = logarithmic mean of the'difference between the temperature of the heating medium and the average water-temperature, and is approximately: 1 * -- temperature of the heating medium, Fahrenheit. '. Equation 2 may be used to cheek the heating coil ratings under tempera tures other than those stated in the manufacturer's published ratihgsl " Example 6: What area of copper transfer surface will be required to heat' 70, gal oi water per hour from 40 to. 180 F with boiler water at 220 F? . ,, , Solution: 1 ^220 - (i80 + 4Q)j 110 A = 70 X 8.33(180 - 40) = 7.42:sq ft 100 X 110 For instantaneous submerged heaters, the surface required will .depend upon (1) the velocity of water in the tubes, (2) the boiler, water, tempera- (3) the inlet water temperature, (4). the outlet water temperature, te) the cleanliness of thei coil surface, and (6) the condition of the boiler water surrounding the coil. If the heater is located in the water of an ac-