Document 9Le7jm091bvVE8QRXLgERbGOp

1012 CHAPTER 48 |"hot water to fabra 1951 Guide Fiq. 9. Indirect Water Heater Mounted on Side op Boiler 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 hot and cold water from mixing, and especially is this an important consideration when large quantities of water are withdrawn.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 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 tubeboilers andsmall residentialboilers. In this casethe heat transfersurface 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 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, Fio. 10. Indirect Water Heater Placed in Boilrh Water Services 1013 and by fittings and restrictions, so that the water attains the desired temperature in one passage through the coil. 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 fife, 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 are important items, as brought out in Chapter 42. COMPUTING HEAT TRANSFER SURFACE The area of the inside surface of a heating coil may be determined from Equation'2. A Q X 8-33(1, - t,) UXln (2) where A = surface area of coil, square feet, Q = quantity of water heated, gallons per hour, f, = hot water outlet temperature, degrees Fahrenheit, h = cold water inlet temperature, degrees Fahrenheit. V = coefficient of heat transmission, Btu per (hour) (square foot) (degree Fahren heit logarithmic mean temperature difference). For copper or brass coils U -- 240 (steam) and 100 (hot water). For iron coils U = 160 (steam) and 67 (hot water). (m -- logarithmic mean of the difference between the temperature of the heating medium and the average water-temperature, and is approximately: l, = temperature of the heating medium, degrees Fahrenheit. Equation 2 may be used to check the heating coil ratings under, tempera tures other than those stated in the manufacturer's published ratings. nxampie o. wnat area of copper transfer surface will be re of water per hour from 40 to 180 F with boiler water at 220 F? Solution. 2 Jtm (180 + 40)1 - 110 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.temperature, (3) the inlet water temperature,, (4) the outlet water temperature, (5) the cleanliness of the coil surface, and (6) the condition of the boiler water surrounding the coil. If the heater is located in the water of an ac-