Document QXBGy0E5KjMDxarLe1q3oEjr6

HEATING VENTILATING AIR CONDITIONING CUIDE 1944 tank. A typical indirect heater using steam is shown in Fig. 2. The coils usually are of copper and are U 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 expanded into a floating head to take care of expansion. The coils should be capable of easy withdrawal for inspection and for Fig. 2. Indirect Water Heater | hot water to fixtures Fig. 3. Indirect Water Heater Mounted on . Side of Boiler Fig. 4. Indirect Water Heater Placed in Boiler 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. 3. 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 circu lates 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 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 water are withdrawn.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. 4 the heat transfer surface is placed inside 806 CHAPTER 46. HOT WATER SUPPLY the boiler instead of in a separate vessel, but otherwise the operation is similar to that of Fig. 3. This arrangement with vertical tank is com monly used for small domestic installations. Sometimes the heating element is located inside of the larger type fire tube boilers and small residential boilers. In this case the heat transfer surface is in the torm of a number of straight copper tubes with rear V 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 without 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 thermo static 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 srale, 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 its dissolved salts.... .COMPUTING AREA OF HEAT TRANSMITTING SURFACE The area of the inside surface of a heating coil may be determined from Equation 1. A = OXS-SSfe-JJ ,(1) where o X mij . - '. ' A: = surface area of coil, square feet. . Q *= quantity of water heated, gallons per hour. . ti = :hot water outlet temperature, degrees Fahrenheit.. /, = cold water inlet temperature, degrees Fahrenheit. . * . k0 -- coefficient of heat transmission, Btu per hour per square foot surface. For copper or brass coils 0 = 240 (steam) and. 100 (hot water). For iron coils fe0. -- 160 (steam) and 67 (hot water). . fa = logarithmic mean of the difference between the temperature; of the .heating medium and the average water temperature and is approximately: - . 7' : ' L:*rJi ; Is.= temperature of the heating medium, degrees Fahrenheit. Equation 1 may be used to check the heating coil ratings under tempera ture conditions differing from those stated in the manufacturer's published ratings.... Example 4. What area of copper transfer surface will be required to heat 70 gal per hour from 40 to 180 F with boiler water at 220 F? -.0220-Safi2j.no A = 70 X 8.33 (180 100 X 110 40)' = 7.39 sq ft. 807