Document dZ9YZqLwnw0X59edJxknR7v6

780 CHAPTER 56 1960 Guide Rg 9.... Monthly Cost of Operation of Electric Water Heaters4 number of occupants, number of bathrooms or plumbing fixtures, and presence of appliances such as laundry equip ment, dishwashers, and the like. Information of this kind is given in Tables 2,10,11 and 12 and in design manuria issued by the electric utility industry and by heater manufacturers. For commercial applications, the hourly hot water require ments must be analyzed and the best combination of tank capacity and heating element rating determined, with due consideration of electric power demand created in relation to the user's other electric demands. A common type of electric service for water heating is the off-peak service under which the charge for energy consumed during designated hours is lower than for normal or unre stricted service. With this system, the operation of heaters is confined to such hourly intervals daring the 24 hours as will (a) avoid creating peak electrical demands by the custo mer in excess of the demand resulting from the customer's other uses of electricity, or (b) keep the water heating load off the electric utility system during peak hours. In general, Case (o) occurs where rate schedules contain a demand charge component, and Case (6) where a special low rate applies to off-peak consumption. Water heater circuits are controlled with relays actuated by an electric-clock mecha nism or by a carrier-current impulse impressed at suitable intervals on the electric distribution system, or by an auto matic load-limiting type of control. For general information on automatic control see Chapter 43. One arrangement of domestic hot water heater for off-peak service, shown in Fig. 10, has the lower heating unit under control of an off-peak switch. Upper unit is connected di rectly to the line, so that if the reserve of hot water becomes depleted, the top thermostat brings its heating unit into op eration only until the upper portion (usually about 25 per cent) of tank contents reaches the thermostat temperature setting. Indirect Water Heaters In 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 the outside of the steam tubes which are submerged within a tank. A typical indirect heater using steam is shown in Fig. 11. The coils usually are of cop per, and are U-shaped to permit expansion and contraction. The shell may be of steel, with a protective coating or with a special inside protective lining, or may be of copper or cop per alloy. 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 with drawal 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 beating boiler to the domestic water is illustrated in Fig. 12. The water heater is generally a cast-iron shell within which there is lo cated a spiral copper coil. Hot water from the boiler circu lates inside the shell and around the coil, and returns to the boiler, while domestic water from the storage tank circulates Off-Peak Service i Water Services 781 inside the coll. The storage tank should be installed with the bottom of the tank as far above the boiler as posable. Hori zontal storage tanks of less than IS- or 20-in. diameter are not recommended because of the difficulty of preventing the hot and cold water from mixing, especially when large quan tities of water are withdrawn. In fig. 13 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. 12. 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. In this case the heat-transfer surface 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 ] Mmkbtni used it should be sized for the same water volume in gallons per minute used for calculating the size of the hot water main. Since there is no mixing of cold water and hot water, as would occur in a storage tank, the instantaneous heater is to be se lected for a temperature rise only sufficient to produce the re quired delivery temperature. COMPUTING HEAT-TRANSFER SURFACE The area of the inside surface of a heating coil may be de termined from Equation 4. Q X 8-33(1, - <t) V X t (4) where A = surface area of coil, square feet. Q -- quantity of water heated, gallons per hour. it * hot water outlet temperature, Fahrenheit. U " cold water inlet temperature, Fahrenheit. U ~ coefficient of heat transmission, Btu per (hour) (square foot) (Fahrenheit degree logarithmic mean tempera ture difference). For copper or brass coils V = 240 (steam) and 100 (hot water). For iron coils IS = 100 (steam) and 67 (hot water). tm -- logarithmic mean of the difference between the tem perature of the heating medium and the average water temperature, and is approximately: Fig. 12 .... Indirect Water Heater Mounted on Side of Boiler coil may be placed in the steam space above the waterline of a steam boiler, it is usually placed below the waterline. Long coils of small diameter 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, and by fittings and restrictions, so that the water attains the de sired temperature in one passage through the coil. Tins ar rangement is frequently 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 miring valve should be used to maintain a uni form temperature of the hot water supply to the plumbing fixtures and prevent danger of scalding. In order to reduce clogging by precipitated solids, water heating plants sometimes develop steam in. a closed circuit, and transfer the heat through a tubular heater to the do mestic service water. The water in the primary heater, ex posed 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 tem perature 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 are important items discussed in Chapter 55. Where plentiful steam supply is available, and sometimes where gas firing is to be used directly, it is advantageous to use instantaneous type heaters. This minimizes first cost and avoids the radiation losses of large storage tanks. Instan taneous heaters have the advantage also where hot water demands are either very steady or very infrequent. Since the instantaneous heater must heat the water as it is t, = temperature of the heating medium, Fahrenheit. Equation 4 may be used to check the heating coil ratings under temperatures other than those stated in the manufac turers' published ratings. Example 9: What area of copper transfer surface will be re quired to heat 70 gal of water per hour from 40 to 180 F with boiler water at 220 F? Solution: tm [*_ga]. A 70 X A33(180 - 40) 7.42 sq ft For instantaneous submerged heaters, the surface required will depend upon (1) the velocity of wateT in the tubes, (2) the boiler water temperature, (3) the inlet water tempera ture, (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 y an actively steaming part of a boiler, the heat transfer may be twice as great as would be obtained if the water surround- Table 17.... Coefficient of Heat Transfer of instantaneous Water Heaters U p*r (hr) bq ffl (fuliifcobeft dtgrea logariiftAic neon tmapmrcfUT* difftiefKa) V..................................... 210 200 180 225 175 150