Document mBoKqj3d93qMx6N64m7bJMb4Q

number of occupants, number of bathrooms or plumbing fixtures, and presence of appliances such as laundry equip ment, dishwashers, and the like. Information of tins kind is given in Tables 2,10,11 and 12 and in design manuals 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 ^ off-peak service under which the charge for energy consume during designated hours is lower than for normal or unr^. stricted service. With this system, the operation of is confined to such hourly intervals during the 24 hours as will (a) avoid creating peak electrical demands by the custo mer in excess of the demand resulting from the custom^' other uses of electricity, or (6) keep the water heating load off the electric utility .system during peak hours. In general Case (a) 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 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 13 of the 1961 Guide And Data Book. 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. I `Indirect Water Heaters In the indirect method, either steam or hot water is 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 alsn be hot water inside the tubes. Another method of transferring heat from a heating 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 Fig. 10 .... Domestic Hot Water Heater for Off-Peak Service ''de the coil. The storage tank should be installed with the tottom of the as far above the boiler as possible. Hori- ` tal storage tanka of less than 18- or 20-in. diameter are *t'recommended because of the difficulty of. preventing.the St and cold water from mixing, especially when large quan- ..q 0f water are withdrawn. In Fig. 13 the heat-transfer surface is placed inside the boiler instead of in a separate fjssel; but otherwise the operation is similar to that of Fig. 22 This arrangement with vertical tank is commonly used for small domestic installations. Sometimes the heating element is located inside of the (siger-type fire tube boilers and small residential boilers. In gus the heat-transfer surface is in the form of a number of straight copper tubes, with rear U-bends or a floating head, loserted through a special opening in the boiler. While, the used it should be sized for the same water volume in gallons per minute used for calculating the size of the hot water main. Rimrft there is no miring 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 hating coil may be de termined from Equation 4. Q X 8.33, - I,) UX ` where A - surface area of coil, square feet.Q = quantity of water heated, gallons per hour. tt = hot water outlet temperature, Fahrenheit. ti -- cold water inlet temperature, Fahrenheit. U -- coefficient of heat transmission, Btuper (hour) (square foot) (Fahrenheit degree logarithmic mean tempera ture difference). For copper or brass coils U -- 240 (steam) and 100 (hot water). For iron coils U = 160 (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: Rg. 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. This ar rangement is frequently found in oil burner installations where the heating boiler, either steam or hot water type, is ased 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 w-aMing In order to reduce clogging by precipitated solids, water heating plants sometimes develop steam in a closed circuit, end 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, tile effect of impurities, and means of improving the quality of tiie water are important items discussed in Chapter 15 ot tile.1961 Guide And Data Book. Where plentiful steam supply is available, and sometimes frtere 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. Instantnneous heaters have the advantage also where hot water detnands are either very steady or very infrequent. Since the instantaneous heater must heat the water as it is t, a temperature of the heating medium, Fahrenheit. Equation 4 may be used to check the heating coil ratings under temperatures other than those stated inthe 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: - 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 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 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 -- ttfu per (hr) (sq ft) (fohretdted degree logarithmic mean temperature difference} 210 200 180 225 175 160