Document KpB0B8rDvJZKvOOKd4orrgj0

1058 CHAPTER 41 , 1958 Guide experience shows that such high heat requirement may result in excessive operating cost. HEATING WATER BY ELECTRICITY Electric water heaters for hot-water service in residences and commercial buildings are predominantly of the automatic storage type. These gener ally have a cylindrical tank mounted vertically, with a primary electric heating element or resistor near the bottom and a secondary unit located within the upper one-quarter of the tank. Water heaters of single-heatingunit type have an element only at the bottom. The first cost of automatic electric heaters is somewhat higher than that of fuel-fired types, owing to the need for larger tank storage to compensate for the limitation of recovery rate, since for economical electric service the connected load should be kept low. Large tank capacity is necessary also with off-peak operation, where there are low electric rates for this class of service. Instantaneous heaters are used in special cases only, because of the high electric power input required, for example, 15 kw.to deliver 1 gpm with 100 deg temperature rise. Capacities of household automatic storage-type water heaters, as par tially standardized within the electrical industry, range from 30 to 140 gal nominal tank capacity with input 1600 to 7000 watts. Wattage input ratings of two-unit heaters are based on approximately 30 watts per gallon of tank capacity for secondary unit and 20 watts for primary unit. Single unit heaters have tank sizes up to 80 gal with an input of 4000 watts, the ratings being based on 50 watts per gallon. The schedule of heater capaci ties shown in ASA Standard C72.1-1949 is reproduced in Table 2. The wattages listed in Table 2 are minimum recommended values for con tinuous service. Some departure from the standardized watts-per-gallon ratios is to be expected, as experience indicates that increased hot-water use with automatic laundry machines and dishwashers may call for larger heating units than have heretofore been found desirable. With off-peak operation the watts-per-gallon ratio likewise must be raised. Rated voltage for water heaters is 240 volts, with a range 220 volts minimum to 248 volts maximum, but specified test performance is required to be met only at the 236 volts taken as normal design voltage. Thermo stats controlling the heating units are normally set for 150 F, with upper limit of adjustment range of 170 to 180 F. Heating units are of three general types, all of which give approximately the same service efficiency if selected to suit the operating conditions, as follows: 1. Immersion element, inserted horizontally through a threaded or flanged opening in side of tank. 2. Strap-on unit, externally mounted directly against the tank shell and held in close contact by a band or strap encircling the tank. 3. Side-arm or outside circulation unit, consisting of a vertical cylindrical husl"6 or heat exchanger which contains an immersion heating element, connected by pipes into top and bottom of the storage tank. For the third type, which is less commonly used, thermostats separate from the heating units are mounted within the storage tank. A fourth type of heater, applied only in exceptional cases and for large installations with alternating current, employs the water itself as a re sistor in which electrodes are immersed. Close temperature regulation and Electric Heating 1059 compensation for the effect of variable amount and composition of solids and gases in the water are difficult to obtain with small unattended heaters of this kind. The electrode type of construction has in the past been used also for generating steam in so-called electric boilers, mainly for industrial plants; however, in only few instances today is electric energy obtainable at cost low enough to make it competitive with fuel-fired boilers for steam pro duction. Tanks of electric heaters are regularly constructed to withstand a 300 psig hydrostatic test pressure and are rated for a safe working pressure of 127.5 psig. Materials in contact with the water are selected to resist corrosion caused by solid and gaseous impurities occurring in some locations. It may be desirable also to provide sacrificial metal anodes of magnesium, zinc or alloys to protect the tank and its piping connections. A safety relief valve of pressure-actuated type must be included; it may optionally be actuated also by internal temperature. A heat trap at the hot-water Table 2. Standard Rated Capacities op Electric Water Heaters Tank Size Gallons Nominal 30 40 52 66 80 110 120 140 ------- ------------------------ Range 30 to 35 35 to 45 45 to 55 55 to 70 70 to 90 90 to 115 115 to 135 135 to 175 Rated Inpot, Watts Two-Unit Heater Primary Secondary Single-Unit Heater 600 1000 1500 750 1250 2000 1000 1500 2500 1250 2000 3000 1500 2500 4000 2000 3000 2500 4000 3000 4000 outlet, if not provided by manufacturer within or outside the tank shell and inside the insulated jacket, may be placed in the external piping. The trap consists of vertical pipe at least 6 in. long having downward flow, to retard circulation of hot water within the distribution piping system under standby conditions. Occasionally, an uninsulated tempering tank is installed ahead of an water heater, enabling the cold inlet water to absorb heat from the basement or room air--if conditions are such that objectionable sweat ing does not occur. The practice of connecting a tempering tank with a re-box coil, the electric heater then furnishing heat only when there is ericiency from the boiler or furnace, is not recommended. t j*`vry Performance efficiency of heaters meeting the electrical-industry s andard 24-hour operation test is 90 percent minimum, which allows 10 percent for thermal loss through the jacket insulation. In service, the sutTM vu -^ wa*,er withdrawn from an automatic storage heater has con ic Cr 'n^IJenco on overall efficiency. With increasing quantities, serv e,e*1'ejency rises and approaches the upper limit fixed solely by the Jacket thermal loss; on the other hand, if little or no water is drawn, ef- a aPProaches zero since no useful work is done. Hence oversizing g a . "eater by having too large a tank impairs the service efficiency. essive heating-unit wattage causes undesirably high electric demand.