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876 CHAPTER 50 1948 Guide - "The conditions given in Example 2 may be cited as average. It is possible to vary the storage and heating capacity by increasing and decreasing one over the other. Such a conditionJs illustrated in Ex ample. 3. Example 8. Assume an apartment house housing 200 people. From the data in Table 5: Daily requirements = 200 X'40 = 8000 gal. Maximum hours demand = 8000 X H = 1140 gal. Duration of peak load = 4 hr. Water required for 4-hr peak = 4"X 1140 = 4560. If a 1000 gal storage tank is used, hot water available from the tank = 1000 X 0.75, = 750. Water, to be heated in 4 hr = 4560 -- 750 - 3710 gal. Heating capacity per hour = 3710/4"=_S30 gal. . If instead of a 1000 gal tank, a 2500 gal tank had been installed, the required heating capacity per hour wou,l_d, be4--5--6-0---------(-2--5^0-0----X----0--.7--5--)-= 671 gal. Table 6 may be used to determine the size of water heating equipment from the number of fixtures. To obtain the probable maximum demand multiply the total quantity for the fixtures by the Demand Factor in line 11. The heater or coil should, have a water heating capacity equal to this probable maximum demand. The storage tank should have a capacity equal to the probable maximum demand multiplied by the storage capacity factor in line 12. Example 4 will illustrate the procedure. Example 4. Determination of heater and storage tank size for an apartment building from number of fixtures. 60 lavatories........... ....... 30 bath tubs._________ 30 showers...................... 60 kitchen sinks.______ 15 laundry tubs. X 2 = 120 gal per hour X 20 = 600 gal per hour X 75 = 2250 gal per hour X 10 = 600 gal per hour X 20 = 300 gal per hour Possible maximum demand_____________________________ = 3870 gal per hour Probable maximum demand__________________ = 3870 X 0.30 = 1161 gal per hour Heater or coil capacity_________________________________ = 1161 gal per hour Storage tank capacity________________________ = 1161 X 1.25 -- 1450 gal METHODS OF HEATING WATER Hot water may be heated either by the direct combustion of fuel, by an intermediate carrier such as steam or hot water, or by electrically heated surfaces. The simplest method is to have the fire on one side of a metal barrier and water on the other. In such a method if the water surfaces of heat transfer are small, and if the water carries a heavy proportion of predpitable salts, the water passages may soon clog and then bum out. A familiar example of such trouble is the water back of the firebox in the kitchen stove or the pipe coil inserted into the firebox of a warm air furnace or small boiler. The critical water temperature, at which the lime, magnesia, etc. collect on hot surfaces, varies with the character and proportions of the solids, but generally such deposits are not a serious trouble with water temperatures lower than 140 F. Coal-burning direct-fired water heaters may be constructed of cored cast-iron sections or of steel. In some cases the external appearance of the cast-iron sections is the same as in heating boilers, but internally the cores are changed to enable the sections to withstand the city water .pressure. In small capacity water heaters, efficiency is not considered so Water Services 877 important as low first cost and ability to maintain a fire/at a low rate of combustion, and consequently such heaters are generally built with a dry section or fire-brick, lining at the base of the fire-pot to prevent too much chilling of the fuel. While mud and scale will eventually clog the water ways of any direct-fired heater, increased life may be obtained by pro viding a three-way cock in the return line between the heater and the bottom of the storage tank, so that water can be blown through the heater or the tank separately at full line pressure to clean out loose sediment. Clean-out openings in the bottom of the heater are advan tageous if used by operators of water heaters for periodic cleaning out of sediment. Oil-burning direct-fired water heaters usually are of steel and operate with higher flame temperature and better efficiency than commensurate sized coal-burning heaters. They have the same tendency as coal boilers Table 6. Hot Water Demand per Fixture for Various Types of Buildings Gallons of mater per hour per fixture, calculated at a final temperature of lJfl F OmcsApabt- BBNT .. exHouse Club GtmNASIUM Hos pital Hotel INDUS' TBIAL Plant Build ing Private Resi School dence yj*. 1. Basins, private lavatory___ 2 2. Basins. public lavatory____ 4 3. Bathtubs- 20 4. Dishwashersi 15 5. Foot basins A Witehpn oinV 3 10 7. Laundry, stationary tuba__ 20 Sr Pantry nk_ 6 0 Rhnwn . 75 10. Hlnp mnk 20 11 npmanrl fartnr 0.30 12. Storage capacity factor*__ L25 2;* 6 20 50-150 3 20 2S 10 150 20 0.30 0.90 2 8 30 12 225 0.40 1.00 222 6 8 12 20 20 30 50-150 50-200 20-100 3 3 12 20 20 20 28 28 ` 10 10 75 75 225 20 30 20 0.25 0.25 0.40 0.60 080 1.00 2 6 .... -- .... 15 0.30 2.00 2 ;-- 20 15 3 10 20 5 75 15 030 070 22 " 15 s 30 20-100 20-100 3 12 10 20 28 10 10 225 225 20 20 0.40 040 1.00 1.00 Ratio of storage tank capacity to probable maximum demand per hour. to lime up, and the water passages should be large in cross-section and accessible for periodic cleaning. Gas-burning direct-fired water heaters may be of the instantaneous or storage type. Instantaneous heaters are generally constructed of spiral water tubes of copper around which the products of combustion circulate upward from high capacity burners. Storage-type heaters may include in one unit an insulated storage tank, a combustion chamber, flues, burner equipment, and controls, or may consist of a separate storage tank and external direct-fired water heater, which may be a so-called side-arm heater for small capacity or a gas-fired boiler for larger capacity. Gas boilers used for direct hot water supply must be able to withstand the city water operating pressure. While direct-fired gas heaters are used generally for residences and small installations of 100 gal storage capacity or less, indirect heaters are recommended for larger installations. Electric water heaters for domestic hot water supply are described in the section Heating Domestic Water by Electricity in Chapter 30.