Document 3Nwror32MxvZeYxwg207D8Z7y

960: CHAPTER 50 1949 Guide' ' tm = logarithmic mean of the difference between the temperature of the heating medium and the average water-temperature and is approximately : - U = temperature of the heating medium, degrees Fahrenheit. Equation 1 may be us^d to check the heating coil ratings under tempera tures other than those stated in the manufacturer's published ratings/ Example 6. What area of copper transfer surface will be required to heat 70 gal of water per hour from 40 to 180'F with boiler water at 220 F? (180 + 40)1 70 X 8.33(180 - 40) 2 J= 110 A = 7.42 sq ft. 100 X 110 For instantaneous submerged heaters the surface required will depend upon (1) the velocity of water in the tubes, (2) the boiler water tempera- Hff Fig. lOi Indirect Water HeaterPlaced in Boiler ' ture, (3) the inlet water temperature, (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 waiter of an ac tively steaming part of. a boiler, the heat transfer may be twice, as great as would be obtained if the water surrounding the coil were circulating slowly. Ratings of instantaneous water heating coils will therefore'vary' greatly, depending upon the assumptions made regarding the conditions of operation. The values of the coefficient of heat transmission for in stantaneous heaters shown in Table 7 are conservative. For a coil in which heat is transferred from steam to water the value'of U = 300 y/v may safely be used (v = velocity of water in feet per second). The rate of heat transfer between steam or water as the carrier'and the domestic water is influenced.by the rate, of movement, of both the-carrier and the water which receives the heat. For this reason, where the transfer occurs.from heating system-water to domestic water, it is good practices to install a circulating pump to insure rapid movement of the boiler wateri In view of the high condensation rates obtained when steam is used with gravity circulation from-the boiler, as when there is a sudden demand followed by an inflow of cold water, the bottom of a steam heating transfer Water Services 961 Table 7. Coefficient of Heat Transfer of Instantaneous Water Heaters U = Btu per (hr) (sqft) (Fahrenheit degree logarithmic mean temperature difference) Boiler Water Temperature.--------....... 210 200 180 V............................................ ............ 225 175 150 element always should be at least 30 in. above the boiler water line, and the steam and condensate return pipes should be of liberal size. Otherwise water hammer and reduced capacity may result due to imperfect drainage of condensate. When connecting a transfer-type hot water .heater below the, waterline of a cast-iron steam boiler having vertical sections! there should be a sepa rate tapping for water circulation into every section of the boiler, as shown in Fig. 9, unless the boiler has large top nipple ports providing inter-sec tional circulation. If the top nipples are entirely within the boiler steam space, no internal circulation occurs between sections and steaming may occur in sections not connected to the heater and further the unconnected sections will not deliver any heat to the heater. CONTROL OF SERVICE WATER TEMPERATURE Coal-fired boilers are usually controlled by an immersion thermostat located in the heated water, which opens or closes draft dampers at the boiler to adjust the rate of fuel combustion. With oil or gas-fired boilers the immersion thermostat controls the oil burner or the automatic gas valve. The gas pilot flame usually bums continuously. With electric heaters the immersion thermostat operates a switch on the source of energy. When steam or hot water is the medium for heating the water in the tank, an immersion thermostat is used to control a valve in the steam or hot water supply line. In small residence installations, using water as the carrier, a combined immersion thermostat and butterfly valve in one simple fitting may be installed in the transmitting circuit to prevent, over-heating of the service water. , In residences heated by pump circulated hot water, the house tempera ture is controlled by operating- the circulating pump intermittently* while domestic hot "water is warmed by transfer from the house boiler, inde pendent of the pump operation. The domestic water is heated from the heating .boiler the year 'round. Under such an arrangement, to prevent overheating the house by thermal circulation when the pump is not running, it is usual to insert a weighted check-valve in the house, heating main, so that no circulation to the house heating system can occur unless the pump operates. In summer the fire may be controlled, to maintain a boiler water temperature lower than when heating, and generally about 20 F warmer than that desired in the domestic hot water system. In buildings which have restaurants it is generally desirable to install two separate service hot water systems so that water at about 180 F mini mum may be available for dish washing, while water at 140 F maximum may be used for lavatory and bath purposes. The immersion thermostat in a hot water storage tank should be located no higher than the center of the tank, and possibly should be even closer to the bottom since water in a tank stratifies proportionally to the tem perature. When hot water is removed,'the cold water entering to replace it quickly reduces the temperature in the lower parts of the tank.