Document mmR2dypdKgqQ9wMLkaknjw86J

1030 CHAPTER 48 1952 Guide 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 11, 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. (t> = 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 practice to install a circulating pump to insure rapid movement of the boiler water. 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 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, Table 11. Coefficient of Heat Transfer of Instantaneous Wateb Heaters U -- Btu per (hr) (sq ft) (Fahrenheit degree logarithmic mean temperature difference) Boiler water'temperature................... U...................:................ ............................... 210 225 200 175 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. Steaming may then occur in the boiler sections not connected to the, heater and, further, the unconnected sections, will not deliver any heat to the water heater. HOT WATER SUPPLY PIPING It is common practice to provide circulating piping in all hot water supply systems in which it is desirable to have hot water available continuously at the. fixture^. In average-sized and small residences and systems, in which the piping from the heater to the fixtures is short, return circulating piping is generally omitted in order to reduce installation cost, and to reduce heat loss from the piping, particularly during periods of no water demand. The hot water supply may be distributed by either an up-feed or downfeed piping system. Three common methods of arranging the circulating lines are shown in Fig. 11. Although the diagrams apply to multi-story buildings, the arrangements (a) and (b) are sometimes used in residential designs. A check valve should be provided in the run-out from each return riser to prevent temporary reversal of flow in the piping when a faucet is open. Proper air venting of a circulating system is extremely important, particu larly if gravity circulationisemployed. InFig. 11 (a) and (6), this is accom- Water Services 1031 plished by connecting the circulating line below the top fixture supply. With this arrangement, air is eliminated from the system each time the top fixture is opened. . Where an overhead supply main is located above the highest fixture as in Fig. 11 (c), an automatic float type air vent is installed at the highest point of the system, or a fixture branch is connected to the top of the main where air venting is desired, and then dropped to the fixture outlet. It is sometimes necessary to make an allowance for pressure drop through the heater when sizing hot water lines, particularly where instantaneous hot water heaters are used and the available pressure is low. The principles involved in the sizing of the" hot water supply pipes are the same ag those for the sizing of cold water supply lines. For small and medium sized installations a j^-in. hot water return will be ample. For larger installations,- the size of the hot water return may be computed from considerations of the heat losses in the hot water piping.4 A throttling. Vent U-'' 11 i *! i / (a) t1 -1 -t t^ . QO.. 3 t L- t ~ / J Q. * . i j -> Q. t3 C * t / 1 3 1 " " 1 aa3.. . aa.. 3 1 1^ sf 1^ j (6) <c> Fig. 11. Methods of Arranging Hot Water Circulation Lines aa.. 3 J Ai? valve should be placed in the hot water return pipe so that the rate of cir culation may be adjusted. Where the hot water piping system is exceedingly long, a water circu lator is frequently installed and controlled from an immersion thermostat (in the return line) set to start and stop the pump over approximately a 20 F deg temperature range. - CONTROL OF SERVICE WATER TEMPERATURE Coal-fired boilers are usually controlled by ah 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 burns 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