Document x1oZRV0yV9e5EenB7dxnbomk1

516 CHAPTER 24, 1049- Guide in the heating system to take place without producing undesirable stresses due to pressure in any part of the system. Expansion tanks may be open, as illustrated in Fig. 15, or closed as shown in Fig. 16. . An open expansion tank has free vent to the atmosphere and consequently the pressure on the surface of the water is always that of, one atriiosphere. - The minimum contents of an open tank should be 0.06 of the .volume, of the water in the system including that in the boiler, heat transmitters, pipes, etc.. This capacity is 50 per cent in excess of the actUal increase ;in; volume of water due to increase in temperature from 40 F to 200 F. The tank should be located at least 3 ft above the highest radiator. Provision must be made , Hot WaterHeating Systems and.Piping'/ .. C 517 to prevent freezing pf the water in the tank as well as in the pipe leading to the tank. When the vent from an open expansion tank is extended through the roof, it should be not less than 4 in. in diameter fronr a'point below the roof* through and beyond the roof line. This will prevent vapor, which sometimes rises from an expansion tank, from closing the vent during outside freezing temperatures. In a gravity circulation system, the pipe to. the open expansion tank should be connected to the supply riser from the boiler, so that the air liberated from the water in the boiler will enter the expansion tank. . . In a forced circulation system, the pipe to an open expansion tank should be connected on the suction side of the circulating pump. . A closed expansion tank is sealed against free venting to the atmosphere. The tank may be above the highest radiator or heat, transmitter, or may be below the lowest one. The minimum contents of a closed expansion tank must be such that the expansion of the water due to increase in temperature will be cushioned against a' reservoir of compressed air above the'w'ater level'in the expansion tank.' The tank mud; provide space not only for the change in water volume, but also for variations in 'air volume within the tank due to changes in air pressure. If the closed expansion tank is below the heat transmitters, the tank should be larger than if it is above them; and the higher the budding, under such circumstances, the larger should be the air capacity in excess of that required for increase in water volume due to temperature rise. The size of an expansion tank for installation in a closed system may be determined by the following formula: 7= B Pi Pi + a.ma Pi p. (3) where V = required tank capacity, gallons. " -'; B expansion of water from cold system.io flow riser temperaturej.gallons. '" Pi " atmospheric pressure, psia. ! Pi = maximum tank pressure specified for heated system, psia. H = height of top of filled system above tank, feet. ' (Notb : Top pf system open to atmosphere when system is filled.). . ... ! .' . . Example 7. Select a closed expansion tank for basement installation on a system containing 5000 gal and operating at 200 F flow temperature, . The static head due to the height of the system is 70 ft. . The maximum pressure should not exceed 100 psig. Solution. Assume that the system is filled at 40 F. Then' E = 0.04 X 5000 = 200 gal Pi = 14.7 psia Pi = 100 + 14.7 = 114.7 paia..: Substituting these values in Equation 3: V" 200 200 14.7 _ 147 " 0.327 - 0.128 14.7 + 0.434(70) 114,7 ; lOOOgal. . :