Document zd0k297bJzKLyOZR1z2zOBeBn

\ American Society of Heating and Ventilating Engineers Guide, 1934 Down-Feed Gravity One-Pipe Air-Vent System In the overhead down-feed gravity one-pipe air-vent system there is no change over the up-feed system in the radiators, the radiator valves, the air valves, or the radiator runouts as far back as the risers. Beyond tjhis point there are basic differences. The steam is taken from the boiler and carried to the top of the building as near the boiler as possible (Fig. 5). If the run to the main riser is long, or if the riser extends several stories in order to reach the top, the bottom of the riser should be dripped into the wet return.' The horizontal main is taken off the top of the riser and, grades down from the riser toward all of the drops, each drop taking its share of the main condensation (Fig. 6),-or all of the drops except the last may be taken from the top of the main (Fig. 7), the last drop being from the bottom and serving as a drain for the entire main. As the overhead Steam pressure at Boiler steam pressure end of main ' / "HrHl Return .water Water line of boiler. - -f line ^-U-^Rtse ------- --Level -- Wet return Fig. 6. Steam Runouts Dripping Main ^7W7777777777777777r577W777 >777OT777777777T Fig. 4. Difference in Steam Pressure on Water in Boiler and at End of Steam Main Runout Steam drop to radiators-" Fig. 7. Steam Runouts with Main Dripped at End Only Fig. 5. Typical Down-Feed Gravity One-Pipe Air-Vent System 422 Chapter 31--Steam Heating Systems main does not carry any condensation from the radiators it is immaterial which method is used. The air vent shown on the main just before the last drop (Fig. 5) may be placed at this point or it may be located at the bottom of the drop under the last radiator connection and sufficiently above the water line of the boiler to prevent flooding. GRAVITY TWO-PIPE AIR-VENT SYSTEM The gravity two-pipe system is now considered obsolete although many of these systems are still in use in older buildings. Separate supply and Fig. 8. Typical Up-Feed Gravity Two-Pipe Air-Vent System return mains and connections are required for each heating unit; Lir valves are installed on the heating units and mains; hand valves are installed on the returns. Up-Feed Gravity Two-Pipe System This system (Fig. 8) has a steam and a return connection to each radiator. The radiator valves for steam, return, and air are the same as those described for the gravity one-pipe air-vent system. The steam main is run and pitched in the same manner as in the one-pipe system, but the returns from each radiator are connected'into a-separate return line system which has its risers carried down and joined to a Wet-return line under the boiler water line level. Where the return has to be kept high to function as a dry return, it is advisable-to connect the. return risers to the dry return main through water seals about 36-in.-deep,-:as shown in Fig. 9, to .prevent steam from one riser entering another.-and .closing the air valves on the nearest radiators. 423