Document 15894kOR1Vq5861KRnKMo4e4j

American Society of Heating and Ventilating Engineers Guide, 1936 water at one end and the steam main pressure on the top of the water at the other end. The difference between these two pressures is the pressure drop in the system, i.e., the friction of the steam in passing from the boiler to the far end of the main. The water in the far end will rise sufficiently to overcome this difference in order to balance the pres sures, and it will rise enough farther to produce a flow through the return into the boiler Steam pressure at Fig. 4. Difference in Steam Pressure on Water in Boiler and at End of Steam Main (usually about 3 in. unless the pipes are small or full of sediment), and it will rise still farther if a check valve is installed in-the return so as to obtain sufficient head to lift the tongue of the check (usually 4 in. will be necessary). If a one-pipe steam system is designed, for example, for a total pressure drop of lb, and utilizes an Underwriters Loop* instead of a check valve on the return, the rise in the water level at the far end of the return due to the difference in steam pressure would be % of 28 in., or 3H in- Adding 3 in. to this for the flow through the return main and 6 in. as a factor of safety gives in. as the distance.the bottom of the lowest part of the steam main and all heating units must be above the boiler water line. The same system, however, installed and sized for a total pressure drop of H lb, and with a check in the return, would require Vi of 28 in., or 14 in., for the difference in steam pressure, 3 in. for *See discussion of.piping details in Chapter 32. 536 Chapter 31--Steam Heating Systems the flow through the return, 4 in. to operate the check, and 6 in. for a factor of safety, making a total of 27 in. as the required distance. Higher pressure drops would increase the distance accordingly. 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 this 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 Runouts Steam drop to radiators Fig. 6. Steam Runouts Dripping Main Steam drop to radiators' Fig. 7. Steam Runouts with Main Dripped at End Only 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 return mains and connections are required for each heating unit; air 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 537