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. Heating Ventilating Air Conditioning Guide 1938 use of the on-and-off method of manual control gives reasonably satis factory results. Improved systems and devices are now available which make it possible to obtain a modulating effect from one-pipe gravity heating systems. It is important that the lowest points of the steam mains and heating units be kept sufficiently above the water line of the boiler to prevent Boiler steam pressure Steam pressure at end of main T-mt ^-Water line of boiler -- Level /(Vet return E'Risewater Bne difference Fig. 4. Difference in Steam Pressure on Water in Boiler and at End of Steam Main flooding. . Usually 18 in. is sufficient but construction limitations fre quently make shorter distances necessary. The distance may be checked in the following manner : Referring to Fig. 4 it will be seen that the water in the wet return is really in an in verted siphon, or U-shaped'container, with the boiler steam pressureon the top of'the water at one emiand 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 mction of the steam in passing from the briiler to the far end of the main and<the pressure reduction in consequence of the condensation occurring in the system. 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 278 Chapter IS. Steam Heating Systems (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 Ye lb, and utilizes an Underwriters' Loop1 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 3Y in. Adding 3 in. to this for the flow through the return main and 6 in. as a factor of safety gives 12in. 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 Y lb, and with a check in the return, would require Y of 28 in., or 14 in., for the difference in steam pressure, 3 in. for 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 Fig. 6. Steam Runouts Dripping Main Fig. 7. Steam Runouts with Main Dripped at End Only 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 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. *See discussion of piping details in Chapter 16: 279