Document yrqp7QN1a1q3oR6L2DNqb4qD3

American Society of Heating and Ventilating Engineers Guidi ^1935 return in the opposite direction. Each radiator has an individ valve. aua` air Up-Feed Gravity One-Pipe Air-Vent System This system is the most common of all methods of steam heating, due largely to its low cost of installation and its simplicity. As will be'seen from Fig. 1, the steam piping rises to a point as high as possible at the boiler and pitches downward from this location until the far end of the main or mains is reached. At the far ends drips are taken off at the low points of the steam mains, are watercsealed below-the boiler water line, and then brbiightfback to the boiler in a wet return. Single pipe risers Chapter 31--Steam Heating Systems head for any distance before dropping, the return should slope down 'll with the flow, phe radiator valves may be of the angle-globe or gate type. They hid not be of the straight-globe type because the damming effect of the ^*d valve seat interferes with the flow of condensation through the ^lye Graduated valves cannot be used, as the steam valves on this tein must be fully open or closed to prevent the radiators' filling with SJ,Ster Air valves may be manual or automatic, with or without a check prevent the re-entrance of expelled air. Usually the automatic type is -stalled. -The.greatest source of difficulty, withpne-pipe steam .systems m that the heat is all on or ail off, with no intermediate position possible. However, intelligent use of the on-and-off method of manual control gives reasonably satisfactory results. It is important that the lowest points of the steam mains and heating l. sufficiently above the water line of the boiler to prevent Fig. 1. Typical Up-Feed Gravity One-Pipe Air-Vent System are branched off the main or mains to feed the radiators, the steam passing up the riser and the condensation' flowing'down it. The steam and con densation flow in opposite directions in the riser but after the condensa tion enters the steam main it flows in the same direction as the steam and is disposed of through the drip connection at the end of the main. In buildings of several stories, it is customary to drip the heel of each riser separately, whereas in one- or two-st'ory buildings this is not necessary. Both types of branches and risers are shown in Fig. 1. ' ' Horizontal branches to radiators and risers should be pitched at least Y<l in. in 10 ft downward toward the riser or vertical pipe, and the hori zontal branches from the steam main should be graded at least this amount toward the main, excelpt where the heel of the riser is dripped, in which' case the branch should pitch down toward'the riser drip (Figs. 2 and 3). The. return line, if wet, may be run without pitch or may be pitched in either direction, but if it is necessary to carry the return main i 504 Fir 2 Typical Steam Runout where Risers are not Dripped Fig. 3. Typical Steam Runout where Risers are Dripped flooding, although proper design will eliminate this danger. Usually 18 in. is sufficient but construction limitations frequently make shorter dis tances 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 pressure on the top of the 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,-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 (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 H 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 12j 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 lb, and with a check in the return, would require )4 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. *See discussion of piping details in'Chapter 32. 505