Document bBo5MboYOwRkrvV06yEm07qXD

HEATINC VENTILATING AIR CONDITIONING GUIDE 1943 There are three general types of mechanical return devices in common use, namely, (1) the mechanical return trap, (2) the condensation return pump, and (3) the vacuum return line pump. CHAPTER 14. STEAM HEATING SYSTEMS condensation flow in- opposite directions. ' In long steam mains it flows in the-same direction as the steam and is removed from the main- through the drip. Short mains may be arranged for the condensate to flow in "a direction opposite the steam by sizing them so the critical velocity is not exceeded. It is customary to drip the heel of each riser in buildings of several stories to avoid counter-flow of the steam and condensate in the riser branch. In buildings of one or two stories the condensate is returned to the steam main instead of being dripped.- Both types of risers are shown, in Fig. 1, and riser connections are shown in Figs. 2 and 3. A typical overhead down-feed system is illustrated in Fig. 4. While wet return mains need not be pitched toward the boiler to maintain steam circulation, they should be pitched for drainage. GRAVITY ONE-RIPE AIR-VENT SYSTEM ` This system is the most common of all methods of steam heating, especially for small size installations, due largely , to its low cost and simplicity. . Up to radiator or riser: . Pitch, in. per ft Steam main ------- 3 ft approximately - Fig. 2. Typical Steam Runout where Risers are Not Dripped Fig. 3. Typical Steam Runout where i Risers are. Dripped - The downward pitch of a one-pipe air-vent system is indicated in Fig. 1. Low points and ends of steam mains pitched down from the boiler should be dripped. 'All drips should be sealed below water line befpre connecting together. In the risers and radiator connections, steam,and 270 To improve steam circulation in one-pipe systems quick vent air valves should be provided at the ends and at intermediate points where-the steam.main is brought to a higher elevation. It is desirable to install the air-vent valves about a-foot ahead of the drips, as indicated in Fig. 1, to prevent possible damage to their mechanisms by wafer. The radiator valves may be the angle-globe, offset-corner pattern or gate type.- Straight-globe and straight-corner type should not be used since the damming effect of the raised valve seat would interfere with the flow of. condensation through the valve. Graduated valves cannot be used since the.steam valves on this system fnust be fully open or fully closed to prevent the radiators filling with water and creating a dangerous water line condition. With a one-pipe system the heat cannot be modu lated at the radiator, the steam being either all on or all off. Systems and devices are available which make it possible to obtain a partial modulating effect from one-pipe heating systems. \