Document kDdBq015wJKavp1wKnYXm34jV

HEATING VENTILATING AIR CONDITIONING GUIDE 1941 There are three general types of mechanical return devices in common use, namely, (1) the mechanical return trap, (2) the condensation return pumpjand (3) the vacuum return line pump. CHAPTER 13. 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-PIPE 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. Fig. 2. Typical Steam Runout where Risers are Not Dripped Fig. 3. Typical Steam Runout where 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 before connecting together. In the risers and radiator connections, steam and 250 Fig. 4. Typical Down-Feed Gravity One-Pipe Air-Vent System 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 water. 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 must 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 aW on or all off. Systems and devices are available which make it possible to obtain a partial modulating effect from one-pipe heating systems. 251