Closed Vapor Systems
A vapor steam with a return line closed to the atmosphere is some- times referred to as a closed vapor system. The condensate returns by gravity flow to a receiving device (an alternating receiver or
boiler-return trap), where it is discharged into the boiler. The condensate from the alternating receiver is discharged against the boiler pressure.
Because air cannot enter a closed vapor system, a moderate vacuum is created by the condensing steam. As a result, steam is produced at lower temperatures, and the system will continue to provide heat after the boiler fire has died down.
Figure 8-26 shows the arrangement of a typical upfeed, two-pipe vapor system with an automatic return trap. The heat-emitting units discharge their condensate through thermostatic traps to the dry return pipe. These systems operate at a few ounces of pressure and above, but those with mechanical condensation return devices may operate at pressures upward of 10 psi. The simplest method of venting the system consists of a 3⁄4-in pipe with a check valve open- ing outward. Most systems employ various forms of vent valves, which allow air to pass and prevent its return. A dry return is pro- vided so that the air will easily go out the vent pipe.