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Heating Ventilating Air Conditioning Guide 1939 Down-Feed Two-Pipe Vapor System In the down-feed two-pipe vapor system the steam is carried to the ton of the building, the top of the vertical riser constituting the high point of the system, and the horizontal supply main is sloped down from this location to the far ends of each branch. The branches are taken off the main from the bottom or at a 45-deg angle downward, with the runouts sloped toward the drops (Fig. 6). Thus each branch from the main forms a drip and no accumulation of water is carried down any one drop. Another method of running the steam main, which is not considered as satisfactory but which is practical, is to take the branches off the top of the main (Fig. 7) and to drip the end of the main through the last riser, as illustrated in the down-feed one-pipe system detail shown in Fig. 6. If this is done, the pipe drop at the end or ends of the mains should be Bottom of *| Graduated valve ' steam drop"* Drip trap )rip i | Dirt pocket- f----VFIoor '//////////A Connected to dry return (where connected to wet return, drip trap may be omitted) Fig. 12. Detail of Drip Connections at Bottom of Down-Feed Steam Drop enlarged one pipe size to provide capacity for this concentration of the main drip. The steam drops are carried down through the building with suitable reductions as the various radiator connections are taken off until the lowest radiator runout is reached. If the drop is only two or three stories high, the portion feeding the bottom radiator should be increased one pipe size to provide for draining the riser, and if the drop is over three stories high it is well to increase the portion feeding the two lowest radi ators one or. two pipe sizes, especially if the two lowest radiators are small and the normal size of drop required is 1 in. or less. The bottom of the steam drops should terminate with a dirt pocket as shown in Fig. 12. The returns on a down-feed vapor system are the same as on an up-feed system except that every steam drop must have a drip at the bottom connected either into the return through a trap dr into a separate watersealed drip line below the boiler water line, as illustrated in Fig. 10, in which case the thermostatic traps may be omitted. The runouts to the radiators and the radiator connections of the down-feed system are the same as those for the up-feed system already described. 290 Chapter 15. Steam Heating Systems ATMOSPHERIC SYSTEM S~The distinguishing features of the atmospheric system are gravity t n to the boiler or to waste, graduated or ordinary radiator valves, no tUmatic air valves on the radiators, thermostatic traps on the radiator aUtms and the venting of all air from the system by means of pipes open ^ the atmosphere. The returns are open to the atmosphere at all times, t0uallv by extending the return risers to the top of the building where thev are either connected together in groups and carried through the roof " ^tended through the roof individually. Atmospheric systems, either p-feed or down-feed, are often used where the condensation is not , Fig. 13. Typical Atmospheric System with Automatic Return Trap3 Proper piping connections are essential with special appliances for pressure equalizing and air elimination. returned to the boiler, as in heating systems supplied by high pressure steam through pressure-reducing valves at locations far from the boilers. The returns may be delivered back to the boiler, if desired, by condensa tion return pumps which are vented to the atmosphere. The return lines in such systems are simply gravity waste lines in which the condensation flows entirely by gravity and is not aided by any pressure difference. Atmospheric systems contemplate maintaining a practically constant pressure in the steam pipe and atmospheric pressure in the return pipe. When graduated steam valves are provided,: they enable the occupant of a room to vary the flow area to the radiator, so as to obtain a greater or lesser heating effect. The steam side may be run as that for either up-feed or down-feed two-pipe vapor systems, as the conditions require, and the radiator con- 291