Document Ra2p2ppxvOj0BXYv0yvnZwd2n

424 CHAPTER 23 1946 Guide system the automatic air valves are of special design to permit the ready release of air and prevent its ready return after it is expelled. The steam radiator valves are a type which, when opened, give a free and unob structed passageway for water. The piping is the same as for the one-pipe gravity system but sized so as to permit operation at a few ounces pressure. - TWO-PIPE VAPOR SYSTEM A two-pipe up-feed vapor, system using separate supply and return pipes is shown in Fig. 8. The radiators discharge their condensation through thermostatic traps to the dry return pipe. These systems operate at a few ounces pressure and above, but those with mechanical condensate return devices may operate at pressures upward of 10 psi. The simplest Non Return Steam Heating Systems and Piping 425 . which is usually located with the bottom about 18 in. above the boiler water line. Some traps are constructed so that they will operate when they are installed with their bottom as close as 8 in. above the boiler.water line. On the other side of this connection a second check valve is installed in the main return just before it enters the boiler. Fig. 9 shows a typical connection for an automatic return trap. Down-Feed Two-Pipe Vapor System In the down-feed two-pipe vapor system the steam is carried to the top 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 runouts are taken off the main from the bottom or at a 45-deg angle downward, and sloped toward the drops. Thus each runout from the main forms a drip and no accu mulation of water is carried down any one drop. Fig. 9. Typical Connections for Automatic Return Trap method of venting the system consists of a %-in. pipe with a check valve opening outward. Most systems employ various forms of vent valves, designed to allow the air to readily pass out df the system and to prevent . its return. These systems permit control of the heat in the radiator by varying the opening of the graduated radiator valves. The' boiler pressure is maintained at substantially constant pressure, slightly above atmos pheric pressure. These systems may be classified as (1) closed systems, consisting of those which have a device to prevent the return of air after it has once been expelled from the system, and which can operate at both super and subatmospheric pressures for a period of four to eight hours depending upon the tightness of the system and rate of firing, and (2) open systems, com prising those which have the return line constantly open to thp atmos phere without a check or other means to prevent the return of air. The open systems are not so popular because they have the disadvantage of not holding heat when the rate of steam generation is diminishing. Closed systems should preferably be equipped with an automatic return trap to prevent water from backing out of the. boiler. In installing the return trap a check valve is inserted in the return main at a point near the boiler and a vertical pipe is run up into the bottom of the return trap, Fig. 10. Detail of Drip Connections at Bottom of Down-Feed Steam Drop The steamdrops 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 each steam drop should terminate with a dirt pocket and be dripped as shown in Fig. 10. The returns on a down-feed vapor system are the same as on an up-feed system. The runouts to the radiators and the radiator con nections of die down-feed system are the same as those for the up-feed system already described. . While many manufacturers of patented vapor heating accessories have their own schedules for pipe sizing, an inspection of these sizing tables indicates that in general as small a drop as possible is recommended. The reasons for. this are: (1) to have the condensation return to the boiler by gravity, (2) to obtain a more uniform distribution of steam throughout the system, especially when it is desirable to carry a moderate or low