Document QJ0NpGVnyogpk0ZzXm21aabjo

American Society of Heating and Ventilating Engineers Guide, 1937 ' risers to the dry return main through water seals about 36 in. deep, ^ shown in Fig. 9, to prevent steam from one riser entering another and closing the air valves on the nearest radiators. Down-Feed Gravity Two-Pipe System The steam main in the down-feed system is carried to the top of the building, and the piping of the steam side is arranged practically asin the down-feed one-pipe gravity system. The drips at the bottoms of the steam drops and the runouts to the radiators are similar to those shown in Fig. 8 for the up-feed gravity two-pipe system. On the return side of the system, the piping is arranged in exactly the same manner as the up-feed gravity two-pipe system. - Chapter 31--Steam Heating Systems TWO-PIPE VAPOR SYSTEM Two-pipe vapor systems may be classified as (1) closed systems con sisting of those which have a device to prevent the return of air after it is nee expelled from the system, and which can operate at sub-atmospheric Pressures for a period of four to eight hours depending upon the tightness of the system and rate of firing, and (2) open systems consisting of those which have the return line constantly open to the atmosphere without a check or other device to prevent the return of air, and which operate at a few ounces above atmospheric pressure. The open systems have the disadvantage of not holding heat when the rate of steam generation is diminishing. Under the first classification the essentials are packless graduated valves on the radiators, thermostatic return traps on the returns, and traps on all drips unless they are water sealed. Such a system, illustrated in Fig. 10, should be equipped with an automatic return trap to prevent the water from backing out of the boiler. In this up-feed arrangement the supply piping is carried to a high point directly at the boiler and is Return from r*di*tors-* PRO) -Pyn Fig. 8. Typical Up-Feed Gravity Two-Pipe Air-Vent System ONE-PIPE VAPOR SYSTEM A vapor system is one which operates under pressures at or near atmospheric and which returns the condensation to the boiler by gravity. The piping arrangement of a one-pipe vapor system is similar to that of the gravity one-pipe steam system; in fact, one-pipe gravity installations may readily be changed to one-pipe vapor systems by making a few simple alterations. The steam radiator valve is a plug cock which when opened gives a free and unobstructed passageway for water. The auto matic air valve is of special design to permit the ready release of air from the radiator and to prevent the return of the air after it is expelled. The air valves on the main are a quick relief type, and the whole system is designed to operate on a few ounces of pressure. _L y-Floor Fig. 9. Method of Connecting Two-Pipe Gravity Returns to Dry Return Main graded down toward the end or ends of the' supply main, each supply main being dripped at the end into the wet return or carried back to a point near the boiler where it drops down below the boiler water line and becomes a wet return. From this main,'runouts are branched off to feed risers or radiators above, these being graded back toward the steam mam if they are not dripped at the bottom of the riser, or toward the riser if the riser heel is dripped. Both conditions are illustrated in Figs. 2 and 3. Return risers are connected to each radiator on its return end through thermostatic traps. Their bottoms are connected to the return main through runouts which slope toward the main. The return main itself is sloped back toward the boiler if it is carried overhead; if run wet, the slope may be neglected, although it is desirable to slope the pipe so that the system may be drained. An air vent is installed at the point at which the return main drops-below the water line.. In the simplest cases this vent consists of a,,^-in. pipe with a check valve opening outward, but certain systems employ special patented forms of vent valves, designed to allow the air readily to pass out of the system and to prevent its return. 551