Document NeVjnDmvrOLDkOmrKQN8wNNyR

HEATINC 'VENTILATINC AIR CONDITIONING CUIDE 1942 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 system's operate at a few ounces pressure and above, but those with mechanical condensate return devices may operate at pressures upward of 10 lb. The simplest CHAPTER 14. STEAM HEATINC SYSTEMS terns of this design 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, 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 thebther 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. Air vent and check Fig. 8. Typical Up-Feed System with Automatic Return Trap3 Proper piping connections are essential with special appliances for pressure equalizing and air elimination. 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 of 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 bo.th 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 the 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.- Sys- 272 Fig. 9. Typical Connections for 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 branches are taken off the main from the bottom or at a 45-deg angle downward, with the runouts sloped toward the drops. Thus each branch from the main forms a drip and no. accumulation of water is carried down any one drop. 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, and1 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 273