Document Lg9ZD2bdK2140z1Z5NpbEGXy7

msgsmam*-',, - ---- -------- -- " Heating Ventilating Air Conditioning Guide 1938 return to the boiler or to waste, graduated or ordinary radiator valves, no automatic air valves on the radiators, thermostatic traps on the radiator returns, and the venting of all air from the system by means of pipes open to the atmosphere. The returns are open to the atmosphere at all times, usually by extending the return risers to the top of the building where they are either connected together in groups and carried through the roof or extended through the roof individually. Atmospheric systems, either up-ieed or down-feed, are often used where the condensation is not 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 Bottom of steam drop' Graduated valve Drip trap Dirt pocket- tt) ,/Floor V////Z////Z ^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 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 nections are the same as for vapor systems in that they .have graduated valves on the radiator supply ends and thermostatic traps on the radiator return ends. All drips from the supply main and the steam side of the system must pass through thermostatic drip traps before entering the return system where only atmospheric pressure exists. -Fig. 13 illustrates a typical scheme of piping used on atmospheric systems. Such systems do not maintain heat in the radiators under declining fires. As the steam supply diminishes, air from the atmosphere re-enters through the open vent pipe retarding the inflow, of steam and cooling the radiator. 284 Chapter IS. Steam Heating Systems VACUUM SYSTEM In the vacuum system, a vacuum is maintained in the return line practically at all times but no vacuum is carried on the steam side, and the usual accessories include graduated valves on the radiator supply and thermostatic traps on the radiator return. The air is expelled from the system by a vacuum pump and all drips must pass through thermostatic traps before connecting to the return side of the system. These systems are often fed from high pressure steam mains through pressure-reducing valves but they may be fed direct from a low-pressure steam heating boiler as shown in Fig. 14, in which a typical up-feed vacuum system is illustrated. The supply main slopes down in the direction of flow; the runouts pitch down toward the riser if the riser is dripped (Fig. 3) or up toward the riser if the riser is not dripped (Fig. 2); both conditions are indicated in Fig. 14. The matter of dripping the risers depends largely on the height of the riser and the judgment of the designer. Ordinarily risers less than three stories high are not dripped and those more than four stories high are dripped, but there is no set rule for this. When risers are dripped the runouts from the steam main may be taken from the bottom if desired and each runout then serves as a .drip for the main. The risers are carried up to the highest radiator connection and are connected to the radiator through runouts sloping back toward the riser. The radiators usually have graduated valves on the supply end, although this is. not absolutely necessary. Angle-globe valves and gate valves may / be used where graduated manual control is not desirable. The return valves must be of the thermostatic type which will pass air and water but which will close against the passage of steam. The return risers are connected in the basement into a common return line,, which slopes downward toward the vacuum pump. The vacuum pump discharges the air from the system and pumps the water back to the boiler, or other receiver, which may be a feed-water tank or a hot well. It is essential on these systems that no connection from the supply side to the return side be made at any point except through a trap. While the best practice demands a return flowing to the vacuum pump in an uninterrupted downward slope, in some cases limitations make it necessary to drop the return below the level of the vacuum pump inlet before the pump can be reached. -In such event one of the advantages of the vacuum system is that the return can be raised by the suction of the vacuum pump to a considerable height, depending on the amount of vacuum maintained, by means of a lift fitting inserted in the return. Best practice dictates that the lift should be limited to a single lift con nection at the entrance to the vacuum pump and preferably that an accumulator tank or receiver with float control be used at the low point of the return main at the entrance to the vacuum pump. When the .'lift is considerable, several lift fittings should be used in steps (Fig. 15), more successful operation being obtained by this method than when the lift is made in one step. If the lift occurs close to the vacuum pump, a special arrangement is used as shown in Fig. 16. It is desirable that means be provided for draining manually the low point of the lift fittings to eliminate from the return piping all water in danger of freezing in case 285