Document dnK1MRg45XzX2kYDJowNbL9ve

370 CHAPTER 26 1960 Guide Fig. 12 .... Typical Down-Feed Two-Pipe System with unit heaters or large built-up fan units, or in which high-pressure steam is required for process work. Fig. 9 illustrates a typical high-pressure system. Because of the high pressures and the great differential between steam and return mains, it is posable to locate returns above the heating units and lift die condensate to these returns. The condensate can be flashed into steam in low-pressure mains if any are available, or passed through an econo mizer heater before being discharged to a .vented receiver. It is, of course, necessary to provide for the elimination of air from high-pressure systems, just as in low-pressure sys tems. Return traps used on high-pressure systems are usually of the bucket, inverted-bucket, float, or impulse type. They are described in a later section Steam Traps. Two-Pipe Low-Pressure Systems Low-pressure systems operate at pressures of 0 to 15 psig. The piping arrangement of both up-feed and downfeed low-pressure systems is identical with those of twopipe vapor systems described in the following section. The only difference between the two systems is in the type of air valve used. The air valves used in low-pressure systems usually do not contain the check discs and hence, the sys tem cannot operate under a vacuum. The low-presure sys tems are not as popular as the vapor systems, because they have the disadvantage of not holding heat when the rate of steam generation is diminishing. They also have the dis advantage of corroding to a greater extent than vapor sys tems, due to the continued presence of new air in the system. Low-pressure systems have the advantage, however, of re turning condensate to the boiler readily and not retaining it in the piping, as may be possible in vapor systems when the system pressure exceeds the static head provided for the gravity return at the boiler. Fig. 10 illustrates the con version of a conventional gravity return system to a typical low-pressure system with a condensation pump. A check valve and a gate valve must always be installed between the pump discharge and the boiler. A check valve on the re ceiver vent will prevent the backflow of air when the system is cooling down. Two-Pipe Vapor Systems Two-pipe vapor systems operate at pressures varying from 20-in. vacuum or more (depending upon the tightness with which the system is assembled) to 25 psig without the use of a vacuum pump. A typical two-pipe up-feed vapor sys tem is shown in Fig. 11, and a typical two-pipe down-feed system is illustrated in Fig. 12. The method of dripping drop Down-Feed Steam Drop fig. 15 .... Typical Two-Pipe Vacuum Pump System Steam Heating Systems 371 Two-Pipe Vacuum Systems Vacuum systems operate under conditions of both low .pressure and vacuum, but employ the vacuum pump to insure maintenance of subatmospheric pressures in the re turn piping for all operating conditions. The pump thus assists the supply steam in overcoming the resistance of the system piping and insures rapid circulation and even heating. 'Hie system may operate transiently with subatmospheric presure in. the supply piping during the time the rate of steam generation is equivalent to or less than the total connected load. A typical two-pipe vacuum system is illustrated in Fig. 15. fig. 16.... Accumulator Tank Installation In Typical Two-Pipe Vacuum Pump System risers in a down-feed system is illustrated in Fig. 13. Radia tors discharge their condensate and air through thermostatic traps to the dry-return main. Air is eliminated, when the system is under pressure, at the ends of the supply and re turn mains just before they drop to the wet return. The vent valves are of the float rather than float and thermo static type since there should be no steam in the return main as long as the thermostatic traps on the radiators or convectors are operating properly. The float element of the valve closes the valve when, due to pressure differences, water rises to the point of overflow in the main. The vent valves are also provided with a small check disc which closes to prevent the inflow of air to the system when the pressure drops below atmospheric pressure. This enables the system to operate under vacuum conditions at lower steam temperatures for a period of four to eight hours, de pending on the tightness of the system. Vapor systems may also be provided with an automatic re turn trap or alternating receiver which automatically re turns condensate to the boiler when the boiler is steaming under pressure conditions which would prevent the return of condensate by gravity. The typical connections for an automatic return trap are illustrated in Fig. 14. The two check valves are necessary for its successful operation. Each heating unit in a vapor system, as in all two-pipe systems, is provided with a graduated or modulating valve which permits the control of heat in the radiator by varying the opening of the valve. fig. 17 .... Method of Constructing a Lift in o Return Line Hie return risers are connected in the basement into a common return main which slopes downward toward the vacuum pump. The vacuum pump withdraws the air and water from the system, separates the air from the water and expels it to atmosphere, and pumps the water back to the boiler or other receiver, which may be a feed-water heater or hot well. It is essential that no connection be made from the supply side to the return side at any point except through a trap. Hie return system should provide unre stricted gravity flow of the condensate and air to the pump receiver so that the vacuum differential may be more uni formly applied throughout the return system. In some in stances, load conditions make it necessary to drop the return below the level of the vacuum pump inlet before this pump can be reached. This can be taken care of when using a vac uum pump by installing a float controlled auxiliary accumu lator tank, pitted if necessary, just ahead of the vacuum pump inlet as shown by Fig. 16. The auxiliary tank as shown still permits an uninterrupted gravity flow from ail return lines, but it does reduce the vacuum on the returns by 1-in. Hg for every foot of lift. The lift should be limited to 5 ft. Lifts should be avoided, but on individual mains near the end of the system a lift fitting may be employed. Fig. 17 shows a single-step lift, which may be used up to 5 ft of lift. When the lift is greater but does not exceed 8 ft, it should be divided into two lifts shown in Fig. 18. It is preferable