Document JGr4gw0peRGmamyRLrqZow46

84 CHAPTER 6 mechanical condensate return devices in common use: (a) the alternating return trap, (6) the condensate return pump, and (c) the vacuum return pump. In systems where pressure conditions in the system vary between that^of a gravity return and a forced return sys tem, a boiler return trap or alternating receiver is employed and the system may be known as an alternating return sys- When condensate is pumped to the boiler under pressures of :the' atmosphere or above,' the system is known as a con densate pump return system. When condensate is pumped to the boiler under vacuum conditions, the system is known as a vacuum return sys tem. In either the condensate or vacuum pump systems it is highly desirable to arrange for gravity flow to a receiver and to the pump. The pump then forces condensate into the boiler against its pressure. ONE-PIPE SYSTEMS One-pipe systems, as previously defined, are systems in which steam and condensate flow in the same pipe. Radia tors and other beating units, in general, have only one pip ing connection from main to unit, although it is possible to employ two connections to the same main as indicated in Fig. 2. Unit heaters in one-pipe systems may also have sepa rate connections to the wet-return. 1962 Guide And Data Boole There are several variations in the piping arrangement of a one-pipe system as follows: 1. Up-feed one-pipe systems where the radiators and other heating units are located above the supply mams. The in this instance convey both steam and condensate. Therefore it is preferable to limit this design to systems with a connected load under 2500 sq ft EDR. Such a system is illustrated in Fig. 3. Typical connection? to radiator cr risen are in Figs. 4 and 5 and method of *h.nging iriean of mains in Fig. 6. Branches from steam in onie-pipe gravity steam systems should use the preferred connection shown in -Fig. 7, but where .radiator condensate does not flow back into the main, the acceptable method shown in the same figure may be used. This acceptable method has the advantage of giving a perfect swing joint when connected to the vertical riser or radiator connection, whereas the preferred connection does not give this swing without distorting the angle of the pipe. Runouts are usually made about 5 ft long to provide flexibility for movement in the main. 2. Up-feed one-pipe system with a connected load m excess of tSQO sq ft EDR, where radiators and other heating units ore located above the mains, as illustrated tn Fig. 8. It will be noted that the mains are dripped at each riser connection to a wetreturn, as indicated in Fig. 5, so that the mains carry a mini mum of the condensation. This feature is always desirable in the design of any one-pipe steam system. Radiators immediately above the mains may be connected as shown in Fig. 4. tJpfeed systems are not recommended for systems higher Ehn^ four or five stories. 3. Down-feed one-pipe systems, where the radiators and other heating units are located below the supply main. In Rg. 4----- Typical Steam Runout where Risers Are Not Dripped Rg. 2....Typical Two-Pipe Connections to Unit Heaters in One-Pipe Air-Vent Systems STEAM MAIN RISER--. yS *H PER | - S FT APPROX.-- Rg. 5.... Typical Steam Runout where Risers are Dripped aBS .no , I KCENRK oesuewe Rg. 6.... Method of Changing Size of Steam Mam when Runouts are Taken from Top steam Hearing Systems 85 OTangement only risers and connections to heating units convey both'steam and condensate, and both are flowing in the same direction. The steam main is kept relatively free of condensate by dripping through the drop risers. Drain valves should.be installed on all wet-return mains and on the return header at the boiler. prh radiator or beating unit in a one-pipe system must be supplied with a thermostatic air valve which functions to relieve air from the heating unit under pressure, and to close when steam itself heats the thermostatic element of the valve. To improve steam circulation in one-pipe systems quick- vent air valves should be provided at the ends and at inter mediate points where the steam mnAn is brought to a higher elevation, or where dropped below the water line. It is de sirable to install the air-vent valves about a foot ahead of the drips, as indicated in Fig. S, to prevent possible damage to their mechanisms by water. Air valves are of two general types, the pressure and the vacuum types. The pressure type permits the inflow of atmospheric air to the system when the steam pressure in the system falls below atmospheric pressure. The vacuum type, which contains a small check- valve, prevents the air from flowing back to the system and thereby maintAina vacuum conditions in the system, and a consequent evapora tion or generation of steam or vapor at stibatmospheric pressures,' and at consequent 'lower temperatures. Systems which use vacuum valves are known as vapor or vacuum one-pipe systems. The vapor or. vacuum systems will main tain a more uniform temperature condition than the pres sure systems. Each heating unit in a one-pipe system may also be pro vided with a valve on the connection to the unit, although this is not essential except to shut the unit off when it is not desired for heating. Valves on one-pipe systems must be either fully opened or fully closed. No throttling or modu lating position can be maintained since, if a valve is par tially closed, condensate will not drain from the unit. This condition is dangerous because it may create a low water condition in the boiler with consequent burning or cracking of the boiler, or create a hazard due to the freezing of the waterlogged heating unit itself. TWO-PIPE SYSTEMS Two-pipe systems, as previously defined, are systems in which steam and condensate flow in separate pipes. Twopipe systems may operate under high-pressure, low-pressure, vapor, or vacuum conditions. Either the up-flow or the down-flow arrangement of mains may be employed. Two-Pipe High-Pressure Systems Two-pipe high-pressure systems operate at pressures above IS psig, usually from 30 to 150 psig. They are usu ally used in large industrial buildings, which are equipped Rg. 3.... TyplcalUp-Feed Gravity. One-Pipe Air-Vent System ACCEPTABLE PREFERRED -` Rg. 7.... Methods of Taking' Branch from Main Rg. U .... Typical Up-Feed Two-Pipe System with Automatic Return Trap*