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472 CHAPTER 20 1952 Guide Fig. 6. Typical Up-Feed Gravity One-Pipe Air-Vent System dr vapdr at sub-atmospheric pressures, and at consequent lower tempera tures. Systems which use vacuum valves are known as vapor or vacuum one-pipe systems. The vapor or vacuum systems will maintain a more uniform temperature condition than the pressure systems. Each heating unit in a one-pipe system may also be provided with a valve on the connection to the unit, although this is not essential except to' shut the unit off in case it is not desired for heating. Valves on onepipe systems must be either fully opened or fully closed. No throttling or modulating- position caii be maintained since, if a valve is partially closed, condensate will not drain from the unit. This condition is dan gerous because it may create a low water condition in the boiler with con sequent burning or cracking of the boiler, or create a hazard due to the freezing of the water-logged heating unit itself. TWO-PIPE SYSTEMS Two-pipe systems, as previously defined, are systems in which steam and condensate flow in separate pipes. Two-pipe systems may operate under high pressure; low pressure, vapor, or vacuum conditions. Either the lip-flow or the down-flow arrangement of mains may be employed. Fig. 7. Typical Steam Runout where Risers are Dripped Steam Heating Systems 473 Two-Pipe High Pressure Systems -r-' Two-pipe high pressure systems operate at pressures above, i5 psig, usually from 30 to: 150 psig. They are usually used in large industrial type buildings, which are equipped with unit heaters or large built-up fan units, or in which high pressure steam is required for process work. Fig. 8 illustrates a typical high pressure system. Because of the high pressures and the great differential between steam and; return mains, it is possible to locate returns above the-heating units and lift the condensate to these returns. The condensate can be flashed into steam in low pressure mains if any are available, or passed through an economizer heater before being dis charged to a vented receiver. It is, of course, necessary to provide for the' elimination of air from high pressure systems, the same as in low pressure systems. . GATE VALVES PIPE COIL ; / . yFLOAT TRAP HECK VALVE --GATE VALVE FLOAT TRAP/y FLOAT TRAP ^ATE VALVE // AIR VENT _/ _____ \\.r* ------ ECONOMIZER \ j0` 1 tRT FINNED RADIATOR FLOATTRAP --TO BOILER \ CHECK VALVE RECEIVER Fig. 8. Typical High Pressure Heating System Return traps used on high pressure systems are usually of the bucket, inverted bucket, float or impulse type*. Two-Pipe Low Pressure Systems Low pressure systems operate at pressures of 0 to 15 prig. The. piping arrangement of both up-feed and down-feed low- pressure systems is iden tical with those of two-pipe vapor systems described in. the following sec tion. 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 system cannot operate under a vacuum. The low pressure systems are not as popular as the vapor sys tems, because they have, the disadvantage' of not holding heat when the rate of steam generation is diminishing. . They also have the disadvantage of corroding to a greater extent than vapor systems,, due, to the continual presence of new air in the system. Low pressure systems have the advantage, however, of returning con densate to the boiler readily and not retaining it in the piping, as may be posrible in vapor systems when the system pressure exceeds the operating