Document OzQeBbrpajvmqrv8Qv5pjnxOe

446 CHAPTER 20 1950 ,Guide or vapor 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 can 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 operate under either high pressure, low pressure, vapor, or vacuum conditions; Either the up-flow or the down-flow arrangement of mains may be employed. . Fio.,7. Typical'Steam Runout ' where Risers are Dripped' ' Steam Heating Systems and Piping, 447 Two-Pipe High Pressure Systems Two-pipe high pressure systems operate at pressures above 15 psig, usually from 30 to 150 psig. Large industrial type buildings, which are equipped with unit heaters or large built-up fan units, usually use high . pressure steam systems. 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. FROM HIGH PRESSURE BOILER GATE VALVE^_ PIPE COIL _/___ _ yFLOAT TRAP CHECK VALVE ' GATE VALVE UNIT I HEATE GATE VALVE ECK VALVE CHECK VALVE FLOAT TRAP FLOAT TRAfy^ _7 FLOAT TRAP ^OATE VALVE AIR VENT ri/^-r--------\ \ W!ECONOMIZER 'o . L ------All I VENTl__cr-! FINNEO RAOIATOR ---TO. BOILER \ FLOAT TRAP CHECK VALVE RECEIVER Fio. 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 psig. 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 do not contain the check discs and hence, the system cannot operate under vacuum conditions. The low pressure systems 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 disadvantage of corroding to a greater extent than vapor systems, due to the continued 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 possible in vapor systeins.