Document k9mYa9yDb22BdpBDxGyGrxKqO

464 CHAPTER;23r ' - :jn:1949. Guide Steam Heating Systems and Piplng-.Q':`-i f. 465 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 systcms. .. Fig. 8. illustrates a typical.high. pressure system. Because.of the high pressures and the great differential between steam andireturh mains, it is posable to locate returns above, the heating units and lift the.condensate to these-returns. > .. ....... ' ibi?--"- 1: 1 The condensate can be flashed into steam in low pressure mains if any are available or passed through an economizer heater before being discharged 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. '. -.............. - -..........r.'J - - or vapor at sub-atmospheric pressures, and at consequent lower tempera tures. Systems which use vacuum1 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 partiallyclosed 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, orof creating a hazard due to the freezingOf.the water logged beating ;unit itself.. . TWO-PIPE SYSTEMS Two-pipe systems, as previously defined, are systems in which steam and condensate flow in separate'pipes. . Twq-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'. Oripto wet return Fio. 7. Typical Steam Runout ' . where Risers are Dripped ' 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 .havethe disadvantage of. corroding to a greater extent than vapor systems due' to the continued presence of new air in the system; . - 'L ' 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 systems. . i: