Document 1evrLyx17XBVMv0k07MkQ35a

538 CHAPTER 24 - " --' '1950Guide used' in summer as' a means of circulating air to give Some' measure; of comfortdue to air motion. In such cases the heating element; should'be shut off from" the source of heat. The thermostat which prevents the fan from-starting'until the heating element is heated, should be provided with a by-pass switch, which, upon being closed, will permit the fanto be operated independently of the heating element. -, PIPING CONNECTIONS FOR STEAM UNIT HEATERS - Piping connections foristeami unit heaters are similar to those for other types of fan blast heaters. The piping of unit heaters must conform strictly Unit Heaters and Unit Ventilators ''.- -. TM 539 is shown.in Fig.fi. - -The traps must pass air and-condensate rapidly to keep the return piping partially full of water. .j.' -*** Since unit heaters are often constructed with sufficient strength, the use of high pressure steam in them is a common practice: As shown in Fig. 7, the condensate and air reach the return overhead through ,traps, and check valves are located in the return piping. It is, however, preferable' to locate the'high pressure return'below the heater. ` I''. -1 - For twp-pipe closed gravity return systems, the. return from each iinit should be fitted with a heavy duty or blast trap, and an automatic air valve should be connected into the return header of each unit heater,,. Provision . ! Fiq. 6. Unit Heater Connection for Vacuum or Vapor System Discharging Condensation into Dry Return l to'the system/requirements while at the same time permitting the heaters theinselves to function as intended. The basic piping principles for steam systemsare discussedinChapter20. must be made to compensate for the pressure drop by elevating the unit heater above the water line of the boiler or of the receiver. S --A In pump and receiver' systems; the air may be eliminated by individual air valves on the heaters, or it may be carried into the returns as in vacuum - Rapid 'condensation of stearn, especially during heating-up periods,'' is systems, and the entire retuM system be free-vented to'the atmosphere; pro :i characteristic-of this type . of equipment. The-returnpiping; must' be planned' to keep the heating coil free of rapid condensation, while the Steam vided all units, drip-points, and'radiation-are properly-trapped to' prevent steam.entering the returns.-' i;. ` piping must be ample td'earry a full supply of steam to-the unit to take the place, of that condensed. Adequate sizes of piping are especially,important where a Unit heater fan is operated under start-and-stop control,: and where all or part of the air is taken from the outside: In such installations the /condensation rate may vary rapidly, and the necessity for ample pipe capac ity is particularly important. . ,/ ; \. "Qn vacuum or open vent systems, the return from each unit-should'be fitted withalarge capacity trap todischMgetfi^ water of condenSatiohy'aiid with a'thenbbstatic air yalyefor eliminating the air^br .with a heavy-dut^ trap for handling both the condensation and the air, provided^he air finalljr can be eliminate at some other point in' the return system; A --: For Viigb pressure systems the same kind of traps may be used as with / A method of connecting a;unit heater to a one-pipe gravity system is .iflustrated/in-Fig.'5. IWhenthe return main is located- above-the.-boiler, j vacuum systems, except that they, must be constructed for the.pressure used. If the air is to be eliminated at'the return header of the uhitjA high . . water!-line,;an -artificial water line must be created- by. providing an equaliz-: S pressure air valve can be used; otherwise the air may be passed'with the ing loop-to.prevent steam passing into.the return, and thus.into other;units.; ! ' condensate, through the high-pressure return trap; arid';then "eliminated'at Vl'A'^ipmg Arrangement where both the air and condensate pass througha | some other point in the system. ' ; ;;V' common return to a boiler, with vent trap or condensate pump and receiver, , Fig; 8 represents the connections to a hot water heating system: The an1