Power Engineering Lectures - BG2 Steam Heating Systems动力工程英文教材教程教案讲义Steam Heating Systems HVAC 6022 1 Learning Outcome When you complete this module you will be able to: Describe the operating principles and maintenance procedures of steam heating systems and the components of these systems. Learning Objectives Here is what you will beable to do when you complete each objective: 1. Describe standard types of piping andequipmentlayoutforsteamheatingsystems.2.Describethegeneraloperationand maintenance of steam heating systems. 3. Apply a steam heating system trouble-shootingguide.INTRODUCTION The piping and heating units of a steam heating system must be so arrangedthat:1.Steam can flowfreelyand insufficientquantitiesto allheatingunits. 2.The condensate can be removed freely from these units and returned to the boiler.3. The aircan be expelledfrom the heatingunitsand piping. Inallsystems the horizontalsteam supplypipes between boilerand heatingunitsare calledthe mains; the verticalpipes to the various floors, the risers. When the supply mains are located below the heating units they serveand the steam is supplied upwards through the risers to the heating units, the system iscalled an upfeed system. In a downfeed system, the mains are located above the heating units. The pipes carrying the condensate back to the condensate tank or boiler are called the return risers and return mains. When the return main of a gravity return system is located belowthe level of the water line in the boiler and, therefore always filled with water, it iscalled a wet return. On the other hand, a return main located above the water line, whichis usually only partly filled with condensate and will drain dry when the flow of condensate ceases, is called a dry return. Many types of steam heating systems have been developed,each having advantages and disadvantages over the others. These systems are classifiedaccordingto pipingarrangement,operatingpressure,and method of returningcondensate to the boiler. In regard to classification by piping arrangement, the heating system may beeither the one pipe or the two-pipe design. In terms of operating pressures, the system may be high pressure, low pressure, vapor, or vacuum. In regard to methods of returning condensate to the boiler, the system may use a gravity return, a return trap, a condensate return pump, or a vacuum pump. Some of these designs, such as the one pipe system, areobsolete and hence are no longer used in modern installations. However,they are described in order to help the student understand the principles involved in steam for heating buildings. HVAC 6022 2ONE PIPE, GRAVITY RETURN SYSTEM The one-pipe system, also called the air vent system,was one of the first steam heating systemsto be developed. A singlemain is used tocarry both the steam supply from the boiler to the radiators and the steam condensate from theradiators back to the boiler. Each radiator has only a single connection and steam is fedfrom the main to the radiatorthrough thisconnection. Asthe steam in the radiatorcondenses, it drains by gravity back to the steam main through the single connection. The end of thesteam supply main is tied into the condensate return main. All piping is sloped downward so that the condensate can flow freely by gravity back to the boiler without interferingwith the flow of the steam. TWO-PIPE, GRAVITYRETURNSYSTEM Toovercome the difficulties可编辑资料 - - - 欢迎下载encountered when steam and condensate flow in opposite directions in the same pipe, the two-pipe steam heating system was developed. In this system, which operates at pressures between 0 - 103 kPa, gauge separate steam and condensate return mains are used. Eachradiator has two connections. The steam flows from the steam main through the radiator valve to the radiator. As the steam condenses, it flows out through another valve into the return main and back to the boiler. Air vents in the upper part of the radiator remove the air.This system, now obsolete, had the disadvantage that during start-up some steam could pass through a radiator without condensing and could enter other radiators via the return main, thus trapping the air in the center.TWO-PIPE, RETURN TRAP SYSTEM In a gravity returnsystem, the boilershould be placed wellbelow the lowest radiatoror convectorin the heating system in order to obtain sufficient static head for the returning condensate to overcomethe boiler pressure. In buildings with low basement ceilings, it was sometimes necessaryto set the boilerin a pitbelow the floorin orderto obtainthe requiredstatichead. This difficulty was overcome by the use of a return trap, which returns the condensate to theboiler. HVAC 6022 3 BG_2_0_2.jpg GFig. 1 shows a diagram of a return trap system while Fig. 2 shows a basic diagram of the return