TWO-PIPE STEAM HEATING SYSTEMS
In two-pipe systems, separate pipes are provide for the steam and the condensation, hence they may be of smaller size than in one pipe systems where a single pipe must take care of both steam and condensation. Various piping arrangements are used in two-pipe systems (e.g. circuit, divided circuit, loop, etc.) as will best meet the requirements of the building. Steam is supplied to the heat emitting units through risers and the condensation re turned through downflow or drip pipes.
TWO-PIPE DIVIDED CIRCUIT SYSTEM
A two-pipe divided circuit system is shown in Fig. 15. In this type of two-pipe system, steam passes from the boiler at the high point to the mains along which risers bring the steam to the heat emitting units. From the opposite side of each unit is connected a drop or drip pipe. The illustration shows a wet return on the left side and a dry return on the right side.
![Fig.-15.-Two-pipe-under-or-upfeed-re[2]](http://machineryequipmentonline.com/hvac-machinery/wp-content/uploads/2020/04/Fig.-15.-Two-pipe-under-or-upfeed-re2.jpg "Fig.-15.-Two-pipe-under-or-upfeed-re[2]")
In the wet return, the condensation is returned to ·the boiler by means of individual drips provided at each connection. There is a different arrangement for the dry return. The drip pipe from each heat emi,tting unit terminates in a loop or siphon which is tapped to the dry return. In operation, the condensation gradually fills the siphons and flows over into the dry return passing into the boiler drip pipe.
A ·two-pipe system should also be provided with a check valve so placed as to allow water to pass into the boiler but prevent undue outflow. Under certain conditions, this prevents the water in the boiler from being driven out into the return system by the boiler pressure. The disadvantage of a check valve is that it some times gets stuck; a problem which can interfere with the operation of the system. An equalizing pipe with a Hartford connection loop may be used in place of a check valve to avoid this situation (see HARTFORD RETURN CONNECTION on page 253 in this chapter).
VAPOR STEAM HEATING SYSTEMS
A vapor steam heating system is one which commonly uses steam at approximately atmospheric pressure or slightly more, and which operates under a vacuum condition without the aid of a vacuum pump (see VACUUM STEAM HEATING SYSTEMS). (Figs. 16, 17 and 18).
![Fig.-16.-A-vapor-type-steam-heating-[1]](http://machineryequipmentonline.com/hvac-machinery/wp-content/uploads/2020/04/Fig.-16.-A-vapor-type-steam-heating-1.jpg "Fig.-16.-A-vapor-type-steam-heating-[1]")
![Fig.-17.-Piping-of-a-vapor-system-to[1]](http://machineryequipmentonline.com/hvac-machinery/wp-content/uploads/2020/04/Fig.-17.-Piping-of-a-vapor-system-to1.jpg "Fig.-17.-Piping-of-a-vapor-system-to[1]")
![Fig.-18.-Piping-of-a-vopor-system-to[1]](http://machineryequipmentonline.com/hvac-machinery/wp-content/uploads/2020/04/Fig.-18.-Piping-of-a-vopor-system-to1.jpg "Fig.-18.-Piping-of-a-vopor-system-to[1]")
The steam pressure at the boiler necessary to operate a vapor system is generally very low (often less than one pound), being no more than is required to overcome the frictional resistance of the piping system. Under most operating conditions, the pressure at the vent will be zero or atmospheric.
Vapor steam heating systems may consist of various combina tions of closed or open, upfeed or downfeed, and one-pipe or two-pipe arrangements, depending on the requirements of the ins·tallation. Any of these combinations will have certain advan tages and disadvantages.