The one special object aimed at with the two-pipe system of steam - heating apparatus is to provide separate pipes for the steam and the condensed water, and so to arrange these pipes that the steam and its water shall naturally seek and keep to their respective channels. There is much difference of opinion as to the advantages - or otherwise - of the one-pipe and two-pipe work ; but in America, which might be called the home of steam heating, the one-pipe system is favoured, though both are used. The conditions of the job often influence the choice.

Fig. 66 illustrates an apparatus which embodies all the usual details customarily appearing in these works. The steam main, it will be seen, like that of the one-pipe system, is made to reach its highest point as quickly as possible on leaving the boiler, and from here it falls in the direction of its work. This, as already explained, is to cause the steam and condensed water to travel in the same direction, and not come in conflict. It should be stated, however, that with this system the steam main is practically dry compared with that of the one-pipe system.

Branches (steam) to radiators are usually carried with a rise, and therefore any water appearing in them comes back in a contrary direction to the steam ; but the quantity is trifling, as the water produced in the radiators does not return by this route. The radiators are tilted up slightly at the steam-supply end, if necessary, to cause the water to flow towards the return connection. With branches extending up to radiators on two or more floors - and called " risers" - the foot of the vertical pipe is usually dripped, as shown on the right side of the illustration, this drip taking any water that may come down the riser and so keeping the main free. In this case the branch from the main is given a fall as shown, and in some instances it is taken with a tee looking downwards. On the left side of the illustration the first radiator is supposed to come over the main, and as straight vertical branches are not allowed in this work the steam supply is taken to the distant end of the radiator and the return from the near end. This obviates all risk of injury by the strains due to expansion and contraction of the main. On the right side of the illustration will be seen a distinctive feature of this system - which is its fault. This is the great number of return pipes that may be necessary. To carry out this work as it should be done, every radiator must have a distinct and separate return pipe down to a point below water-line. It may there join any water-sealed pipe, but may not join above water-line.

The Two Pipe System of Steam 98

Fig. 66.

The main return is carried wholly below water-line, and is connected to the steam main by a small drip or relief pipe at the extremity of the run. This pipe should have an air vent in it above water line. This return is known as a "wet" or "submerged " return, owing to its being below water-line and always containing water. Its use is to receive the water of the return branches, but particularly to water-seal these pipes and prevent steam getting from one return to another. It needs no rise or fall, but can lie on the floor or be carried below floor line.

In the front of the illustration there appears a "relay drip or relief." As explained previously, a steam main may be run so far that its gradual fall brings it too low to, say, pass over a doorway. It may then be jumped-up, which rise is known as a relay ; but the low point must, of course, be dripped. The illustration shows this, the drip joining the wet return.

Sizes of Main Pipes for the Two-pipe System of Low-Pressure Steam-Heating Apparatus.

Size of Steam Supply.

Size of Return.

Will serve this Area of Radiation

Inches.

Inches.

Square Feet.

1

3/4

140

1 1/4

1

200

1 1/2

1 1/4

300

2

1 1/2

500

2 1/2

2

800

3

2 1/2

I20O

3 1/2

2 1/2

1800

4

3

2500

Sizes Of Radiator Branches And Connections

Size of Steam Supply.

Size of Return.

Will serve this Area of Radiation.

Inch.

Inch.

Square Feet.

1

3/4

48

1 1/4

1

96

1 1/2

1 1/4

150

If the returns bore the same porportion to the steam-supply pipes that the condensed water bears to the steam they would be exceedingly small. In practice nothing like such a proportion will do, and the customary practice is to use one less size of pipe for the returns. Some engineers use two sizes less, and say that the results are satisfactory, but it is not the general practice.