two valves is equivalent to 2,240 feet of straight pipe, and the -addition of another elbow would be equivalent to 350 feet of straight pipe and would reduce the delivery in the ratio of V2,240 -f- 2,590.
Draining pipes. In laying out the piping system for a heating plant, besides the proper size of pipes there are two points which must 'be very carefully considered : (1) That pipes as well as radia- tors are properly drained so that the water of condensation will flow off easily and uniformly to its proper receptacle; and (2) that proper provision be made for the expansion of pipes, so that such expansion shall not interfere with the flow of steam or water or disturb the setting of the radiators.
Pipes for an ordinary one-pipe system, which are run around the basement of a building, should be pitched toward the extreme ends, from which the return connection should be taken and run back to the receiver below the water-line. If the mains are very long they should be drained at intervals into this pipe.
In a two-pipe system in which the mains are similarly run, they should be drained into the return pipes, and in making these drip connections care should be taken that the return pipes into which they drain are lower than the supply mains, so that there will be no opportunity for water to flow from the returns into the supply mains. The return pipes are generally, where possi- ble, run under the basement floor, and should be lower than the supplies. Figures 31 and 32 represent typical connections from mains to risers. Where, on account of economy of space, it is necessary to run the supply and return mains on nearly the same level, as in Figure 32, the supply main must be dripped into a separate pipe run back under the floor or along the wall and con- nected into the principal return main below the water-line of the system. In two-pipe systems where the supply mains are short and properly covered, so that there is not much condensa- tion, they may be given a slight rise from the boiler or source of supply and the water of condensation allowed to flow back. Riser connections should be taken out of the top of the mains so as to prevent any water of condensation that may be in the mains from getting into the risers.
In the one-pipe overhead system there is always a main supply riser running to the branch mains in the attic, and this should have a drain pipe from its lowest point extending back to the receiving
