Steam Piping

STEAM PIPING Size of Main for Circuit System. Since the main of a circuit system, as described in Part I (Heating and Ventilation), must carry both steam and water of condensation, it should be made considerably larger in proportion to the surface supplied than mains which are drip ped at intervals or which carry only the condensation from the main itself.

Sizes, ample for circuit mains of ordinary length, are indicated in the accompanying table: Dry Return System. In many cases it is desirable to run the supply and return mains overhead. Such systems contain less water than wet return systems, and are therefore more susceptible to changes in the fire, because of the smaller quantity of water in the apparatus. The return mains must be made larger than when they are placed below the water line, since they are filled with steam, except the space occupied by the return water running along the bottom. The pipes should have a greater pitch than wet returns.

With dry returns, if certain supply risers are of inadequate size, steam is apt to back up into the radiator through the dry returns and to cause a holding-back of the water in the radiators. To prevent this, check valves are sometimes introduced in the branch returns. If the piping is properly proportioned, however, this is unnecessary. Siphon drips are frequently used, as explained in Part I (Heating and Ventilation).

Wet Return Systems.

This system, illustrated in Fig. 20, provides for water sealing all returns and drips, and avoids the backing-up action men tioned above. Suppose, f o r example, the pressure in one of the vertical returns is pound less than in the others; then, since a column of water 2.3 feet high corresponds to 1 pound pressure, the water will back up this particular return about 1.15 feet higher than in the others and thus equalize the difference in pressure. Where the mains must be long, the wet return system affords the opportunity to rise and drip the supply main as often as necessary; whereas, with the dry return system, the main and return have a gradual pitch from start to finish. This often

brings the return so low as to interfere with head room.

With the wet return system the return may be dropped below the floor line at door ways without interfering with the circulation. The sizes of wet returns may be made considerably smaller than dry returns for a given radiating surface, as shown in Table III.

Overhead Feed System. The overhead feed system (see Fig. 21) is most commonly used in connection with exhaust steam plants, since in such systems the exhaust pipe from the engines must be car ried to the roof, and the steam supply to the building may conven iently be taken from a tee near the upper end of this pipe. The main should be pitched down, and outlets taken from the bottom, to drain the condensation through the risers (see Fig. 22). With this system the water of condensation always flows in the same direction as the steam; hence the horizon tal pipes and the risers may be made somewhat smaller than in up-feed systems.

This system has the ad vantage of placing the big pipes in the attic, where their heating effect is less objectionable than in the basement. As the pipes gradually decrease in size from top to bottom, this gives small pipes on the lower floors, which in modern buildings generally contain a few large rooms and little space for concealing pipes. It is fre quently advisable to combine with this system the up-feedmethod of heating the first floor, which is generally high-studded and requires a large amount of radiation. Relieving the down feed system of this load means smaller risers throughout the building, which, in the modern sky-scraper, results in a saving that more than offsets the cost of the separate up-feed system for the lower floor. Another reason why it is advisable to put the lower floor on a separate system, is that the steam is dry, whereas the steam from an overhead system becomes pretty wet from condensation by the time it reaches the lower floor.