Cyclopedia of heating, plumbing and sanitation; a complete reference work . t mains of ordinary length, are indicatedin the accompanying table: TABLE II Sizes of Circuit Mains of Circuit M mn Djkect Radiattnc Surface 2 in dies 200 sq. ft. 2i :;.-»() :; ooo :;i 900 4 1,200 4£ 1,700 5 2,100 6 3,000 Dry Return System. In many cases it is desirable to run thesupply and return mains overhead. Such systems contain less waterthan wet return systems, and are therefore more susceptible to changesin the fire, because of the smaller quantity of water in the return mains must be m
Cyclopedia of heating, plumbing and sanitation; a complete reference work . t mains of ordinary length, are indicatedin the accompanying table: TABLE II Sizes of Circuit Mains of Circuit M mn Djkect Radiattnc Surface 2 in dies 200 sq. ft. 2i :;.-»() :; ooo :;i 900 4 1,200 4£ 1,700 5 2,100 6 3,000 Dry Return System. In many cases it is desirable to run thesupply and return mains overhead. Such systems contain less waterthan wet return systems, and are therefore more susceptible to changesin the fire, because of the smaller quantity of water in the return mains must be made larger than when they are placedbelow the water line, since they are filled with steam, except the spaceoccupied by the return water running along the bottom. The pipesshould 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 tocause a holding-back of the water in the radiators. To prevent this. 240 STEAM VXD HOT WATER FITTE\TG 17 If theSiphon. check valves are sometimes introduced in the branch returns. piping is properly proportioned, however, this is unnecessary. 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, for example, the pressure in one of the vertical returns is § pound less than in the others; then, since a column of water feet high corresponds to 1 pound pressure, the water will back up this particular return about 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 havea gradual pitch from startto finish. This oftenbrings the
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