Cyclopedia of heating, plumbing and sanitation; a complete reference work . s are made forthis purpose, in which the bulb is im-mersed in a bath of oil or mercury (seeFig. 108). FORCED HOT=WATER CIRCU=LATION While the gravity system of hot-water heating is well adapted tobuildings of small and medium size,there is a limit to which it can be car-ried economically. This is due to theslow movement of the water, whichcalls for pipes of excessive size. Toovercome this difficulty, pumps areused to force the water through themains at a comparatively high velocity. The water may be heated in aboiler i
Cyclopedia of heating, plumbing and sanitation; a complete reference work . s are made forthis purpose, in which the bulb is im-mersed in a bath of oil or mercury (seeFig. 108). FORCED HOT=WATER CIRCU=LATION While the gravity system of hot-water heating is well adapted tobuildings of small and medium size,there is a limit to which it can be car-ried economically. This is due to theslow movement of the water, whichcalls for pipes of excessive size. Toovercome this difficulty, pumps areused to force the water through themains at a comparatively high velocity. The water may be heated in aboiler in the same manner as forgravity circulation, or exhaust steammay be utilized in a feed-water heaterof large size. Sometimes part of theheat is derived from an economizer placed in the smoke passagefrom the boilers. Systems of Piping. The mains forforced circulation are usuallyrun in one of two ways. In the two-pipe system, shown in Fig. L09,the supply and return are carried side by side, the former reducingin size, and the latter increasing as the brandies are taken Fig. 108. Thermometer Attached toPeed-Pipe near Beater, to Deter-mine Temperature of Water. 12? 118 HEATING AND VENTILATION The flow through the risers is produced by the difference inpressure in the supply and return mains; and as this is greatestnearest the pump, it is necessary to place throttle-valves in the risersto prevent short-circuitinjg and to secure an even distribution throughall parts of the system. Fig. 110 shows the single-pipe or circuit system. This is similarto the one already described for gravity circulation, except that it canbe used on a much larger scale. A single main is carried entirely around the building in thiscase, the ends being connected with the suction and discharge of thepump as shown. As the pressure or head in the main drops constantly throughoutthe circuit, from the discharge of the pump back to the suction, it is 1 1 i J tl 1 i- \ -1 1*1 ^ 1 01 A^_ i 1 P-iaw-i-p Fig. 109.
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