. Power, heating and ventilation ... a treatise for designing and constructing engineers, architects and students. in back pressureis easily obtained. Computations for an engine having a clearance of 5 per cent,supplied with steam at 80 pounds pressure and cutting off at ^4stroke, show that raising the back pressure 3 pounds calls for acorresponding increase of only 5 pounds in the boiler pressure tomaintain the same power of the engine. The indicator card shows a back pressure of about 3 poundswhen an engine is exhausting into the atmosphere, so that an in-crease of 3 pounds would bring the p
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. Power, heating and ventilation ... a treatise for designing and constructing engineers, architects and students. in back pressureis easily obtained. Computations for an engine having a clearance of 5 per cent, supplied with steam at 80 pounds pressure and cutting off at ^4stroke, show that raising the back pressure 3 pounds calls for acorresponding increase of only 5 pounds in the boiler pressure tomaintain the same power of the engine. The indicator card shows a back pressure of about 3 poundswhen an engine is exhausting into the atmosphere, so that an in-crease of 3 pounds would bring the pressure up to a total of 5 172 HEATING AND VENTILATING PLANTS pounds, which should be more than ample to circulate the steam, through any well-designed heating system, even though it be ofcomparatively large size. A modern heating plant with ample mains and branches shouldcirculate freely on a pressure not exceeding 2 pounds, and goodresults are often obtained with 1 pound or even less. Improvements in the apparatus employed for vacuum heating-have caused this system, in different forms, to be quite generally. Fig. 122. Pratt Return Tiap. used in exhaust heating, especially in plants of large size. Imthis case there is no increase in the back-pressure on the engine:and in most instances it is somewhat reduced. Special Apparatus. Steam Traps are used for draining the water of condensation-from steam pipes, coils, heaters, etc., without allowing the steamito escape at the same time. There are a great many differentforms, most of them, however, coming under the heads of float, , bucket, and expansion traps. Fig. 120 shows the Curtis trap, a, simple form of the first type. When the water line reaches a.certain point, the float rises and opens the discharge valve A, which is placed near the bottom of the trap. As the water flows;out, the float falls, and the valve closes before being uncovered, , so that no steam is allowed to escape. EXHAUST STEAM HEATING 173 Bucket Traps.—A bucket trap