| Injector feeding through a | | | | heater in which the water is | | | | heated from 150 to 200 | | | | degrees | .938 | 6.2 | | | | | | Direct-acting Pump feeding | | | | water through a heater in | | | | which it is heated from 60 | | | | to 200 degrees | .879 | 12.1 | | | | | | Geared Pump run from the | | | | engine, feeding water | | | | through a heater in which it | | | | is heated from 60 to 200 | | | | degrees | .868 | 13.2 | |______________________________|_____________________|____________________| The injector, considered only in the light of a combined heater and pump, is claimed to have a thermal efficiency of 100 per cent, since all of the heat in the steam used is returned to the boiler with the water. This claim leads to an erroneous idea. If a pump is used in feeding the water to a boiler and the heat in the exhaust from the pump is imparted to the feed water, the pump has as high a thermal efficiency as the injector. The pump has the further advantage that it uses so much less steam for the forcing of a given quantity of water into the boiler that it makes possible a greater saving through the use of the exhaust from other auxiliaries for heating the feed, which exhaust, if an injector were used, would be wasted, as has been pointed out. In locomotive practice, injectors are used because there is no exhaust steam available for heating the feed, this being utilized in producing a forced draft, and because of space requirements. In power plant work, however, pumps are universally used for regular operation, though injectors are sometimes installed as an auxiliary method of feeding. Table 18 shows the relative value of injectors, direct-acting steam pumps and pumps driven from the engine, the data having been obtained from actual experiment. It will be noted that when feeding cold water direct to the boilers, the injector has a slightly greater economy but when feeding through a heater, the pump is by far the more economical.