BOILER, a process of power gen eration which uses mercury instead of water in a boiler. The boiling point of mercury as compared with water is given below. It will be seen that with mercury saturated vapour can be obtained with low pressures and with much higher temperatures than is possible with water steam.
The highest pressure at which saturated water vapour can exist is 3,200 lb. pressure per square inch, and at that pressure the tem perature is only 706° F. In the mercury cycle, mercury is boiled in a specially designed boiler at whatever pressure is desirable. In the Hartford, Conn., installation this pressure has been chosen at 7o lb. gauge. The vapour is then carried through a turbine where it does useful work. The exhaust pressure is kept at what ever vacuum is desirable to obtain the proper steam pressure. In Hartford the steam is generated at 35o lb. pressure, which means a mercury vacuum of about 28 inches.
In the mercury cycle all the latent heat of the mercury vapour is turned into steam at any steam pressure desirable, and this steam can be used either for power as in Hartford or for heating purposes. The mercury cycle can thus be considered as a steam producer, where a large amount of power can be obtained from the mercury vapour at practically theoretical rate, and the amount of steam produced is only slightly less than that produced by a steam boiler for the same amount of fuel.
Basis of Design.—The following list gives data as to designed conditions for the i o,000 kw. unit and its substantial accuracy is fully established by experience with apparatus now operating: Output of mercury turbine, i o,000 kw. ; speed of mercury tur
bine, 720 r.p.m.; steam produced per hour, 125,000 lb.; steam pressure, 35o lb. gauge; steam temperature, 700° F; mercury vapourized per hour, 1,150,000 lb. ; temperature mercury vapour, 884° F; pressure vapour at mercury turbine, 70 lb. gauge; vacuum in mercury condenser, 28 in.; temperature mercury exhaust, 458° F; gas temperature beyond mercury liquid heater, 95o° F; gas temperature beyond steam superheater, 65o° F; gas temperature beyond water economizer, 48o° F; gas temperature to stack, 28o° F; air temperature entering furnace, 39o° F; coal burned per hour, 14,500 lb. ; heat in coal per pound 14,500 B.T.U. It will thus be seen that when 54,500 lb. of coal (with a heat value of 14,500 B.T.U. per pound) is burned under the mercury boiler signed for the following results: Full-load output of mercury turbine, io,000 kw.; coal burned per hour, 14,500 lb.; steam pro duced per hour, 35o lb. gauge, 700° F, 125,000 pounds. The fuel is burned in the furnace, as in a steam boiler, and the products of combustion pass successively through a mercury boiler, a mer cury liquid heater, a steam superheater, a feed water heating economizer and an air preheater. The first two of these give heat to the mercury. The others give it to the steam or return it to the furnace.
The mercury vapour made in the boiler passes first through a turbine driving at io,000 kw. generator at 72o r.p.m. and then exhausts into a pair of surface condensers. The tubes of these condensers contain water, so that the heat delivered to them by the condensation of the exhaust mercury vapour causes them to make steam which collects in drums above the tubes at which point it is available for use. In the equipment being described, this steam passes from these drums through the superheater men tioned. The condensed mercury is drained through a cleaning sump and then runs by gravity through the liquid heater and into per hour, io,000 kw. are generated by the mercury boiler and 125,000 lb. of steam are generated at 35o lb. gauge pressure and with a temperature of 700° F (264° F superheat).