134 SECTIONAL ADDRESSES. 
which appeals strongly to the central-station engineer is its wide range 
of economical working both above and below its normal load ; this specially 
fits it for the peaks and variations of demand with which power stations 
have to cope. 
It is in the great power stations equipped with large turbines and 
coal-fired boilers, using steam of high pressure and high superheat, that 
we find, beyond any question, the most economical production of power. 
The very bigness of the units tends towards efficiency, but that is not all. 
The turbine, as a thermodynamic machine, has permitted a far closer 
approach to the ideal cycle of Carnot than was possible in the reciprocating 
steam-engine, which, as Lord Armstrong said, skimmed the cream and 
threw away the milk. In the turbine the steam expands right down to 
the lowest vacuum that the condensing water will produce, doing effective 
work all the way, and thereby saving a most valuable and previously 
wasted portion of the whole heat-drop. Moreover, with the turbine there 
is a complete escape from the alternate heating and cooling of metallic 
surfaces which was a source of much loss in engines of the older type. 
In still another notable respect the turbine cycle approaches the cycle of 
Carnot: it allows a method of regenerative feed-heating to be adopted 
in which some steam is ‘ bled’ at successive stages of the expansion to 
restore heat to the condensed water on its way back to the boiler. Finally, 
the steam turbine has immensely widened the range of the thermodynamic 
cycle by raising the upper limit of temperature through the use of higher 
pressure and higher superheat. Pressures of 600 or 700 1b. per square 
inch are now commonplace; 1,200 lb. is becoming familiar; 2,000 lb., 
or even more, is not unknown. Superheating is often carried to 750° 
or 800° Fahr.—sometimes to 850°, and in rare cases to 900°—and is 
limited only by the ability of the metallurgist to supply metal that will 
not ‘creep’ too seriously under the combined influence of high pressure 
and high temperature. The more superheating the better, in this respect, 
that it tends to reduce the wetness of the steam in the late stages of the 
expansion and so avoid not only a loss of useful energy but also a tendency 
on the part of the turbine blades near the exhaust end to be pitted as a 
result of their impact against water particles. Another cure for wetness 
is to reheat the steam at one or more stages in its expansion ; with very 
high initial pressure this becomes necessary, but opinions differ somewhat 
as to the conditions that make it worth while to carry out so troublesome 
an operation. We have no time to discuss moot points or to dwell on 
details, but enough has perhaps been said to show why the steam turbine 
in fact achieves an efficiency far greater than was known to the steam 
engineers of Bramwell’s day. A modern turbine can generate one 
electrical unit with a consumption of barely 1 lb. of cheap coal, which 
means that it converts into electrical energy fully 30 per cent. of the 
potential energy of the fuel. It is not surprising that the internal-com- 
bustion engine finds little favour in power stations, save as an occasional 
stand-by to assist in meeting the peak load. 
1 Among other merits of steam turbine plant is its comparatively low capital 
cost. Published figures show that in recently-equipped British power stations the 
cost, including land, buildings, boilers, turbines, and all electrical machinery is from 
£14 to £16 per kilowatt of plant installed. For Diesel plant the corresponding capital 
cost is stated to be from two to four times as great. 
