ng the steam causes
condensation of a portion of the steam, so that even were the steam dry
on entering the turbine, it would contain water on leaving the turbine.
By superheating the steam the water that exists in the low pressure
stages of the turbine may be reduced to an amount that will not cause
trouble.

Again, if saturated steam contains moisture, the effect of this moisture
on the economy of a steam turbine is to reduce the economy to a greater
extent than the proportion by weight of water, one per cent of water
causing approximately a falling off of 2 per cent in the economy.

The water rate of a large economical steam turbine with superheated
steam is reduced about one per cent, for every 12 degrees of superheat
up to 200 degrees Fahrenheit of superheat. To superheat one pound of
steam 12 degrees requires about 7 B. t. u. and if 1050 B. t. u. are
required at the boiler to evaporate one pound of the saturated steam
from the temperature of the feed water, the heat required for the
superheated steam would be 1057 degrees. One per cent of saving,
therefore, in the water consumption would correspond to a net saving of
about one-third of one per cent in the coal consumption. On this basis
100 degrees of superheat with an economical steam turbine would result
in somewhat over 3 per cent of saving in the coal for equal boiler
efficiencies. As a boiler with a properly designed superheater placed
within the setting is more economical for a given capacity than a boiler
without a superheater, the minimum gain in the coal consumption would
be, say, 4 or 5 per cent as compared to a plant with the same boilers
without superheaters.

The above estimates are on the basis of a thoroughly dry saturated steam
or steam just at the point of being superheated or containing a few
degrees of superheat. If the saturated steam is moist, the saving due to
superheat is more and ordinarily the gain in economy due to superheated
steam, for equal boiler efficiencies, as compared with commercially dry
steam is, say, 5 per cent for each 100 degrees of superheat. Aside from
this gain, as already stated, superheated steam prevents erosion of the
turbine buckets that would be caused by water in the steam, and for the
reasons enumerated it is standard practice to use superheated steam for
turbine work. The less economical the steam motor, the more the gain due
to superheated steam, and where there are a number of auxiliaries that
are run with s