at in the large and economical
plants using turbines or steam engines, in which there is a large ratio
of expansion, to from 10 to 25 per cent for 100 degrees of superheat for
the less economical steam motors.

Though a properly designed superheater will tend to raise rather than to
decrease the boiler efficiency, it does not follow that all superheaters
are efficient, for if the gases in passing over the superheater do not
follow the path they would ordinarily take in passing over the boiler
heating surface, a loss may result. This is noticeably true where part of
the gases are passed over the superheater and are allowed to pass over
only a part or in some cases none of the boiler heating surface.

With moderate degrees of superheat, from 100 to 200 degrees, where the
piping is properly installed, there will be no greater operating
difficulties than with saturated steam. Engine and turbine builders
guarantee satisfactory operation with superheated steam. With high
degrees of superheat, say, over 250 degrees, apparatus of a special
nature must be used and it is questionable whether the additional care
and liability to operating difficulties will offset any fuel saving
accomplished. It is well established, however, that the operating
difficulties, with the degrees of superheat to which this article is
limited, have been entirely overcome.

The use of cast-iron fittings with superheated steam has been widely
discussed. It is an undoubted fact that while in some instances
superheated steam has caused deterioration of such fittings, in others
cast-iron fittings have been used with 150 degrees of superheat without
the least difficulty. The quality of the cast iron used in such fittings
has doubtless a large bearing on the life of such fittings for this
service. The difficulties that have been encountered are an increase in
the size of the fittings and eventually a deterioration great enough to
lead to serious breakage, the development of cracks, and when flanges
are drawn up too tightly, the breaking of a flange from the body of the
fitting. The latter difficulty is undoubtedly due, in certain instances,
to the form of flange in which the strain of the connecting bolts tended
to distort the metal.

The Babcock & Wilcox Co. have used steel castings in superheated steam
work over a long period and experience has shown that this metal is
suitable for the service. There seems to be a general tendency toward
the use of steel