The Locomotive - Lighthouse Survival Blog

HO THE LOCOMOTIVE. [July,
on the continent, also true to their characteristics, seem to have been plodding along in
the direction which they knew to be meritorious, gradually working out the problems,
and slowly overcoming the obstacles, until we now find them in a state very much in
advance of ourselves. In this matter great credit is due to that able Alsatian engineer,
G. A. Hirn, and to his successor, Emil Schwoerer, and also to the German engineer, W.
Schmidt, for keeping the subject alive. We find them with records of several thousand
successful installations of large and small plants, and European engineers educated up
to a point where no economical steam plant is seriously contemplated without including
the superheater. Most of the records of these tests show a saving in steam and fuel
ranging from 6 to over 20 per cent.
What is the reason for this increased efficiency of steam when superheated ? Ran-
kine discusses the question in his work on the steam engine.
1. We raise the temperature at which the steam receives its heat, and so increase
its efficiency without producing a dangerous pressure. According to the law of effi-
ciency of thermo-dynamic engines, the heat transformable into mechanical energy bears
the same ratio to the whole heat received by the fluid that the range of temperatures
bears to the absolute temperature at which the heat is received, as follows:
T,-T9 Tx +461
The more heat supplied per unit of volume of steam to the engine, the more work can
be obtained from the engine; and the increase of pressure having a practical limit, this
extra heat is to be obtained by superheating the steam.
2. The diminished density of the steam employed to do the work lessens the back
pressure, or, as it is commonly expressed, improves the vacuum.
3. The prevention of condensation during expansion without the use of steam
jackets, and in a much more effective manner. This is the most important advantage
to be attached to the use of superheated steam. Steam jackets are wasteful and ineffi-
cient when applying heat to the engine [?]. Inefficient, because the contact is only
with the steam immediately adjacent to the walls of the cylinder, with probably little
effect being derived by the steam in contact with the piston and in the interior of the
cylinder out of reach of the radiation from the walls; wasteful, because of the full effect
of heating the exhaust steam after it has done its work in the cylinder during the wan-
ing stages of expansion and throughout the exhausting stroke. This is particularly true
of engines which have a pause at the end of the stroke, which pause always finds the
cylinder full of steam which has completed its work in the cylinder and is ready to be
exhausted. Ample evidence of the truth of this statement is found in the high ratio of
water condensed in the jackets of direct acting pumping engines, to the amount of
steam used in the cylinders.
The use of superheated steam is bound to have an important effect on the question
of jacketing cylinders. If the superheating is sufficient to carry the steam through the
engine to the point of final exhaust without reaching the saturation limit, there is
evidently no use for steam jackets. This, however, necessitates the careful covering of
the cylinder walls and steam passages, to prevent as much radiation as possible; and the
logical development of this question would seem to be to abandon the low pressure
jacket first, and follow by giving up the intermediate, and finally the high pressure
jackets, and substitute for these ample intermediate reheaters, between the cylinders, so
proportioned that each cylinder would exhaust its steam at a point just sufficiently
above the temperature due to the pressure, to insure the absence of any condensation.