The Locomotive - Lighthouse Survival Blog

1901.] THE LOCOMOTIVE. m
The use of perfectly dry steam, and the elimination of condensed water, would
appear to be a very practical advantage of superheating. Water is a disturbing element
in steam engines and pipes at all times. It increases the friction of the wearing sur-
faces, interferes with the lubrication, and chokes up the discharge; it produces
unequal strains in the metal, due to different temperatures, and often gives leaks at
joints which remain perfectly tight under dry steam. Dripping at stuffing boxes is also
avoided in this way.
It has always been difficult to determine the actual amount of moisture which is
contained in steam ; some light is thrown on the subject by the use of superheated
steam. For instance, at a working pressure of 125 pounds, 1 per cent, moisture would
represent about 20 degrees of superheat. We find under the best conditions of properly
proportioned steam pipes, and well covered, a loss of £ to J of a degree Fahr. of super-
heat per foot of steam pipe. It would not be out of reason to assume that with the
ordinary large and indifferently covered steam pipe the loss would reach even as much
as 1 degree Fahr. for every foot of pipe; consequently a pipe 100 feet long might lose
100 degrees of superheat, corresponding to 5 per cent, of moisture in the steam, pro-
vided the steam was dry on leaving the boiler. Even assuming £ degree loss per foot,
we should have a condensation of 2£ per cent, in a steam pipe of this length.
As the friction for superheated steam is much less than for saturated steam in
passing, there- is not so much loss in ports and passages, and the size of the pipes may
be considerably reduced. The tendency in this country has been to use steam pipes
which are much too large. Good practice with superheated steam recommends that
the velocity should not be less than 100 feet per second, in passing from the boiler to
the engine. Of course, this means ample steam receivers in proximity to the engine,
and a very good plan would be to have these receivers fitted with reheating tubes containing
steam which is being circulated through the superheater, or else a portion of
the hot furnace gases, diverted from their course for this purpose, and returning again
to the boiler flue by means of an induced draft.
As to the present effect of superheated steam on lubricating oils and stuffing-box
packings, it may be said that the present practice provides, and, in fact already
demands, oil and packing which will easily withstand these conditions. Minerals oils
have supplanted the old vegetable oils and animal fats which were formerly used with
machinery, and metallic steam packings have been universally adopted. Both of these
were necessitated by the temperatures due to the steam pressures in the vicinity of 150
pounds to the square inch, the use of which is now common; hence we already have
the way thoroughly paved for the introduction of superheated steam.
We have mentioned European practice in this direction. There we find no prominent
central station designed without providing for superheated steam; engine builders
make guarantees contingent upon it giving an advantage averaging about 12 per cent,
in pounds of steam per horse power, based upon the steam being superheated. The
range of temperature has resolved itself into recommending the use of steam at between
500 and 600 degrees Fahr. Good practice abroad may be said to lie in the vicinity of
570 degrees. Experimenters have observed that steam expands more rapidly during
the first 20 degrees of superheating than when further raised in temperature ; consequently
the first effects are proportionately more beneficial.
The method of constructing the superheater naturally presents the most important
aspect of the situation, after deciding that the results are beneficial and desirable.
Dismissing the subject of superheating by means of wire drawing, as being inefficient
and impracticable, the methods resolve themselves into a system of tubes or pipes,
through which the steam is passed after leaving the boiler on its way to the engine;
these pipes being subjected to hot furnace gases, either in connection with the steam
boiler itself, or in a separate setting having a fire of its own. In large plants it is usual
and even more desirable to set the superheater by itself; and it is placed alongside of
the boiler as a continuation of the battery, with fronts and setting similar to the
boilers, and fired in the regular manner as the boilers. — Ernest H. Foster. (Quoted
in Steam Engineering.)