[Footnote: A paper read by title at a recent stated meeting of the
Franklin Institute]
By WILLIAM BARNET LE VAN.
The most economical application of steam power can be realized only by
a judicious arrangement of the plant: namely, the engines, boilers, and
their accessories for transmission.
This may appear a somewhat broad assertion; but it is nevertheless one
which is amply justified by facts open to the consideration of all those
who choose to seek for them.
While it is true that occasionally a factory, mill, or a water-works
may be found in which the whole arrangements have been planned by a
competent engineer, yet such is the exception and not the rule, and such
examples form but a very small percentage of the whole.
The fact is that but few users of steam power are aware of the numerous
items which compose the cost of economical steam power, while a yet
smaller number give sufficient consideration to the relations which
these items bear to each other, or the manner in which the economy of
any given boiler or engine is affected by the circumstances under which
it is run.
A large number of persons--and they are those who should know better,
too--take for granted that a boiler or engine which is good for one
situation is good for all; a greater error than such an assumption can
scarcely be imagined.
It is true that there are certain classes of engines and boilers which
may be relied upon to give moderately good results in almost any
situation--and the best results should _always_ be desired in
arrangement of a mill--there are a considerable number of details which
must be taken into consideration in making a choice of boilers and
engines.
Take the case of a mill in which it has been supposed that the motive
power could be best exerted by a single engine. The question now is
whether or not it would be best to divide the total power required among
a number of engines.
_First_.--A division of the motive power presents the following
advantages, namely, a saving of expense on lines of shafting of large
diameter.
_Second_.--Dispensing with the large driving belt or gearing, the first
named of which, in one instance under the writer's observation, absorbed
_sixty horse-power_ out of about 480, or about _seven per cent_.
_Third_.--The general convenience of subdividing the work to be done,
so that in case of a stoppage of one portion of the work by reason of
a loose coupling or the changing of a pulley, etc., that portion only
would need to be stopped.
This last is of itself a most important point, and demands careful
consideration.
For example, I was at a mill a short time ago when the governor belt
broke. The result was a stoppage of the whole mill. Had the motive power
of this mill been subdivided into a number of small engines only one
department would have been stopped. During the stoppage in this case
the windows of the mill were a sea of heads of men and women (the
operatives), and considerable excitement was caused by the violent
blowing off of steam from the safety-valves, due to the stoppage of the
steam supply to the engine; and this excitement continued until the
cause of the stoppage was understood. Had the power in this mill been
subdivided the stoppage of one of a number of engines would scarcely
have been noticed, and the blowing off of surplus steam would not have
occurred.
In building a mill the first item to be considered is the interest on
the first cost of the engine, boilers, etc. This item can be subdivided
with advantage into the amounts of interest on the respective costs of,
_First_. The engine or engines;
_Second_ The boiler or boilers;
_Third_. The engine and boiler house.
In the same connection the _form_ of engine to be used must be
considered. In some few cases--as, for instance, where engines have to
be placed in confined situations--the form is practically fixed by the
space available, it being perhaps possible only to erect a vertical or a
horizontal engine, as the case may be. These, however, are exceptional
instances, and in most cases--at all events where large powers are
required--the engineer may have a free choice in the matter. Under
these circumstances the best form, in the vast majority of cases where
machinery must be driven, is undoubtedly the horizontal engine, and the
worst the beam engine. When properly constructed, the horizontal engine
is more durable than the beam engine, while, its first cost being less,
it can be driven at a higher speed, and it involves a much smaller
outlay for engine house and foundations than the latter. In many
respects the horizontal engine is undoubtedly closely approached in
advantages by the best forms of vertical engines; but on the whole we
consider that where machinery is to be driven the balance of advantages
is decidedly in favor of the former class, and particularly so in the
case of large powers.
The next point to be decided is, whether a condensing or non-condensing
engine should be employed. In settling this question not only the
respective first costs of the two classes of engines must be taken into
consideration, but also the cost of water and fuel. Excepting, perhaps,
in cases of very small powers, and in those instances where the exhaust
steam from a non-condensing engine can be turned to good account for
heating or drying purpose, it may safely be asserted that in all
instances where a sufficient supply of condensing water is available
at a moderate cost, the extra economy of a well-constructed condensing
engine will fully warrant the additional outlay involved in its
purchase. In these days of high steam pressures, a well constructed
non-condensing engine can, no doubt, be made to approximate closely to
the economy of a condensing engine, but in such a case the extra cost of
the stronger boiler required will go far to balance the additional cost
of the condensing engine.
Having decided on the form, the next question is, what "class" of engine
shall it be; and by the term class I mean the relative excellence of the
engine as a power-producing machine. An automatic engine costs more than
a plain slide-valve engine, but it will depend upon the cost of fuel at
the location where the engine is to be placed, and the number of hours
per day it is kept running, to decide which class of machine can be
adopted with the greatest economy to the proprietor. The cost of
lubricating materials, fuel, repairs, and percentage of cost to be put
aside for depreciation, will be less in case of the high-class than in
the low-class engine, while the former will also require less boiler
power.
Against these advantages are to be set the greater first cost of the
automatic engine, and the consequent annual charge due to capital sunk.
These several items should all be fairly estimated when an engine is
to be bought, and the kind chosen accordingly. Let us take the item of
fuel, for instance, and let us suppose this fuel to cost four dollars
per ton at the place where the engine is run. Suppose the engine to be
capable of developing one hundred horse-power, and that it consumes five
pounds of coal per hour per horse-power, and runs ten hours per day:
this would necessitate the supply of two and one-half tons per day at
a cost of ten dollars per day. To be really economical, therefore, any
improvement which would effect a saving of one pound of coal per hour
per horse-power must not cost a greater sum per horse-power than that on
which the cost of the difference of the coal saved (one pound of coal
per hour per horse-power, which would be 1,000 pounds per day) for, say,
three hundred days, three hundred thousand (300,000) pounds, or one
hundred and fifty tons (or six hundred dollars), would pay a fair
interest.
Assuming that the mill owner estimates his capital as worth to him ten
per cent, per annum, then the improvement which would effect the above
mentioned saving must not cost more than six thousand dollars, and so
on. If, instead of being run only ten hours per day, the engine is run
night and day, then the outlay which it would be justifiable to make to
effect a certain saving per hour would be doubled; while, on the other
hand, if an engine is run less than the usual time per day a given
saving per hour would justify a correspondingly less outlay.
It has been found that for grain and other elevators, which are not run
constantly, gas engines, although costing more for the same power,
are cheaper than steam engines for elevating purposes where only
occasionally used.
For this reason it is impossible without considerable investigation to
say what is really the most economical engine to adopt in any particular
case; and as comparatively few users of steam power care to make this
investigation a vast amount of wasteful expenditure results. Although,
however, no absolute rule can be given, we may state that the number
of instances in which an engine which is wasteful of fuel can be used
profitably is exceedingly small. As a rule, in fact, it may generally be
assumed that an engine employed for driving a manufactory of any kind
cannot be of too high a class, the saving effected by the economical
working of such engines in the vast majority of cases enormously
outweighing the interest on their extra first cost. So few people appear
to have a clear idea of the vast importance of economy of fuel in mills
and factories that I perhaps cannot better conclude than by giving an
example showing the saving to be effected in a large establishment by an
economical engine.
I will take the case of a flouring mill in this city which employed two
engines that required forty pounds of water to be converted into steam
per hour per indicated horse-power. This, at the time, was considered a
moderate amount and the engines were considered "good."
These engines indicated seventy horse power each, and ran twenty-four
hours per day on an average of three hundred days each year, requiring
as per indicator diagrams forty million three hundred and twenty
thousand pounds (40 x 70 x 24 x 300 x 2 = 40,320,000) of feed water to
be evaporated per annum, which, in Philadelphia, costs three dollars
per horse-power per annum, amounting to (70 x 2 x 300 = $420.00) four
hundred and twenty dollars.
The coal consumed averaged five and one-half pounds per hour per
horse-power, which, at four dollars per ton, costs
((70 x 2 x 5.5 x 24 x 300) / 2,000) x 4.00= $11,088
Eleven thousand and eighty-eight dollars.
Cost of coal for 300 days. $11,088
Cost of water for 300 days. 420
-------
Total cost of coal and water. $11,503
These engines were replaced by one first-class automatic engine,
which developed one hundred and forty-two horse-power per hour with a
consumption of _three pounds_ of coal per hour per horse-power, and the
indicator diagrams showed a consumption of _thirty_ pounds of water per
hour per horse-power. Coal cost
((142 x 3 x 24 x 300) / 2,000) x 4.00 = $6,134
Six thousand one hundred and thirty-four dollars. Water cost (142 x
3.00= $426.00) four hundred and twenty-six dollars.
Cost of coal for 300 days. $6,134
Cost of water for 300 days. 426
------
Total cost of coal and water. $6,560
The water evaporated in the latter case to perform the same work was
(142 x 30 x 24 x 300 = 30,672,000) thirty million six hundred and
seventy-two thousand pounds of feed water against (40,320,000) forty
million three hundred and twenty thousand pounds in the former, a saving
of (9,648,000) nine million six hundred and forty-eight thousand pounds
per annum; or,
(40,320,000 - 30,672,000) / 9,648,000 = 31.4 per cent.
--_thirty-one and four-tenths per cent_.