[Extracts from a Lecture delivered by George H. Babcock, at Cornell
University, 1887[10]]
The chemical compound known as H_{2}O exists in three states or
conditions--ice, water and steam; the only difference between these
states or conditions is in the presence or absence of a quantity of
energy exhibited partly in the form of heat and partly in molecular
activity, which, for want of a better name, we are accustomed to call
"latent heat"; and to transform it from one state to another we have
only to supply or extract heat. For instance, if we take a quantity of
ice, say one pound, at absolute zero[11] and supply heat, the first
effect is to raise its temperature until it arrives at a point 492
Fahrenheit degrees above the starting point. Here it stops growing
warmer, though we keep on adding heat. It, however, changes from ice to
water, and when we have added sufficient heat to have made it, had it
remained ice, 283 degrees hotter or a temperature of 315 degrees
Fahrenheit's thermometer, it has all become water, at the same
temperature at which it commenced to change, namely, 492 degrees above
absolute zero, or 32 degrees by Fahrenheit's scale. Let us still
continue to add heat, and it will now grow warmer again, though at a
slower rate--that is, it now takes about double the quantity of heat to
raise the pound one degree that it did before--until it reaches a
temperature of 212 degrees Fahrenheit, or 672 degrees absolute (assuming
that we are at the level of the sea). Here we find another critical
point. However much more heat we may apply, the water, as water, at that
pressure, cannot be heated any hotter, but changes on the addition of
heat to steam; and it is not until we have added heat enough to have
raised the temperature of the water 966 degrees, or to 1,178 degrees by
Fahrenheit's thermometer (presuming for the moment that its specific
heat has not changed since it became water), that it has all become
steam, which steam, nevertheless, is at the temperature of 212 degrees,
at which the water began to change. Thus over four-fifths of the heat
which has been added to the water has disappeared, or become insensible
in the steam to any of our instruments.
It follows that if we could reduce steam at atmospheric pressure to
water, without loss of heat, the heat stored within it would cause the
water to be red hot; and if we could further change it to a solid, like
ice, without loss of heat, the solid would be white hot, or hotter than
melted steel--it being assumed, of course, that the specific heat of the
water and ice remain normal, or the same as they respectively are at the
freezing point.
After steam has been formed, a further addition of heat increases the
temperature again at a much faster ratio to the quantity of heat added,
which ratio also varies according as we maintain a constant pressure or
a constant volume; and I am not aware that any other critical point
exists where this will cease to be the fact until we arrive at that very
high temperature, known as the point of dissociation, at which it
becomes resolved into its original gases.
The heat which has been absorbed by one pound of water to convert it
into a pound of steam at atmospheric pressure is sufficient to have
melted 3 pounds of steel or 13 pounds of gold. This has been transformed
into something besides heat; stored up to reappear as heat when the
process is reversed. That condition is what we are pleased to call
latent heat, and in it resides mainly the ability of the steam to do
work.
[Graph: Temperature in Fahrenheit Degrees (from Absolute Zero)
against Quantity of Heat in British Thermal Units]
The diagram shows graphically the relation of heat to temperature, the
horizontal scale being quantity of heat in British thermal units, and
the vertical temperature in Fahrenheit degrees, both reckoned from
absolute zero and by the usual scale. The dotted lines for ice and water
show the temperature which would have been obtained if the conditions
had not changed. The lines marked "gold" and "steel" show the relation
to heat and temperature and the melting points of these metals. All the
inclined lines would be slightly curved if attention had been paid to
the changing specific heat, but the curvature would be small. It is
worth noting that, with one or two exceptions, the curves of all
substances lie between the vertical and that for water. That is to say,
that water has a greater capacity for heat than all other substances
except two, hydrogen and bromine.
In order to generate steam, then, only two steps are required: 1st,
procure the heat, and 2nd, transfer it to the water. Now, you have it
laid down as an axiom that when a body has been transferred or
transformed from one place or state into another, the same work has been
done and the same energy expended, whatever may have been the
intermediate steps or conditions, or whatever the apparatus. Therefore,
when a given quantity of water at a given temperature has been made into
steam at a given temperature, a certain definite work has been done, and
a certain amount of energy expended, from whatever the heat may have
been obtained, or whatever boiler may have been employed for the
purpose.
A pound of coal or any other fuel has a definite heat producing
capacity, and is capable of evaporating a definite quantity of water
under given conditions. That is the limit beyond which even perfection
cannot go, and yet I have known, and doubtless you have heard of, cases
where inventors have claimed, and so-called engineers have certified to,
much higher results.
The first step in generating steam is in burning the fuel to the best
advantage. A pound of carbon will generate 14,500 British thermal units,
during combustion into carbonic dioxide, and this will be the same,
whatever the temperature or the rapidity at which the combustion may
take place. If possible, we might oxidize it at as slow a rate as that
with which iron rusts or wood rots in the open air, or we might burn it
with the rapidity of gunpowder, a ton in a second, yet the total heat
generated would be precisely the same. Again, we may keep the
temperature down to the lowest point at which combustion can take place,
by bringing large bodies of air in contact with it, or otherwise, or we
may supply it with just the right quantity of pure oxygen, and burn it
at a temperature approaching that of dissociation, and still the heat
units given off will be neither more nor less. It follows, therefore,
that great latitude in the manner or rapidity of combustion may be taken
without affecting the quantity of heat generated.
But in practice it is found that other considerations limit this
latitude, and that there are certain conditions necessary in order to
get the most available heat from a pound of coal. There are three ways,
and only three, in which the heat developed by the combustion of coal in
a steam boiler furnace may be expended.
1st, and principally. It should be conveyed to the water in the boiler,
and be utilized in the production of steam. To be perfect, a boiler
should so utilize all the heat of combustion, but there are no perfect
boilers.
2nd. A portion of the heat of combustion is conveyed up the chimney in
the waste gases. This is in proportion to the weight of the gases, and
the difference between their temperature and that of the air and coal
before they entered the fire.
3rd. Another portion is dissipated by radiation from the sides of the
furnace. In a stove the heat is all used in these latter two ways,
either it goes off through the chimney or is radiated into the
surrounding space. It is one of the principal problems of boiler
engineering to render the amount of heat thus lost as small as possible.
The loss from radiation is in proportion to the amount of surface, its
nature, its temperature, and the time it is exposed. This loss can be
almost entirely eliminated by thick walls and a smooth white or polished
surface, but its amount is ordinarily so small that these extraordinary
precautions do not pay in practice.
It is evident that the temperature of the escaping gases cannot be
brought below that of the absorbing surfaces, while it may be much
greater even to that of the fire. This is supposing that all of the
escaping gases have passed through the fire. In case air is allowed to
leak into the flues, and mingle with the gases after they have left the
heating surfaces, the temperature may be brought down to almost any
point above that of the atmosphere, but without any reduction in the
amount of heat wasted. It is in this way that those low chimney
temperatures are sometimes attained which pass for proof of economy with
the unobserving. All surplus air admitted to the fire, or to the gases
before they leave the heating surfaces, increases the losses.
We are now prepared to see why and how the temperature and the rapidity
of combustion in the boiler furnace affect the economy, and that though
the amount of heat developed may be the same, the heat available for the
generation of steam may be much less with one rate or temperature of
combustion than another.
Assuming that there is no air passing up the chimney other than that
which has passed through the fire, the higher the temperature of the
fire and the lower that of the escaping gases the better the economy,
for the losses by the chimney gases will bear the same proportion to the
heat generated by the combustion as the temperature of those gases bears
to the temperature of the fire. That is to say, if the temperature of
the fire is 2500 degrees and that of the chimney gases 500 degrees above
that of the atmosphere, the loss by the chimney will be 500/2500 = 20
per cent. Therefore, as the escaping gases cannot be brought below the
temperature of the absorbing surface, which is practically a fixed
quantity, the temperature of the fire must be high in order to secure
good economy.
The losses by radiation being practically proportioned to the time
occupied, the more coal burned in a given furnace in a given time, the
less will be the proportionate loss from that cause.
It therefore follows that we should burn our coal rapidly and at a high
temperature to secure the best available economy.
[Illustration: Portion of 9880 Horse-power Installation of Babcock &
Wilcox Boilers and Superheaters, Equipped with Babcock & Wilcox Chain
Grate Stokers at the South Side Elevated Ry. Co., Chicago, Ill.]