In the industries, there are often considerable quantities of liquid to
be evaporated in order to concentrate it. Such evaporation is very often
performed by burning fuel in sufficient quantity to furnish the liquid
the heat necessary to convert it into steam. This process is attended
with a consumption of fuel such as to form a very important factor in
the cost of the product to be obtained. In order to vaporize, at the
pressure of the atmosphere, 1 kilogramme of water at 0°, 637 heat units
are required, and of these, 100 are employed in raising the water from
0° to 100° and 537 in converting the water at 100° into steam at 100°.
This second quantity is called the _latent heat_ of the steam at 100°.
The sum of the two quantities is called the _total heat_ of the steam at
100°. The total heat of the steam remains nearly constant, whatever be
the temperature at which the vaporization occurred.
[Illustration: THE WEIBEL-PICCARD EVAPORATION APPARATUS.]
In order to utilize the steam as a means of heating, it is necessary to
condense it, that is to say, to cause it to pass from the gaseous to a
liquid state. This conversion disengages as much heat as the passage
from the liquid to the gaseous state had absorbed.
It results from this that if we could condense the steam that is given
off by a liquid that we are vaporizing, in contact with another liquid
that it is also a question of vaporizing, we should utilize all the heat
contained in the steam that was being given off from the first.
This object can be practically attained by two means, viz., by (1)
putting the disengaged steam in contact with the sides of a vessel that
contains a liquid colder than the one that produced it; (2) by raising
the temperature and pressure of the disengaged steam in order to
condense it in contact with the sides of the vessel which contains the
very liquid that has produced it.
The first of these means is realized in the apparatus called multiple
acting, that are at present so generally employed in sugar works. The
second means, which permits of a greater saving in fuel being made than
the other does, is realized by compressing the disengaged steam. This
compression, which raises the temperature and pressure of the steam,
permits of condensing the latter in contact with the vessel wherein it
has been produced. By such condensation we continuously restore to the
liquid which is being vaporized the heat of the steam which it gives
off.
This solution of the question, which has been partially seen at
different epochs, has but recently made its way into the industries. It
is being operated at present with complete success at the salt works of
France and Switzerland, at those of Austria and Prussia, in the sugar
of milk factories of France and Switzerland, and, finally, in 1882, the
first application of it in the sugar industry was made at Pohrlitz, in
Moravia.
The saving of fuel that has been made in these different applications
has always been great.
We shall now, for the sake of explaining the system, give a brief
description of the apparatus as used at the Pohrlitz sugar works
mentioned above. These works treat 255 tons of beets per 24 hours, and
obtain 4,000 hectoliters of juice, which is reduced to about 1,000
hectoliters of sirup. Up to the present, the concentration has been
effected in a double acting apparatus partly supplied by exhaust steam
from the motive engines and partly by steam coming directly from the
generators.
In order to diminish the consumption of direct steam, these sugar works
put in a Weibel-Piccard apparatus designed to concentrate only a third
of their juice, or about 1,350 hectoliters per day.
This apparatus (see engraving) consists of a steam compressor, 0.835 m.
in diameter, actuated directly by a driving cylinder of 0.5 m. diameter
and 0.8 m. stroke, and of three evaporating boilers of the ordinary
vertical tube type, the first of which has a surface of 150 square
meters, the second 60, and the third 80.
The steam, at the ordinary pressure of the generators, say 5
atmospheres, is taken from the connected generators of the works, and is
led to the driving cylinder, where it expands and furnishes the power
necessary to run the compressor. It then escapes at a pressure of l.4
atmospheres and enters the intertubular space of the first evaporator.
The compressor sucks up the steam from the juice of the first evaporator
(which is boiling at the pressure of the atmosphere, without vacuum or
effective pressure), compresses it to 1.4 atmospheres, and forces it
likewise into the intertubular space. The ebullition of the first
evaporator, then, is kept up not only by the exhaust from the motive
cylinder, but also by the steam from the juice itself, which has been
rendered fit to serve as a heating steam by the pressure that it has
undergone in the compressing cylinder.
In this first application of the new system to sugar making, it became
a question of ascertaining whether the advantage resulting from
compression was of great importance, and, in the second place, whether
the apparatus could be run with certainty and ease. In truth, the
applications of the system for some years past in other industries
permitted a favorable result to be hoped for, and the result turned out
as was expected.
With this apparatus it has been found that the work furnished by one
kilogramme of steam passing through the motive cylinder, from a
pressure of 5 atmospheres to one of 1.4, is sufficient to compress 2.5
kilogrammes of steam taken from the juice, led into the compressor
at one atmosphere and escaping therefrom at 1.4. In other words, one
kilogramme of motive steam is sufficient to convert into heating steam
for the first evaporator 2.5 kilogrammes of steam taken from the juice
in this same evaporator. Besides, this same kilogramme of motive steam
produces three effects, one in this same evaporator, and the other
two in the two succeeding ones. The effect obtained, then, from one
kilogramme of motive steam is, in round numbers, 5.5 kilogrammes of
steam removed from the juice.
It must not be forgotten that the motive steam was at the very moderate
pressure of 4 effective atmospheres. Had the use of steam at high
pressure (7 atmospheres for example) been possible, it is easy to
conclude from the above results that more than 6 kilogrammes of water
would have been vaporized with one kilogramme of steam.
The results here cited were ascertained by accurately measuring the
quantities of water of condensation from each evaporator, they soon
received, moreover, the most important of confirmations by the decrease
in the general consumption of fuel by the generators which occurred
after the new apparatus was set in operation.
The mean consumption of coal per 24 hours for the twenty days preceding
the 18th of November was 86,060 kilogrammes. After this date the regular
consumption was as follows:
Nov. 19.................31,800 kilogrammes.
" 20.................33,800 "
" 21.................33,800 "
" 22.................32,000 "
" 23.................31,400 "
" 24.................31,600 "
" 25.................30,500 "
" 26.................30,500 "
" 27.................28,600 "
" 28.................30,300 "
It must be remarked that in the perfectly regular running of the sugar
works, nothing was changed saving the setting of this evaporating
apparatus running. The same quantity of beets was treated per 24 hours,
and the general temperature remained the same. This remarkable result in
the saving of fuel was brought about notwithstanding the new apparatus
treated but a third, at the most, of the total amount of the juice, the
rest continuing to be concentrated by the double action process.
As for the running of the apparatus, that was perfectly regular, and the
deviations in temperature in each evaporater were scarcely two or three
degrees. The following are the mean temperatures:
First evaporator: heating steam 110° C.; juice steam 100° C. Second
evaporator: juice steam 83° C. Third evaporator: juice steam 62° C. As
regards facility of operating the apparatus, the experiment has proved
so conclusive that the plant will be considerably enlarged in view of
the coming crop, in order that a larger quantity of juice may be treated
by the new process. The effect of this will be to still further increase
the saving in coal that has already been effected by the present
apparatus. The engraving which accompanies this article represents the
Weibel-Piccard apparatus as it is now working in the Pohrlitz sugar
works. What we have said of it above we think will suffice to make it
understood without further explanation.--_Le Genie Civil_.
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