[Footnote: A paper read before the Franklin Institute.]
By GEORGE S. STRONG.
In order to properly understand the requirements of an effective
feed-water purifier, it will be necessary to understand something of the
character of the impurities of natural waters used for feeding
boilers, and of the manner in which they become troublesome in causing
incrustation or scale, as it is commonly called, in steam boilers. All
natural waters are known to contain more or less mineral matter, partly
held in solution and partly in mechanical suspension. These mineral
impurities are derived by contact of the water with the earth's surface,
and by percolation through its soil and rocks. The substances taken
up in solution by this process consist chiefly of the carbonates
and sulphates of lime and magnesia, and the chloride of sodium. The
materials carried in mechanical suspension are clay, sand, and vegetable
matter. There are many other saline ingredients in various natural
waters, but they exist in such minute quantities, and are generally so
very soluble, that their presence may safely be ignored in treating of
the utility of boiler waters.
Of the above named salts, the carbonates of lime and magnesia are
soluble only when the water contains free carbonic acid.
Our American rivers contain from 2 to 6 grains of saline matter to the
gallon in solution, and a varying quantity--generally exceeding 10
grains to the gallon--in mechanical suspension. The waters of wells and
springs hold a smaller quantity in suspension, but generally carry a
larger percentage of dissolved salts in solution, varying from 10 to 650
grains to the gallon.
When waters containing the carbonates of lime and magnesia in solution
are boiled, the carbonic acid is driven off, and the salts, deprived of
their solvent, are rapidly precipitated in fine crystalline particles,
which adhere tenaciously to whatever surface they fall upon. With
respect to the sulphate of lime, the case is different. It is at best
only sparingly soluble in water, one part (by weight) of the salt
requiring nearly 500 parts of water to dissolve it. As the water
evaporates in the boiler, however, a point is soon reached where
supersaturation occurs, as the water freshly fed into it constantly
brings fresh accessions of the salt; and when this point is reached,
the sulphate of lime is precipitated in the same form and with the same
tenaciously adherent quality as the carbonates. There is, however,
a peculiar property possessed by this salt which facilitates its
precipitation, namely, that its solubility in water diminishes as the
temperature rises. This fact is of special interest, since, if properly
taken advantage of, it is possible to effect its almost complete removal
from the feed-water of boilers,
There is little difference in the solubility of the sulphate of lime
until the temperature has risen somewhat above 212° Fahr., when it
rapidly diminishes, and finally, at nearly 300°, all of this salt,
held in solution at lower temperatures, will be precipitated when the
temperature has risen to that point. The following table[1] represents
the solubility of sulphate of lime in sea water at different
temperatures:
Temperature. Percentage Sulph.
Fahr. Lime held in Solution.
217° 0.500
219° 0.477
221° 0.432
227° 0.395
232° 0.355
236° 0.310
240° 0.267
245° 0.226
250° 0.183
255° 0.140
261° 0.097
266° 0.060
271° 0.023
290° 0.000
[Footnote 1: _Vide_ Burgh, "Modern Marine Engineering," page 176 _et
seq._ M. Cousté, _Annales des Mines_ V 69. _Recherches sur Vincrustation
des Chaudières a vapour_. Mr. Hugh Lee Pattison, of Newcastle-on-Tyne,
at the meeting of the Institute of Mechanical Engineers of Great
Britain, in August, 1880, remarked on this subject that "The solubility
of sulphate of lime in water diminishes as the temperature rises. At
ordinary temperatures pure water dissolves about 150 grains of sulphate
of lime per gallon; but at a temperature of 250° Fahr., at which the
pressure of steam is equal to about 2 atmospheres, only about 40 grains
per gallon are held in solution. At a pressure of 3 atmospheres, and
temperature of 302° Fahr., it is practically insoluble. The point
of maximum solubility is about 95° Fahr. The presence of magnesium
chloride, or of calcium chloride, in water, diminishes its power of
dissolving sulphate of lime, while the presence of sodium chloride
increases that power. As an instance of the latter fact, we find a
boiler works much cleaner which is fed alternately with fresh water and
with brackish water pumped from the Tyne when the tide is high than one
which is fed with fresh water constantly."]
These figures hold substantially for fresh as well as for sea water, for
the sulphate of lime becomes wholly insoluble in sea water, or in soft
water, at temperatures comprised between 280° and 300° Fahr.
It appears from this that it is simply necessary to heat water up to a
temperature of 250° in order to effect the precipitation of four fifths
of the sulphate of lime it may have contained, or to the temperature of
290° in order to precipitate it entirely. The bearing of these facts on
the purification of feed-waters will appear further on. The explanation
offered to account for the gradually increasing insolubility of sulphate
of lime on heating, is, that the hydrate, in which condition it exists
in solution, is partially decomposed, anhydrous calcic sulphate
being formed, the dehydration becoming more and more complete as the
temperature rises. Sulphate of magnesia, chloride of sodium (common
salt), and all the other more soluble salts contained in natural waters
are likewise precipitated by the process of supersaturation, but owing
to their extreme solubility their precipitation will never be effected
in boilers; all mechanically suspended matter tends naturally to
subside.
Where water containing such mineral and suspended matter is fed to a
steam boiler, there results a combined deposit, of which the carbonate
of lime usually forms the greater part, and which remains more or less
firmly adherent to the inner surfaces of the boiler, undisturbed by the
force of the boiling currents. Gradually accumulating, it becomes harder
and thicker, and, if permitted to accumulate, may at length attain such
thickness as to prevent the proper heating of the water by any fire that
may be maintained in the furnace. Dr. Joseph G. Rogers, who has made
boiler waters and incrustations a subject of careful study, declares
that the high heats necessary to heat water through thick scale will
sometimes actually convert the scale into a species of glass, by
combining the sand, mechanically separated, with the alkaline salts. The
same authority has carefully estimated the non-conducting properties
of such boiler incrustations. On this point he remarks that the evil
effects of the scale are due to the fact that it is relatively a
nonconductor of heat. As compared with iron, its conducting power is
as 1 to 37½, consequently more fuel is required to heat water in an
incrusted boiler than in the same boiler if clean. Rogers estimates that
a scale 1-16th of an inch thick will require the extra expenditure of
15 per cent. more fuel, and this ratio increases as the scale grows
thicker. Thus, when it is one-quarter of an inch thick, 60 per cent.
more fuel is needed; one-half inch, 112 per cent. more fuel, and so on.
Rogers very forcibly shows the evil consequences to the boiler from the
excessive heating required to raise steam in a badly incrusted boiler,
by the following illustration: To raise steam to a pressure of 90 pounds
the water must be heated to about 320° Fahr. In a clean boiler of
one-quarter inch iron this may be done by heating the external surface
of the shell to about 325° Fahr. If, now, one-half an inch of scale
intervenes between the boiler shell and the water, such is its quality
of resisting the passage of heat that it will be necessary to heat the
fire surface to about 700°, almost to a low red heat, to effect the same
result. Now, the higher the temperature at which iron is kept the more
rapidly it oxidizes, and at any heat above 600° it very soon becomes
granular and brittle, and is liable to bulge, crack, or otherwise give
way to the internal pressure. This condition predisposes the boiler to
explosion and makes expensive repairs necessary. The presence of such
scale, also, renders more difficult the raising, maintaining, and
lowering of steam.
The nature of incrustation and the evils resulting therefrom having been
stated, it now remains to consider the methods that have been devised
to overcome them. These methods naturally resolve themselves into
two kinds, chemical and mechanical. The chemical method has two
modifications; in one the design is to purify the water in large tanks
or reservoirs, by the addition of certain substances which shall
precipitate all the scale-forming ingredients before the water is fed
into the boiler; in the other the chemical agent is fed into the boiler
from time to time, and the object is to effect the precipitation of the
saline matter in such a manner that it will not form solid masses of
adherent scale. Where chemical methods of purification are resorted to,
the latter plan is generally followed as being the least troublesome. Of
the many substances used for this purpose, however, some are measurably
successful; the majority of them are unsatisfactory or objectionable.
The mechanical methods are also very various. Picking, scraping,
cleaning, etc., are very generally resorted to, but the scale is so
tenacious that this only partially succeeds, and, as it necessitates
stoppage of work, it is wasteful. In addition to this plan, a great
variety of mechanical contrivances for heating and purifying the
feed-water, by separating and intercepting the saline matter on its
passage through the apparatus, have been devised. Many of these are of
great utility and have come into very general use. In the Western States
especially, where the water in most localities is heavily charged
with lime, these mechanical purifiers have become quite indispensable
wherever steam users are alive to the necessity of generating steam with
economy.
Most of these appliances, however, only partly fulfill their intended
purposes. They consist essentially of a chamber through which the
feed-water is passed, and in which it is heated almost to the boiling
point by exhaust steam from the engine. According to the temperature
to which the water is heated in this chamber, and the length of time
required for its passage through the chamber, the carbonates are more or
less completely precipitated, as likewise the matter held in mechanical
suspension. The precipitated matter subsides on shelves or elsewhere in
the chamber, from which it is removed from time to time. The sulphate
of lime, however, and the other soluble salts, and in some cases also a
portion of the carbonates that were not precipitated during the brief
time of passage through the heater, are passed on into the boiler.
Appreciating this insufficiency of existing feed-water purifiers to
effectually remove these dangerous saline impurities, the writer in
designing the feed-water heater now to be described paid special
attention to the separation of all matters, soluble and insoluble; and
he has succeeded in passing the water to the boilers quite free from any
substance which would cause scaling or coherent deposit. His attention
was called more particularly to the necessity of extreme care in this
respect, through the great annoyance suffered by steam users in the
Central and Western States, where the water is heavily charged with
lime. Very simple and even primitive boilers are here used; the most
necessary consideration being handiness in cleaning, and not the highest
evaporative efficiency. These boilers are therefore very wasteful, only
evaporating, when covered with lime scale, from two to three pounds of
water with one pound of the best coal, and requiring cleansing once
a week at the very least. The writer's interest being aroused, he
determined, if possible, to remedy these inconveniences, and accordingly
he made a careful study of the subject, and examined all the heaters
then in the market. He found them all, without exception, insufficient
to free the feed-water from the most dangerous of impurities, namely,
the sulphate and the carbonate of lime.
Taking the foregoing facts, well known to chemists and engineers, as the
basis of his operations, the writer perceived that all substances likely
to give trouble by deposition would be precipitated at a temperature of
about 250° F.
His plan was, therefore, to make a feed-water heater in which the water
could be raised to that temperature before entering the boiler. Now, by
using the heat from the exhaust steam the water may be raised to between
208° and 212° F. It has yet to be raised to 250° F.; and for this
purpose the writer saw at once the advantage that would be attained by
using a coil of live steam from the boiler. This device does not cause
any loss of steam, except the small loss due to radiation, since the
water in any case would have to be heated up to the temperature of the
steam on entering the boiler. By adopting this method, the chemical
precipitation, which would otherwise occur in the boiler, takes place
in the heater; and it is only necessary now to provide a filter, which
shall prevent anything passing that can possibly cause scale.
Having explained as briefly as possible the principles on which the
system is founded, the writer will now describe the details of the
heater itself.
In Figs. 1 and 2 are shown an elevation and a vertical section of
the heater. The cast-iron base, A, is divided into two parts by the
diaphragm, B. The exhaust steam enters at C, passes up the larger tubes,
D, which are fastened into the upper shell of the casting, returns by
the smaller tubes, E, which are inside the others, and passes away by
the passage, F. The inner tube only serves for discharge. It will be
seen at once that this arrangement, while securing great heating surface
in a small space, at the same time leaves freedom for expansion and
contraction, without producing strains. The free area for passage of
steam is arranged to be one and a half times that of the exhaust pipe,
so that there is no possible danger of back pressure. The wrought iron
shell, G, connecting the stand, A, with the dome, H, is made strong
enough to withstand the full boiler pressure. An ordinary casing, J,
of wood or other material prevents loss by radiation of heat. The
cold water from the pump passes into the heater through the injector
arrangement, K, and coming in contact with the tubes, D, is heated; it
then rises to the coil, L, which is supplied with steam from the boiler,
and thus becomes further heated, attaining there a temperature of from
250° to 270° F., according to the pressure in the boiler. This high
temperature causes the separation of the dissolved salts; and on the way
to the boiler the water passes through the filter, M, becoming thereby
freed from all precipitated matter before passing away to the boiler at
N. The purpose of the injector, K, and the pipe passing from O to K, is
to cause a continual passage of air or steam from the upper part of the
dome to the lower part of the heater, so that any precipitate carried up
in froth may be again returned to the under side of the filter, in order
more effectually to separate it, before any chance occurs of its passing
into the boiler.
[Illustration: FIG. 1.--Elevation. FIG. 2.--Vertical Section]
The filter consists of wood charcoal in the lower half and bone black
above firmly held between two perforated plates, as shown. After the
heater has been in use for from three to ten hours, according to the
nature of the water used, it is necessary to blow out the heater, in
order to clear the filter from deposit. To do this, the cock at R is
opened, and the water is discharged by the pressure from the boiler. The
steam is allowed to pass through the heater for some little time, in
order to clear the filter completely. After this operation, all is ready
to commence work again. By this means the filter remains fit for use for
months without change of the charcoal.
Where a jet condenser is used, either of two plans may be adopted. One
plan takes the feed-water from the hot well and passes the exhaust from
the feed pumps through the heater, using at the same time an increased
amount of coil for the live steam. By this means a temperature of water
is attained high enough to cause deposition, and at the same time to
produce decomposition of the oil brought over from the cylinders. The
other plan places the heater in the line of exhaust from the engine to
the condenser, also using a larger amount of coil. Both these methods
work well. The writer sometimes uses the steam from the coil to work the
feed pump; or, if the heater stands high enough, it is only necessary
to make a connection with the boiler, when the water formed by the
condensation of the steam runs back to the boiler, and thus the coil is
kept constantly at the necessary temperature.
In adapting the heater to locomotives, we were met with the difficulty
of want of space to put a heater sufficiently large to handle the
extremely large amount of water evaporated on a locomotive worked up to
its full capacity, being from 1,500 to 2,500 gallons per hour, or from
five hundred to one thousand h.p. We designed various forms of heaters
and tried them, but have finally decided on the one shown in the
engraving, Fig. 3, which consists of a lap welded tube, 13 inches
internal diameter, 12 feet long, with a cast-iron head which is divided
into two compartments or chambers by a diaphragm. Into this head are
screwed 60 tubes, one inch outside diameter and 12 feet long, which
are of seamless brass. These are the heating tubes, within which
are internal tubes for circulation only, which are screwed into the
diaphragm and extend to within a very short distance of the end of the
heating tube. The exhaust steam for heating is taken equally from both
sides of the locomotive by tapping a two-inch nipple with a cup shaped
extension on it in such a way as to catch a portion of the exhaust
without interfering with the free escape of the steam for the blast, and
without any back pressure, as it relieves the back pressure as much as
it condenses. The pipe from one side of the engine is connected with
the chamber into which the heating tubes are screwed, and is in direct
communication with them. The pipe from the other side is connected with
the chamber into which the circulating tubes are screwed. The beat of
the exhaust, working, as it does, on the quarters, causes a constant
sawing or backward and forward circulation of steam without any
discharge, and only the condensation is carried off.
The water is brought from the pump and discharged into the lower side of
the heater well forward, and passes around the heating tubes to the end,
when it is discharged into a pipe that carries it forward, either direct
to the check or into the purifier, which is located between the frames
under the boiler, and consists of a chamber in which are arranged a live
steam coil and a filter above the coil. The water coming in contact
with the coil, its temperature is increased from the temperature of the
exhaust, 210°, to about 250° Fahr., which causes the separation of the
lime salts as before described, and it then passes through the filter
and direct to the boiler from above the filter, which is cleansed by
blowing back through it as before described.
One of these heaters lately tested showed a saving in coal of 22 per
cent, and an increase of evaporation of 1.09 pounds of water per pound
of coal.--_Franklin Journal_.
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