CONSTRUCTION AND MAINTENANCE OF TANNERIES.
As few architects have specially studied the construction of tanneries,
and in most cases much of the arrangement depends on the knowledge of
the tanner himself, a short chapter on the subject will not be out of
place.
In the selection of a site, a clay or loamy soil is to be preferred
to a gravelly or sandy one, as lessening the liability to leakage,
and waste of liquor. Perhaps, however, the first consideration of all
is the water supply, since for manufacturing purposes town water is
generally very expensive. With regard to quality and impurities of
water, information may be found on p. 83; but, as a general rule, the
softer and purer the supply the better. It is also of great advantage
when the source is at such a level as to flow into the tan-yard, or
at least into the beam-house, without pumping. Filtration too, when
needed, is much facilitated by a sufficient head of water.
Of scarcely less importance than the water supply is the drainage of
the yard. It not unfrequently happens that tanneries are prohibited
from discharging their refuse liquors, limes, and soaks into rivers
and watercourses, and it is sometimes a matter of extreme difficulty
to find any other way of getting rid of them. In default of an outlet,
recourse must be had to precipitation and filtration, but this is a
costly expedient, and in fixing a site for a new yard it is far better
to provide against such a possible contingency. Should, however, such
means become necessary, it may be borne in mind that limes and liquors
in great measure mutually precipitate each other, and that if all the
various refuse is run into one tank, mixed, and settled, much is
accomplished in the direction of purification. The further treatment of
the effluent water must be determined by its nature and composition.
The site chosen, the next question is the arrangement of the buildings.
It is very doubtful, where ground is not inordinately expensive,
whether it is wise to erect drying-sheds over the pits. In case of
fire, very serious damage is done to liquor and leather by the heat
and burning timber. If the turret form of drier be decided on, strong
foundations are required, and the ground-floor or basement is occupied
with heating apparatus; and, on the other hand, the tan-house may
be easily and cheaply covered with slated roofs, with sections of
glass, to the north, if possible, like a weaving-shed, through which
sufficient light for convenient work and cleanliness is admitted. The
direct rays of the sun should be avoided, but in the writer's opinion
the balance of advantage is largely in favour of a liberal supply of
light. Iron roofs are unsuitable, since the moisture condenses on, and
rusts them; and particles of oxide fall into the liquors, and cause
iron-stains.
Good ventilation along the ridge of the roof should be provided,
wherever there is any steam or hot liquor used; or the condensed
moisture soon leads to decay.
As regards the general plan of the buildings, much depends on local
circumstances; but as far as possible, they must be so arranged that
the hides and leather work straight forward from one department to
another with as little wheeling or carrying as possible; that the
buildings where power is used be near to the engine, so as to avoid
long transmissions, which are very wasteful of power; and that the
different buildings be so isolated as to diminish the risk of the whole
being destroyed in case of fire.
As regards the first of these conditions, if the various soaks,
limes, bates, and handlers are well arranged, it is hardly necessary
to do more than draw the goods from one pit into the next throughout
the whole of the process. To, and from the layers, the goods must
generally be carried or wheeled. In the sheds, if it be a sole-leather
tannery, the butts should first come into turrets or open sheds for
the rough drying; then into a room sheltered from draughts to temper
for striking. The striking machines or beams should be in an adjoining
room, or immediately below; then a small shed-space for drying before
rolling; next the roller room; and then the warm stove for drying
off. If two of these can be provided to be used alternately, it will
allow the goods to be aired off without taking down, and they may then
be immediately handed or lowered into the warehouse, without fear of
over-drying, which is sometimes difficult to avoid where leather must
be taken direct out of the hot drying-room.
To fulfil the second condition named, the engine should be at the
centre of the main range of buildings, with perhaps the grinding
machinery on one side, and the leather-finishing on the other; but
this would be rather contrary to the third requirement. A very good
plan would be to have the engine-house in the centre as suggested, but
separated from the buildings on each side by brick gables; and with the
boiler-house behind it, and under a separate roof, say of corrugated
iron. Figs. 61, 62, from Eitner's book on American Tanning, show the
arrangement of a sole-leather tannery in the United States. If it be
impossible to have the engine near its work, it is in most cases better
to employ a separate high-pressure engine, which may be within a glass
partition, and will work all day with scarcely any attention. The loss
of power in carrying steam for moderate distances through sufficiently
large and well-clothed pipes is much smaller than that of long lines
of shafting. The writer has known cases where fully half the indicated
power of the engine was consumed in friction of the engine, shafting,
and belts. High-pressure engines are as a rule to be preferred to
condensing for tannery use, since the waste steam can generally be
employed for heating, and both the first cost and that of maintenance
are smaller. Where much fuel is used, it is quite worth while to have
the cylinders indicated occasionally, both running light, and driving
the machinery; much information is gained in this way as to the power
spent on the various machines, and very frequently large economy is
effected by proper adjustment of the valves. To work economically, an
engine should be of ample power for all it has to do; and adjusted to
its work, not by lowering the pressure of steam, or by checking it
at the throttle-valve, but by setting the slide-valves to cut off as
early in the stroke as may be. As to how early this is possible, an
indicator-diagram will at once give information. In arranging shafting,
moderate speeds, say 100-150 rev. per min., should be chosen for main
lines, and when higher speeds are necessary, they should be got up by
light and well-balanced counter-shafts, with wrought-iron pulleys. In
calculating speeds, it must be remembered that they vary inversely as
the size of the pulleys. Thus a 3 ft. pulley running at 100 rev., will
drive a 2 ft. one at 150 rev., and a 12 in. one at 300. Of course the
higher its speed, the more power any shaft will transmit, but increased
friction and wear and tear soon limit this advantage. The velocity
of a belt in feet per min. is obtained by multiplying the number of
revolutions per minute by the girth of the pulley in feet or by its
diameter multiplied by 3-1/7, or more accurately, 3·1416.
[Illustration: Fig. 61.]
[Illustration: Fig. 62.]
Pulleys should always be of ample breadth for the power they have to
transmit; and it is more economical both in power and cost, to use
broad single belting than the same strength in double. If the pulley
will not take a belt broad enough for the work it has to do, a second
belt may be made to run on the top of the first, and will do its share
of the work. Belts should be washed occasionally with soap and tepid
water, and oiled with cod-oil; but if of sufficient breadth, should
not require the use of rosin, or adhesive materials, to make them grip
the pulley. Makers of machines often err in constructing their driving
pulleys too small both in breadth and diameter.
The horse-power which a belt is capable of transmitting obviously
varies extremely with circumstances, but may be approximately
calculated by the formula
(_a_ · _v_)/66000,
where _a_ is the area of contact of the belt with the smallest pulley,
and _v_ its velocity in feet per minute. Another rule is, that at a
velocity of 1000 ft. per min. each inch of breadth of belt should
transmit 2-1/2 horse-power on metal pulleys, or 5 on wooden ones,
on which the adhesion is greater. Adhesion may also be increased
by covering the pulleys with leather or india-rubber. Both rules
assume that the belt is of ample strength. One horse-power would be
transmitted by a belt running 1000 ft. per min. with a pull of 33 lb. A
good single belt should not break with a much less strain than 1000 lb.
per inch of breadth, and should stand about 1/10 as much as a working
strain.
Countershafting and high-speed machinery, such as disintegrators,
striking machines of the Priestman type, &c., should run without
material jar or vibration. If this occurs, it is generally a sign that
the running part is not equally balanced. In this case, the shaft must
be taken out of its bearings, and supported on two exactly horizontal
straight-edges, when it will roll till the heaviest part is downwards;
and weight must be taken off or added till it will lie in any position.
In this way, the writer had recently to add fully 2 lb. of iron to the
drum of a striking machine before equilibrium was secured, and a most
troublesome vibration prevented. Of course all machinery should be
supported as solidly as possible; and if circumstances permit, most
machines are better on a ground-floor. In placing bark mills, however,
it is frequently convenient to fix them in the top of a building, so
that the ground material may be sent down shoots by its own weight to
the required places. An alternative plan is to set the mill on the
ground-floor, and to raise the ground material with a bucket-elevator.
This may be done successfully by letting the material fall directly
from the mill into the buckets; but otherwise it must be thrown in
with a shovel, as buckets will not pick up ground bark, even from
a hopper; and in any case such elevators are often troublesome. In
a grinding plant designed by the writer, the unground material is
filled on the basement floor into an iron barrow, which may be wheeled
into an iron bow working between upright guide-rails. On pulling a
brake-line, the barrow is raised to the top of the building, and its
contents are tipped into a large hopper, after which the barrow rights
itself, and descends for another load. In the bottom of the hopper is a
sliding shover, which forces the material on to vibrating screens, by
which it is guided either into a disintegrator, or crusher rolls, at
pleasure. Both these discharge through iron spouts into large hoppers
on the outside of a brick gable, from which, powdery materials like
myrabolanes and valonia, can be run direct into barrows or trucks.
It is very desirable that such hoppers should be separated from the
main building by a fireproof partition. The writer is glad to say, he
does not know of a case of fire from disintegrators grinding tanning
materials, but he is informed that a Carter's disintegrator employed in
grinding bones in a manure works has repeatedly set fire to the flannel
bag into which the dust was allowed to escape. If this were to occur
with a dry and dusty tanning material, it is not unlikely that it might
result in an explosion such as sometimes happens in flour-mills from
a similar cause. On the whole, however, mills of the coffee-mill type
are probably more dangerous than disintegrators; since if they become
partially choked, the heat caused by friction is very great.
For lubricating purposes, mineral oils of high density are not more
dangerous than animal or vegetable, but rather the reverse; as, though
they are possibly more inflammable, their mixture with cotton-waste
and other porous vegetable materials is not spontaneously combustible,
while vegetable and animal oils occasionally are. Heavy mineral oils
should always be used as cylinder oils in high-pressure engines, in
preference to other oils or tallow, since they are not decomposed
by steam, and do no harm if blown into the feed-water, but serve to
loosen and prevent scale and deposit. Ordinary oils and tallow, on the
other hand, when submitted to the action of high-pressure steam, are
separated into glycerin and fatty acids (see p. 60), and the latter
corrode the valve faces and seatings, and in combination with temporary
hardness in the boilers form a very dangerous porous deposit, which
often leads to overheating of the tubes.
Next to the machinery, the pits demand special consideration. The
chapter on the subject in Mr. Schultz's book on 'Leather Manufacture,'
is well worth attentive study as giving American practice on the
subject.
The old-fashioned method of sinking pits is to make them of wood, and
carefully puddle them round with clay, which should be well worked
up before use. Such pits, if made of good pine and kept in constant
use, are very durable, some of the original pits at Lowlights Tannery,
constructed in 1765, being still in use. Loam mixed with water to the
consistence of thin mortar may also be employed, the pits being filled
up with water, to keep them steady, at the same rate as the loam is run
in. Probably the best materials for pit-sides are the large Yorkshire
flagstones. Where these are not attainable, very durable pits may be
made of brick, either built with Lias lime, and pointed with Portland
cement, or built entirely with the latter. Common lime cannot be used,
as it spoils both liquors and leather; and even cements with too large
a percentage of lime are unsatisfactory. Brick and common mortar are,
however, suitable for lime-pits.
The writer has constructed wooden pits in two ways. In the one case,
after making the excavation, beams were laid in a well-puddled bed of
clay; on these a floor of strong tongued and grooved deals was laid,
and on this the pits were constructed of similar wood to the floor, and
puddled round with clay. In the second case the pits were built like
large boxes above ground, and when finished, lowered on to a bed of
clay prepared for them, and then puddled both around and between. It
may have been from defective workmanship in the first case, but those
made on the last-named plan, which is that adopted from very early
times, have certainly proved the tightest and most satisfactory. Mr.
Schultz describes a plan as the Buffalo method, in which a floor is
laid as just described, and grooves cut with a plane for the reception
of the sides, which are formed of perpendicular planks, each end and
side being finally tightened up by the insertion of a "wedge plank."
If bricks be used, great care must be taken that the cement is not
merely laid so as to fill the joints towards the two surfaces of the
wall, as is the habit of modern bricklayers, but actually floated into
all the joints so as to make the wall a solid mass; or leaks can hardly
be avoided. Cement pits are very good, and, though not particularly
cheap in material, which must be of the best, are readily made by
intelligent labourers under good supervision. The first step is to lay
a level floor of good concrete, in which glazed pipes for emptying
the pits may be embedded; care being also taken that all joints in
these are thoroughly tight, since future repairs are impossible. The
next step is to make frames, the exact length and breadth of the pits
required, and perhaps 15 in. deep. These are arranged on the floor
where the pits are to be, and the intervening spaces are filled with
concrete of perhaps 1 of cement to 3 or 4 of crushed stone or brick.
Rough stones and bricks may also be bedded in the concrete as the work
goes on, to help to fill up. After the first layer has set, the frames
may be raised and a second added, and so on. The work is generally
finished by floating over it, while still damp, a little pure cement,
to give a smooth surface. Before using, the cement should be tried
on a small scale, to be sure that it does not discolour leather or
liquors, and the pits should always be seasoned with old or cheap
liquor before actual use.
If possible, both latches and handler-pits should be provided with
plugs and underground pipes, communicating with a liquor-well some
feet below their levels. Glazed fire-clay is very suitable both for
pipes and plug-holes, which should be in the pit corners. Some means
should also be provided for the ready clearing of the pipes when choked
with tanning materials. A good plan is to let each line of pipes end
in a liquor-well large enough for a man to go down. As it is almost
impossible to make plugs fit without occasional leakage, it is not
well to run pits with very different strengths of liquors to one well,
but the layers, handlers, and different sets of leaches should each
have their own, so as to avoid mixture. A good means of clearing pipes
consists in a series of iron rods 3-4 ft. long, connected by hooks
fitting into double eyes, as shown in Fig. 63. It is obvious that in a
narrow pipe or drain, these cannot become disconnected.
[Illustration: Fig. 63.]
It is, as Schultz points out, of questionable advantage to lay wooden
troughs for supplying liquor to each pit under the alleys, since it is
almost impossible to preserve them from decay; but the same objection
would not apply to glazed pipes, well clayed or cemented. A very good
and cheap plan in practice, is to let the liquor-pump, or a raised
liquor-cistern, discharge into a large and quite horizontal trough
raised 5 or 6 feet above the level of the yard, and provided with
plug-holes at intervals, under which short troughs may be set to run
the liquor into the various pits.
[Illustration: PL. VIII.
_E. & F. N. Spon, London & New York._
"INK-PHOTO." SPRAGUE & CO. LONDON.
RUBBING-DOWN DYED SKINS.]
In tan-yard construction, iron should, as far as possible, be avoided
wherever it can come into contact with liquor, as it discolours the
leather. In default of underground pipes, india-rubber suction hose
may be employed. Direct-acting steam pumps without fly-wheels are not
suitable for tanneries, as they "hammer" when the pit is nearly sucked
up. Steam-jet elevators and the pulsometer are very useful for some
purposes, but slightly warm, and dilute the liquors with condensed
steam.
[Illustration: Fig. 64.]
Much that has been said about pits applies also to leaches. They may
be constructed either of wood, or brick and cement, and where heat is
employed the latter is the better. They are also to be provided with
plugs and pipes leading to a liquor-well. About 6 in. from the bottom
of the pit is a false bottom B made of boards, perforated with holes
or set a little distance apart; and in the corner is an "eye" C (Ger.
_Pfaff_) consisting of 2 boards set at right angles, so as to preserve
a vertical channel communicating with the space under the false bottom.
This serves, in pits provided with pipes, for the insertion of the
plug; and where this is absent, for that of a suction hose to pump
off the liquor. In the American Press-leck System, the eye of one
pit communicates by a horizontal spout with the top of the next (see
D, Fig. 64). The Allen and Warren Sprinkler Leck (Fig. 65) has very
much superseded this arrangement in America, though it is doubtful
if it spends the bark so completely. The round tubs, however, have
several advantages and may well be used for many purposes in English
yards. Their construction is described in some detail in Mr. Schultz's
book above cited. Some details will also be found on p. 209 of the
'Manufacture of Leather' by Davis. The rule for finding the capacity
of a round tub with perpendicular sides in cubic feet is to square the
diameter and multiply by ·7854, and by the depth in feet; or roughly,
to square half the diameter and multiply by the depth and by 3-1/7.
[Illustration: Fig. 65.]
Leaches and liquors are generally heated by blowing in steam direct. In
this case, the condensed water mixes with the liquor, and in heating
a liquor to boiling point it may be taken that about 20 per cent. of
water will be thus added. Where strong liquors are to be heated, it
is therefore obviously much better to pass the steam into a closed
copper coil in the liquor. Such a coil, with steam at 30 lb. pressure,
will heat about 27-1/2 gal. per hour per square foot of surface from
46° F. to boiling, and evaporate about half that quantity of liquor
already at boiling temperature. (See Box, 'Treatise on Heat,' p. 176.)
Heating coils must of course be provided with steam traps to carry off
condensed water; and in boiling by open steam it is very desirable
to let the steam pass through such a trap before use, to stop water
condensed in the pipes, which usually contains iron, and discolours the
liquors.