DRYING-SHEDS FOR LEATHER.
The primitive way of drying leather was to hang it on poles in the
open air, but this in our uncertain climate has become quite obsolete.
The oldest plan now actually in use is to hang on poles in a shed
generally raised some height above the ground, so as to catch the wind,
and provided on all sides with louvre boards arranged so as to open
and shut as required. These sheds, to give good results (especially on
mixed tannages, which need much more care in drying than bark), demand
very watchful management. In windy weather, and with wet leather at
all times, the louvres must be kept nearly or quite closed, and on the
sunny side of the shed the same precaution is generally necessary.
Again, in very damp weather the leather does not dry at all, and in
frosty seasons it is apt to freeze, by which sole leather is made soft
and spongy, and dressing leather, though whitened, is said to be less
capable of carrying grease. To prevent freezing, and to enable leather
to be dried in damp or cold weather, it became customary to provide
sheds with ranges of steam-pipes on the floor; this, though decidedly a
valuable addition, has not proved by any means an entirely satisfactory
solution of the problem of leather drying. No sufficient means are
provided for controlling the ventilation, and the upward currents of
hot air dry the leather irregularly, and produce bad colour. A much
more satisfactory shed is the American turret drier.
This consists of a lofty building, 3 to 8 stories high, without
louvres, but with latticed floors. J. S. Schultz recommends 5 stories,
of 7 ft. clear between beams, as a convenient height, and the building
should be divided by partitions from top to bottom into 4 or more
series of chambers one above another, each of which is capable of
having the heat and ventilation separately regulated. The Americans
usually fill one of these series at once, and dry off the whole in
about 10 days, so that as many will be required for a tannery as will
hold a 10 days' production. For ventilation, each of these sets of
chambers is provided with a lantern ventilator at the top for the exit,
and shutters or dampers on the bottom floor for the admission of air.
The bottom floor is also provided with steam-pipes, of which those for
each set of compartments are controlled by a separate cock. When warmth
is applied at the bottom, the tall building acts like a chimney, and a
continuous current of air passes from the ventilators at the base up to
those at the top. The usual American practice is, after filling one of
these ranges of compartments, to apply no steam-heat for the first 3
or 4 days, and, if the weather be dry or windy to keep the ventilators
also closed. After the third or fourth day, a moderate degree of heat
is given, and this is increased so that at the end of about 10 days the
stock is fully dry.
This is in accordance with a common American practice, in which the
leather is fully dried before rolling, in order to fix the soluble
colour, and prevent it striking out to the surface in the finishing.
The wet leather is raised by an elevator, consisting of an endless
chain provided with hooks, to which the leather is attached at the
bottom, and from which it is taken at the top. Various ways are adopted
to lower the leather from these tall turrets to the room where it is
stored prior to damping down for rolling. In some cases, the lattice
floors are made movable, and the whole contents of the room, including
the sticks from which the leather is hung, are allowed to fall into the
lowest room. This method is of very questionable advantage, if we take
into account the labour of separating the sticks and carrying them back
to their places. Another plan is to have shoots from each loft, down
which the sides are slid to the rolling-room. The floors should have
what light is necessary supplied through glass windows, so arranged as
not to admit direct sunlight.
To adapt the turret drier for English requirements, some slight
modification is needed, since we do not dry our leather right off,
and then damp back, but, when it is suitably dry, lay it in a pile to
"sammy" for striking; then, perhaps, after striking, hang up again for
a short time to temper for rolling, possibly again between rollings,
and finally to dry off at a temperature of, say, 68-77° F. (20-25° C.).
Perhaps on this account, the writer has seen no complete turret-driers
in use in England, though a portion of one of the large sheds at
Dartford belonging to Messrs. Hepburn was converted by them some years
since into a very good turret, which gave excellent results both
for sole leather and kip butts in stuff. This turret is represented
in section in Fig. 66, and is about 56 ft. × 24 ft. in area, and
50 ft. high from the ground-line to the top of the roof, which is
ventilated by a dormer, _a_, with fixed louvres at the top, while air
is admitted at the bottom through ventilators with sliding flaps, _b
b_. It is heated by 10 rows of 4-in. steam-pipe, _c c_, each 54 ft.
long, making a total of 540 ft. run, or about 640 ft. superficial (a
4-in. pipe being about 4-5/8 in. diameter outside). I am informed
by Mr. J. G. Hepburn that he considers 4-in. pipes inferior for the
purpose to smaller ones, giving too much heat in one place, and without
sufficiently distributing it, and were he constructing a new turret he
would replace them by 1-1/2 in. wrought-iron, using about 3 of 1-1/2
in. to replace 2 of 4 in., small pipes being much more effective (as
will be seen by table, p. 250) than larger ones, in proportion to
their surface. He considers, however, that the best way of heating
drying-sheds, though more expensive in first cost, is by means of hot
water, which is much more constant in temperature than steam. Mr.
Hepburn, to whom I am much indebted for the above information, informs
me that the turret still acts very well, drying kip butts on the
upper floor a good colour in all weathers in about a week. He finds,
however, that the steam-pipes as described are hardly sufficient in
very cold weather, and intends to increase them, or replace with
1300-1400 ft. of hot water pipe heated by a saddle boiler. At Lowlights
tannery, a shed arranged on the turret principle (though much less
completely carried out from want of height in the buildings) has been
for many years in operation, principally for drying off sole-leather,
with the most satisfactory results.
[Illustration: Fig. 66.]
It is noted by Box ('Practical Treatise on Heat,' p. 166) that an exit
for the moist air should not be placed at the top of a drying-chamber,
but at the bottom, since in the first case, the hot dry air tends to
rise at once to the opening, and pass away unsaturated with moisture,
while that cooled by evaporating water from the goods, being heavier,
tends to form downward currents and remain in the chamber. To this it
may be objected that aqueous vapour is much lighter than air; this
is true, other things being equal, but in practice the evaporation
of a given quantity of water cools the air and makes it heavier in a
materially greater degree than the admixture of aqueous vapour lightens
it. This source of waste of heat exists in the turret drier, but is
there, from its great height, reduced to a minimum. In lower sheds it
becomes very material, and the air currents formed are productive of
much harm by causing irregular drying. This difficulty has been met by
Mr. Edward Wilson, of Exeter, to whom the leather trade owes several
very useful inventions, by an ingenious drying-room constructed on
the lines indicated by Box, though I do not know that he was in any
way indebted to that writer for the idea. In this Mr. Wilson arranges
the steam-pipes, instead of on the floor, in a vertical compartment
partitioned from the chamber, through which air is admitted and heated.
This hot air fills the top of the chamber and from its lightness floats
in a horizontal layer, only descending and escaping by apertures in the
floor as it becomes cooled by evaporating the moisture of the hides.
Mr. Wilson states that the method answers well in practice, and it is
certainly the most scientific in conception, but it might be feared
that, as applied to a single floor, the upper parts of the butts,
suspended near the ceiling, would dry more rapidly than those near
the floor. If applied to a double-floored building, this disadvantage
would, from the stronger draught, and consequent larger supply of air,
be less likely to show itself, and the upper floor with its uniform
warm air would be well adapted for drying off finished sole-leather,
while the cooler and milder drying of the ground floor would be fitted
both in character and situation for that wet out of the yard. Special
precaution would be needed to prevent the heated air escaping by doors
opening into the upper floor. There is little doubt that as regards
heat this is the most economical system which has yet been invented.
A method has been introduced in the United States of drying wet and
finished leather all together, in drying-rooms heated to a considerable
temperature, and closely shut up. This is found to answer fairly on
leather from sour liquors, but that from strong and sweet liquors is
darkened, as might be expected. The drying is accomplished in much
shorter time than by the turret drier. The mixture of wet and dry
leather, and the lack of ventilation produce an atmosphere nearly
saturated with moisture, and hence the drying is not nearly so harsh as
might be supposed from the considerable temperatures made use of. There
does not, however, seem anything in the principle to recommend its
general adoption.
Another invention, of which we have as yet heard little definite in
England, consists in drying at a low temperature by air artificially
deprived of its moisture. This may be accomplished in several ways.
Experiments have been made in drying in a closed chamber provided
with trays of calcium chloride to absorb the moisture evaporated. Air
when artificially cooled by compression and subsequent expansion, as
in the case of ice-making machines, parts with a large portion of its
moisture, which is condensed in the form of ice in the tubes of the
machine. Such air, if subsequently warmed, would dry powerfully and
rapidly.
Before leaving the subject of drying-sheds, a few words on the
mechanics of drying in general may not be out of place. Air-drying
is dependent on the condition that the air must be capable of taking
up more moisture than it already contains. It is a matter of common
experience that there are warm days when the air is so saturated with
moisture in the form of invisible vapour, that scarcely any drying
takes place; and similarly, cool dry days, when leather dries rapidly.
The relative amount of moisture in the air is easily ascertained by
the simple instrument known as the wet and dry bulb hygrometer; an
instrument which ought to be in every drying-shed, especially where
steam heat is used. It consists of two similar thermometers, side by
side, of which one has the bulb covered with muslin and kept wet by a
piece of lamp-cotton attached to it, and dipping in a cup or bottle of
water. This water evaporates more or less rapidly, according to the
dryness of the air; and as heat is consumed by it in passing into the
gaseous condition, the wet thermometer falls more or less below the dry
in proportion to the rapidity of the evaporation. On a summer's day,
the difference may amount to 9°-12° F. (5°-7° C.), and this is about
the extreme dryness permissible in a drying-room for finished leather.
Wet leather should of course be dried much more slowly. The influence
of heat on drying is two-fold. It increases the capacity of the air
for moisture, and it replaces the heat consumed by evaporation. The
following tables give the capacity of air for moisture at different
temperatures, and the percentage of saturation as shown by the wet and
dry thermometer. At Greenwich, the mean humidity for the year is 82 per
cent.; or for the day-time only 76 per cent., varying from 62 in summer
to 86 in winter:--
Table I.--Capacity of Air for Moisture.
───────────┬──────────────────┬──────────────────────┐
│ Weight in Pounds │ Weight in Pounds of │
Temp. Fahr.│ of a Cub. Ft. of │ Moisture contained in│
│ Dry Air. │ a Cub. Ft. of │
│ │ Saturated Air. │
───────────┼──────────────────┼──────────────────────┤
32° │ ·0807 │ ·000304 │
42 │ ·0791 │ ·000440 │
52 │ ·0776 │ ·000627 │
62 │ ·0761 │ ·000881 │
72 │ ·0747 │ ·001221 │
82 │ ·0733 │ ·001667 │
92 │ ·0720 │ ·002250 │
102 │ ·0707 │ ·002997 │
───────────┴──────────────────┴──────────────────────┘
Table II.--Hygrometer Table.
───────────┬───────────────────────────────────────────────┐
Temperature│ Degrees between Wet and Dry Thermometers. │
of Air. ├───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┬───┤
│ 1 │ 2 │ 3 │ 4 │ 5 │ 6 │ 7 │ 8 │ 9 │ 10│ 11│ 12│
───────────┼───┼───┼───┼───┼───┼───┼───┼───┼───┼───┼───┼───┤
32° F. │ 87│ 75│ ..│ ..│ ..│ ..│ ..│ ..│ ..│ ..│ ..│ ..│
42 │ 92│ 85│ 78│ 72│ 66│ 60│ 54│ 49│ 44│ 40│ 36│ 33│
62 │ 94│ 88│ 82│ 77│ 72│ 67│ 62│ 58│ 54│ 50│ 47│ 44│
82 │ 95│ 90│ 85│ 80│ 76│ 72│ 68│ 64│ 60│ 57│ 54│ 51│
│ │
│ Per cent. of moisture, saturation being 100.│
───────────┴───────────────────────────────────────────────┘
As regards the heat consumed in evaporation; it requires about 1000
times as much heat to convert 1 lb. of water into vapour, as it does
to raise the temperature of the same quantity 1° F. At least as much
heat as this must be supplied if the air which has been used in drying
is to retain the same temperature it had at the outset, and therefore
if a turret is to keep at a higher temperature than the air, which is
necessary to create a draught, this is the minimum amount of heat which
must be supplied per pound of water to be evaporated. In practice much
more will be needed.
The following table shows the heat given out by different sizes of
pipes at different temperatures, and steam pressures, in units equal to
the heat required to raise 1 lb. of water 1° F., and the cubic feet of
air which they will heat.[V]
[Footnote V: To illustrate the use of such tables, the following
example may be given. To dry 100 butts in a turret, each containing 20
lb. of moisture, at least 20 × 1000 × 100 = 2,000,000 units of heat
will be required to replace the loss by evaporation alone. As a 4-in.
pipe at 300° gives off 669 units per foot per hour (see Table III.),
about 125 ft. would give off 2,000,000 units per day. If we compare
this with Mr. Hepburn's practical experience, supposing the 4 working
floors of his turret to hold 100 butts each (a low estimate), and to
dry in 10 days; we have 540 ft. for 40 butts or 1350 ft. for 100 butts
a day; showing that more than 10 times the minimum is required in
practice. Of course this allows for weather in which the air must be
heated considerably before it will dry at all, for heat that escapes
uselessly at the top and sides of the building, and for the fact that
the pipes are not heated the whole time, and probably, on the average,
to a much lower temperature.]
Table III.--Heating Effect of Pipes freely exposed to Air at 60° F.
──────┬─────────┬──────────────────────────┬──────────────────────────────┐
│ │Units of Heat per Ft.-run │ Cub. Ft. of Air at 60° F. │
│ │ of Pipe per Hour. │ (15-1/2° C.) heated 1° per │
Temp. │ Pressure│ │ Ft.-run of Pipe per Hour. │
of │ of Steam├──────┬─────┬──────┬──────┼───────┬───────┬───────┬──────┤
Pipe.│ per In. │2 in. │3 in.│ 4 in.│ 6 in.│ 2 in. │ 3 in. │ 4 in. │ 6 in.│
──────┼─────────┼──────┼─────┼──────┼──────┼───────┼───────┼───────┼──────┤
° F. │ lb. │ │ │ │ │ │ │ │ │
300 │ 53 │ 403 │ 545│ 669 │ 938 │ 22235 │ 28713 │ 36919 │ 51760│
280 │ 35 │ 355 │ 480│ 587 │ 825 │ 19582 │ 26490 │ 32387 │ 45521│
260 │ 21 │ 312 │ 421│ 515 │ 723 │ 17218 │ 23233 │ 28421 │ 39952│
240 │ 10 │ 271 │ 366│ 448 │ 627 │ 14946 │ 20199 │ 24717 │ 34594│
220 │ 2·5 │ 233 │ 313│ 384 │ 537 │ 12858 │ 17271 │ 21184 │ 29629│
200 │ .. │ 195 │ 263│ 322 │ 452 │ 10775 │ 14507 │ 17780 │ 24967│
180 │ .. │ 160 │ 216│ 264 │ 369 │ 8830 │ 11920 │ 14573 │ 20368│
160 │ .. │ 128 │ 172│ 210 │ 295 │ 7070 │ 9487 │ 11590 │ 16300│
──────┴─────────┴──────┴─────┴──────┴──────┴───────┴───────┴───────┴──────┘
It may be taken that 1/20 of the above volumes may be heated 20°, from
50° F. to 70° F., and so on; but if the average temperature is higher
than 60° F., the duty will be less, and to obtain the same effect the
pipe must be heated so much hotter as to keep the same difference as
before between the pipe and air. Thus a pipe at 300° F. will only heat
as much air at 80° F. as one of 280° F. will of air at 60° F.
[Illustration: Fig. 67.]
It will be noted that the efficiency of small pipes is much greater
than that of larger ones, and in these days of high-pressure steam,
much may be said in favour of the use of comparatively small
wrought-iron steam-pipes instead of the larger cast metal ones. The
first cost is small, the pipes are easily obtained ready screwed, and
in the lengths required, and may be put together by any intelligent
workman. The risk of fracture by the concussion of condensed water
is very trifling, as compared to that of metal, and much lighter
pipes are safe for high pressures. Steam-pipes must always be laid
with an incline of say 1 in. in 10 ft. from the end where the steam
is admitted, so that the condensed water may get away, and at the
lowest point a steam-trap must be provided for its escape. In the
writer's experience, the best form is that of Holman, made by Tangye
of Birmingham, of which the principle will readily be understood from
Fig. 67. The cup-shaped vessel _a_ floats on the water in the outer
casing, and so closes the valve _b_ until _a_ gets full, when it
sinks and allows the water to escape until it floats up again. It is
important that this trap should be set level, or the valve will not
close properly. Each pound of condensed water is equivalent to about
1000 units of heat given off (see Table III.). In planning steam-pipes,
it is not necessary that they should be arranged in a single line.
Even if in gridiron form the steam will still reach every part, in
proportion to the condensation which takes place. A series of large
pipes may be supplied by small pipes from a common main, and discharge
their condensed water into a common waste-pipe with branch from each.
A 1/2-in. pipe from a high-pressure boiler will supply a considerable
range, say 100 ft. of 4-in. pipe, though a larger size is advisable. At
the farther end of a range of steam-pipes a small tap must be provided
to let out the air which accumulates in them. In employing the exhaust
steam of an engine for heating purposes, the pipes must be of ample
size and freely open at the ends to avoid back-pressure. For this
purpose the gridiron form is a very good one.
The planning of hot-water pipes is much more difficult than that of
steam-pipes, but the general principle is that the pipes must rise
all the way from the boiler to the farther end, where there must be
an expansion-box or supply-cistern to allow the water to rise and
fall and dissolved air to escape. From this the pipes must fall more
or less, throughout the distance, back to the boiler, entering it at
the bottom. If at any point the pipe has to fall, leaving an upward
bend, a tap must be provided for the escape of air, but such upward
bends are a fertile source of difficulty and failure of action. With
long runs of either steam or water pipes, arrangements must be made
to allow of expansion and contraction, which will amount to 1-2 in.
per 100 ft., according to the temperature employed. If one end of the
system can be left free, all that is needed is to support the pipes on
rollers (pieces of old pipe may be used); if not, stuffing-boxes must
be provided.
The air heated by boilers, and other sources of waste heat, may often
be utilised for heating purposes, but generally requires to be driven
by a fan, unless the drying-room can be arranged directly above the
source of heat. If air has to be conveyed, the air-ways must be of
ample size, and if the ascending force of heated air be relied on,
passages less than 2 ft. sq. are seldom of much use. This ascending
force is generally much overrated where the differences of temperature
are so small as those employed in a drying-room. In a boiler chimney,
where the temperature of the escaping gases is 552° F. (289° C.), the
specific gravity of the air is about half that outside, and a chimney
of 50 ft. in height gives a draught equal to the pressure of a column
of about 1/3 in. of water, and the hot gases theoretically have a
velocity of about 80 ft. per second; whereas the same chimney with a
difference of temperature of 30° F. would have a draught equal to 1/300
in. of water only, and a velocity of 8 ft. per second.
The following table will enable the reader to calculate approximately
loss in friction in air-passages and the pressure required to pass a
given volume of air. The pressure needed increases in proportion to the
length of the pipe and the square of the velocity of the current of air
to be passed. Thus if we double the length of the pipe we must double
the pressure to pass the same quantity; and in order to double the
quantity of air through a given pipe, the pressure must be quadrupled.
Table IV.
Head, or Difference of Pressure at the two ends of a Circular Pipe 1
yd. long in inches of water required to pass 1000 cub. ft. of air per
minute.
────────────┬────────────┬─────────┬─────────────────────────────────
Velocity in │ Diameter of│ Head. │
Ft. per Sec.│ Pipe. │ │
────────────┼────────────┼─────────┤
│ in. │ │
84·8 │ 6 │ ·186 │ }
37·7 │ 9 │ ·02442 │ } To pass 100 ft. per min. these
21·2 │ 12 │ ·00579 │ } figures must be divided by
9·4 │ 18 │ ·000763 │ } 100. To pass 10,000 ft. they
5·3 │ 24 │ ·000181 │ } must be multiplied by 100.
3·4 │ 30 │ ·0000593│ }
────────────┴────────────┴─────────┴─────────────────────────────────
To calculate the head required for a long pipe, multiply the head given
by the table by the length in yards. The air passed by square pipes of
the same diameters will be 1·273 times greater with the same heads.
To be added to the pressure required to overcome friction is that
needed to force the air out at the end of the pipe. This varies with
the shape of the tube, &c., but for our purpose may be taken as given
in the following table:--
Table V.
Approximate Pressure needed to force Air out of a Pipe with a Velocity
of--
Ft. per Sec. Head in Inches,
Water.
84·8 1·8
37·7 0·36
21·2 0·13
9·4 0·02
5·3 under 0·01
3·4 under 0·005
Air-passages should be, as far as practicable, of uniform area
throughout their length, as much velocity is lost in passing even from
a smaller to a larger tube. Of course sharp bends must be avoided.