DUST AND FUME RECOVERY
FLUES, CHAMBERS AND BAG-HOUSES
DUST CHAMBER DESIGN
BY MAX J. WELCH
(September 1, 1904)
Only a few years ago smelting companies began to recognize the
advantage of large chambers for collecting flue dust and condensing
fumes. The object is threefold: First, profit; second, to prevent law
suits with surrounding agricultural interests; third, cleanliness about
the plant. It is my object at present to discuss the materials used in
construction and general types of cross-section.
Most of the old types of chambers are built after one general pattern,
namely, brick or stone side walls and arch roof, with iron buckstays
and tie rods. The above type is now nearly out of use, because it is
short-lived, expensive, and dangerous to repair, while the steel and
masonry are not used to good advantage in strength of cross-section.
With the introduction of concrete and expanded metal began a new
era of dust-chamber construction. It was found that a skeleton of
steel with cement plaster is very strong, light and cheap. The first
flue of the type shown in Fig. 29 was built after the design of E.
H. Messiter, at the Arkansas Valley smelter in Colorado. This flue
was in commission several years, conveying sulphurous gases from the
reverberatory roaster plant. The same company decided, in 1900, to
enlarge and entirely rebuild its dust-chamber system, and three types
of cross-section were adopted to meet the various conditions. All three
types were of cement and steel construction.
The first type, shown in Fig. 27, is placed directly behind the blast
furnaces. The cross-section is 273 sq. ft. area, being designed for a
10-furnace lead smelter. The back part is formed upon the slope of the
hillside and paved with 2.5 in. of brick. The front part is of ribbed
cast-iron plates. Ninety per cent. of the flue dust is collected in
this chamber and is removed, through sliding doors, into tram cars.
There is a little knack in designing a door to retain flue dust. It is
simply to make the bottom sill of the door frame horizontal for a space
of about 1 in. outside of the door slide.
The front part of the chamber, Fig. 27, is of expanded metal and
cement. The top is of 20 in. I-beams, spanning a distance of 24 ft.
with 15 in. cross-beams and 3 in. of concrete floor resting upon
the bottom flanges of the beams. This heavy construction forms the
foundation for the charging floor, bins, scales, etc.
[Illustration: FIG. 27.—Rectangular form of Concrete Dust Chamber.]
While dwelling upon this type of construction I wish to mention a
most important point, that of the proper factor of safety. Flue dust,
collected near the blast furnace, weighs from 80 to 100 lb. per cubic
foot, and the steel supports should be designed for 16,000 lb. extreme
fiber stress, when the chamber is three-quarters full of dust. If the
dust is allowed to accumulate beyond this point, the steel, being well
designed, should not be overstrained. Discussions as to strains in
bins have been aired by the engineering profession, but the present
question is “Where is a dust chamber a bin?” Experience shows that bin
construction should be adopted behind, or in close proximity to, the
blast furnaces.
Fig. 28 shows the second type of hopper-bottom flue adopted. It is
of very light construction, of 274 sq. ft. area in the clear. The
beginning of this flue being 473 ft. from the blast furnaces removes
all possibility of any material floor-load, as the dust is light in
weight and does not collect in large quantities. The hopper-bottom
floor is formed of 4 in. concrete slabs, in panels, placed between 4
in. I-beams. Cast-iron door frames, with openings 12 × 16 in., are
placed on 5 ft. centers. The concrete floor is tamped in place around
the frames. The side walls and roof are built of 1 in. angles, expanded
metal, and plastered to 2.5 in. thickness. At every 10 ft. distance,
pilaster ribs built of 2 in. angles, latticed and plastered, form the
wind-bracing and arch roof support.
[Illustration: FIG. 28.—Arched form of Concrete Dust Chamber.]
Fig. 29 shows the beehive construction. This chamber is of 253 sq. ft.
cross-sectional area. It is built of 2 in. channels, placed 16 in.
centers, tied with 1 × 0.125 in. steel strips. The object of the strips
is to support the 2 in. channels during erection. No. 27 gage expanded
metal lath was wired to the inside of the channels and the whole
plastered to a thickness of 3 in. The inside coat was plastered first
with portland cement and sand, one to three, with about 5 per cent.
lime. The filling between ribs is one to four, and the outside coat one
to three.
The above types of dust chamber have been in use over three years at
Leadville. Cement and concrete, in conjunction with steel, have been
used in Utah, Montana and Arizona, in various types of cross-section.
The results show clearly where not to use cement; namely, where
condensing sulphur fumes come in contact with the walls, or where
moisture collects, forming sulphuric acid. The reason is that portland
cement and lime mortar contain calcium hydrate, which takes up sulphur
from the fumes, forming calcium sulphate. In condensing chambers, this
calcium sulphate takes up water, forming gypsum, which expands and
peels off.
[Illustration: FIG. 29.—Beehive form of Concrete Dust Chamber.]
In materials of construction it is rather difficult to get something
that will stand the action of sulphur fumes perfectly. The lime mortar
joints in the old types of brick flues are soon eaten away. The arches
become weak and fall down. I noted a sheet steel condensing system,
where in one year the No. 12 steel was nearly eaten through. With a
view of profiting by past experience, let us consider the acid-proof
materials of construction, namely, brick, adobe mortar, fire-clay, and
acid-proof paint. Also, let us consider at what place in a dust-chamber
system are we to take the proper precaution in the use of these
materials.
At smelting plants, both copper and lead, it is found that near the
blast furnaces the gases remain hot and dry, so that concrete, brick
or stone, or steel, can safely be used. Lead-blast furnace gases will
not injure such construction at a distance of 6 or 8 ft. away from the
furnaces. For copper furnaces, roasters or pyritic smelting, concrete
or lime mortar construction should be limited to within 200 or 300 ft.
of the furnaces.
Another type of settling chamber is 20 ft. square in the clear, with
concrete floor between beams and steel hopper bottom. This chamber
is built within 150 ft. distance from the blast furnaces, and is one
of the types used at the Shannon Copper Company’s plant at Clifton,
Arizona. After passing the 200 ft. mark, there is no need of expensive
hopper design. The amount of flue dust settled beyond this point is so
small that it is a better investment to provide only small side doors
through which the dust can be removed. The ideal arrangement is to have
a system of condensing chambers, so separated by dampers that either
set can be thrown out for a short time for cleaning purposes, and the
whole system can be thrown in for best efficiency.
As to cross-section for condensing chambers, I consider that the
following will come near to meeting the requirements. One, four, and
six, concrete foundation; tile drainage; 9 in. brick walls, laid in
adobe mortar, pointed on the outside with lime mortar; occasional
strips of expanded metal flooring laid in joints; the necessary
pilasters to take care of the size of cross-section adopted; the top
covered with unpainted corrugated iron, over which is tamped a concrete
roof, nearly flat; concrete to contain corrugated bars in accordance
with light floor construction; and lastly, the corrugated iron to have
two coats of graphite paint on under side.
The above type of roof is used under slightly different conditions over
the immense dust chamber of the new Copper Queen smelter at Douglas,
Arizona. The paint is an important consideration. Steel work imbedded
in concrete should never be painted, but all steel exposed to fumes
should be covered by graphite paint. Tests made by the United States
Graphite Company show that for stack work the paint, when exposed to
acid gases, under as high a temperature as 700 deg. F., will wear well.
CONCRETE IN METALLURGICAL CONSTRUCTION[45]
BY HENRY W. EDWARDS
The construction of concrete flues of the section shown in Fig. 31
gives better results than that shown in Fig. 30, being less liable
to collapse. It costs somewhat more to build owing to the greater
complication of the crib, which, in both cases, consists of an interior
core only. For work 4 in. in thickness and under, I recommend the
use of rock or slag crushed to pass through a 1.5 in. ring. Although
concrete is not very refractory, it will easily withstand the heat
of the gases from a set of ordinary lead-or copper-smelting blast
furnaces, or from a battery of calcining or roasting furnaces. I have
never noticed that it is attacked in any way by sulphur dioxide or
other furnace gas.
[Illustration: FIGS. 30 and 31.—Sections of Concrete Flues.]
Shapes the most complicated to suit all tastes in dust chambers can
be constructed of concrete. The least suitable design, so far as the
construction itself is concerned, is a long, wide, straight-walled,
empty chamber, which is apt to collapse, either inwards or outwards,
and, although the outward movement can be prevented by a system of
light buckstays and tie-rods, the tendency to collapse inwards is
not so simply controlled in the absence of transverse baffle walls.
The tendency, so far as the collection of mechanical flue dust is
concerned, appears to be towards a large empty chamber, without
baffles, etc., in which the velocity of the air currents is reduced to
a minimum, and the dust allowed to settle. In the absence of transverse
baffle walls to counteract the collapsing tendency, it seems best to
design the chamber with a number of stout concrete columns at suitable
intervals along the side and end walls—the walls themselves being made
only a few inches thick with woven-wire screen or “expanded metal”
buried within them. The wire skeleton should also be embedded into the
columns in order to prevent the separation of wall and the columns.
This method of constructing is one that I have followed with very
satisfactory results as far as the construction itself is concerned.
[Illustration: FIG. 32.—Concrete Dust Chamber at the Guillermo Smelting
Works, Palomares, Spain. (Horizontal section.)]
Figs. 32 and 33 show a chamber designed and erected at the Don
Guillermo Smelting Works at Palomares, Province of Murcia, Spain.
Figs. 34 and 35 show a design for the smelter at Murray Mine, Sudbury,
Ontario, in which the columns are hollow, thus economizing concrete
material. For work of this kind the columns are built first and the
wire netting stretched from column to column and partly buried within
them. The crib is then built on each side of the netting, a gang of men
working from both sides, and is built up a yard or so at a time as the
work progresses. Doors of good size should be provided for entrance
into the chamber, and as they will seldom be opened there is no need
for expensive fastenings or hinges.
[Illustration: FIG. 33.—Concrete Dust Chamber at the Guillermo Smelting
Works, Palomares, Spain. (End elevation.)]
_Foundations for Dynamos and other Electrical Machinery._—Dry concrete
is a poor conductor of electricity, but when wet it becomes a fairly
good conductor. Therefore, if it be necessary to insulate the
electrical apparatus, the concrete should be covered with a layer of
asphalt.
[Illustration: FIG. 34.—Concrete Dust Chamber designed for smelter at
Murray Mine, Sudbury, Ontario, Can. There are eight 9 ft. sections in
the plan.]
_Chimney Bases._—Fig. 36 shows the base for the 90 ft. brick-stack at
Don Guillermo. The resemblance to masonry is given by nailing strips of
wood on the inside of the crib.
[Illustration: FIG. 35.—Concrete Dust Chamber designed for smelter at
Murray Mine, Sudbury, Ontario, Can. (End elevation.)]
_Retaining-Walls._—Figs. 37, 38, and 39 show three different styles
of retaining-walls, according to location. These walls are shown
in section only, and show the placing of the iron reenforcements.
Retaining-walls are best built in panels (each panel being a day’s
work), for the reason that horizontal joints in the concrete are
thereby avoided. The alternate panels should be built first and the
intermediate spaces filled in afterward. Should there be water behind
the wall it is best to insert a few small pipes through the wall, in
order to carry it off; this precaution is particularly important in
places where the natural surface of the ground meets the wall, as
shown in Figs. 37 and 38. If a wooden building is to be erected on the
retaining-wall, it is best to bury a few 0.75 in. bolts vertically in
the top of the wall, by which a wooden coping may be secured (see Figs.
37, 38, and 39), which forms a good commencement for the carpenter work.
[Illustration: FIG. 36.—Concrete Base for a 90 ft. Chimney at the
Guillermo Smelting Works, Palomares, Spain.]
Minimum thickness for a retaining-wall, having a liberal quantity
of iron embedded therein, is 20 in. at the bottom and 10 in. at the
top, with the taper preferably on the inner face. In the absence of
interior strengthening irons the thickness of the wall at the bottom
should never be less than one-fourth the total hight, and at the top
one-seventh of the hight; unless very liberal iron bracing be used,
the dimensions can hardly be reduced to less than one-seventh and
one-tenth respectively. Unbraced retaining-walls are more stable with
the batter on the outer face. Dry clay is the most treacherous material
that can be had behind a retaining-wall, especially if it be beaten
in, for the reason that it is so prone to absorb moisture and swell,
causing an enormous side thrust against the wall. When this material is
to be retained it is best to build the wall superabundantly strong—a
precaution which applies even to a dry climate, because the bursting
of a water-pipe may cause the damage. In order to avoid horizontal
joints it is best, wherever practicable, to build the crib-work in its
entirety before starting the concrete. In a retaining-wall 3 ft. thick
by 16 ft. high this is not practicable. The supporting posts and struts
can, however, be completed and the boards laid in as the wall grows,
in order not to interrupt the regular progress of the tamping. A good
finish may be produced on the exposed face of the wall by a few strokes
of the shovel up and down with its back against the crib.
[Illustration: FIGS. 37, 38, and 39.—Retaining-Walls of Concrete.]
In conclusion I wish to state that this paper is not written for the
instruction of the civil engineer, or for those who have special
experience in this line; but rather for the mining engineer or
metallurgist whose training is not very deep in this direction, and who
is so often thrown upon his own resources in the wilderness, and who
might be glad of a few practical suggestions from one who has been in a
like predicament.
CONCRETE FLUES[46]
BY EDWIN H. MESSITER
(September, 1904)
Under the heading “Flues,” Mr. Edwards refers to the Beehive
construction, a cross-section of which is shown in Fig. 31 of his
paper. A flue similar to this was designed by me about six years
ago,[47] and in which the walls, though much thinner than those
described by Mr. Edwards, gave entire satisfaction. These walls, from
2.25 in. thick throughout in the smaller flues to 3.25 in. in the
larger, were built by plastering the cement mortar on expanded-metal
lath, without the use of any forms or cribs whatever, at a cost of
labor generally less than $1 per sq. yd. of wall. Of course, where
plasterers cannot be obtained on reasonable terms, the cement can be
molded between wooden forms, though it is difficult to see how it can
be done with an interior core only, as stated by Mr. Edwards.
In regard to the effect of sulphur dioxide and furnace gases on the
cement, I have found that in certain cases this is a matter which
must be given very careful attention. Where there is sufficient heat
to prevent the existence of condensed moisture inside of the flue,
there is apparently no action whatever on the cement, but if the
concrete is wet, it is rapidly rotted by these gases. At points near
the furnaces there is generally sufficient heat not only to prevent
internal condensation of the aqueous vapor always present in the gases,
but also to evaporate water from rain or snow falling on the outside
of the flue. Further along a point is reached where rain-water will
percolate through minute cracks caused by expansion and contraction,
and reach the interior even though internal condensation does not occur
there in dry weather. From this point to the end of the flue the roof
must be coated on the outside with asphalt paint or other impervious
material. In very long flues a point may be reached where moisture will
condense on the inside of the walls in cold weather. From this point
to the end of the flue it is essential to protect the interior with an
acid-resisting paint, of which two or more coats will be necessary.
For the first coat a material containing little or no linseed oil is
best, as I am informed that the lime in the cement attacks the oil. For
this purpose I have used ebonite varnish, and for the succeeding coats
durable metal-coating. The first coat will require about 1 gal. of
material for each 100 sq. ft. of surface.
In one of the earliest long flues built of cement in this country, a
small part near the chimney was damaged as a result of failure to apply
the protective coating, the necessity for it not having been recognized
at the time of its construction. It may be said, in passing, that other
long brick flues built prior to that time were just as badly attacked
at points remote from the furnaces. In order to reduce the amount of
flue subject to condensation, the plastered flues have been built with
double lath having an intervening air-space in the middle of the wall.
In building thin walls of cement, such as flue walls, it is
particularly important to prevent them from drying before the cement
has combined with all the water it needs. For this reason the work
should be sprinkled freely until the cement is fully set. Much work of
this class has been ruined through ignorance by fires built near the
walls in cold weather, which caused the mortar to shell off in a short
time.
The great saving in cost of construction, which the concrete-steel flue
makes possible, will doubtless cause it to supersede other types to
even a greater extent than it has already done. If properly designed
this type of construction reduces the cost of flues by about one-half.
Moreover, the concrete-steel flue is a tight flue as compared with
one built of brick. There is a serious leakage through the walls of
the brick flues which is not easily observed in flues under suction
as most flues are, but when a brick flue is under pressure from a
fan the leakage is surprisingly apparent. In flues operating by
chimney-draft the entrance of cold air must cause a considerable loss
in the efficiency of the chimney, a disadvantage which would largely be
obviated by the use of the concrete-steel flue.
CONCRETE FLUES[48]
BY FRANCIS T. HAVARD
In discussion of Mr. Edwards’s interesting and valuable paper, I
beg to submit the following notes concerning the advantages and
disadvantages of the concrete flues and stacks at the plant of the
Anhaltische Blei-und Silber-werke. The flues and smaller stacks at the
works were constructed of concrete consisting generally of one part of
cement to seven parts of sand and jig-tailings but, in the case of the
under-mentioned metal concrete slabs, of one part of cement to four
parts of sand and tailings. The cost of constructing the concrete flue
approximated 5 marks per sq. m. of area (equivalent to $0.11 per sq.
ft.).
_Effect of Heat._—A temperature above 100 deg. C. caused the concrete
to crack destructively. Neutral furnace gases at 120 deg. C., passing
through an independent concrete flue and stack, caused so much damage
by the formation of cracks that, after two years of use, the stack,
constructed of pipes 4 in. thick, required thorough repairing and
auxiliary ties for every foot of hight.
_Effect of Flue Gases and Moisture._—The sides of the main flue, made
of blocks of 6 in. hollow wall-sections, 100 cm. by 50 cm. in area,
were covered with 2 in. or 1 in. slabs of metal concrete. In cases
where the flue was protected on the outside by a wooden or tiled roof,
and inside by an acid-proof paint, consisting of water-glass and
asbestos, the concrete has not been appreciably affected. In another
case, where the protective cover, both inside and outside, was of
asphalt only, the concrete was badly corroded and cracked at the end
of three years. In a third case, in which the concrete was unprotected
from both atmospheric influence on the outside, and furnace gases on
the inside, the flue was quite destroyed at the end of three years.
That portion of the protected concrete flue, near the main stack, which
came in contact only with dry, cold gases was not affected at all.
Gases alone, such as sulphur dioxide, sulphur trioxide, and others,
do not affect concrete; neither is the usual quantity of moisture
in furnace gases sufficient to damage concrete; but should moisture
penetrate from the outside of the flue, and, meeting gaseous SO₂ or
SO₃, form hydrous acids, then the concrete will be corroded.
_Effect of the Atmosphere Alone._—For outside construction work,
foundations and other structures not exposed to heat, moist acid gases
and chemicals, the concrete has maintained its reputation for cheapness
and durability.
_Effect of Crystallization of Contained Salts._—In chemical works,
floors constructed of concrete are sometimes unsatisfactory, for the
reason that soluble salts, noticeably zinc sulphate, will penetrate
into the floor and, by crystallizing in narrow confines, cause the
concrete to crack and the floor to rise in places.
BAG-HOUSES FOR SAVING FUME
BY WALTER RENTON INGALLS
(July 15, 1905)
One of the most efficient methods of saving fume and very fine dust in
metallurgical practice is by filtration through cloth. This idea is by
no means a new one, having been proposed by Dr. Percy, in his treatise
on lead, page 449, but he makes no mention of any attempt to apply it.
Its first practical application was found in the manufacture of zinc
oxide direct from ores, initially tried by Richard and Samuel T. Jones
in 1850, and in 1851 modified by Samuel Wetherill into the process
which continues in use at the present time in about the same form as
originally. In 1878 a similar process for the manufacture of white
lead direct from galena was introduced at Joplin, Mo., by G. T. Lewis
and Eyre O. Bartlett, the latter of whom had previously been engaged
in the manufacture of zinc oxide in the East, from which he obtained
his idea of the similar manufacture of white lead. The difference
in the character of the ore and other conditions, however, made it
necessary to introduce numerous modifications before the process became
successful. The eventual success of the process led to its application
for filtration of the fume from the blast furnaces at the works of the
Globe Smelting and Refining Company, at Denver, Colo., and later on for
the filtration of the fume from the Scotch hearths employed for the
smelting of galena in the vicinity of St. Louis.
In connection with the smelting of high-grade galena in Scotch hearths,
the bag-house is now a standard accessory. It has received also
considerable application in connection with silver-lead blast-furnace
smelting and in the desilverizing refineries. Its field of usefulness
is limited only by the character of the gas to be filtered, it being
a prerequisite that the gas contain no constituent that will quickly
destroy the fabric of which the bags are made. Bags are also employed
successfully for the collection of dust in cyanide mills, and other
works in which fine crushing is practised, for example, in the
magnetic separating works of the New Jersey Zinc Company, Franklin,
N. J. , where the outlets of the Edison driers, through which the ore
is passed, communicate with bag-filtering machines, in which the bags
are caused to revolve for the purpose of mechanical discharge. The
filtration of such dust is more troublesome than the filtration of
furnace fume, because the condensation of moisture causes the bags to
become soggy.
[Illustration: FIG. 40.—Bag-house, Globe Smelting Works.]
The standard bag-house employed in connection with furnace work is a
large room, in which the bags hang vertically, being suspended from
the top. The bags are simply tubes of cotton or woolen (flannel)
cloth, from 18 to 20 in. in diameter, and 20 to 35 ft. in length, most
commonly about 30 ft. In the manufacture of zinc oxide, the fume-laden
gas is conducted into the house through sheet-iron pipes, with suitably
arranged branches, from nipples on which the bags are suspended, the
lower end of the bag being simply tied up until it is necessary to
discharge the filtered fume by shaking. In the bag-houses employed in
the metallurgy of lead, the fume is introduced at the bottom into brick
chambers, which are covered with sheet-iron plates, provided with the
necessary nipples; or else into hopper-bottom, sheet-iron flues, with
the necessary nipples on top. In either case the bags are tied to the
nipples, and are tied up tight at the top, where they are suspended.
When the fume is dislodged by shaking the bags, it falls into the
chamber or hopper at the bottom, whence it is periodically removed.
The cost of attending a bag-house, collecting the fume, etc., varies
from about 10c. per ton of ore smelted in a large plant like the Globe,
to about 25c. per ton in a Scotch-hearth plant treating 25 tons of ore
per 24 hours.
No definite rules for the proportioning of filtering area to the
quantity of ore treated have been formulated. The correct proportion
must necessarily vary according to the volume of gaseous products
developed in the smelting of a ton of ore, the percentage of dust and
fume contained, and the frequency with which the bags are shaken.
It would appear, however, that in blast furnaces and Scotch-hearth
smelting a ratio of 1000 sq. ft. per ton of ore would be sufficient
under ordinary conditions. The bag-house originally constructed at
the Globe works had about 250 sq. ft. of filtering area per ton of
charge smelted, but this was subsequently increased, and Dr. Iles,
in his treatise on lead-smelting, recommends an equipment which would
correspond to about 750 sq. ft. per ton of charge. At the Omaha works,
where the Brown-De Camp system was used, there was 80,000 sq. ft. of
cloth for 10 furnaces 42 × 120 in., according to Hofman’s “Metallurgy
of Lead,” which would give about 1000 sq. ft. per ton of charge
smelted, assuming an average of eight furnaces to be in blast. A
bag-house in a Scotch-hearth smeltery, at St. Louis, had approximately
900 sq. ft. per ton of ore smelted. At the Lone Elm works, at Joplin,
the ratio was about 3500 sq. ft. per ton of ore smelted, when the
works were run at their maximum capacity. In the manufacture of zinc
oxide the bag area used to be from 150 to 200 sq. ft. per square foot
of grate on which the ore is burned, but at Palmerton, Pa. (the most
modern plant), the ratio is only 100:1. This corresponds to about 1400
sq. ft. of bag area per 2000 lb. of charge worked on the grate. In the
manufacture of zinc-lead white at Cañon City, Colo., the ratio between
bag area and grate area is 150:1.
Assuming the gas to be free, or nearly free, from sulphurous fumes, the
bags are made of unbleached muslin, varying in weight from 0.4 to 0.7
oz. avoirdupois per square foot. The cloth should have 42 to 48 threads
per linear inch in the warp and the same number in the woof. A kind of
cloth commonly used in good practice weighs 0.6 oz. per square foot and
has 46 threads per linear inch in both the warp and the woof.
The bags should be 18 to 20 in. in diameter. Therefore the cloth should
be of such width as to make that diameter with only one seam, allowing
for the lap. Cloth 62 in. in width is most convenient. It costs 4 to
5c. per yard. The seam is made by lapping the edges about 1 in., or
by turning over the edges and then lapping, in the latter case the
stitches passing through four thicknesses of the cloth. It should be
sewed with No. 50 linen thread, making two rows of double lock-stitches.
The thimbles to which the bags are fastened should be of No. 10 sheet
steel, the rim being formed by turning over a ring of 0.25 in. wire.
The bags are tied on with 2 in. strips of muslin. The nipples are
conveniently spaced 27 in. apart, center to center, on the main pipe.
The gas is best introduced at a temperature of 250 deg. F. Too high
a temperature is liable to cause them to ignite. They are safe at 300
deg. F., but the temperature should not be allowed to exceed that point.
The gas is cooled by passage through iron pipes of suitable radiating
surface, but the temperature should be controlled by a dial thermometer
close to the bag-house, which should be observed at least hourly, and
there should be an inlet into the pipe from the outside, so that, in
event of rise of temperature above 300 deg., sufficient cold air may be
admitted to reduce it within the safety limit.
In the case of gas containing much sulphur dioxide, and especially any
appreciable quantity of the trioxide, the bags should be of unwashed
wool. Such gas will soon destroy cotton, but wool with the natural
grease of the sheep still in it is not much affected. The gas from
Scotch hearths and lead-blast furnaces can be successfully filtered,
but the gas from roasting furnaces contains too much sulphur trioxide
to be filtered at all, bags of any kind being rapidly destroyed.