424

THE COLLIERY GUARDIAN.

August 27, 1915.

CURRENT SCIENCE

Deterioration of Fireclay.

A question that has excited no little interest is that of
the damage to retorts or firebricks caused by the
p» esence of “ salt ” in the coals carbonised. In the Gas
World, Thomas Holgate has recently presented a useful
review of the investigations carried out within late
years, quoting the researches of Schreiber, Simmers bach,
Blasberg and others. Amongst other suggestions that
have been made is one that the corrosion is much more
marked where firebricks containing a high percentage
of alumina are employed. On the other hand, it has
been contended that potassium and sodium disintegrate
the bricks by the production of a fusible silicate.

Mr. Holgate cites experimental data to show that
under certain circumstances alumina is undoubtedly
lost from firebricks, but in the tests of bricks from the
Birmingham Corporation coke ovens it was found that
a brick relatively rich in alumina, which had suffered
more than one relatively poor in alumina, had perished
not by loss of alumina, but by loss of silica. The
author suggests that, judging from the large accession
of chlorides and the well-known chemical activity of
aluminium chloride, the last-named substance may be
an intermediary in the mischief. The oxides of carbon, the
hydrocarbons and water present in the coke oven or the
heating flue should not be excluded from consideration,
and towards these gases aluminium chloride is a
powerful catalyst. It is pointed out that the accele-
rating effect of aluminium chloride is roughly in
proportion to the quantity present, and that if it enters
into certain combinations its effect upon the surrounding
substances is diminished. Its behaviour depends, in
some cases, upon its being anhydrous

Simmersbach has referred to the influence of metallic
carbonyls, showing the variation in iron contents in
different parts of a brick examined. Mr. Holgate thinks
that the loss of iron and the accession of carbon may be
due to the agency of iron carbonyl. It has become
known to clay workers that a given amount of ferric
oxide would promote fusibility in a different degree
according to the presence of other fluxes, and also to the
ratios of the concomitant alumina and silica.. If a
silica brick, either by its original or acquired iron
content, contained as much thereof as a more aluminous
brick, fusibility would be increased to a greater extent.

The author comes to the conclusion that, whilst silica
can be lost from coke-oven bricks by fusion, yet it may
be accompanied by disintegration brought about by
depositions within the pores of the material.- If, then,
these hostile forces can be kept outside the brick by
means of a close, homogeneous surface, much would be
gained. The admixture of alumina may, at certain
concentrations, introduce disturbing factors. He also
refers to the deposition of carbon due to imperfections
and heating arrangements, and to the discharge of solid
particles carrying electric charges from hot bodies,
which, according to temperature, may vary from
positive to negative. The ionisation of the gas^s may
also be affected by the constituents of the water present
in the coal being distilled, and be accompanied by
sympathetic action on the brickwork. In this connec-
tion attention is directed to the use of pit water’ for
washing coking coal, which may retain its radio-active
properties.

Storage of Powdered Coal.

Some notes on this subject, by Edward J. Kelly,
appear in the Coal Age. The following precautions, he
says, should be observed in storing powdered coal:—

(1)	Limit the height of the coal pile to 10 or 12 feet;
(?) isolate the coal from all sources of heat such as
steam pipes, flues, reflecting surfaces and direct sun-
light. In regard to the conditions of storage, two
different methods may be cited :—(1) Storing in piles in
the open air; (2) storing in covered bins.

There seems to be very little to choose between these
methods, save that all other things being equal, the
covered bin would protect the coal from direct sunlight,
which was shown to be a contributing factor in spon-
taneous ignition. If covered bins a,re used, added
overhead charges will be entailed, while open storage on
the other hand gives low costs. A novel storage bin
constructed for a firm in Maine contained several rows
of upright beams. Saw-tooth projections were built on
two sides of each upright, thereby forming air spaces,
which led into the main flue, the latter extending from
the bottom of the bin to the top. When the bin was
filled with coal the heat generated and the gases given
off escaped through the flues to the top of the bin and
out into the air. It is perfectly obvious, however, that
should the coal in the bin become ignited by any
chance, the flues would produce a draught and so increase
the rapidity of the combustion.

Bins should be constructed of stone, brick or con-
crete ; if ironwork has entered into the construction, it
should be covered with concrete.

Foundry Coke Specifications.

In a long article on recent proceedings and findings
of the American Society for Testing Materials, the
Iron Trade Review deals with the subject of foundry
coke specifications. Only tentative specifications for
foundry coke were presented, owing to the wide
variation in the physical and chemical requirements of
coke used for various purposes. It was recognised that
a rational and accurate method of sampling coke
should involve taking regular increments while loading
or unloading the car, to secure a several hundred-
pound gross sample, which should then be reduced by
suitable methods of crushing, mixing and quartering to
a convenient size for transportation to the laboratory.
This method of sampling, however, presents several
difficulties, including the crushing and reduction of
several hundred pounds of coke, necessitating the use
of mechanical crushers. Furthermore, sampling while

AND TECHNOLOGY.

unloading the coke is feasible only when the coke is
produced under a system of premiums and penalties.
The report further states that, since the only acceptable
form of specification is one based on maximum and
minimum limits of chemical composition, with the
privilege of rejection when these limits are exceeded, it
seemed essential to take the sample from the exposed
surface of the car, so that the car could be rejected
before unloading, in case the coke did not conform to
the specifications. The committee recognised that a
minimum gross sample of two pecks of walnut size is
too small to be strictly representative. However, in
view of the fact that a sample taken from the top of a
car is only approximately representative of all the coke,
it seemed unnecessary to impose the extra expense
entailed by crushing down a larger sample.

Satisfactory progress has been made toward the
adoption of uniform standard methods of analysing
coke, by co-operation with the joint committee on
coal analysis of the American Chemical Society and
the American Society for Testing Materials, and the
committee on coke analysis of the American Foundry-
men’s Association. It is expected that uniform standard
methods will be available for incorporation in coke
specifications by the end of another year. The com-
mittee gave considerable attention to the various
physical tests of coke, such as crushing strength,
shatter test, porosity, specific gravity, &c., and, as a
result of this investigation, concluded that the com-
pressive strength test, as determined on 1 in. cubes, is
worthless as a means of comparing different cokes, and
that in general the relation between results of physical
tests of coke and its value in practical use is too
indefinite to include quantitative expressions of physical
properties in standard specifications at present.

The Valuation and Estimation of Coke.

A recent issue of Feuerungstechnik, quoted by the
Gas World, contains the results of an investigation of
this subject by Thaler. ’ Cokes for foundry and blast-
furnace use are differentiated in practice, but in reality
very little difference can be found between them. Coke
for foundries should be the largest and best pieces, and
should, as a rule, contain less moisture and sulphur
than blastfurnace coke. A table showing the quality of
coke required by the German Foundrymen’s Association
is also given. This allows a moisture content of 5 per
cent., but in the author’s opinion this is too high. The
moisture content is largely dependent upon the physical
structure of the coke, a good hard quality being
preferred, as this carries less moisture than a softer
coke. The phosphorus existing in the coke is almost
entirely taken up by the iron, and it is desirable, there-
fore, that the phosphorus should not exceed 0’04 to
0'05 per cent.

In classifying cokes, the physical properties as well
as the chemical composition should be taken into
account. The principal points to be considered are as
follows:—

(1)	First-class Coke. — Silver-white lustre, sinewy
structure, dense, firm, with a metallic ring, resistant to
pressure; it should not break into small pieces on
falling, and should not contain more than 8 per cent,
ash, 3'5 to 4 per cent, moisture, and 0*8 per cent,
sulphur.

(2)	Second-class Coke.—Faint white to dark lustre,
structure more bony and porous than (1), less resistant
to pressure and heavier than coke of Class I. It should
not have more than 10 to 11 per cent, ash, 8 to 9 per
cent, moisture, and 1 to IT per cent, sulphur.

(3)	Third-class Coke.—Lustreless, grey-black fracture,
soft, breaking easily when struck; the individual coal
particles are easily distinguished; it is highly porous,
and holds water tenaciously. It has a low density,
9 to 9’5 per cent, ash, 13 to 15 per cent, moisture, and
1 per cent, sulphur.

The importance of proper sampling is strongly
emphasised.

The Calibration of Anemometers.

A recent number of Engineering includes an article
by D. A. Hackett on the above subject. He points out
that an absolute check on the correctness of air-
measuring instruments is, practically speaking, outside
the reach of anyone engaged in manufacturing or
commercial work, and hence any process by which a
calibration can be made and corrections obtained with
accuracy and despatch is welcomed, particularly if the
cost be not excessive. With the anemometer especially,
it is necessary to have some means of calibration if
consistent results are required.

Generally, in order to calibrate an anemometer it is
placed at the end of an arm revolving at uniform speed,
so that the velocity of the air relative to the instrument
is known. A comparison of this velocity with the
instrument reading gives the necessary correction.
Another method is the comparison of the readings of
the anemometer with the figures obtained by a Pitot
tube and water-gauge at a point in a duct in which a
steady current of air is maintained by means of a
centrifugal or other fan. Both these methods have
their disadvantages, but the author describes a method
of calibration which, whilst requiring the Pitot tube and
gauge, does not depend for the accuracy of the results
obtained on the careful observation of minute scale
readings and careful legulationof apparatus.

When a fan delivers air to a given system of ducts, or
operates on a given resistance, it has been shown that
the volume discharged is a linear function of the speed.
This law has been confirmed for peripheral speeds of
fans up to 120 m. per second, and for outlet air velocities
up to 27 m. per second. At the National Physical
Laboratory, Threlfall’s discovery of the fact that for any
given pipe the law of distribution of the axial velocity
over any section is independent of the rate of flow, has

been confirmed, provided the section of the pipe under
examination is sufficiently far from the origin of the
pipe. This latter proviso is necessary owing to the
disturbing effect of abrupt changes of section, and
therefore of velocity, on the flow along the pipe for
some distance from the seat of the disturbance.

If, then, a fan be coupled to a variable-speed motor
and arranged to draw or discharge air through a duct,
it is obvious that if the velocity at any speed of the
motor be known, then the velocity at any other speed is
obtainable. As the air velocity decreases, the difficulty
of obtaining the true velocity head is increased, thus
giving rise to a greater probability of error where Pitot-
tube determinations are depended upon solely. It is,
however, a simple matter to, measure the revolutions
per minute with an accuracy which, at any rate, does
not decrease with diminishing speed, and hence the
reliability of this method. It is only necessary to take
one or two Pitot-tube observations at a point in the
duct for velocities of the order of the maximum for
which the anemometer is suitable, or even higher if
convenient. The velocity for one or two such points
and the corresponding motor speeds having been
obtained, from the relation V = Kn, a curve can be
drawn which gives all the information required for
checking the readings of the anemometer at other
velocities at the same point in the duct. It is only
necessary then to note motor speeds and the anemometer
readings over the range required.

It is desirable, if the observations be made at the
outlet side of the fan, as is generally most convenient,
that the position chosen should be some distance away
from the fan; and, in addition, screens composed of,
say, coarse-mesh wire gauze or perforated metal should
be inserted in the duct to produce a uniform flow. If
it is possible to take the observations in a duct leading
to the inlet of the fan, such precautions are generally
unnecessary, as a uniform flow can be obtained with a
bell- mouthed inlet.

A precisely similar method may be applied to the
calibration of water meters, or, in fact, of any instru-
ment intended to be used for the measurement of the
flow of fluids.

Electric Blasting and Lightning.

A controversy of considerable interest is being waged
by the members of the South African Institute of
Electrical Engineers. The discussion began at the
March meeting of the institute on the subject of blasting
accidents in the shafts from lightning. One speaker,
Mr. A. E. Vai Davies, considers that the predisposing
cause is the use of weak primers. Connected to these
primers, he says, are 10, 20, or 150 ft. of more or less
insulated wire ready to receive energy as the result of
one or more of the following conditions:—Conduction,
induction, electro-static influence, the reception of
electro-magnetic waves, local electrolytic action. The
penultimate of these he considers the crux of the whole
question. The existing form of primer was designed for
use with a hand magneto, or “ igniter.” To detonate a
number of charges simultaneously with the small hand
machine it was therefore essential that fulmination of
the primer should be easy and rapid. If a primer
demanding, say, 50 watts for fulmination was used, this
would at least minimise the risks of prematme explosion
in normal weather.

Refuse as Fuel.—Dealing with the value of refuse as fuel,
Dr. Whitehouse, the medical officer of health for Deptford,
states, in the course of his report, that the composition of
ashbin refuse varies considerably, but quotes the subjoined
as a representative example of what is obtained in towns :—

Weight
per cent.

Breeze and cinder ........................... 50*0

Paper, straw, fibrous and vegetable refuse ... 13'0
Coal ......................................... 0'7

Bones and offal .............................. 0'6

Bags ......................................... 0*4

Coke ......................:	............. 0'3

Ash ......................................... 12'0

Dust and dirt................................20 0

Bottles, 1 per cent.; tins, 0'7 per cent.; metals,

0'2 per cent.; crockery, 0'6 per cent.; broken
glass, 0'5 per cent............................ 3	0

100'0

The quantity of ashbin refuse produced varies very largely in
different districts. It usually ranges from about 4 cwt. to
7 cwt. per head per annum. A table is given showing that
the average calorific power of 11b. of ordinary ashbin refuse,
when dry, is 3,696 units of heat, and if fully utilised would
evaporate 3-826 lb. of water from and at 212 degs. Fahr. :—

Calorific power, or units
of heat per pound of
the combustible.

Evaporative power of—	When dry.	taining the average amount of moisture.
Coal 		14,000	...	9,334
Coke 		12,000	...	8,000
Bones and offal			8,000	...	5,334
Breeze and cinder 		6,000	...	4,000
Rags 		5,000	...	3,334
Paper, straw, fibrous ma- terial and vegetable refuse	3,800	...	2,534

The quality of refuse from a combustion point of view natur-
ally varies very much, but the methods of dealing with it
have much in common, combustion being assisted by hot
air delivered into the furnace ashpit. The furnace is of
ordinary construction, having movable firebars. From 1-8
to 2 lb. of water are usually evaporated from and at 212 degs.
Fahr, per lb. of refuse, and about 50 to 701b. of refuse are
consumed per sq. ft. of grate area per hour. It will, of
course, be understood that, owing to the large varieties of
refuse, and the particular conditions as to transport, etc., to
be met, it is a difficult matter to lay down any rules as to
refuse destruction, or to draw any comparisons between
-different types of destructors. The clinker remaining after
combustion is completed may be used for making mortar,
paving blocks, etc.