March 15, 1918.

THE COLLIERY GUARDIAN.

535

this massive crystalline appearance was found even in
places where there was no coloration, so the change is
not due to the presence of impurity. Nevertheless, some
difference still exists.

Though the author feels sure that the Austrians
calcine in rotary kilns, instead of the shaft kilns
formerly used, he believes that the modern gas-fired
shaft kiln can best answer the needs of magnesite brick
makers whose raw materials are in the crypto-crystalline
state. He suggests co-operative action for the purpose
of making trials in Greece (where very modern gas-fired
shaft kilns could be made available) on good practical
lines under experienced management, with a scientific
expert to make accurate records. In this way doubtful
points could be definitely settled, and the future of the
industry made safe.

The main conclusions drawn by the author from
microscopic examinations include the following:—With
proper application of heat, crystal growth can take
place, and such growth proceeds more quickly and
regularly if the impurity be distributed uniformly
throughout the magnesite. For the best results the
impurity-magma should be able to interpenetrate the
finest interstices of the magnesite particles, and not
merely form coatings round them; failing this, the
impurities should be correspondingly less in amount, as
they will only occupy the interspaces between the larger
particles. Particles in the crypto-crystalline state in the
bricks of furnaces working at white heats are continually
being altered from the small to the large crystal state
with greater porosity between these crystals, until by
continued spalling of the face of the brick next the
furnace, where the greatest contraction has taken place,
the new section of the brick, with altering particles, is
faced with the direct action of the slag, which can
penetrate the pores, causing frothing with greatly
increased porosity. Since, in specimens of Greek
magnesite calcined at about 1,750 (legs. Cent, in modern
gas-fired shaft kilns, it was found that the conversion of
crystalline particles had taken place almost throughout,
but without perfect regularity of crystal growth, it is
probable that the works’ management could attain more
uniform and complete conversion, if made aware of this.
In such specimens as contained ferric oxide, the conver-
sion of the crystal growth from crypto-crystalline to
massive crystalline was much more uniformly attained
within a certain distance from that impurity, and the
material was closely comparable to the Austrian product.
The British-made Greek magnesite bricks which most
closely correspond in micro-section to the Austrian
magnesite brick, as regards both regularity of growth
and the microscopic appearance of the magma, have been
found to give the best results in furnace linings.

In the course of a short discussion, testimony as to the
great improvements recently effected in British-made
magnesite bricks was offered by the manager of a local
steel works.

RECRUITING OF COAL MINERS.

The Government has decided to proceed with the
recruitment of 50,000 coal miners who are fit for general
service, and a communication to that effect has been
addressed to the secretary of the Miners’ Federation of
Great Britain. In order to make available for recruit-
ment the necessary men, the following decertification
Order has been made by the Home Secretary, dated
7th March, 1918 :—

“I hereby withdraw all certificates of exemption
issued on grounds of employment to persons employed
in or about coal mines who were on the 2nd November,
1915, unmarried or widowers without any child de-
pendent upon them, and had on the 1st January, 1918,
attained the age of 18 years and 8 months, but had not
on that date attained the age of 25 years. Provided
that this Order shall not be held to affect any certificate
of exemption issued on grounds of employment since
the 22nd November, 19/7, to a person whose certificate
of exemption was withdrawn by my Order of that
date. This Order shall take effect on and after the
21st March, 1918.”

The Directors of National Service in the regions con-
cerned will get into touch with representatives of
employers and of men so that the men selected may be
chosen either by age groups or by ballot, as may be
found more suitable to meet local conditions. Adequate
safeguards will be arranged so as to permit of the
retention in the mines of persons who are considered
indispensable for the safe working of the mines.

Applications for the renewal on personal grounds of
the exemption withdrawn by the decertification Order
may be made to local tribunals within the limits
prescribed by the regulations and instructions for
applications for renewal.

The Miners’ Federation has received from the Minister
of National Service a letter dealing with the scheme.

The letter states that the Government intend to
return from the Army for work in the coal mines a
number of pre-war miners who3 owing to wounds,
sickness, or physical condition, are unfit for combatant
duties either in this country or oversea. The proposed
scheme of recruitment of 50,000 men will accelerate the
process of returning to the mines a number of men
who have already done several years’ service with the
colours, as, until the 50,000 men are recruited, the Army
Council cannot entertain the idea of releasing from the
colours any pre-war miners. The recruitment will be
carried out by the Directors of National Service.

Sir Auckland Geddes adds that the national emergency
and the vital and immediate demand for recruits make
it necessary that the steps indicated in the letter shall
be taken.

The voting will be on two questions—the withdrawal
of the 50,000 men, and the use of the Federation
machinery for such a purpose. The figures should be
in the hands of the executive by next Monday. A
special conference of delegates will be held in London
on Wednesday, March 20.

NOTES ON COLLIERY TURBO PLANT.

By L. Fokes.

In the extensive literature dealing with turbo plant,
very little is to be found which would be helpful to the
large number of non-technical men who are engaged in
looking after this class of machinery throughout the
coal fields, or to those mining students who desire a
general knowledge of the principles upon which a
turbine works. There is an abundance of steam con-
sumption tests and various performances of interest
from the economic standpoint, but this is of little use
to men whose duty it is to look after plant and in some
measure preserve the conditions of operation so that
they shall not fa1! appreciably below those obtained on
the official tests.

For men to take an intelligent interest in the plant
under their care, they should at least understand the
general principles of operation, which would enable
them immediately to connect cause and effect, and so
maintain the plant in an efficient condition; and, with
this end in view, the following remarks will be of an
entirely non-technical character.

“First Principles.”

The fact which makes the steam turbine possible, is
that when steam is expanded from a high to a low
pressure it acquires very high velocities, which depend
upon the degree of expansion allowed. The steam
pressure in the boiler represents only potential energy,
which has neither movement or direction but is capable
of doing work if released—z.e., by allowing it to expand
or blow into a chamber of lower pressure than that of
the boiler. The high velocity attained by the steam
and the kinetic energy contained therein enable useful
work to be done upon any obstacle in its path.

To enable this to be accomplished effectually, the
steam must necessarily be directed in an efficient
manner during expansion, in order that the maximum
kinetic energy may be utilised.

Impulse Reaction Turbine.

• This is usually known as the reaction type as
differing from that of the impulse type, and was the
first. to be of real commercial value. The rapidly-
moving steam was made to travel through a number of
rings of moving and tixed blades spaced alternately.

to

Fig. 1.

rn

T E A M

70/

0
0
b

steam Velocity
relative to a-
fixed point

■

Fig. 2.

Fig. 1 shows a section of five rows of such*blades.
The fixed blades were for the purpose of directing the
steam, after leaving a moving row, in such a direction
as to again come in contact with the next row of moving
blades in the direction of rotation. The peculiar shape
of the blades will be noticed, this being designed to
give both a direct impulse and a reactive effort in the
direction of rotation.

At point A on each of the moving blades, part of the
turning effort or torque was produced by direct impact
of the steam as it left the fixed blades, while the straight
portions of the blades provided a nozzle effect which
re-established the velocity lost by friction in the bucket
or impulse portion of the previous row of blades, so that
the steam was directed with its original velocity against
point B on the fixed blades, the reaction of the
expanding steam exerting a force in the opposite
direction and in the direction of rotation.

This action and reaction occurred throughout the
whole of the blades, which were made larger in section
and length as the exhaust end of the turbine was
approached. This was in order to deal with the larger
volume of steam due to expansion, and to provide more
area of blade surface for it to act upon.

In this form of turbine the steam expanded from the
point where it entered the first row of fixed blades, right
through the successive rows of fixed and moving blades
to the exhaust end of the turbine. This set up a con-
siderable end thrust necessitating the use of balancing
pistons, on the high pressure end, consisting of discs
against which the steam pressure acted in a direction
opposite to the direction of steam flow through the
turbine, and in order to balance the end thrust.

Again, the continuous expansion of steam throughout
the turbine resulted, of course, in the maintenance of
the maximum boiler pressure between the inlet and the
exhaust. This circumstance in turn was responsible for
considerable leakage of steam, across the ends of the
blades, which did no useful work, and necessitated very
fine clearances being provided to reduce the leakage.

The fine clearances, however, presented mechanical
trouble, due to blades stripping through fouling the
casing or the rotor.

The leakage was also minimised by increasing the
pressure stages, which, however, made the turbine
casing long ; and in the larger capacities difficulties

were experienced through unequal expansion of the
casing. Moreover, great care had to be exercised in
starting up, to see that the turbine had been thoroughly
warmed before turning steam on for starting.

Bearing in mind that the live steam was directed
straight on to the blades, it is obvious that if the blade
temperature was much lower than that of the steam,
considerable condensation would take place; and this
was very harmful to the blading, as the water encountered
such enormous resistance that sometimes the blading
became buckled, and serious damage was caused.

Impulse Turbine.

In this type of machine, the steam was expanded in
stationary nozzles down to any predetermined point,
and directed by them on to an impulse wheel. Taking
a simple case, the action can be understood from fig. 2.

It is assumed that the velocity of the steam, as it
leaves the nozzle and comes in contact with the blades,
is 1,000 ft. per second, and that the speed of the blades
on the periphery of the wheel is 500 ft. per second.

The shape of the blades shown in fig. 2 is such as to
turn the impinging steam in a backward direction.

Now, if the wheel were stationary, the velocity of the
steam leaving the blades in a backward direction would
be about 1,000 ft. per second, and it is easy to see that
if the blades are travelling in the direction of the steam
at a velocity of 500 ft. per second, the velocity of the
steam relative to that of the blades is 500 ft. per second ;
but since, as already noted. the steam is directed back-
wards, the velocity in this direction will be 5o0 ft. per
second, whilst the blades are travelling forward at the
same velocity, so that 500 — 500 — 0, and the velocity
of the steam relative to a fixed point is zero, or only just
sufficient to enable it to clear the wheel.

From this example it is evident that the most efficient
speed of a single impulse wheel is half that of the steam
driving it.

The chief object, of course, is to use the whole of the
kinetic energy in the steam after expansion, and this
can be said to have been accomplished when the flow of
steam has been brought to rest, as in the case just
referred to.

It should be noted in passing that, in this case, the
steam has been expanded from boiler pressure to that
of the condenser, before entering the wheel so that the

pressure on^each[side‘bf the^wheel was the same ; there-
fore no end thrust exists and balancing pistons are
unnecessary.

Velocity Stages.

As steam at 165 lb. per sq. in. absolute, when
expanded to 28 in. vacuum reaches a velocity of about
4,010 ft. per second, it is impracticable to use blade speed
of half that velocity, owing to mechanical considerations.
If the impulse wheel just considered were run com-
paratively slowly, the whole of the velocity of the steam
would not be absorbed; but considerable kinetic energy
would still be left, in the form of velocity, to do further
useful work. It may therefore be arranged for the
steam issuing from the nozzles, to pass through the first
row of blades and be then redirected by the fixed blades
on to the next row and so on, as shown in fig. 3, until
the whole of the kinetic energy has been used, and the
steam has just sufficient velocity to clear the last row of
moving blades.

This complete operation of reducing the velocity of
the steam by passing through several rows of blades, is
known as a “ velocity stage ” and enables the speed of
the blades, and hence the revolutions per minute, to be
reduced, and so bring the speed down to a useful value.

Pressure Stages.

If, after the steam had been expanded in the nozzles,
it were passed through a sufficient number of rows of
blades, the speed of the turbine might be sufficiently
reduced by the use of one velocity stage; but it has
been found inadvisable to use too many rows of blades
after expansion takes place, so that another method is
resorted to, as follows :—

Instead of the first row of nozzles completely expand-
ing the steam, they expanded it only partially; the
corresponding steam velocity being dealt with by the
blades in that stage at the end of which the steam had
been brought to rest and was further expanded in
another row of nozzles which imparted to the steam
about the same velocity as did the first row of nozzles,
this velocity being again dealt with in a number of rows
of blades as in the first case.

This series of expansions coniinued until the last row
of nozzles expanded the steam to the condenser pressure.

Each of the expansions referred to was like a number
of small turbines enclosed in the one casing. Each one