February 4, 1916.

THE COLLIERY GUARDIAN.

217

Typical Lay-out of Turbo-Blower.

Fig. 6 shows a typical lay-out of a turbo blower based
on the installation at the Norton Works of Messrs.
R. Heath and Company. The blower is of the double-
flow type rated at 30,000 cu. ft. per minute at a pressure
of 8 to 1’2 pounds per square inch.

The air is drawn from outside the station through
the air ducts A, A, which may with advantage be kept
separate. After passing through the blower the air is
discharged at I) passing through the non return valve
NR V into the distributing main E. The surging valve
is shown at SV immediately above the non-ieturn
valve, and is operated either by the non-return valve
spindle or by a hand wheel in the engine-room. The
discharge from the surging valve E is preferably taken
to a silencer S.

In this particular installation the blower is driven by
a mixed pressure steam turbine which is supplied with
a Leblanc multi-jet condenser, the pumps being engine-
driven.

TURBO-COMPRESSORS.

The tut bo compressor is a rotary air pump which
delivers air at a pressure usually between 50 and 100
pounds per square inch. In principle it is identical with
the turbo blower, and the remarks which have been
made on blowers apply in general to the turbo com-
pressor. Turbo compressors are made with capacities
varying from 3,000 to 50,000 cubic feet per minute, a
popular unit being 7,500 cu. ft. per minute. As compa- ed
with a blower, the smaller air output results in com-
pressors being made usually of the single-flow type, and
the high pressures with a correspondingly increased
temperature rise in the air, necessitates some provision
being made to cool the air during compression, so that
an efficient machine may be obtained.

Fig. 7 shows a seel ion of a compressor supplied by the
British Westinghouse Company and installed at
Welbeck pit, rated at 7,500 cu. ft. of free air per minute
at 80 lb. per square inch. The compressor consists of
20 stages, of which 10 form the L.P. cylinder and 10
the H.P. cylinder. The two sets of impellers are
mounted so that the unbalanced
thrust of the one opposes that of
the other, the residual thrust being
taken up by the balance piston L.

The cooling water enters at Gr,
passes into chamber D, then through
the hollow guide vanes F to the
chamber E ; it returns through the
neighbouring hollow guide vanes to
the chamber D. This path is
repeated until the outlet L is
reached, the water having passed
backwards and forwards three times-

The whole of the surface other
than the impeller is water jacketed,
and as the air brushes across the
surface at a high speed, the rate of
heat transmission is very high.

When sufficient surface cannot
be provided inside the compressor,
external coolers of the surface con-
denser type are resorted to.

For maximum efficiency a definite
relation exists between the radial
and peripheral velocities of the air

at outlet from impeller, necessitating a reduction in its
diameter in the successive stages of a compressor.

Surging Point.—The surging point in a compressor
having curved leaning-back blades in the impellers is
reached when tbe output is reduced to 30 per cent, of
that for which it is designed. Compressors as a rule
are required to work over a greater range of volume
than blowers, so that particular care has to be taken in
tbe shape of the impeller blade, the design of which
materially affects the minimum volume of air which
the compressor can discharge. For this reason the
blades of a turbo compressor impeller should be sloped
back at a considerable angle, necessitating a curved
blade. The construction, though more expensive than
the adoption of straight blades, is justified by the
lowering of the minimum possible output. By operating
on butterfly valves placed in the air ducts on the
suction side of the compressor cylinders, the surging
point can be reduced to 20 per cent, of the designed
output; this device is an invention of Prof. Rateau.

A surging valve is provided which opens when the
output is reduced to the minimum just referred to, and
through it sufficient air is passed to keep the total
discharged by the compressor above the critical value.

Such a valve is shown in fig. 8, where B is the surging
valve and A a non-return valve placed in the discharge
main. The height of the non-return valve from its seat
depends upon the amount of air passing through it.
At full discharge A is at the top of its lift, the end of
the spindle is above the lever E to which it is not
attached, and the surging valve B is closed by the
spring C. The non-return valve spindle is of such a
length that it comes into contact with E when the
discharge has decreased to a value, a little above the
surging point; any further decrease in output will
cause A to fall and the surging valve to open.

Governing.

Constant Pressure.—A compression can be arranged
with constant pressure governing, in which case a
maximum pressure variation of ± 3 per cent, is a
reasonable one to specify. The governing in the case of
British Westinghouse compressors is effected by means
of an oil relay. Tbe under side of a spring loaded
piston is connected to the discharge of the compressor,
so that with a fall in pressure it moves a pilot piston
down from its mid position, thus admitting a supply
of oil under pressure. The power piston is then lifted,
raising the turbine governor valve and returning the
pilot valve to its mid-position. The turbine then

speeds up, increasing the air pressure, and a condition
of equilibrium is reached with the pressure sensibly
at its normal value and the turbine running at an
increased speed.

When the speed of the turbine rises above a pre-
determined value, a centrifugal governor comes into
action, the constant pressure having reached the limit
of its range. The air pressure for which the gear is set
can be varied ± 10 per cent, by means of a hand-wheel.

Constant Speed.—As in the case of blowers, constant
speed governing can be adopted with the advantage that
the machine is simpler and the governing more direct.
In such a case the governor would control the speed
within 2 per cent, from normal air discharge down to
the surging point, so that tbe pressure would vary 4 per
cent, more than is given by the constant speed charac-
teristic. This variation in pressure is corrected by a
hand adjustment of speed on the governor.

Typical Lay-out of Turbo-Compressor.

Fig. 9 shows a typical lay-out of a turbo compressor
based on an installation which was supplied by the
British Westinghouse Company to Messrs. Angus Scott
and Partners for Brodsworth Main Colliery.

The compressor has twenty stages, arranged in two
cylinders, and is normally rated at 7,500 cubic feet of
free air per minute against a terminal pressure of 80 lb.
per square im-h gauge, the normal speed being 4,100
revolutions per minute.

Air is drawn in through the inlet duct A passing
through the butterfly valve Bi to I1? the inlet of tbe
L.P. cylinder. After passing through ten stages, it is
discharged at Bypassing through butterfly valve B2
into the H.P. cylinder at I2, and then through 10 H.P.
stages to tbe final discharge D9. The air then flows
through a non-return valve NRV and sluice valve SV
into the distributing main. Butterfly valves B, and
B2 are operated at light loads to reduce the surging
point. The surging valve SV discharges into the
silencer S. The compressor is driven by a mixed
pressure turbine which is supplied with a Leblanc

Fig. 9.—Typical Turbo-compressor Installation.

multi-jet condenser, the pumps being engine-driven.
Water is supplied to the compressor cooling jackets by
means of a centrifugal pump P, passing via pipes E and
return pipe F.

Russian Coal Mines and War Prisoners. — A note from

South Russia says that of the total number of workmen
engaged at the end of the year at the Donetz coal mines,
numbering 168,845, prisoners of war accounted for 15,765,
and the total number of prisoners of war occupied in the
collieries, iron works, etc., made about 25,000.

Hull Coal Exports.-—The official return of the exports of
coal from Hull to foreign countries for the week ending
Tuesday, January 25, is as follows :—Calais, 2,130 tons,
Christiania, 737; Dunkirk, 1,812; Genoa, 4,332; Gothenburg,
3,269; Malaga, 1,203; Rotterdam, 2,049; Rouen, 16,242;
West Coast Africa, 703; total, 32,477 tons. These figures
do not include bunker' coal, shipments for the British
Admiralty, or the Allies’ Governments. Corresponding
period, January 1915, total 57,459 tons.

The Government and Coal Supplies.—It is understood that
the Government will shortly announce that, in view of the
difficulties experienced in certain areas in obtaining adequate
supplies of coal, they are takipg steps to appoint local com-
mittees for the purpose of securing such adequate supplies
of fuel to important consumers. Nothing in the shape of
commandeering or controlling mines or output is meant.
There will be, it is stated, a central committee, with Mr.
Marwood, C.B. (Board of Trade) and Sir R. Redmayne
(Home Office) as chairman and vice-chairman respectively.
The Munitions Department, the Admiralty, the Railway
Executive, coal owners, merchants, and consumers will be
represented by the Central Committee; 11 mining areas in
the country will have local committees, and these will
act in concert with the Central Committee. The supply of
coal and coke to munitions factories will have special con-
sideration, the munitions department being represented by
Mr. Leonard Llewellyn and Mr. K. W. Price (Explosives
Department). The committees for Lancashire, Cheshire,
North Wales, and the Forest of Dean have not yet been
appointed. An unofficial report states that the committee
for South Wales will be Messrs. T. E. Watson (Bute Docks),
AV. R. Hann, Powell Duffryn), Ben Nicholas (Pontypool),
and Evan Williams (Llandennach). The committees for
West and South Yorkshire are stated to be as follow :—
West : Messrs. C .B. Crawshaw, chairman (Dewsbury);
F. J. Warrington, vice-chairman (Wakefield), A. W. Archer
(Pontefract), W. Hargreaves (Normanton), R. Holiday
(Featherstone) ; and A. B. Blakely (Wakefield). South
Yorkshire: Messrs. F. Parker Rhodes (Rotherham), J. J.
Addey (Barnsley), R. Richardson (Barnsley), and F. J.
Jones (Rotherham).

MINE ACCIDENTS AND UNIFORM
RECORDS.*

By Albert H. Fay, U.S. Bureau of Mines.

The magnitude of the mining industry may be realised
when it is understood that there are 6,000,000 men
employed in the various mines of the world. Of this
number about 3,800,000 are engaged in coal mining, and
2,200,000 in other mining. In addition to those there
are hundreds of thousands employed in the allied
industries, such as the manufacture of iron and steel,
copper wares, coke and chemicals, all of which are
directly based upon the mining industry.

The conservation of human life in industrial plants is
a question of paramount importance, and is being con-
sidered by legislative bodies, labour organisations,
captains of industry, and individual employers. Every
fatal accident leaves its impress upon the community in
the loss of a useful citizen and the provider for a family,
with the result that many widows and orphans are
rendered public charges, for which tax payers must con-
tribute support. The sufferings and privations borne by
many of the dependants cannot be measured by words,
nor compensated by a money equivalent. Here is an
industry employing 6,000,000 men, of which three out
of every 1,000 are killed each year. A reduction of
50 per cent, in the number of fatalities would result in
an annual saving of 9,000 human lives, to -say nothing
of the injuries and sufferings sustained by hundreds of
thousands of unfortunates. From a humanitarian point
of view, no greater good for the industry could be accom-
plished than to effect this reduction. Accidents will
occur, yet experience is showing that they may be
reduced in number, although it is impossible to eliminate
all accidents.

Principal Causes of Accidents.

Falls of Roof.—The United States Bureau of Minesf
has tabulated 24,291 fatalities due to falls of roof and
pillar coal in and about the coal mines of the United
States covering a period of 44 years. This represents
48 per cent, of the total number of fatalities, or a
fatality rate of 1-55 per 1,000 men employed. In the
metal mines of the United States fatalities due to this
cause represent about 35 per cent, of the total.

It is not always in the mines having the strongest roof
that the least number of fatalities occur. When a
mine has a bad roof, the miner, foreman, and all others
concerned will be cautious, and use plenty of timber
to keep the roof in place. Furthermore, the roof will be
tested frequently, and the miner will be on the look-
out at all times when he knows that roof conditions are
bad. With a strong roof, however, such precautions are
not usually taken. The miner and the foreman con-
sider the roof safe, and give it no further thought. This
leads to negligence, and, as a result, many of the
fatalities due to this cause occur where roof conditions
are considered the best.

Falls of roof being the largest principal cause of acci-
dents in both coal and metal mines, is one that should
command the serious attention of the inspectors,
operators, mine foremen, and the miners. Falls are
bound to occur, yet with proper precautions, use of
sufficient timber, and care on the part of the foremen
and miners, they should be reduced to a considerable
extent. Since 1909 this group of accidents in coal mines
has shown a gradual decline in the fatality rate from
1-84 per 1,000 men employed to 1-48 in 1914, the latter
figure being below the general average for 45 years, as
cited above.

Explosives.—Fatalities due to the use of explosives
in the coal mines of the United States represent 5 to 7-5
per cent, of the total number killed. The handling and
use of explosives deserve much attention on the part of
the operator, inspector, and the miner. Of these
fatalities, about 25 per cent, are caused by premature
blasts. The premature blasts are usually a result of
short fuses, the use of metal tamping bars, sparks falling
from matches, or from open lamps. About 17 per cent,
of the fatalities due to explosives are the result of
handling and transportation. The introduction of per-
missible explosives into the coal mines (1901) has had a
marked effect on the reduction of fatalities. In 1903
the total amount used was 288,661 lb., at which time the
fatality rate, due to explosives, was 0-339 per 1,000 men
employed. The quantity increased rapidly from the
above figure to 15,213,2571b. in 1914, with a fatality
rate of 0-096 per 1,000 men employed, showing the
advantages obtained (72 per cent, reduction) by the use
of this class of explosives.

Haulage Systems.—Fatalities due to the various types
of haulage systems represent about one-eighth of the
total number killed in and about the coal mines of the
United States. The percentage of fatalities from this
cause in 1871 was 13-33, while in 1914 it was 15-48. The
number of men killed per 1,000 employed (based on the
total number of men employed at the mines) was 0’747
in 1871 (the highest during a 44-year period), and showed
a decrease until about 1897, at which time it was 0’247;
since when there has been a gradual increase both in
percentage and in the number killed per 1,000 employed.
The use of electric and compressed-air haulage neces^
sarily means more rapid transit, and, furthermore, the
haulage systems • of the present day are much more
extensive and longer than they were in the early days of
the coal mining industry. This increase of the hazard
due to haulage systems should be combated by all means
possible for the prevention of such accidents.

Electricity.—The first fatality reported as due to
electricity in the coal mines of the United States was
in 1895. The number of fatalities so reported has

* From a paper read before the Second Pan-American
Scientific Congress, held in Washington, on December 31,
1915.

f Coal Mine Fatalities in the United States, 1870 to 1914.
By Albert H. Fay. Bulletin 115, U.S. Bureau of Mines,
Washington, D.C.