THE COLLIERY GUARDIAN
AND
JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXI.	FRIDAY, JANUARY 21, 1916.	No. 2873.

High=Speed Air Compressors for Mining Work.*

By J. M. WAL5HE, A.M.I.E.E

Air compressing machinery appears to be developing
along the same lines,as the production of electricity, the
slow-speed compressor giving place to the high-speed
type, with its greater economy of material and space;
and, where very large units are required, the turbo-
compressor is supplying the need.

In the very large air supply stations on the Rand,
turbo-compressor units of a capacity of 20,000, and
even 50,000 cu. ft., requiring 4,000 to 13,000 horse-
power, are quite a commercial proposition, and. have
proved a practical success.

Air compressors of this size are, however, very excep-
tional, and the high-speed, reciprocating compressor is
quite capable of dealing with the highest outputs so far
required in this country; and, in the hands of manu-
facturers experienced in the design of high-speed steam
engines, a very high standard of reliability and efficiency
in operation is obtained.

■ Units of 1,000 to 1,300 horse-power are being success-
fully worked, and, owing to the relatively high speed of
revolution, the weight of the parts and the space taken
up are comparatively small. The speed, too, in the
case of electrically-operated compressors, assists in
keeping down the dimensions of the motor, and permits
of the motor being.directly connected., .

The so-called “ high-speed,” or, more correctly,
“ quick revolution ’’ type: of compressor is now exten-
sively used in mines and collieries, and any information
concerning it should be of general interest.

For the air. pressures required- for power purposes in
mining work, two-stage compression, with an inter-
cooler between stages, is the design usually adopted in
best practice.

Representative types of high-speed air compressors
were very fully dealt with in a recent paper read by
Mr. G, Blake Walker before the Midland Institute of.
Mining, Civil, and Mechanical Engineers.f The writer,
proposes to select as an example of this type of com-
pressor the one with which he is most familiar, and
which is probably more largely used than any other, at
any rate, in-England and the colonies, namely, the
double-acting two-cylinder two-crank type, with auto-
matic light plate air valves, and with the working parts
lubricated on the forced lubrication principle.

In this type the air valves are placed on the side of
the air cylinder, in which position they are readily
accessible, and leave the ends free for the water, jackets;
and, seeing that the compression is mainly done at the
ends of the stroke, this is a very desirable arrangement.

In this design it is impossible for oil from the crank
chamber to be carried up into, the air cylinder, as there
is a distance in 'excess of the full stroke of the com-
pressor between the cylinder - and casing stuffing boxes.
Another good feature is that,-piston rods are used, and
not trunks. Although the. differential trunk; is a-,very
usual design, and may be necessary in the case of small'
single crank compressors, it. is - .one which should be
avoided whenever possible. It will be appreciated that
a trunk is much more difficult to keep airtight than a
piston rod, and that a trunk working in and out of a
crank chamber is Hable to carry up a great deal of oil,
whilst at the same time permitting, air to pass down.
Apart from the waste of oil, the oil thus carried up may
become partly carbonised on the air valves, or distri-
' buted throughout the air system, to the detriment of the
- hoses. An even more serious objection 'is that, if the
air becomes heavily charged with oil vapour, dangerous
explosions may occur in the air discharge piping or air
’ receiver. .

If a trunk is used (and its use is almost imperative in
the case of single-crank two-stage compressors), the

* From a paper read before the North Staffordshire Insti-
tute of Mining and Mechanical Engineers.

f Trans. Inst. M.E., 1913, vol. xliv., p. 629.

the use of cleaner- and drier air offers advantages suffi-
cient to warrant the extra expense of an after-cooler is
a question which the writer would like the members to
discuss.

In point of compactness it may be observed that a
vertical high-speed compressor of a capacity of 4,000
cu. ft. occupies less than one-third of the floor space
than is required for a slow-speed horizontal compressor
of the same capacity. There .is also a great disparity,
in size and cost of foundations required by the two
types. It follows, too, that a smaller and less costly
building is required to accommodate the vertical com-
pressor, with a lower rental and taxation, and a travelling
crane of less span and lifting capacity.

A great point in favour of the adoption of the turbo
type of compressor for colliery or mining work is that
exhaust steam can be used in the turbine operating the
compressor; but it may be pointed out that the exhaust
steam can be utilised more economically by coupling the
exhaust steam turbine through gearing to a high-speed
reciprocating, compressor. • Such a plant-has'been in
successful operation for three years past at the Glass
Houghton Colliery, of the Glass Houghton and-Castleford
Collieries Limited, and has proved a practical and com-
mercial success. Before this novel combination wTas
adopted, the direct driven turbo-compressor received
full consideration,-but, from estimates received from a
large number of makers, both British and Continental,
it was apparent that a turbo-compressor capable of deal-
ing with 6,600 cu. ft. of free air, and delivering that
quantity at a gauge pressure of 701b. per square inch,
would require at least 21 per cent, more power to operate
it than a reciprocating set.

The efficiency of turbo-compressors may have been
improved of late years; mechanically, they' are - greatly
improved, but. they are still , far from being as efficient
as reciprocating.compressors of equal output. Moreover,
in mining work, where necessarily compressors often run
during some portion of the 24 hours at loads much below
the full load, the reciprocatbr possesses still greater
advantages, as,-for reasons which will be stated later
on, the efficiency of turbo-compressors is'very .low .at the
lighter loads.- ..	•	• -

In the .plant now in use, the air compressor lis of the
vertical two-crank double-acting type, suitable for the
specified duty when running at a speed of 163 revolutions
per minute, and fitted with automatic plate valves of the
kind previously described.. The turbine is of the mixed
■ pressure type, and. the gearing; of’the double helical type
supplied by the Power Plant/Ebm^any, the bearings
..being forced lubricated from the'’system which supplies
the turbine. Flexible couplings’ja-re fitted on both sides
of the gearing. The gearing- first ..supplied was not
entirely satisfactory; but, as a re stilt of certain modifica-
tiori^tfiat were made, the- initial difficulties were
overcome,. and the success .of the gear drive is now
beyond, question. •	.. . . •	■	• .	- '

In consequence of the satisfactory performance of this
plant,' another similar, plant of slightly' smaller dimen-
sions is now in course, of ■construction, and when the
advantages of this combination become more widely
known, it.is likely to be generally.adopted, although for
units of considerably larger output the turbo-compressor
will hold the field.

As is the case with steam turbines, the efficiency of
turbo-compressors improves with the size, and .for the
very large sizes in use on the Rand, the efficiency, if not
equal to that of a reciprocating compressor, is, still not
much below it, and their low first cost is greatly in their
favour.

Efficiency. >

When the performances of compressors is contrasted,
the only satisfactory, basis of .comparison is the. over-all
efficiency. When a comparison is made between steam-
driven compressors running under similar conditions of
steam supply and air -output, the number of cubic feet
of free air compressed per pound of steam consumed -is a
. very - convenient figure . for the over-all efficiency.
The equivalent figure for an electrically-driven com-

compressor should be so designed that the trunk does
not pass into the crank chamber. With the differential
trunk piston design, the final compression takes place
in the annular' space between the trunk and the air
cylinder, and thus not only are the trunk packings sub-
jected to the final air pressure, but also the first stage
piston packing rings. This increases greatly the possi-
bility of serious air leakage, and impairs the efficiency
by increasing the friction. In the two-crank design, the
circumference for leakage of piston ring and packing is
only a quarter of that in the differential trunk design,
and, moreover, the difference in pressure on the two
sides of the pistons tends to reduce the leakage. For the
rest, the forced lubrication system is too well known to
need description, but a word may be said concerning the
ai£ valves, a most important part of any compressor.

Automatic Air Valves.

In one very efficient valve of this type, the valve seat
and valve guard are made of cast iron. In the valve
guard are recesses for four closing springs. Next to the
seat comes the valve plate, made of thin tempered steel
perforated to give a multiple opening. Parts in the
slotted, plate are ground down and especially tempered ■■
to form springs, which enable the valve plate to rise and
fall through the small amount of its lift uniformly and
without friction. Next above the valve plate .is the lift
washer, and then the cushion plate, made of thin steel,
its function being to soften the blow of the valve plate
when lifting.

- As the working part pf the valve is made of a tempered
steel plate only -^in. or leSiS thick, it has the advantage -
of extreme lightness, especially when considered in pro-
portion to its surface area. • As the lift is only about
equal to the thickness, and. the- valve plate is guided
in a perfectly frictionless manner, it is obvious that it
will not be liable to hammer itself or its seat to pieces,
and can be depended upon to continue to work for an
indefinite period without failure, and without noise.

As previously stated, this type of compressor is
especially adapted for direct . connection to electric
motors. The use of spur or helical,gearing between,
the motor and the compressor should be avoided when-
ever possible, as (even if the cost of the motor is thereby
reduced) the resulting loss of efficiency and possibility of .
noise and trouble with the gear' discount largely any-
saving in price effected. In the case of motor driven
slow-speed compressors, the use of ropes, belts, or other/
speed reduction gearing is a common practice, and. the
consequent loss in over-all efficiency is one of the draw-
backs inherent to that type of machine.' •	.	. , '

The intercooler is provided with a central-diaphragm
plate supporting the tubes at, a point mid-way of their
length.' The air that enters divides on this plate, and
follows the course of the arrows, passing four'times both -
upwards and-downwards. , -	-

The cooling water ds also arranged ;to pass four times
upwards and downwards in a direction opposite to the
air. The result is a very efficient, intercooler, the air -
usually passing away to the second stage -at a lower
temperature than that at which the cooling water leaves
the intercooler. The tubes and tube plate are of brass,
similar tin design to the best surface condensers, -and can
be removed for cleaning. The allowance of cooling sur-
face depends to some extent, of course, on the tempera- •
ture of the cooling water, but is usually at the rate of
1 sq. ft. for every 4 or 5 cu. ft. of free air compressed
per minute.

The same design with a strengthened casing makes a
very efficient lafter-cooler. There is a growing tendency,
doubtless based on experience gained, to use after-
coolers, presumably to improve the quality of the air.
A great deal of the moisture in the air is condensed in
the after-cooler, and any oil or dirt which may have come .
through the compressor is also deposited there. Whether