572

THE COLLIERY GUARDIAN

September 17, 1915.

the credit side this volume of compressed air, the whole
of it, must be accounted for. In order to complete
the credit side of the balance, it is necessary to analyse
•the expenditure of air; to state the volumes spent in
productive work, and the volumes absorbed by unavoid-
able losses, and to set out in ruthless figures the volumes
uselessly squandered.

Two examples of air balances are given here. The
collieries to which they refer were not selected as in any
way exceptional, but solely because at each of them
compressed air was used for coal cutting only, and the
construction of a balance was thus comparatively easy.

80

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Diagram Showing Distribution of Air Volumes
Relative to Balance A

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Fig. 3.—Distribution of Volume.

Data for Air Balance.—In the course of the tests
referred to, it was not convenient to measure by meter
the total volume of air delivered by the compressors at
the various collieries. The method adopted was to take
indicator-diagrams from the compressor cylinders, and
from these to estimate the volumetric efficiency. This
having been obtained, a revolution counter was attached
to the compressor to record the total number of com-
pressing strokes made in the course of eight hours or
a longer period, and the total volume of air compressed
during the test period was thus ascertained. Neither
was it convenient to attach meters temporarily for
measuring the volume of leakage. The amount oi leak-
age as a proportion of the total volume compressed was,
therefore, ascertained by closing the stop valves of every
air-using appliance, and then counting the number of
compressor strokes per minute required to maintain
normal pressure. The results obtained by this method
were in some cases checked by stopping the compressor,
plotting the fall of pressure against time, and computing
the rate of leakage from the curve; the cubic contents
of the piping and receivers having been calculated.

When the consumption of the coal cutters was being
measured, the meter was connected at the gate-end
valves, and leakage from the hose pipes is therefore
included in the consumption of each machine. In the
tests upon which the balances are based, as recorded
on Tables II. and III., the hose leakage was not serious,
except in the cases where leakage is specially noted.
In order to arrive at the total efficiency of the system,
it is necessary to estimate or ascertain the power
delivered by the coal cutter motors; an exact determina-
tion of this is not practicable, but the empirical method
adopted is described in the following paragraph.

Diagram Showing Distribution of Energy

Relative to Balance A

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Fig. 4.—Distribution of Energy.

Power of Coal Cutters.—Estimate of the brake horse-
power delivered by the coal cutter motor per square yard
cut is based upon a long series of observations, recorded
during many years, of the energy expended by electric
coal cutters with motors of known efficiency, when
cutting in materials of various hardness. It is ascer-
tained that, in a very easy coal holing, the power
developed by a coal cutter motor is about 0’20 brake
horse-power X hour (=12 brake horse-power X minute)
per square yard cut, and for the hardest materials in
which it is customary to use machines about 0-80 brake

horse-power x hour (= 48 brake horse-power x
minute). The powers required to cut other materials
vary between these extremes. Examination of hundreds
of specimens in the light of the known power expended
in cutting them, enables one to place any sample
approximately in its true position in the scale of hard-
ness, and to estimate the power required to cut it.

It was not feasible on the same day to measure
separately the consumption of all the coal cutters, and
the average consumptions of the individual machines
were ascertained by tests on successive shifts. On the
day when the total volume of air compressed was
observed, the area undercut by each machine was noted
for entry on the credit side of the balance.

A noticeable feature on the credit side of the balance
is the allowance of 12 per cent, for no load and light
load running. The machine is run on light load (a)
while being manoeuvred into position by its haulage
rope; (b) while progress is arrested during incidental
attentions to the roof, adjusting props, etc.; (c) while
shifting the haulage post and drawing in slack rope; and
(d) always before the machine is stopped for any reason,
the haulage gear is disengaged and the machine is run
for a sufficient time to ensure that the cutter picks cut
clearance to allow the machine to start freely and
unloaded. These are all necessary operations; the
amount of air required by them is governable to a con-
siderable extent by the skill and care of the driver, but
for this item 12 per cent, is not an over-estimate.

Diagrams of Volume and Energy.—With reference to
the example treated in balance “ A,” a graphic repre-
sentation of the distribution of the air compressed is
presented by fig. 3, and fig. 4 represents the distribution
of energy in the same case.

Efficiency Deduced from Balance “ A.”—Assuming
that the materials cut require the development by the
machine of 0-5 brake horse-power hour per square yard

1  Coal  CUTTEf REFER ENCErf	2 NATURE OF Holins Matcrin	i o j o 	a £	4  Length  OF Trailing HoStYos	5 number OF Obser- vations	6 Air temper ATURE fah’	7  Press at Gate Machine sTahOik	8 press at Gate Machine Cuttine	9 Press Drop in hose lbs	10  PRESS at Coal Cutter LBS	II  INCHES PEfrMIN	12  Depth of Cut in  INCHES	13  Cub.ft  FREE  Air PER MIN	14  Cub ft  FREE  Air per ypcut
1	Milo Firecla*	2	50	17	49°	30	19 4	7 4	12	7 8	32	342	1800
2		2	35	18	49	29	18	56	124	8 0	32	313	1597
3	-	2	30	18	49’	38	33	13 8	I8	19	36	645	1155
4	A	2	20	17	50°	357	29	Gauge	Broken	12 5	30	376	1325
5		i	36	17	50°	39-3	343	16 3	I8	136	30	619	I960
6	Coal	2	36	17	48°	20	13	5	8	4 0	36	267	2427
7	Coal	2	72	17	49°	27 5	22 5	12 4	I0	10 0	38	508	1780
8	Soft  COAL	2	58	17	48°	43	25	II	I4	19 6	40	566	923
9	Coal	2	40	17	48°	40	32 5	*16	I6 5	13	43	650	1484
10	Coal		30	17	48° 	1	355  1		19	3	16 3	4	33	552	5465

The figures given in columns 6 to 14 are averages of the number of
-- OBSERVATIONS STATED IN COLUMN S DAMAGED & LEAKING AIR HOSE,-
f Average. Pressure

Table IV.—Tests of Coal-cutters Referred
to in Air-balance “B” (Table V). Air-
pressure at Surface 35 lb.

cut, a figure which in 'this case is certainly a liberal
estimate, the following results are arrived at :—

Total area undercut.................. 502 sq. yds.

Total volume of air compressed ...... 1,605,323	cu. ft.

Volume of air compressed per sq. yd. cut 3,200 cu ft.

Energy required to compress 3,200 cu. ft. to 50 lb. =

7’83 brake horse-power hours.

0‘5 brake horse-power hour

_	.	delivered by coal-cutter motor

Efficiency = 7.83 brake horse.power hours = 6’38 per cent,
delivered to compressor

To those familiar with the actual conditions of the in-bye
use of compressed air in collieries, there should be
nothing startling in that figure, which, let it be
remembered, refers only to the efficiency of the trans-
mission of the power of the steam engine on the surface,
to the gearing of the coal cutter, and has no-thing to
do with the efficiency, or inefficiency, of the steam plant.

The tests revealed that No. 1 machine (Table II.),
which was old, was badly in need of overhaul, and that
No. 2, although lately sent underground, was in a very
defective condition, and it also was withdrawn.

Efficiency Deduced from Balance “ B.”—Assuming
•that the materials cut require 0-4 brake horse-power per
square yard, under the conditions a liberal estimate, the
approximate efficiency of the compressed-air system may
be stated as follows :—

Total area undercut ................. 1,145 sq. yds.

Total volume of air compressed ...... 2,716,740	cu. ft.

Volume of air compressed per sq. yd. cut 2,372 cu. ft.

Energy required to compress 2,372 cu. ft. to 35 lb. =

4'66 brake horse-power hours.

0‘4 brake horse-power hour

„ .	delivered to coal-cutter

Efficiency = 4.6K brake horse-power hodJs’ = 8’6 per cent,
delivered to compressor

If the average pressure at the surface be taken as only
35 lb., and on this the brake horse-power delivered to
the compressor, and also the leakage, is based, it is
probable that the efficiency is overstated at 8-6 per cent.

Notes on Balance “ B.”—The machines tested were
all of the same type and size, and by the same maker,
and, with one exception, were cutting material of the
same order of hardness. Machine No. 8, which used
the smallest volume of air per square yard, also cut the
greatest area. This is to be attributed partly to the
material being more easily cut, but chiefly to the valves
and pistons of the machine being in good condition—
for the air, instead of blowing through the machine,
was devoted to useful work. The machine that con-

sumed the largest volume of air per square yard was
the one that cut the smallest area. The variations in
air consumption per minute by the machines were due
to two principal causes, namely :—(a) Low air pressure;
and (b) leaking valves and pistons. That low air con-
sumption per minute when due to low air pressure does
not imply economical use of air may be seen from No. 6
in Table IV.; and No. 10 shows that a rate of con-
sumption, not in itself abnormal if the machine were
developing full power, may be associated with excessive
extravagance. In both cases the high air consumption
per square yard cut is due to the machine being unable

Dr	Air Balance B	Cr								
I L	DESCRIPTION	commuho :	T  E «	DESCRIPTION	SOUA»E YAWD3			TOTAL
A	SlNOLE S’ASt OOuBLt CVLINOLP DOUGuE ACTIN® AlR compressor Cylinders 26 oia.  - 60* STROKE Volumetric crr* 71% Revolutions OF COMPRESSOR >N 15 HOURS* 38.700	2,027.880	1	Coal cutter  Running light	127-6	1.800	229,680  27.561	257,241
			2	COal Cutter Running light l24	2C-2	1 597	41 841 5,020	46.861
			3	Coal Cutter Running light 12	134-0	(! 155	(54,770  18.572	173.342
			4	Coal Cutter  Running Light	90	I 325	HO.250 14,310	133.560
			5	Coal Cutter Running Light flA	HO	J.960	215,600  25.872	241.472
B	Single stasc Single Cvunocr DOUBLE, actin® Air compressor Cylinder 22’pi a * 56’ stroke Volumetric srro 75% Revolutions or compressor in 15 hours. 38,7O(	688.860						
			6	Coal Cutter Running Li.ht l2^	152 i	12.427	320,364  38.443	358.807 .
			7	Coal Cutter Running LiSHT 1 tA	141 i	il. 780	250,980 30.117	281.097
			8	Coal Cutter running Light 12 i	222	923	204,906  24.588	229.494
			9	Coal Cutter  Running LIGHT	132	1.484	195.888  23. 506	219.394
			10	Coal Cutter Running Lisht 124	30	5.465	163.950  19.674	>85.624
				LOSS 6Y LEAKAGE from joints.opaim  - 19% or TOTAL				518.400-
				Unallocated losses including Air Blown off At safety Valve •2-7z;or total.				30,090
		2,716.740						2,716,740
								

Table V.—Air-balance “B.”

to develop sufficient power, and to the consequent low
cutting speed, with proportionately long time required
to cut a square yard. No. 10 machine was in a very
bad state, and with the stop-cock partly open, a large
volume of air escaped through the machine without
moving it. As a result of the test, this machine was
withdrawn for repair. The penalties of using a machine
in such condition are referred to in a later paragraph.

The compressing plant at this colliery was overloaded
on account of abnormal air consumption of some of the
machines, and the consequence was frequent fall of
pressure, high air consumption per square yard cut, and
unsatisfactory performance of the machines generally.
As will be readily understood, many points of much
local importance emerged from the test. The leakage
of 19 per cent, does not include the leakage from hose
pipes, which, in this case, was probably equivalent to
about 4 per cent, of the total volume; the leakage test
was made at a pressure of 35 lb.

Air Leakage.

Testing Rate of Leakage.—Considering the facility
with which the volume of air lost by leakage—as a pro-
portion of the total volume compressed — may be
approximately ascertained, the general neglect to make
periodic tests, as a check upon waste, can only be
attributed to lack of appreciation of the extent to which
leakage is responsible for loss of energy and low
efficiency. The simplest checking test for leakage is to
shut the stop valves at all the air using appliances, and
to note the number of strokes of the compressor required
to maintain the working pressure, no air being utilised.

Examples of loss by Leakage .						
1 H P OF COMPRESSORS	1	2	3	4	5	6
	1434	670	532	1250	317	440
8hp of Compressors at 80% Efficiency	1144	S34	426	1000	254	3S2
Fotal volume of free air  Compressed per minute in Cubic feet	6400	2800	2900	6486	17 30	3000
forking Air pressure	64 LBS	74 LBS	SO LBS	54L8S	SOlbs	35 LBS
volume of Free Air Compressed pep 8h p Per Minute inCuBic. Ft	SS	S-25	6-8	6*5	6-8	8>S
Volume LOST PER minute by leakage in Cub ft	1900	717	72 S	3350	7 70	S7O
proportion of loss to total	29*5/o	25*7/®	257®	52/.	44-6X	197.

Table VI.—Examples of Loss by Leakage.

This test is crude, but is of great value; it requires no
instruments, and may be made at every week-end or
in any half-hour when the plant is not in operation, by
anyone who knows that it is worth while to do so.
The actual leakage during working conditions and at
full load must be somewhat less than under the test
conditions, because the pressure in the pipes is not so
well maintained; but the difference is discounted by the
fact that usually a considerable proportion of the lost
air escapes from serious leaks at the joints of the main
piping, where the pressure is highest. It should be
noted that the lower the load factor is, the less is the
ratio of air utilised to air lost by leakage. In the case
of example (4) (Table VI.), 11 per cent, of the total
volume compressed was lost from the joints of the large
pipe between the receivers and the shaft; with the addi-
tion of the shaft piping (the valve at the shaft bottom
being shut), the loss was 30 per cent, of the total volume
compressed. These percentages are based on the