June 20, 1913

THE COLLIERY GUARDIAN.

1329

5
«0
<0

*»

I

S’

isssi

SSSKI

SsiiiE

'sssm

liihiiliiEHsihiii

■■■■■■■■■■■■■■■■■■■■

ISSSSSS

s

IHSHII

H!

i®...

gs
■■■■■■■■■I

<■■■■■ ■■■■■■
issssflsas:
!»■■■ esses:
rs»s —

■■■■■■

■■■■■■
IE

Fig. 4.

																																																								
																																																								
								—																													—																			
													T	‘3...															—																											
																																																		/E 1	N° (					
																																																								
																													■ -													- -														
																																																								
																													—																											
																																																								
																																																										
																													—																									4		
																																																								
																													—																											
																													---																											
																			—													(T	iGf	<inc													—									
																				—r											4						t-f											—-	...					"T		
																					- -										4		.-I									;4								S.-+						
																														1	ITT										5|-t															
																														?/T7 sneb			: jstdon or																							
														_ a j															; lometans.			y?e	i '0																							
																																										1	1														
																																/4a																								
																																			ci?/			<444 Lrr?																		
																															1 l‘-t										e															
																															1 1^	jnys	VP5.		Thin dptt																					
																																																								
																—																Jcye	V?	6V	THjn coat				JS									4-4-								
																											---																										4-			
																													- -																											
																													---																											
														---																																		-U-								
				---																														-r1-																						
																													—																											
																																																								
												“7"																	■ -										-								---						■“b			
																																																					--ft4-		- -4-	

50 •	60	70	80	90

Times.—Hundredths of a second (reckoned from the time of ignition).



The rapidity with which explosive combustion (as dis-
tinct from inflammation) is propagated in coaldust and
air mixtures is hardly compatible with the notion that
distillation of gas must necessarily first take place, and
that it is the heat-energy derived from the combustion
of that gas which is chiefly responsible for the explosive
effects. Such a distillation requires an appreciable time
and absorbs energy. If, in the propagation of the flame,
the time required to raise the dust and air mixture to
the ignition temperature allows appreciable distillation
to take place, and the inflammable gases thus evolved
are burnt in preference to the entire dust particles, then
analysis of the products of explosive combustion should
afford proof of it. Preferential burning of the “ volatile
constituents ” of the coaldust would result in the pro-!
duction of a smaller proportion of permanent gases >
(carbon dioxide and carbon monoxide) than would be ■
given by combustion of the dust particles as a whole,
for the “ volatile constituents ” contain a larger propor-
tion of hydrogen than does the coal as a whole.

No evidence has been obtained of such a preferential
burning of the “ volatile constituents ” in those experi-
ments where explosive combustion, has been obtained. On
the contrary, the gases withdrawn (just after the flame
has passed*) have always shown a ratio

r___oxygen present as oxides of carbon

oxygen present as steam

in agreement with the ratio required for combustion of
the dust particles as a whole, as determined from the
chemical composition of the dust.

Samples withdrawn during the preliminary period of
comparatively slow inflammation do indeed show that,
in this stage, there is a preliminary distillation of gas
from the dust, and, as explained in a previous report, it
is the ease with which such distillation is effected that
mainly determines whether a particular dust is capable
of propagating inflammation more or less readily.

The transition from comparatively slow inflammation
(during which distillation of gas takes place as a pre-
liminary to burning) to explosive combustion (during
which each dust particle that aids in propagating the
flame is burnt as a whole) appears to be well defined.
It is characterised by a sudden rise of pressure, the
change from low to high pressure taking place within a
few feet distance of travel of the flame (vide curve No. 5,
fig. 4). When no such “ peak of pressure ” occurs, as,
for example, when the distance of travel has been in-
sufficient, analysis of the products of combustion shows
that preferential burning of the volatile constituents of

* Subsequent to the passage of the explosion along the
gallery, the heated products of combustion effect the distil-
lation of gas from excess coaldust—ooaldust which has not
assisted in the propagation of flame—and if an auxiliary
supply of air enters the gallery and mingles with this gas,
a secondary explosion (of gas and air) may take place.

ISO

IOO

20

120

60

140

80

90

N?

100

Al

>ej

---Curve,

&

perimeht

120

JV

’734

'30

6-
<0

7</. Curve N. 3:
Curved?4.

---Curve

These curves should be com-
pared with those reproduced
in fig. 4. They were obtained
from m anometers fixed along
the gallery in the same posi-
tions as in the Experiment
(No. 119) of which the curves
in fig. 4 are records.









Time.—Hundredths of a second.

Fig. 7.—Showing the Effect of Constrictions
in the Path of the Explosion.

the dust has taken place; after this rise of pressure has
occurred, no such preferential burning can be detected.

It is for this reason that we consider it necessary to
draw a line of demarcation between “inflammation”
and “ explosive combustion ” of coaldust and air mix-
tures, the difference, though mainly, of course, one of

Distances from Point of Ignition.



Experiment N? H 3,

kJ

o

Uj

102

109

103

104-

106

108

I I O

IO

105

I I I

100

IOJ

I I

98

I I 9

96

97

99

I I 2

I 3

4

to

Fig. 5.

/oo

70/

703.

i04-

/06,

102

[07.

/os

US

96

IQ9.

95

H2

97

H4-

98

113

JJ5

US

117

JI9

J/8

250 300'	350' <400'

To open
end of
gallery
50 ft.
further
on.



degree of rapidity of combustion, being also one of
character.

The Influence of Constrictions in the Gallery on the
Propagation of Inflammation.

Many experiments, of which that described (No. 119)
is an example, were made with the gallery perfectly
clear, so that the inflammation was free to travel from
one end to the other without encountering any obstruc-
tion which might cause irregularity in its development.

Previous to the commencement of a systematic study
of the manner of development of the explosions, how-
ever, the gallery had been used for demonstrating the
fact that coaldust was capable of forming an explosive
mixture with air. In these demonstrations the inside of
the gallery was furnished with props and timber set to
represent the manner in which the roof of the roads in
coalmines is supported. The timbering consisted of
pairs of wooden props 6 in. in diameter fixed on either
side of the gallery, with a tightly-wedged bar at the top
holding them in position. These pairs of props with
bars were fixed every 9 ft. throughout the gallery ; the
force of the explosion usually dislodged all of them,,
projecting some of them 300 or 400 feet out of the open
end of the gallery, and giving an effective proof of the
power of an explosion of coaldust and air to do damage.
(Vide Record of the Mining Association, p. 91, et seq.)

The pressures recorded during these demonstratione
sometimes amounted to as much as 501b. per square
inch for a distance of travel of the explosion of only
225 ft. In contrast to this, it has already been shown
that when the gallery was clear a pressure of only 16 lb.
per square inch was developed after a distance of travel
of 450 ft.

Several series of experiments were therefore made
with a view to determining the influence of constrictions
in the gallery on the propagation of inflammation.
These experiments bring out very clearly the salient
difference between “inflammation” and “explosive
combustion,” and show that the change from the one type
of propagation to the other is susceptible of control.