Parr Instrument 1108P - Manual (Page 11)

w w w . p a r r i n s t . c o m 11
1108P Oxygen Combustion Vessels
Operating Suggestions
Poor Combustion
The difference in combustion characteristics of the
wide variety of materials which may be burned in an
oxygen bomb make it difficult to give specific direc-
tions which will assure complete combustions for all
samples. However, two fundamental conditions may
be stated. First, some part of the sample must be
heated to its ignition temperature to start the com-
bustion and, in burning, it must liberate sufficient
heat to support its own combustion regardless of
the chilling effect of the adjacent metal parts. Sec-
ond, the combustion must produce sufficient turbu-
lence within the bomb to bring oxygen into the fuel
cup for burning the last traces of the sample.
An incomplete combustion in an oxygen bomb is
nearly always due to one or more of the following
causes:
1. Excessively rapid admission of gas to the bomb
during charging, causing part of the sample to
be blown out of the cup.
2. Loose or powdery condition of the sample which
will permit unburned particles to be ejected
during a violent combustion.
3. The use of a sample containing coarse particles
which will not burn readily. Coal particles which
are too large to pass a 60-mesh screen may not
burn completely.
4. The use of a sample pellet which has been made
too hard or too soft. Either condition sometimes
causes spalling and the ejection of unburned
fragments.
5. The use of an ignition current too low to ignite
the charge.
6. Insertion of the cotton thread below the surface
of a loose sample. Best results are obtained by
barely touching the surface or by having the
cotton thread slightly above the sample.
7. The use of insufficient oxygen to burn the
charge, or conversely, the use of a very high
initial gas pressure which may retard the
development of sufficient gas turbulence within
the bomb.
8. Insufficient space between the combustion
cup and the bottom of the bomb. The bottom
of the cup should always be at least one-half
inch above the bottom of the bomb, or above
the liquid level in the bomb, to prevent thermal
quenching.
9. Excessive moisture or non-combustible material
in the sample amounts to approximately 20
percent or more of the charge it may be difficult
to obtain complete combustion. This condition
can be remedied by adding a small amount of
benzoic acid or other combustion aid.
Oxygen Charging Pressure
Operators sometimes disagree as to the most desir-
able oxygen charging pressure. As a rule, it is best
to use the lowest gas pressure that will give com-
plete combustion. Lower pressures permit higher
gas temperatures and greater turbulence, both of
which help to secure better combustion. The range
of charging pressures for Parr oxygen bombs usu-
ally falls between 25 and 35 atmospheres, and it
should never exceed 40 atmospheres.