Page 530 - Dust Explosions in the Process Industries

Assessment of lgnitability 497

7.7.2.5
Storage of Bulk Powder Samples in a Heated Atmosphere

The apparatus used for this test is shown in Figure 7.21. The sample is suspended in a
metal gauze basket in a heating chamber through which a given flow of preheated air
circulates.The ambient air temperature and the temperatureinside the powder sample are
measured and the difference recorded. If the temperature in the powder sample rises
beyond that of the air but no higher than 400°C, the phenomenon is named self-heating.
Temperature rises beyond 400°C are named self-ignition.
Figure 7.22 illustrates the typical linear relationshipbetween the minimum ignition tem-
perature and the sample size. Further details concerning self-heating and self-ignition
processes are given in Chapter 5.

t

!TIME

TEMPERATURE
DIFFERENCE RECORDER
THERMOCOUPLES
PRE- METAL GAUZE POWDER BASKET
HEATED - HEATING CHAMBER
AIR

Figure 7.21 Apparatus for storage of bulk powder samples in a heated atmosphere (From Verein
deutscher Ingenieure, 1988).

- 250
- - 00020
u

z 0
'3 '.
LY '0
150 - ..-
I
w s
[L
2c '0 - 0 0025
a \ .\ $2-
b
E5
2 100 ~ 0,
c
rr \\
4
x \ Figure 7.22 A typical series of results from the
3
+: \ determination of the minimum ambient air tem-
z \\ - 00030 peratures T, leading to ignition of different sizes
50 1 1 I I 1 1 1'-