Glow temperature

The glow temperature is the lowest temperature, for which a dust layer with a thickness of 5 mm is ignited. It is determined under well-defined testing conditions on a hot plate (vid. [31]). The glow temperature decreases with increasing layer thickness. Additionally, grain size and bulk powder density influence the heat balance and hence the glow temperature. Table 2.24 contains values for selected materials. 

Dust-air mixtures are flammable only within a certain range of concentrations just like gas-air mixtures. This range is marked by the lower explosion limit (LEL) and the upper explosion limit (UEL). 

Material Maximum particle size in pm Main particle size in pm Bulk powder density in kg/1 Glow temperature in °C 

The lower limit is of special practical interest. For many technical dusts it lies between 15 und 60 gW the UEL is very high, viz. 2-6 kg/m. Investigations have shown that the LEL is largely independent of the ignition energy employed [3]. The temperature dependence of the LEL is described by [3] 

37) LEL(T) is the lower explosion limit at temperature T and LEL(To) that at the reference temperature Tq. Table 2.25 gives values for selected materials. 

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For hot surfaces a maximum permissible surface temperature Tmas must be specified, with the help of the minimum ignition temperature of a dust cloud MITC and the minimum ignition temperature of a dust layer MITL (usually 5-mm dust layer = glow temperature), so that the temperature of all surfaces is not exceeded. Independent of the zone the Tmax for dust clouds is % MITC and for dust layers MITL - 75 K. 

Osmium Oxysulphide, Os2Os(SH)a, is obtained 4 as a brown powder by the action of hydrogen sulphide on dry osmic acid, H20s04. The reaction is very vigorous, the mass being raised to glowing temperature, water and sulphur vapour being emitted ... 

Table 1.4 Ignition temperatures T, and glow temperatures TG for combustible dusts in air with a pressure of 1.013 105Pa. TG refers to a layer thickness of 5 mm...

Figure 2.6 Surface temperature derating diagram for dust layers with a thickness exceeding 5mm according to practice A/IEC 61241-1-2. For dust layers s5mm, the surface temperature Ts shall be limited tolG -75K (Tg = glow temperature for a 5 mm dust layer). The surface temperature of an electrical apparatus normally refers to an ambient temperature of +40°C.

Despite the high ignition temperature of methane-air mixtures of 595°C/650°C (see Table 1.3), the maximum surface temperature is subjected to a very strong limitation with respect to the unavoidable presence of coal dust with relatively low glow temperatures (see Table 1.4)... 

As explained in Section 2.5 and Table 2.5, the relevant standards ask for a certain tightness of the enclosure against the ingress of dust and a surface temperature ensuring a certain margin of safety related to the glow temperature and ignition temperature of the combustible dust referred to. 

Theories of Elementary Homogeneous Electron-Transfer Reactions GLow-Temperature Electrochemistry at... 

In the study of the oxidation of the higher alcohols, the concentrations of air and alcohol and the velocity of flow of gas over the catalyst required in each case to maintain the spontaneous glow of the catalyst was experimentally determined. In the oxidation of propyl alcohol the copper catalyst was kept at a dark red heat. Air at the rate of 2.3166 liters per minute carried 1.165 grams of alcohol per liter. The products consisted of aldehyde, equal to about 50 per cent of the alcohol used, hydrocarbons equal to 11.75 per cent, carbon monoxide equal to 1.4 per cent and carbon dioxide equal to 3.6 per cent. The rate of flow was about the same as that required with methanol to maintain the catalyst at glowing temperature. 

From time to time samples were removed for analysis, surface area determination (B. E. T.), x-ray, susceptibility, and glow-temperature study. Particular care was taken to prevent atmospheric oxidation, and... 

The term glow-temperature means the temperature at which a vigorous exothermic change occurs spontaneously. 

When the iron oxide gel was heated, water was, of course, lost, but the susceptibility per gram of iron remained constant until the glow temperature ( 270°) was reached, after which the susceptibility fell to about 32 X 10 per gram of iron at 25°, and began to vary directly, rather than inversely, with the absolute temperature. 

The glow temperature and the minimum ignition temperature of a powder-air mixture are very high (450-600 C)... 

Fig. 3.78 Measurement of glowing temperature of a dust layer according to the Hungarian...

The glowing temperature of the powder layer in open air is measured by an electric heating plate. The powder sample is distributed on a circular plate of 150 mm dia. at a thickness of 5 mm (Fig. 3.78). The minimum temperature of the metal plate at which the powder layer will glow up within 3 hours is determined. The glowing temperatures of some plastics powders are presented in Table 3.14... 

Glowing temperatures of powder layers, ignition temperatures and minimum explosible concentration of dust clouds measured by the BAM method according to the Hungarian Standard MSZ 1600/8-77... 

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