Dust in air

Vanadium and its compounds are toxic and should be handled with care. The maximum allowable concentrahon of V2O5 dust in air is about 0.05 (8-hour hme-weighted average -40-hour week). 

Many finely divided metal powders in suspension in air are potential e] losion hazards, and causes for ignition of such dust clouds are numerous [Hartmann and Greenwald, Min. MetalL, 26, 331 (1945)]. Concentration of the dust in air and its particle size are important fac tors that determine explosibility. Below a lower Umit of concentration, no explosion can result because the heat of combustion is insufficient to propagate it. Above a maximum limiting concentration, an explosion cannot be produced because insufficient oxygen is available. The finer the particles, the more easily is ignition accomplished and the more rapid is the rate of combustion. This is illustrated in Fig. 20-7. 

These weaker discharges may nevertheless ignite some dusts in air since their effective energies include the range between ordinary brush discharges (5-10 mJ) and PBDs (100-1000 mJ). 

Corona and brush discharges from compacted beds will not ignite dusts in air. 

To apply these relationships to the hybrid mixture MIE problem, it is noted that only two points need be defined on the Y axis the MIE of the dust in air and the LMIE of the gas in air. The first on the x axis corresponds to zero gas, so X[ = 0, and the second to the optimum gas concentration. All the unknowns are experimental quantities. 

LEL (lower explosive, OR FLAMMABLE, LIMIT) The minimum eoneentration of a gas, vapour, mist or dust in air at a given pressure and temperature that will propagate a flame when exposed to an effieient ignition souree. Generally expressed as % by volume for gases and vapours, and as mg/m for mists or dusts. 

Hybrid mixture A suspension of dust in air/vapor. Such mixtures may be flammable below the lower explosive limit of the vapor and can be ignited by low energy sparks. 

Maximum Charge (Charge Umite in Fr) is the max charge of an expl that can be fired in a coal mine without causing ignition of firedamp and/or coal dust in air Ref Marshall 2, 585 (1917)... 

Here, we concentrate on the gas pressures developed for high explosive detonations within vented and unvented enclosures, and these explosives plus nearby combustible materials. There is a voluminous literature on pressures and the effects of venting for confined explosions with only combustible gases and dusts in air, but that topic seems outside the scope of this book, and is not discussed here. 

Flammable mixture A mixture of a gas, vapor, mist, or dust in air which is within its flammable range. 

Following values of Table 2 for lower combustion limits of dusts in air and temperatures required for ignition are abstracted from Table 4, p 152 of Ref 14 ... 

Dust in Air of Temperature of Ignition, °C Lower Limit in g/m of Mixture... 

The theoretical calculation of the detonation temperature as a safety criterion was adopted only in France. In all other countries (and more recently also in France) a practical criterion has been introduced based on an experimental evaluation of the effect of explosion of a sample of the explosive on a mixture of air and methane, or on a suspension of coal-dust in air, under conditions similar to those existing in mines. For this purpose testing galleries were devised, simulating the conditions of mine galleries. 

After the explosion at Courriere experiments were carried out in testing galleries on blasting explosives and on suspensions of coal-dust in air. As the coal-dust from various mines showed different inflammability, the most inflammable kind, i.e., that with a high content of volatile matter, was commonly used for testing. 

Miscele di polvere suspense in gas Mixtures of powders suspended in ga s ( Al, Mg or coal dust in air) ... 

Asbestos dust in air can be trapped with an impinger dust-sampling apparatus using 25% aq ethanol as the collecting medium (Ref 4). The safe threshold value for asbestos dust exposure is considered 5 million particles per cubic foot... 

Some forms of these products may contain sufficient fines to be considered nuisance dust and present dust explosion potential if sufficient quantities are dispersed in air. Unformulated PVB and PVF resins have flash points above 370°C. The lower explosive limit (lei) for PVB dust in air is about 20 g/m2. 

L. Pasteur, J. Tyndall, E. T. Chapman, P. Miquel, W. Spring, F. Schulz, etc. Air may be freed from dust particles, etc., in suspension by filtration through biscuit earthenware, asbestos, or cotton wool. When a beam of sunlight is passed through unfiltered air, it reveals a multitude of motes constantly in motion. Lucretius, in his Be natura remm (2.113, 60 b.c.), has given a very vivid description of the phenomenon. With filtered air, there is no such eflect, and J. Tyndall said that such air is optically empty. F. O. Rice showed that in a number of reactions— e.g. the oxidation of soln. of sodium arsenite or sulphite, the decomposition of hydrogen dioxide, etc.—the suspended dust in air acts as a catalytic agent. 

---