Surface temperature dust layers

Special provisions are laid down in lEC 60079-0 and lEC 60079-1 for motors required for such locations in view of fluctuating degrees of humidity and temperature. Such locations are defined with a surface temperature limit of 150°C where coal dust can form a layer, or 450°C where it is not expected to form a layer. Otherwise, other details are generally the same as for flameproof motors type Ex d , according to lEC 60079-1. For variations in length of paths, gaps, widths, creepage and clearance distances, the reader should consult these Standards. 

Minimum ignition temperature of a dust layer (MITi) The lowest temperature of a hot surface on which a dust layer is ignited under specified test conditions. 

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. 

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.

Besides the dust clouds, changes of contents of carbon dioxide and certain other components in the air can also affect the mean temperature of the atmosphere close to the earth s surface. The assumed increase of the surface temperature induced by increasing concentration of certain trace components of the atmosphere is shown in Table 5.6. The problem of effects of increasing CO2 emissions is discussed in more detail in Section 5.2.4.3, where possible effects of CF2CI2 and CFCI3 on the thickness of the ozone layer in the stratosphere are also considered. 

A fire hazard may be safely avoided if the temperature of the material is lower than the relevant minimum ignition or exotherm onset temperature. The greatest risk exists in the part of the drying system where the product moisture content is lowest, such as the separation units (e.g., cyclones and bags). Where dust deposits can accumulate, the requirements are that the maximum permissible temperature of the plant surfaces should not exceed two thirds of the layer MIT. Also, for the dust layer, the time the material is exposed to heat should be taken into account as a controUing factor that will determine cleaning and removal of deposits. 

The resistivity of a layer of dust precipitated in an electrofilter is determined not only by the inherent resistivity of the material at the given temperature, but also by the particle size, density of the dust layer, and surface conductivity of the particles, which may vary as a result of adsorption of gases or vapors. It was found in [316] that the resistivity of copper oxide powder decreases with increasing particle size and with increasing layer density, the layer density being the more important factor. The resistivity of a dust layer is usually 2-3 orders of magnitude greater than the resistivity of the material itself, if the comparison is made under the same ambient conditions. 

The apparatus should be labelled with the IP eategory, maximum surface temperature and minimum temperature for use or storage if this is lower than —20°C. The reason for this is that low temperatures can embrittle some materials and lead to a eraeking risk. The maximum surfaee temperature allows for a 5 mm layer of dust. 

The relationship to the more well-known Zones 0, 1 and 2 is clear. However, the new definitions do not relate to layers or accumulations of dust, which must be avoided or, where they cannot be avoided, the surface temperature of equipment must be reduced to a safe level. 

The specific resistance of a layer of dust deposited in an electric filter is determined not only by the characteristic specific resistance of the material at the specified temperature, but also by the particle size, the density of the dust layer, and the surface conductivity of the particles, which varies as a result of the adsorption of gases and vapors. 

In almost all dryers, there is a buildup of deposits on the inner surface of the drying chamber or an accumulation of dust layer in some dead areas. The material may also be stored in bulk form at the bottom of some dryers and in bins, hoppers, silos, or bags. Ignition of such a material can occur not only by typical high-temperature sources (i.e., electrical sparks or incandescent particles), but also by... 

It is assumed that the dust layer is eroding at the same rate as it is growing owing to the movement of its bottom (sublimation front of water ice) toward the center of cometary nucleus. The thickness of the dust mantle is therefore constant. The surface temperature is calculated according to the energy balance equation accounting for the absorption of solar radiation, the IR emission, and heat conduction through the dust mantle. 

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