In many industrialised countries, during manufacturing, treatment, transport and in the storage of goods, gases, vapours or mists are produced or leaked into the environment.
In industrial manufacturing processes, flammable dust can also be produced which in combination with the oxygen in the air can create a potentially explosive atmosphere that can cause an ignition inducing an explosion. Other sources of ignition are common and can happen as a result of electronic failure, for example in switches or mechanical failure, for example the friction of an impeller with the inlet.
An explosive atmosphere is defined as a mixture in atmospheric conditions caused by the activity of manipulating of the air and flammable substances (gas, vapour or dust). These explosive atmospheres can occur in many industrial activities that surround us such as the chemical industry, power plants, landfills, food processing factories and metallurgical industries.
There are two types of ATEX atmosphere:
Explosive Gas Atmosphere
Mixture of the flammable substance with the air. Combustion is spread to the entire unburned mixture on ignition.
Explosive Dust Atmosphere
Mixture of the air with flammable substances in the form of dust or fibres. On ignition the combustion propagates through the rest of the unburned mixture.
Depending on the level of presence of explosive gas or dust, these are classified into different zones and categories as detailed in a previous article here.
Groups, Ignition temperatures & Maximum Surface Temperature
The group determines the explosive level of the gas while the type of temperature determines the highest acceptable surface temperature on the motors surface. If the temperature on the surface of the motor exceeds this level, ignition of either the gas or dust is possible. This is a required indication that the customer should communicate before selecting an industrial fan for use in an explosive atmosphere. Hot surfaces become dangerous around 135 degrees Celsius so for that reason all ATEX equipment should be selected as having a T class of T4, T5 or T6.
Maximum surface temperature in case of failure for equipment in contact with dust:
Temperature limit 1 = 2/3 of the minimum ignition temperature for the existing dust.
Temperature limit 2 = Minimum ignition temperature for a 0.19 inches powder layer less 75 Kelvin.
The lowest limit temperature in both cases has to be higher than the maximum temperature on the devices surface.
For example in the case of wheat flour:
Temp limit 1 = 2/3 x 480 = 320° C
Temp limit 2 = 450 – 75 = 375°C
Maximum temperature of devices surface = 320°C
According to the below table, the wheat flour example temperature class is T2.
Lower explosion limit (LEL) is in this case 125g/m3. Below this concentration there’s no explosion risk.
See the table below for indicative ignition temperatures and ATEX temperature classes: