The minimum ignition energy - Minimum Ignition Energy (MIE) - is the smallest amount of electrical energy that, when discharged, can ignite the dust-air mixture with the most ignition-prone concentration. The ignition energy of powders can be determined with a modified Hartmann tube.

The modified Hartmann tube is a quasi-open cylindrical body made of vertical 1.2-liter glass. During the test, the dust sample resting on the bottom of the device is brought to a floating state by means of compressed air, initiated by high-voltage spark generators introduced into the side of the vitreous body. In the case of a sufficiently large spark energy, the dust cloud will burst into it and the resulting flue gases will escape at the top of the pipe. The measurement is very spectacular, the flaring of the dust cloud is accompanied by both light and sound phenomena.

KÉPET ILLESZTENI !

An RLC circuit is used to create an electric spark with a specific energy. The pair of electrodes, which can be interpreted as high resistance from an electrical point of view, are formed on the terminals of the capacitance and inductance connected in parallel. The current flowing through the test circuit is limited by a resistor installed directly after the power source.

KÉPET ILLESZTENI !

The DC supply voltage can be varied between 10 kV and 30 kV. The energy of the discharge spark can be calculated by knowing the set voltage and the total capacity measured on the electrodes: E = 0.5 C*U2, where

E - the spark energy [J]

C - capacity measured on electrodes [F]

U – supply DC voltage [V]

After switching on the voltage, a spark discharge is observed between the electrodes, while the powder sample filled in the bottom of the device is atomized into the glass tube with compressed air. An ignition is considered when the fire between the electrodes exits the spark gap.

Measurement procedure:

Factors affecting the ignition of the dust cloud must be taken into account when evaluating the measurement and, if applicable, must be included in the protocol:

 delay time

 measuring circuit inductance

 measuring voltage

 alkalmazott kondenzátor kapacitása

 ohmic resistance of measuring circuit

 dimensions of measuring equipment, applied spark gap

1. The first test should typically be performed with a spark energy value that reliably causes the sample to ignite. With a given dust concentration, the spark energy must be reduced step by step until the sample does not ignite once out of 10 measurements.

2. The test is continued with the lowest spark energy determined in the first point, varying the powder concentration (the charge is typically 750 mg, 1200 mg, 2000 mg, 3000 mg). If ignition is detected at any concentration, the measurement with the given charge mass is continued with a reduction in spark energy as specified in point (1).

3. As a last step, with the spark energy and concentration determined in points (1) and (2), we change the spark delay time (typically 60 ms, 120 ms and 180 ms). The value of the minimum ignition energy is between the highest energy E1, where no ignition occurs, and the lowest energy E2, at which ignition occurs.