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Powder Mixer ATEX speed

Maximum safe loading and discharging rotation speed

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Section summary
1. Definition
2. Calculation of ATEX speed

1. Definition

What is referred here as "ATEX" speed for a mixer of bulk solids is the rotation speed of the shaft for which the tip speed of the mixer agitator reaches 1 m/s.

1 m/s is commonly accepted, for stainless steel equipment, as the limit below which a metal metal contact will not create a spark susceptible to inflame a cloud of dust.

It is very important to run the mixer below the "ATEX speed" when loading and discharging the mixer, which are the process steps where a dust cloud can be formed at concentration that can lead to explosion. During mixing, the concentration of dust is normally too high to create an explosion - some exceptions, operators to review each case - thus allowing to run a mixing speed.

2. Calculation of "ATEX speed"

The "ATEX speed" can be calculated by expressing the tip speed of the agitator as a function of the agitator radius and its rotation speed. Equalling utip = 1 m/s allows to calculate the "ATEX speed"

Equation 1 : Tip Speed calculation

Calculation of Tip Speed of Mixer

With :
- R is the radius of the mixing tool (center of shaft to tip of paddle) in m
- n is the mixing speed in rpm

Equation 2 : "ATEX speed"

Examples for different radius

Table 1 : "ATEX speed" for different mixer agitating tool radius

Radius (m) ATEX speed rpm
0.2 47.7
0.3 31.8
0.5 19.1


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