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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", it is thus necessary to program the mixer speed so that it is at all time less than this limit during loading and discharge. This requires a frequency drive.

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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