1. Definition

How to prevent dust explosions in mixers ?

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 concentrations that can lead to explosion. During mixing, the concentration of dust is normally too high to create an explosion - some exceptions exist, operators MUST review each case - thus allowing to run a mixing speed, which is higher than the value calculated below.

2. Calculation of "ATEX speed"

What is a safe mixing speed in powder mixers ?

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 u_tip = 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

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