Separators

By mist we understand the dispersion of a liquid within a carrier gas. Mist or fog consists of droplets with fairly widely differing diameters. The definition lays down that these droplets should have a diameter more than 1 micron. Mist droplets exist in compressed air

through condensation of water vapour
through condensation of oil vapour

The size of the mist droplets is of great importance where separation is concerned. The droplet diameter, the viscosity of the liquid droplet and the density of the air determine the setting speed of the liquid droplets in the air at rest. The following guidance values apply:

0.1mm diameter 24 m/s
0.01mm diameter 0.3 m/s
0.001mm diameter 0.003 m/s

In order to force mist droplets into motion, considerable forces have to be applied. In order to achieve this, the mist laden air is caused to change direction, allowing strong inertial forces to affect the mist droplets. These forces drive the droplets through the air with elevated relative speed. The velocities so achieved are directly proportional to the forces, i.e. a force of 10 times the weight of the droplet increases the velocity to 10 times the setting speed. The high acceleration values needed for droplet separation from compressed air can, for instance, be achieved through a sharp diversion of fast flowing air, causing larger liquid particles to be projected out and separated.

Depending on the purpose of the compressed air application, the separator is selected to achieve optimum degree of purity in line with requirements. The most important mechanical separators used by industry are:

Cyclone or vortex separators
Impingement separators
Spin separators
Wire mesh