Penetrability of a gas jet into a slag and metal bath depends on the both jet mass (flow rate) and velocity of the jet at the bath surface.

The jet exits the properly designed nozzle with the maximum possible velocity. Then, due to tangential separation of the jet and to ambient gas parameters (velocity, temperature, density, composition), eddies are generated at the boundary between the jet and ambient gas. These eddies move in all directions including across the jet. This movement results in exchange (transfer) of matter, momentum, and heat between the jet and ambient gas. As a result, the cross section of the jet increases while its velocity is reduced. This reduction in the jet velocity lowers its penetrability into the metal bath.

Shrouding reduces the negative influence of interaction between the jet and environment.

Firstly, by reducing the difference between the velocities values of the central supersonic jet and surrounding gas, shrouding reduces the generation of eddies.

Secondly, flame shrouding increases the temperature and, therefore, reduces the density of the gas surrounding the jet. This means the ambient eddies are lighter and “steal” less energy from the supersonic jet.

Flame shrouded tools have been patented by ACI since 1986.