In BOF steelmaking oxygen is blown through a laval nozzle. A Laval nozzle also called a convergent-divergent nozzle and is characterized by a flow passage whose cross sectional area decreases in the direction of flow and attains a minimum cross section area and then increases further in the direction of flow as shown in the figure 13.1(B). The minimum cross section area of the flow passage is called throat of the nozzle. The laval nozzle can accelerate the gas to the supersonic velocity The details of the design can be seen in the references given at the end of the lecture

Velocity of gas can be expressed in terms of Mach number
 

Thus at M= 1 gas exits the nozzle with the sonic velocity. Sonic velocity can be attained at the throat of the nozzle.

M<1 gas exits the nozzle at subsonic velocity

M>1 gas exits the nozzle at supersonic velocity.

Convergent nozzle can accelerate gas at

Convergent – Divergent nozzle can accelerate gas at

In converter steelmaking Laval nozzles are used. Gas velocity at the exit corresponds to  flow rate of oxygen is  at pressures 10-14 bar. Multi –hole nozzles are used. Number of nozzles varies from 3 to 6.