We can now trace the process gas flow within the converter. In the AB cooled converter of Figure A-4, the process gas enters one end of the converter at the center and is directed to the outside of the shell by a series of baffles. It flows through the gas cooler and is directed to the outer section, or pass, of the barrier tubes. All of the flow enters the outer, or first pass, and flows through the tubes. Part of the flow diffuses through the barrier tube walls and the remainder of the flow passes through the tubes and is directed by a crossover to the second pass of tubes. This crossover is sometimes called a doughnut because of its shape. The process gas flow which does not diffuse through the barrier tube walls of the second pass is directed by a second crossover to the third pass. The first crossover is sometimes refereed to as the 1-2 crossover because it directs gas flow from the first pass to the second pass. Similarly, the second crossover is called the 2-3 crossover. The undiffused process gas which passes through the third pass is directed to the "B" outlet of the converter and flows to the stage below. The process gas which was diffused through the barrier tubes in all three passes is collected and flows to the "A" outlet and on to the stage above.

Since some of the process gas flowing into the first pass will diffuse through the tube walls, a reduced amount of process gas flow will enter the second pass. In order to maintain the same flow velocity, the second pass contains fewer tubes than the first pass. Similarly, the third pass will contain still fewer tubes. The velocity of process gas through the tubes affects the flow through the tube walls and thus affects the separation efficiency. The efficiency is highest when the flow velocity is the same in all of the tubes.

In the Badger stage, about 50% of the process gas entering a converter diffuses through the tubing and flows to the stage above. This fraction is called the "cut." A cut of 50% has been found to yield the best separation. As mentioned previously, the "cut" in a Badger Cluster stage may be more or less than 50% depending upon its position in the cluster.

Diffusion

It is necessary to have a higher pressure inside the barrier tubes than outside in order to have a flow through the walls of the tubes. This inside pressure is called the fore pressure or high side pressure (H.S.P.), and is measured at an arbitrary point inside the tubes near the middle of the second pass. The pressure outside the tubes is the back pressure or low side pressure (L.S.P.), and is measured at an arbitrary point outside the tubes near the middle of the second pass.

The fore pressure is regulated by a control valve in the "B" stream, or down flow, from the converter. Actually, the control valve regulates the pressure immediately above it. This is called the control pressure and is the pressure which is indicated at the cell panel. It is slightly lower than the fore pressure due to the pressure drops in the converter and piping.

The rate of diffusion through the tubing walls for any given pressure drop across the barrier is determined by the permeability of the barrier. Mathematically, this is a dimensionless quantity which is the ratio of the rate of gas flow through the barrier to the rate of gas flow through the same area which would take place if the barrier were not there. The term, usually called permeability, is used to relate the actual permeability to the design permeability. For example, if the flow through the barrier has decreased 10% due to plugging of the barrier holes, the permeability would be 90%.