As we obtain a value for x 2 we can continue with the next stage and calculate again y 2 with the use of the equilibrium-condition. We can extend the control volume by one stage (depicted in Figure by a dashed line around stage 1 and 2). This way we receive fixed point iterations for the variables V 2 , L 3 , x 3 . In general for a stage k with 1 ≤ k < n s the fixed point iteration is given for s ∈ [0, ∞) by: