An interesting feature of the calculation is the emergence of the choked flow concept. As you reduce the suction pressure, you will eventually reach a point when a further increase in mass flow is impossible. This appears as a negative incremental length, clearly an impossible situation. If this occurs, we terminate the calculation and report the suction temperature when the calculation was stopped. The best way out of this is to increase the tube diameter. The idea is to find the smallest possible tube without getting choked flow. This would result in the shortest tube length.