The physical measurements of a fluid bed granulator can be exploited in construction of an operating window, a design space, for process performance. The purpose of this study was to determine the influence of inlet air humidity changes on temperature in different parts of a granulator system, on fluidisation behaviour and on the particle size of the final granules. A humidifying setup was constructed on a bench-scale fluid bed granulator that enabled elevated humidity levels and sharp humidity changes of the inlet air. Ibuprofen granules were produced at the various inlet air humidity levels classified as low, intermediate and high. A novel fluidisation parameter was developed. The more improperly the particles were fluidising the smaller was the relationship of airflow rate and fan speed. Four different failure modes were identified and classified, based on the fluidisation parameter: over-fluidisation, risk of improper fluidisation, improper fluidisation and collapsed bed. It was possible to construct process trajectories for smooth fluidisation, which the optimal granulation process should follow.