Complex sentence: These computations are only valid when considering a diffused spray which penetrates the entire volume; this is not possible in the case of a high ceiling: the spray is short and does not reach the upper layers of air.
Split into simpler sentences: These computations are only valid when considering a diffused spray that penetrates the entire volume. This is not possible in the case of a high ceiling: the spray is short and does not reach the upper layers of air.