Conventional air-system design processes result in an ideal fan type, size, and speed for ideal conditions. But in the field, actual conditions are often less than ideal. Obstacles to duct runs lead to sharp turns or changes in elevation, and then another correction to resume the planned path. Or, there might not be enough room for the ideal length of inlet or outlet duct to establish fully developed airflow. The results of less-than-ideal fan conditions are summarily called system effect.