It's a personal preference on where to place a fan in a tent. However, the one thing to keep in mind is that fans need airflow to be able to cool anything down. In other words, the fan needs an opening of some sort for it to blow air out of. For example, if you have a screen house with windows at the top and bottom--that's an ideal area for placement. If you have two screens put together without any windows open, then chances are it will end up creating more heat than cooling down your space since there isn't any airflow coming into or leaving your tent!