Boeing 737 ground operations question
First of all, hello all. Aside from having a deep admiration and love of aviation, I am not a professional in anything regarding the industry. I do however have a deep fascination with how aircraft work. My apologies in advance if this question is dumb in anyway.
I have been doing some research on this topic but I cannot really find a solid answer anywhere. I understand how bleed systems work. I also have a really BASIC understanding of how the air conditioning system works off of bleed air supplied to the packs. My confusion comes from the recirculation fans.
My questions are as follows:
1.) What exactly is the function of these fans?
2.) If an aircraft is parked and is being fed pre conditioned air directly into the mix manifold, do these fans need to be on? Or do these fans only get turned on during pack/bleed operations?
Thanks so much for your time!