OK this was my first spiral job. The square things are 1400 CFM variable speed booster fans. this was kind of a weird deal. They are using the supply from a RTU 160 feet away as a MAU for two small rooms that have a large dust collector exhaust system. The return for the RTU that these come from is in the main plant. They have back draft dampers where they go through the wall, the a big round diffuser. Any way I don't think I posted these yet.