Clair,We have 18 the tubes - maybe 0.5m each to later time hook big fans to them. Engineer required as I would not have
thinkedthought of this. (spell?) Tubes can linked at top for in and out air flow. Getting all the same fans and getting the electricity is the problem but we are thinking on it. All tubes will stay so can reach the way placed. We know we must have many places for catching the air on the outside. The problem is air to outside because there are many government helicopter and drone that catch heat to see. To hide the heat is problem because we have field.
Hopefully your tubes are PVC or something you can cut without it crumbling in case you want to create connections or outlets mid-pipe?
Also, maybe having fans at both the top and bottom will help airflow but your engineer will know what can work best for you. Yes getting rid of heat signatures can be a real problem especially with FLIR and Thermal imagers mounted on helicopters and drones.. They can be a real pain when you don't want to be seen. Our solution is to make it look like someone else.
There's a dry cleaner/laundromat on the far end of the shopping center and we just pipe our heat to come out near their place so we appear the same as the background temperature. (We have also developed means to have drones take a nose dive if they get too bothersome - darn real estate companies!)