So move a lot of air in summer. More catalogue browsing then... Who makes a good, quiet powerful fan? I guess it's better to get something reasonably profesional, it will be working pretty hard. Any thoughts?
Depends on the volume of the airmass you need to shift. The rule o' thumb for ventilation means your exhaust blower (and intake blower, if used) needs to be able to shift the room's air in 3-5 mins. A 500cu ft room would thus need a minimum 100CFM blower. However, in practise, that's enough to supply fresh air for the plants but not generally enough to remove the heat from big lights to keep temps under 28C.
If you have a long exhaust duct run (more than 2-3m) or a carbon filter, you'll want a centrifugal blower. Axials don't cope well pushing air into a high static pressure created by obstructions in the air path, those obstructions including a big column of air. When axials encounter more static pressure than they like, air leaks backward between the fan blades. Centrif blowers are not air compressors but they can push air lots better than an axial into high static pressure.
I use
this 250mm unit as my main exhaust blower; moves 875CFM for my 500cu ft room. It's like a freaking wind tunnel. I use a couple of old 200mm axials (formerly used as exhausts in other applications) as intake blowers. Those old axials combined push about 90% of the capacity of the 250mm centrif, meaning the room remains at a slightly negative pressure. The axial intake fans are not working very hard when they are pushing into air pressure slightly lower than atmosphere. You ask, why bother with intakes, then? Simple- passive (fanless) intake is very inefficient, requires the intake opening be around double the area of the exhaust blower's annulus and thus makes light-trapping the intake difficult.
Keeping your grow at a slightly negative pressure compared to atmosphere is important. If there are any air leaks in your room construction (and there WILL be), air will be sucked into those leaks. All air leaving your grow will go through the exhaust blower. If your room is at positive pressure compared to atmosphere, air will be pushed out of those little leaks, making scents difficult to impossible to control.
Fletch's suggestion is OK if you can cope with scent leakage. His method will keep the room at positive pressure compared to atmosphere. If one uses an intake blower attached to the home heating system duct instead of temporarily repurposing the exhaust blower, the room would remain at negative pressure.