I have found that when air is pulled out through ductwork, it is less problematic with having bends in the duct than if the air is being pushed. That is why I mount my exhaust fans closer to the final exit to outside and let it pull the air through the rest of the duct and lights, (and filters) first. The centrifugal fans are like vacuum cleaner blowers in that they produce a very strong suction(aka negative pressure). The duct fans are worse than worthless for ventilation of grows. Be sure to match the exhaust fan with the cubic space so that you are able to get close to the ability to replace the total air mass in the grow room at least once every minute for good heat removal.