A novel collapsible aeration system is retrofitted into a grain bin with a discharge opening in the bottom. This grain bin may have a sloped or conical hopper or a flat bottom. The aeration system is formed from a set of connected (e.g. hinged) duct sections that define a round or multi-sided duct having a minimum of three sides or faces. The sides may be flat, curved or corrugated. The collapsible aeration system is folded (collapsed), inserted into the bin via a hole, and then unfolded inside the bin. An air duct is connected to the aeration system to enable the aeration system to dry the grain in the grain bin. This technology enables existing grain bins to be retrofitted with a dryer in an easy and efficient manner.