U.S. Pat.  No.  4,817,828 discloses an apparatus including an inflatable air bag for restraining a vehicle occupant.  The apparatus also includes an inflator for inflating the air bag.  The inflator has a cylindrical housing containing an ignitable gas generating material which, when ignited, generates gas for inflating the air bag. 
The housing has a tubular inner body wall and a tubular outer body wall.  The inner body wall defines a cylindrical combustion chamber in which the gas generating material is contained, and has gas flow openings for directing the gas to flow outward from the combustion chamber.  The housing also contains a tubular filter which extends circumferentially around the inner body wall.  The outer body wall surrounds the filter, and has gas flow openings for directing the gas to flow outward from the inflator. 
When the inflator disclosed in the '828 patent is actuated, the gas generating material is ignited and rapidly generates the gas for inflating the air bag.  The gas flows radially outward from the combustion chamber through the gas flow openings in the inner body wall and into the surrounding filter.  The gas is filtered as it flows radially through the filter.  It then flows radially outward toward the air bag through the gas flow openings in the outer body wall. 
When the gas flows through the inflator in the foregoing manner, it flows from the filter to the outer body wall around the entire inner circumference of the outer body wall.  However, the gas flow openings in the outer body wall extend over limited portions of the circumference of the outer body wall.  The outer body wall thus has major portions which extend circumferentially between the gas flow openings and which block the gas from flowing radially outward from the inflator other than at the locations of the gas flow openings.  As a result, the outer body wall cools the gas, and contains the gas under pressure, as the gas flows against and around the outer body wall before it flows outward from the inflator.  Moreover, since the gas flows outward from the inflator only at the locations of the gas flow openings in the outer body wall, the flow of gas moving outward through the filter tends to concentrate in the portions of the filter that are adjacent to the gas flow openings in the outer body wall.