process and device for cooling a tubular sleeve produced by hot extrusion - blowing of a thermoplastic material through an annular die . in the process , a stream of primary air is flowed under pressure substantially parallel to and spaced from the tubular sleeve and reaches the tubular sleeve at a second stage . the stream of primary air induces a stream of secondary air which flows substantially parallel and proximate to the tubular sleeve at a first stage and whose flow is substantially laminar over most of its path . the device includes a first cylindrical sleeve member that , in combination with the tubular sleeve , defines a secondary air flow space . a second cyclindrical sleeve member and the first sleeve member define a primary air flow space therebetween . a cylindrical ring disposed proximate to the annular die directs air into the secondary air flow space . a source of pressurized air is in flow communication with the primary air flow space .