control apparatus for automatically maintaining the desired blown tube diameter in the blown plastic film process where the blown tube is internally cooled by a gas flow into and out of the interior of the tube . a by - pass duct having a by - pass damper is provided on either the inlet or exhaust duct by - passing the inlet or exhaust duct damper respectively and this by - pass damper is controlled by tube diameter sensors . by thus controlling only a small percentage of the inlet or exhaust flow a very fine and stable control is achieved . control apparatus is also provided to maintain the by - pass damper in its optimum operating range .