AFTER REPLACING DUST COLLECTOR BAG, AS HE AND ANOTHER EE WERE EXITING COMPARTMENT CONTROL ROOM OPERATOR MISTAKENLY ACTIVATED SUCTION DAMPER WHICH CAUSED DOORS TO SLAM SHUT. SUPERVISOR WAS AT LOCATION AND PULLED DOOR OPEN SO HE COULD EXIT. INSIDE DOOR CLOSED AND BANGED HIS LEFT KNEE. FAILED TO PLACE PIN THROUGH DAMPER CYLINDER SO IT COULD NOT BE ACTIVATED.