What was found
FLT RETURNED TO DEN DUE TO HEAVY SMOKE ODOR IN THE COCKPIT & FIRST CLASS. NO SMOKE WAS SEEN. AT FL250, THERE WAS A SHIFT OFF THE ENGINE, AT FL270 THERE WAS AN ACRID SMELL AND AT FL310-320 THE SMELL WAS VERY HEAVY IN 2A/2B. CAPT DECLARED AN EMERGENCY. F/O WORE THE 02 MASK FOR SAFETY. *S/D* BOTH PACKS WERE RUN AT HIGH POWER, BUT THE SMOKE ODOR COULD NOT BE DUPLICATED. AS A PRECAUTION, LT AND RT OZONE CONVERTERS WERE REPLACED. AIRCRAFT WAS RETURNED TO REVENUE SERVICE WITH NO FURTHER REPORTS OF SMOKE OR SMELL. THE 2 OZONE CONVERTERS WERE SENT TO VENDOR FOR EFFICIENCY TEST. EACH OF THEM PASSED WITH AN AVERAGE EFFICIENCY HIGHER THAN 90 PERCENT (COMPLIED WITH FAR 121.578).