Ask about particulate contamination in drying and sterilizing tunnel

Our company produce aseptic injection drug. We use drying and sterilizing tunnel to sterilize the vial. This tunnel made in China, but now the ratio particulate contamination very much (about 20 -30% product) although we have replaced new everything ( the belt, the HEPA filter, Our machine look like that:[attach]549[/attach]

The HEPA filter we used right code by Camfil ( Infeed booth, cooling section : HEPA H14 (temp. working <700C); Heating section: HEPA H13 ( temp. working <3500C).
Particle test before heating : passed, particle test after working: infeed and heating section: failed ( particle >5um : about 10 -50).
Could you tell me where is the cause of particulate contamination? thanks.

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