Selection and position of filters

Filter types and their position within the freeze-drier should satisfy two broad criteria  

The advantages and disadvantages of the possible installation positions of filters may be summarised as follows. 

Installing a filter after the condenser also has the advantage of isolating the pump from the product, and by including the filter in the steam sterilisation cycle the slight risk of contaminating the product from the vacuum pump (which cannot be steam sterilised) is eliminated. 

Liquid nitrogen traps or traps containing chemical absorbents such as activated charcoal or alumina may be interposed between the condenser and vacuum pump either to remove non-aqueous solvents or to reduce 

Oil-free vacuum pumps are now available which will eliminate the risks of oil back-migration and associated hazards. 

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