R 407C, R 410A and R 410B may replace R 22. Blends containing R 32 demand new requirements for the filter drier. Filter driers meeting this challenge include the Danfoss DU filter drier with a core of pure molecular sieve, and the Danfoss DN filter drier with a combination core of molecular sieve and activated alumina.

Why filter driers?
The filter drier adsorbs moisture from the refrigerant to minimise the risk of acid formation via hydrolysis of the ester oil. Danfoss DN filter driers with activated alumina also adsorb acid formed during normal operation.

R 32 in blends
R 32, making up 23% of R 407C, consists of small molecules which resemble water in several ways. R 32 might therefore be adsorbed by the molecular sieve. To avoid this, a suggestion could be to use a smaller filter pore size, but this would reduce the water adsorption capacity and adsorption speed would decrease significantly. Danfoss makes the absolute requirement of molecular sieves in DN and DU filter driers that the water adsorption capacity should not be significantly affected and that the R 32 molecules should not be cracked.

Tests of Danfoss DN or DU filter driers have shown that capacity is reduced only by between 2 and 8%. This reduction is modest compared to the high water adsorption capacity of the filter driers. Due to the adsorption forces between the water molecules and the molecular sieve, the water simply displaces the R 32 molecules in the Danfoss type molecular sieves.

Another very important aspect is the catalytic decomposition of R 32 causing HF (hydrofluoric acid) to be formed. Tests using a Scanning Electron Micro-scope (SEM) have shown that fluoride cannot be found in Danfoss DN or DU filter driers after test.

The Danfoss DU and DN filter driers achieve the highest water adsorption capacities in the market and the same filter core can be used for all refrigerants, making DU or DN the perfect choice.

REFRIGERATION AND AIR CONDITIONING CONTROLS
By Arne H. Blom, Product Manager
Page 12, THE DANFOSS JOURNAL 2-1996