Until a few years ago no special requirements were given as to the
filtration of the process air for the spray drying process. Today, however,
very strict requirements are presented by local authorities in order to ensure
a cleaner operation. Filter standards are referred to in Fig. 40a, and it is
important to refer to the test method when specify-ing the filter efficiency in
%. Common for the different requirements is that:
- The air should be prefiltered and supplied by a separate fan to the
fan/filter/heater room. This room must be under pressure to avoid unfiltered
air to enter.
- Filtration degree and filter position depend on the final temperature of
the process air as follows:
- For main drying air to be heated above 120șC
only coarse filtration up to 90% is needed. The filter should be placed on the
pressure side of the fan.
- For secondary air to be heated below 120șC or
not heated at all, the filtra-tion must be 90-95%, and the filter must be
placed after the heater/cooler. Some countries have even stricter requirements
demanding a filtration of up to 99.995%, corresponding to EU13/14 (or H13/14).
- Current practice is as follows:
- Dairy-like products, equal to or
better than 3A: Test method:
-Prefiltration EU4 (or G4) ≈35% Dust-spot
efficiency
-Main air filtration EU7 (or F7) ≈90% Dust-spot efficiency
-Secondary air filtration EU7 (or F7) ≈90% Dust-spot efficiency
-Baby
food products, equal to or better than IDF:
-Prefiltration EU6 (or F6) ≈70%
Dust-spot efficiency
-Main air filtration EU7 (or F7) ≈90% Dust-spot
efficiency
-Secondary air filtration EU9 (or F9) >95% Dust-spot
efficiency