where there is exposed product are best
located inside the building or within an
enclosed structure located on the roof
of the factory. AHUs exposed to the
outdoor elements tend to experience
more frequent hygiene failures.
Another positive hygienic design is
the use of fabric ducting as opposed to
rigid ducts. Fabric ducts are less expensive to install and to keep clean. Retro-fitting these fabric ducts into existing
systems may be cost-effective compared
with routine cleaning of metal ducts.
The design of fabric ducts may eliminate isolated drafts. Finally, the ease of
customizing the duct location within
the room serves to create more thorough air exchanges within the production area.
Ductwork installed directly over
an exposed product line should be
avoided. Condensation and other contaminants could pose a risk to exposed
product below the ducts.
Avoid “shortcutting” the supply
and return of air by having too short a
distance between the two ducts. This is
common in roof-mounted systems that
simply have straight drops of rigid ducting into the manufacturing space below.
Because of the short distance between
the supply and the return ducts, much
of the air supplied to the room simply
travels the short distance to the return
duct and does not disperse or circulate
throughout the entire room.
Air Parameters for Consideration
If airborne contaminants are identified as a potential risk to product safety
and/or quality, then the filtration level
must be adequate to control the identified risk. The engineering group at a
factory, as well as the AHU and filter
suppliers, must help the factory make
the best possible decision for the level
of air filtration needed based on the specific AHU capability and the identified
risk to be controlled.
The most common rating systems for
air filters are the European filter class
standard (EN779), known by its use of
“G” and “F” ratings, and the American
filter class standard (ASHRAE 52.2-
1999), known by its use of “MERV”
ratings. Air filters range from very coarse
to very fine. Many AHUs can utilize
a coarse prefilter followed by a fine
secondary filter. Coarse prefilters can
capture insects and many types of for-
eign materials before they reach the fine
secondary filters. These configurations
can greatly reduce cost by extending the
life of fine secondary filters.
One may think that the finer the
filtration, the better the air quality will
be within the room. This is not always
the case. Well-meaning people may
insist on installing finer filters to im-
prove air quality in a space without un-
derstanding that finer filtration causes