contamination'.Unfortunately this has proven to be true whenyou lookat the
SaraLee hot dogListeriaincident (USEPA, 2003). The use of air pressure
relationships and filtration becomesmoreand more importantas the sensitivity
or risk of productsto air contaminationincreases. Airflow withina facility
shouldflowfromcleanto dirty.Thehighestfiltered air pressureshouldbe
wherethe product is last in touchwiththe environment. Thisis normallythe
packaging room. In many plants the packaging and final process (before
packaging)are locatedin the samearea.The air pressurethenflows outward to
preprocessingareassuchas raw material preparation,raw material receivingand
finallyto the outside. In the otherdirectionthe air flows fromprocessing/
packagingto the casingroom, warehouse and to shipping. Again the air should
flowoutward when the dockdoorsare opened.The pressure differentialis not
large.It is generally acceptedpractice to havea pressuredifferential of 0.01±
0.02inches (0.25±0.5 mm)of watercolumnbetween rooms. Thisequatesto
about250±300feet (76±91m) per minute air velocityfromroomto room.If the
plantoperatesundernegative pressure,that is, air flowsinto the plantwhen the
doorsare openedor comes in throughcracks and crevices,thenthe plant has
absolutely no control overthe qualityof the air cominginto the facility. The rule
of thumbfor plantprocessing areasis a minimumof 6±12air turnsper hour, i.e.
the volumeof air in the roomis changed 6±12times per hour.
In areasthat operateunderrefrigerationthe coldair can be recycled through
filterunits.The recommended procedurefor doingthat is to put an air-handling
unit outsidethe process room(on the roofor beside an outside wall)that has
heatingand refrigeration capacities. Air is drawnfromthe room,passedthrough
a filter of 30±50and 40±50%efficient.At this pointany make-upair is added
fromthe outside(usually 5±10%of the totalvolume). The coldair fromthe
processroomhelpsto precondition the outside make-upair. The mix then passes
overa refrigerationcoiland thenthrougha final filter.Thefinalfilter is
recommendedto be 95%efficientat 5m. The 5 m size has beenrecom-
mendedsincemostmicrobes existin air as passengers (on dust particles), within
droplets, and a veryfew as isolatedorganisms.Bacteriaare on averageabout
1.5m whilemold is 2.5±20m. Yeastare 4±12m. Compared witha human
hairat 50±100m. A magna-helix gage (or similar airflowgage)should be
installed at the finalfilterstageso whenthe pressure dropgets too greatan alarm
will soundin the maintenanceoffice, indicating that the filter requires changing.
A clogged air filtercan becomea positive contamination source.The filtered air
is thenducted into the roomand flowsalong the ceiling,downthe wallsand
backto the exhaust vent for recycling.Thismovingair overthe ceilingand wall
surfaces will greatly reduce the potentialfor condensate formation, especially on
overhead fixtures.
Air ductsare designedeitherroundor rectangular.Rectangularducts should
be sealedagainstthe ceilingif theyare belowthe ceiling surface. If it is not
sealedto the surfacethe flat surfacecan becomea depository for dust and dirt
and make a goodrunway for rodents.Otherductsare roundand presenta
smalleroverheadsurfacethat has to be cleaned.All seamsshouldbe weldedor
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