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Certifying Facilities Certification of Atmospheric Fumigation Chambers
Construction and Performance Standards
10/2002-04 Treatment Manual 6-3-3
PPQ
◆ Should be provided with heating or refrigeration units when they
are required for fumigation efficiency or to prevent product
injury.
◆ Should be equipped with removable, slatted floors unless all
material placed in the chamber is on pallets or carts.
The criteria listed above deal primarily with the efficiency of the
fumigation chamber itself. In determining the ultimate design and
construction, it is essential to give consideration to the safe and
practical operation of the facility.
Gas-Tight Construction
Interior surfaces must be impervious to the fumigant. Joints must be
sealed with proper compound, solders, or welds. All doors and vents
must be provided with proper gaskets. All openings such as for wiring,
thermometer, tubing, ports for pressure-leakage tests, etc., must be
made gas-tight.
Interior surfaces whether metal, cement, concrete block, tile, or
plywood must be painted with epoxy resin, vinyl plastic, or asphalt
base paints. Such paint coverings make the surfaces less sorptive, an
important factor in maintaining gas concentrations.
When wood or wood and sheet metal are used in construction, it is
critical that all joints and seams be sealed with nonhardening mastic.
This makes a gas-tight seal and allows for expansion and contraction
without leakage. In masonry construction, the mortar between all
courses of cement blocks should be jointed to produce a smooth
compact surface. Poured concrete structures should also have smooth
compact surfaces.
The construction and fastening of chamber doors is most critical.
Doors may be hinged from the top or side, or on a davit. A chamber
door hinged at the top is less apt to sag. Refrigerator hinges should be
used if the door is hinged at the side. A high quality gasket, such as
neoprene must be provided along the entire perimeter of the chamber
facing. To obtain maximum tightness, the doors must be fastened
uniformly against the gaskets.
Circulation and Exhaust Systems
Various methods can be used to circulate the fumigant within the
chamber. Equipment should be capable of circulating air at the rate of
at least one-third the volume of the chamber per minute. The rate of
Important
Aluminum base paints are not acceptable because of the corrosive effect
caused by a reaction between such paints and the fumigant. A list of sources
for approved paints is available from the Riverdale office.
Certifying Facilities Certification of Atmospheric Fumigation Chambers
Construction and Performance Standards
6-3-4 Treatment Manual 10/2002-04
PPQ
airflow of the blower should give approximately one complete change
of air per minute, based on the volume of the empty chamber. For
smaller chambers, a suitable circulating fan will usually provide the
necessary air movement. For larger chambers, effective gas
distributions can be obtained by using a circulating or squirrel cage
fan which picks up the air/gas mixture from a duct reaching near the
floor and blowing it across the top of the load. A blower located outside
the chamber may also be used, but this method considerably
increases the possibilities of leakage. Non-sparking explosion-proof
units are required with some fumigants.
Exhaust blowers should also be sized according to the volume of the
chamber. Volume of enclosure (in cubic feet) divided by the sum of
cubic feet per minute (cfm) of the exhaust fan(s) or exhaust blower
equals the number of minutes required per complete gas volume
exchange. (2) Sixty minutes divided by the number of minutes per gas
volume exchange equals the number of complete gas exchanges per
hour. The result should be in the range of 4 to15. The faster the rate of
aeration the better, particularly for perishable commodities. If the
exhaust flow is connected to a methyl bromide recovery system, this
device must not impede the flow rate to less than 4 volumes per hour.
Frequently, circulation and exhaust systems are designed to utilize
the same blower. Venting to the outside is essential, and the exhaust
stack should extend well above all nearby structures. Compliance with
local safety ordinances is essential.
Fumigant Dispensing System
The dispensing system needed will vary with the types of fumigants in
use. The fumigant MB is usually introduced into the chamber through
a tube extending from the supply cylinder. Within the chamber this
tube should be provided with properly spaced openings through which
the fumigant is dispersed. Fumigants in small quantities are generally
　
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