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19
The main duct may be any shape or cross sectional area, so long as the minimum cross section area is provided.
The illustrations on
page 19 and page 20 show the minimum cross section area for multiple dryer round or
square venting. These figures must be increased 10 square inches when rectangular main ducting is used, and
the ratio of duct width to depth should not be greater than 3-1/2 to 1. These figures must be increased in
proportion if the main duct run to the last dryer to where it exhausts to the outdoors is unusually long (over twenty
[20] feet) or has numerous elbows (more than two [2]) in it. In calculating duct work size, the cross section area
of a square or rectangular duct must be increased twenty (20) percent for each additional twenty (20) feet. The
diameter of a round exhaust must be increased ten (10) percent for each additional twenty (20) feet. Each 90°
elbow is equivalent to an additional forty (40) feet, and each 45° elbow is equivalent to an additional twenty (20)
feet. .
IMPORTANT: For extended duct work runs, the cross section area of the duct work can only be
increased to an extent. Maximum proportional duct work runs cannot exceed
twenty (20) feet more than the original limitations of twenty (20) feet with two
(2) elbows. When the duct work approaches the maximum limits as noted in
this manual, a professional HVAC firm should be consulted for proper
venting information.
IMPORTANT: Exhaust back pressure measured by a manometer at each dryer exhaust duct area
should not exceed 0.3 inches of water column.
The duct should be smooth inside with no projections from sheet metal screws or other obstructions which will
collect lint. When adding ducts, the duct to be added should overlap the duct to which it is to be connected. ALL
duct work joints must be taped to prevent moisture and lint from escaping into the building. Inspection doors
should be installed at strategic points in the exhaust duct work for periodic inspection and clean-out of lint from
the duct work.
NOTE: Where the exhaust passes through a wall, ceiling, or roof made of combustible materials,
the opening must be 2-inches larger (all the way around) than the duct. The duct must
be centered within this opening.
a. Outside Duck Work Protection
1) To protect the outside end of horizontal duct work from the weather, a 90° elbow bent downward
should be installed where the exhaust exits the building. If the exhaust duct work travels vertically
up through the roof, it should be protected from the weather by using a 180° turn to point the opening
downward. In either case, allow at least twice the diameter of the duct between the duct opening and
nearest obstruction.
IMPORTANT: DO NOT use screens, louvers, or caps on the outside of opening of exhaust
duct work.