American Dryer Corp. ML-82 III - Manual (Page 17)

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2. Single Dryer Venting
When possible, it is suggested to provide a separate exhaust duct for each dryer. The exhaust duct should
be laid out in such a way that the ductwork travels as directly as possible to the outdoors with as few turns
as possible. It is suggested that the use of 90° turns in the ducting be avoided; use 30° and/or 45° angles
instead. The shape of the exhaust ductwork is not critical as long as the minimum cross section area is
provided.
IMPORTANT: For a dryer that is vented horizontally, the minimum duct size for a gas dryer is
14-inches (35.56 cm) for a round duct or 12-1/2” (31.75 cm) for a square duct.
THE DUCT SIZE MUST NOT BE REDUCED ANYWHERE
DOWNSTREAM OF THE DRYER.
IMPORTANT: Exhaust back pressure measured by a manometer at each basket (tumbler) exhaust
duct area must be no less than 0 and must not exceed 0.3 inches (0.74 mb) of water
column (W.C.).
It is suggested that the ductwork from each dryer not exceed 15 feet (4.57 meters) with no more than two
(2) elbows (excluding dryer connections and outside exhaust outlets). If the ductwork exceeds 15 feet
(4.57 meters) or has numerous elbows, the cross section area of the ductwork must be increased in
proportion to the length and number of elbows in it. In calculating duct size, the cross section area of a
square or rectangular duct must be increased by twenty percent (20%) for each additional 15 feet
(4.57 meters). The diameter of a round exhaust duct should be increased ten percent (10%) for each
additional 15 feet (4.57 meters). Each 90º elbow is equivalent to an additional 30 feet (9.14 meters), and
each 45º elbow is equivalent to an additional 15 feet (4.57 meters).
Outside Ductwork Protection
To protect the outside end of the horizontal ductwork from the weather, a 90º elbow bent downward should
be installed where the exhaust exits the building. If the ductwork travels vertically up through the roof, it
should be protected from the weather by using a 180º turn to point the opening downward. In either case,
allow at least twice the diameter of the duct opening and the nearest obstruction.
IMPORTANT: DO NOT use screens, louvers, or caps on the outside opening of the exhaust
ductwork.