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64 American Dryer Corp. 450268-1
To obtain the most accurate static pressure readings of a dryer, the following guidelines should be
conformed to. The reading should be taken in a straight section of duct, a minimum of ten (10) times the
diameter of the duct (i.e. 10-inches [25.4 cm] diameter duct, a straight section 100-inches [254 cm] in
length). The reading should be taken a minimum of 8-1/2 times the diameter upstream of the dryer or
nearest elbow, and 1-1/2 times the diameter downstream of the Pitot tube. The following is an example
showing the proper location to take a static pressure reading.
In designing a dryer exhaust duct system, it is not necessary to compute the friction loss of the ductwork.
The friction loss (static pressure) is predetermined to not exceed 0.3 inches (0.74 mb) of water column
(W.C.). This maximum allowable static pressure is the same for ALL model dryers and is calculated to
provide maximum dryer efficiency. It is necessary to determine the total equivalent length of the duct
system. The total equivalent length of the duct system is the actual length of the duct added to the
equivalent length of ALL the fittings. The equivalent length of a fitting, is the length of straight duct of the
same size as the fitting in which the friction losses are equal. Equivalent lengths must be determined for
ALL elbows, as well as, any entries which may be incorporated into a common duct system for multiple
dryers. The total equivalent length is then used with the total cfm exhaust rating of the dryer(s) to
calculate the minimum round duct diameter. When designing a single common duct system for multiple
dryers, each individual dryer exhaust duct enters the main common duct.