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62 American Dryer Corp. 450268-1
E. STATIC PRESSURE
The pressure of the air in sheet metal ducts is of great importance for safe and efficient operation of the
dryer. The air pressures, as small as they are, have a large influence upon the performance of the dryer.
There are three (3) pressures present within a duct. The velocity pressure, which is the pressure due to
the moving stream of air, the static pressure, which is the outward push of the air against the walls of the
duct, and the total pressure, which is the sum of the static and velocity pressures. Only the total and
static pressures can be measured. The velocity pressure is then calculated from the difference between
the two. The static pressure is the most influential and determining factor in the design of a dryer duct
system. ADC recommends that the dryer exhaust duct static pressure not exceed 0.3 inches (0.74 mb)
of water column (W.C.).
Gages used to measure such small air pressures utilize the elevation of a column of water. This form of
gage is called a manometer. One form of this gage is known as a U-tube and is shown in Illustration #1.
When one end of the tube is connected to the duct, the static (outward) pressure within the duct forces
the water column to be depressed in one leg of the U-tube and up in the other. The elevation of the water
column in Illustration #1 is 3.2 inches (8 mb).
ILLUS. #1