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TRG-TRC013-EN 3
period one
Fan Performance
notes
When the pressure within the ductwork is positive, that is, greater than
atmospheric, the water column is forced downward in the closed leg and forced
upward in the open leg. Conversely, a negative pressure within the ductwork
causes the water column to drop in the open leg and to rise in the closed leg.
In this illustration, a positive pressure in the ductwork forces the water in the
closed leg 3 in. [76.2 mm] lower than the water in the open leg. A pressure of 1
psi will support a 27.7-in. column of water. [A pressure of 1 kPa will support a
102-mm column of water.] Therefore, this length of water column is equivalent
to a pressure of 0.11 psi [758 Pa].
However, to avoid having to convert units each time a reading is taken,
inches (in.) or millimeters (mm) of water (H
2
O) are often used to measure the
pressures generated by fans. Other common expressions of this measurement
are “water gage” (wg) and “water column” (wc).
Positive Duct Pressure
duct
duct
pressure
pressure
atmospheric
atmospheric
pressure
pressure
3 inches
3 inches
[76.2 mm]
[76.2 mm]
Figure 5