Duct System Calculations
An ideal duct system would be one in which total pressure remained constant. In other words, there would be no pressure losses any- where in the system. Under ideal operating conditions, any change in velocity pressure would be compensated for by an equal change in static pressure, thereby maintaining a constant total pressure (that is, the sum of the velocity and static pressures). A drop in velocity pressure would trigger a corresponding rise in static pressure, and vice versa. Unfortunately, this is not the way things work in actual practice because pressure losses very definitely do occur.
In the ductwork, pressure losses result from the resistance of the ducts to the passage of the air. This resistance occurs as a result of two effects: (1) friction loss and (2) dynamic loss. The former is caused primarily by the friction of the moving air against the surface of the duct. Dynamic loss results from sudden changes of direction (for example, in sharp elbows) in the air stream.
An important aspect of duct system calculations is determining the total external static pressure drop (that is, total resistance) of the duct system. In large part, a blower or fan is selected on the basis of its capacity to operate against the total resistance of the ductwork. This resistance of the ductwork to the flow of air is referred to as the external static pressure (or external static pressure drop), because it represents the pressure drop occurring out- side the heating or cooling unit. In addition to external static pressure, a blower or fan must also overcome resistance due to internal static pressure drop caused by the passage of air through heaters, coils, filters, and washers. This data can be obtained from rating tables provided by manufacturers. Duct resistance (that is, external static pressure drop) must be calculated for each duct system.
The equal friction method is frequently used to calculate the external static pressure of a duct system (see Equal Friction Method in this chapter). Two other methods used for making these calculations are (1) the velocity-reduction method, and (2) the static-regain method. Detailed descriptions of both of these methods can be found in the ASHRAE Guide.