Handbook of air conditioning and refrigeration / Shan K

20.32 CHAPTER TWENTY
FIGURE 20.13 Modulation curve of a VAV system installed with duct static pressure control.
For a variable-air-volume system with duct static pressure control, the pressure loss of the air
system can be divided into fixed part �p fix and variable part �p var when the volume flow rate varies.
This is described in detail in Chap. 21.
Figure 20.13 shows a typical modulation curve of a VAV system with duct static pressure control
using inlet vanes to modulate fan capacity. This VAV system has many fan curves F t at various
volume flow rates and a system curve for the variable pressure loss S var.
During reduction of the volume flow rate of a VAV system, the dampers in the VAV boxes close
partially. The variable part of the pressure loss of the air system �p var from the AHU or PU up to the
pressure sensor of duct static pressure control varies as the volume flow is reduced. Its pressure-volume
characteristics are indicated by the system curve S var. However, the pressure loss of the duct
system, branch takeoffs, and VAV boxes beyond the duct static pressure sensor remains constant,
and is equal to the set point of duct static pressure control �p fix. The modulation curve is actually
the system curve of the part of VAV air system with variable pressure loss S var.
At the same time, the inlet vanes at the fan inlet also reduce their opening in response to the
sensed higher duct static pressure. Therefore, a new fan curve F t2 is formed. The intersection of F t2
and system curve S var at the reduced volume flow rate is the system operating point B. Similarly, at
a further reduction of volume flow rate, the system operating points are C, D, E ....
As the volume flow rate of the air system approaches zero, theoretically, the system total pressure
will equal the fixed part of pressure loss �p fix, which is equal to the set point of duct static
pressure control, as shown in Fig. 20.13.
The modulation curve of a fan-duct system should not enter the fan surge area or stall region, or
unstable operation will result. However, a forward-curved centrifugal fan may be operated in the
surge area if the system total pressure is below 1.5 in. WC. Refer to the manufacturer’s data for detailed
analysis.