System Effects
Whenever we have changes in the duct system that might affect the total static pressure of the system, such changes are termed as system effects. Some of these changes are –> modulating the position of damper blades so as to vary the amount of air flowing or dirty air filters that might restrict airflow. These system effects change the system characteristic curve since the ΔP no longer remains the same.
CHANGING DAMPER POSITION
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Dampers are often used in many HVAC system in order to regulate the amount of air flowing through the vents. This modulation of damper position is accompanied with a change in the total static pressure which then causes the system characteristic curve to change.
DAMPER BLADES OPEN
When the damper is closed from its mean position, the system curve appears to move slightly upwards.
According to the figure, when there is 5 (m^3 /s) of air flowing through the duct, the static pressure in the system is 750 Pa at POINT 1 and the damper is at the mean position.
If we close the damper, the system curve moves upward indicating that the system experiences a static pressure of 1000 Pa for the same airflow rate (POINT 2).
This can also be verified logically. In other words, closing a damper means reducing the area through which air can flow and thus creating a resistance for it. Now the fan has to push the air with a greater force to supply the same airflow.
DAMPER BLADES CLOSED
Looking the figure above we can say that opening the damper blades completely causes the system curve to shift downwards. Now in this case the fan supplies 5 m^3/s at 600 Pa (POINT 2) compared to 750 Pa at the mean position (POINT 1). Thus, we can say that air faces a much less resistive path and the fan has to work against less static pressure to achieve the required airflow.
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The system curve behaves in a manner similar to the first case when it comes to dirty filters. This is also due to the fact that the air faces more resistance when the filter becomes dirty and hence the system characteristic curve moves upwards.