CSLB C-20 HVAC ContractorPlanning and LayoutMedium
A residential HVAC system design requires a total airflow of 1,200 CFM. The system is designed with a constant volume fan. If the static pressure in the supply plenum increases due to a partially closed damper, what is the most likely immediate effect on the system's airflow?
- AThe airflow will oscillate rapidly, causing system instability.
- BThe airflow will remain constant, as the fan automatically compensates.
- CThe airflow will increase, requiring more power from the fan.
- DThe airflow will decrease, as the fan struggles against higher resistance.
Show answer & explanationAnswer & explanation
Correct answer: D. The airflow will decrease, as the fan struggles against higher resistance.
In a constant volume fan system, an increase in static pressure represents higher resistance to airflow. The fan's performance curve dictates that as static pressure rises, the volume of air it can move (CFM) will decrease, assuming no change in fan speed or motor power.
Why the other options are wrong
- A. While extreme conditions could lead to instability, a partially closed damper primarily causes a measurable decrease in airflow due to increased resistance.
- B. Constant volume fans maintain airflow only within their design range; beyond that, increased static pressure will reduce airflow.
- C. Increased static pressure represents higher resistance, which would typically decrease, not increase, airflow for a given fan speed.
Fan Performance Curve
A graphical representation showing the relationship between a fan's airflow (CFM), static pressure, power consumption, and efficiency at various operating points.
- Shows how CFM changes with static pressure.
- As static pressure increases, CFM typically decreases.
- Used to select the correct fan for specific system requirements.
Memory trick: Fan pushes air, but resistance slows the flow.