Tennessee Wastewater Operator Practice Exam

What is the most efficient method to control the output airflow rate of a centrifugal blower?

1. Adjusting the inlet size

2. Using a throttle valve

3. Adjusting the blower speed with a VFD

4. Increasing the impeller size

The correct answer is: Adjusting the blower speed with a VFD

The most efficient method to control the output airflow rate of a centrifugal blower is by adjusting the blower speed with a Variable Frequency Drive (VFD). Using a VFD allows for precise control over the speed of the motor that drives the blower. This capability is particularly beneficial because airflow generated by a centrifugal blower is proportional to the speed of the motor. Reducing the speed decreases the airflow in a controlled manner without increasing system resistance, which can occur when other methods are applied. Additionally, utilizing a VFD can lead to energy savings because it allows the system to operate at just the right speed needed for a given application, avoiding excess energy consumption from running at full speed when it is not necessary. In contrast, methods such as using a throttle valve can lead to increased energy losses due to the restriction of airflow and the associated pressure drop across the valve. Adjusting the inlet size could impact performance but is generally less efficient than speed control. Increasing the impeller size would also not provide the desired flexibility in controlling airflow rates, as it is a permanent alteration to the blower's design. Thus, adjusting the blower speed with a VFD stands out as the most effective and efficient approach for controlling airflow rates.