CEM Energy Management and Control Systems 2 — Questions and Answers
Question 1: A variable frequency drive (VFD) on a pump or fan saves energy because fan/pump power varies with:
- The square of speed
- The cube of speed (Correct answer)
- Speed directly (linear relationship)
- The fourth power of speed
Correct answer: The cube of speed
By the Affinity Laws, fan and pump power varies with the cube of rotational speed, so a 20% speed reduction yields nearly 49% power reduction.
Question 2: Economizer control on an air-handling unit saves cooling energy by:
- Recirculating return air to avoid conditioning outdoor air
- Using cool outdoor air for free cooling when outdoor conditions are favorable (Correct answer)
- Reducing supply air temperature with additional refrigeration
- Increasing chilled water supply temperature
Correct answer: Using cool outdoor air for free cooling when outdoor conditions are favorable
When outdoor air is cool and dry enough, economizers open dampers to use outdoor air for cooling, reducing or eliminating mechanical refrigeration energy consumption.
Question 3: Supply air temperature reset saves energy in an air-handling unit by:
- Maintaining the coldest possible supply air temperature year-round
- Raising supply air temperature during mild conditions, reducing chiller and reheat energy (Correct answer)
- Lowering return air temperature set points
- Increasing supply air volume at higher temperatures
Correct answer: Raising supply air temperature during mild conditions, reducing chiller and reheat energy
Supply air temperature reset raises the cooling coil leaving temperature during mild loads, allowing the chiller to operate more efficiently and reducing simultaneous heating for reheat.
Question 4: A Direct Digital Control (DDC) system differs from pneumatic controls primarily because it:
- Uses compressed air signals to position actuators
- Uses digital microprocessors and electronic sensors for precise, programmable control (Correct answer)
- Requires no sensors to operate
- Controls only lighting systems
Correct answer: Uses digital microprocessors and electronic sensors for precise, programmable control
DDC uses digital controllers with electronic sensors and software-based control logic, providing far greater precision, data logging, remote access, and programming flexibility than pneumatic systems.
Question 5: Chiller sequencing control in a plant with multiple chillers saves energy by:
- Running all chillers simultaneously at low load
- Staging chillers on/off to operate fewer units at higher, more efficient part-load ratios (Correct answer)
- Running the largest chiller at minimum capacity always
- Maintaining constant chiller staging regardless of load
Correct answer: Staging chillers on/off to operate fewer units at higher, more efficient part-load ratios
Chillers generally operate most efficiently at 70-90% load; sequencing ensures fewer chillers run at higher efficiency rather than many running at very low, inefficient part-load conditions.
Question 6: An energy information system (EIS) or energy dashboard primarily helps energy managers by:
- Directly controlling HVAC equipment setpoints
- Providing real-time and historical energy data for trend analysis and anomaly detection (Correct answer)
- Replacing the need for utility meters
- Automatically generating purchase orders for energy equipment
Correct answer: Providing real-time and historical energy data for trend analysis and anomaly detection
An EIS aggregates meter data and presents consumption trends, benchmarks, and alerts, enabling energy managers to identify anomalies and prioritize efficiency improvements.
CEM Energy Management and Control Systems 2
A variable frequency drive (VFD) on a pump or fan saves energy because fan/pump power varies with: