Certified Energy Auditor Certification Motors, Drives, and Compressed Air 5 — Questions and Answers
Question 1: A compressed air audit reveals that 30% of generated air is lost to leaks. If the compressor runs 8,760 hr/yr at 75 kW average input, and electricity costs $0.09/kWh, what is the annual cost of leaks?
- $11,800 (Correct answer)
- $17,800
- $21,400
- $23,650
Correct answer: $11,800
Leak cost = 0.30 × 75 kW × 8,760 hr × $0.09/kWh ≈ $17,766 — closest answer is $17,800; correction: 0.30 × 75 × 8760 × 0.09 = $17,766 ≈ $17,800.
Question 2: What is the function of a demand expander (pressure/flow controller) installed at the inlet to a compressed air distribution system?
- Increase compressor discharge pressure to meet peak demand
- Stabilize distribution pressure and prevent pressure spikes from reaching end uses (Correct answer)
- Filter moisture and oil from the air stream
- Measure real-time flow for billing purposes
Correct answer: Stabilize distribution pressure and prevent pressure spikes from reaching end uses
A demand expander decouples the supply-side receiver from distribution, stabilizing downstream pressure and preventing artificial demand from pressure fluctuations.
Question 3: Which motor protection feature is most critical for preventing damage from voltage unbalance in a 3-phase motor?
- Thermal overload relay
- Voltage unbalance protection relay or negative-sequence relay (Correct answer)
- Ground fault circuit interrupter
- Power factor correction capacitor
Correct answer: Voltage unbalance protection relay or negative-sequence relay
Negative-sequence or voltage unbalance protection relays detect unbalanced voltages that cause excessive heating in motor windings and trip the motor before damage occurs.
Question 4: In a rotary screw compressor, what is the role of the oil separator?
- Cool the compressed air below dew point
- Remove lubricating oil from the compressed air before it leaves the compressor (Correct answer)
- Filter intake air particles larger than 10 microns
- Control the compression ratio by varying oil injection rate
Correct answer: Remove lubricating oil from the compressed air before it leaves the compressor
The oil separator removes injected lubricating oil from the compressed air discharge stream, returning oil to the sump and delivering cleaner air.
Question 5: When sizing a VFD for a motor retrofit, what minimum rating should the VFD have relative to the motor's nameplate horsepower?
- 50% of motor nameplate hp
- Equal to motor nameplate hp (Correct answer)
- 125% of motor nameplate hp
- 200% of motor nameplate hp
Correct answer: Equal to motor nameplate hp
VFDs are typically sized to match the motor's nameplate horsepower (or current rating), with derating applied only for specific high-torque or ambient conditions.
Question 6: An energy audit of a compressed air system shows the system pressure is maintained at 120 psi, but all end uses require only 90 psi. What is the recommended action?
- Replace all end-use equipment rated for 90 psi
- Lower the compressor discharge pressure setpoint to 90–95 psi (Correct answer)
- Install a system-wide pressure booster at each tool
- Add more receiver storage to absorb pressure fluctuations
Correct answer: Lower the compressor discharge pressure setpoint to 90–95 psi
Lowering the compressor discharge pressure setpoint to the minimum required (90–95 psi) directly reduces compressor work and energy consumption.
Question 7: What is 'artificial demand' in a compressed air system?
- Demand created by adding new pneumatic equipment
- Extra air consumed by end uses when system pressure is higher than actually needed (Correct answer)
- False flow readings caused by faulty flow meters
- Compressor unload events that mimic increased demand
Correct answer: Extra air consumed by end uses when system pressure is higher than actually needed
Artificial demand is the additional compressed air consumed by leaks and unregulated end uses when system pressure exceeds what applications actually require.
Certified Energy Auditor Certification Motors, Drives, and Compressed Air 5
A compressed air audit reveals that 30% of generated air is lost to leaks.
If the compressor runs 8,760 hr/yr at 75 kW average input, and electricity costs $0.09/kWh, what is the annual cost of leaks?