Certified Energy Auditor Certification Motors, Drives, and Compressed Air 4 — Questions and Answers
Question 1: A compressed air dryer is rated at 500 scfm at 100°F inlet temperature. The actual inlet temperature is 120°F. What happens to the effective drying capacity?
- Capacity increases due to higher vapor pressure differential
- Capacity remains the same as temperature is compensated internally
- Capacity decreases because warmer air holds more moisture (Correct answer)
- Capacity doubles due to higher thermal energy
Correct answer: Capacity decreases because warmer air holds more moisture
Higher inlet temperatures increase the moisture load on the dryer, reducing its effective capacity below the rated 500 scfm.
Question 2: What is meant by the term 'specific power' of a compressor as used in energy auditing?
- Horsepower per cubic foot of compressor volume
- kW per 100 scfm of compressed air delivered (Correct answer)
- BTU/hr per ton of refrigeration equivalent
- Watts per RPM of shaft speed
Correct answer: kW per 100 scfm of compressed air delivered
Specific power (kW/100 scfm) is the standard metric for comparing compressor efficiency and system performance.
Question 3: Which of the following best describes the purpose of a receiver tank in a compressed air system?
- Remove oil from compressed air
- Provide capacitance to buffer demand swings and reduce compressor cycling (Correct answer)
- Increase system pressure above compressor discharge pressure
- Filter particulates larger than 5 microns
Correct answer: Provide capacitance to buffer demand swings and reduce compressor cycling
Receiver tanks store compressed air to buffer short demand peaks and reduce compressor start/stop cycling, improving system stability.
Question 4: A 3-phase induction motor is operating with a measured power factor of 0.72. What is the most likely cause?
- Motor is overloaded beyond its rated capacity
- Motor is operating at a fraction of its rated load (Correct answer)
- Supply voltage is higher than nameplate voltage
- Motor windings have a ground fault
Correct answer: Motor is operating at a fraction of its rated load
Induction motors have poor power factor at light loads; power factor improves and peaks near full-load operation.
Question 5: When applying the affinity laws to a VFD-controlled pump or fan, which quantity does NOT scale with speed ratio?
- Flow rate
- Head or pressure
- Power
- Rotational speed (Correct answer)
Correct answer: Rotational speed
Rotational speed is the independent variable that drives the affinity law relationships — it is the input, not a derived scaled quantity.
Question 6: An auditor discovers a compressed air system using air at 90 psi for a blowoff application that only needs 15 psi. What is the recommended corrective measure?
- Install a larger receiver tank upstream
- Replace the compressor with a lower-pressure unit
- Install a point-of-use pressure regulator to reduce pressure at that application (Correct answer)
- Add a refrigerated dryer at the blowoff nozzle
Correct answer: Install a point-of-use pressure regulator to reduce pressure at that application
A point-of-use pressure regulator reduces pressure to the minimum required, avoiding energy waste from over-pressurizing low-pressure applications.
Question 7: For a premium efficiency (IE3) motor versus a standard efficiency motor of the same rating, which statement is generally true regarding motor operating temperature?
- IE3 motors run hotter due to higher power density
- IE3 motors run cooler because they have lower losses (Correct answer)
- Operating temperature is identical since rated output is the same
- IE3 motors run cooler only when oversized by 25%
Correct answer: IE3 motors run cooler because they have lower losses
Premium efficiency motors have lower copper, iron, and stray losses, generating less heat and thus running at lower temperatures than standard motors.
Certified Energy Auditor Certification Motors, Drives, and Compressed Air 4
A compressed air dryer is rated at 500 scfm at 100°F inlet temperature.
The actual inlet temperature is 120°F.
What happens to the effective drying capacity?