Certified Energy Auditor Certification Motors, Drives, and Compressed Air 3

Question 1

What is the approximate rule of thumb for energy savings in a compressed air system for every 2 psi reduction in system pressure?

Accepted Answer

1% energy savings

Answer

A commonly accepted rule of thumb is that reducing compressed air pressure by 2 psi saves approximately 1% in compressor energy.

Question 2

A 100 hp motor operates at 75% load for 6,000 hours/year. At $0.08/kWh and 92% efficiency, what is the approximate annual energy cost?

Accepted Answer

$29,200

Answer

Input power = (100 × 0.75 × 0.746) / 0.92 = 60.8 kW; cost = 60.8 × 6,000 × $0.08 ≈ $29,200.

Question 3

Which type of positive-displacement compressor is best suited for applications requiring oil-free air at high pressure with low flow rates?

Accepted Answer

Reciprocating (piston) compressor

Answer

Reciprocating compressors can achieve high pressures with small flows and are available in oil-free configurations for clean air applications.

Question 4

An energy auditor finds a compressed air system where the compressor unloads frequently. What does this most likely indicate?

Accepted Answer

System demand is significantly less than compressor capacity

Answer

Frequent unloading indicates the compressor capacity exceeds actual system demand, causing it to cycle between load and unload states.

Question 5

What is the primary advantage of a variable-speed drive (VSD) compressor over a fixed-speed compressor with inlet valve modulation?

Accepted Answer

Lower energy consumption at part-load conditions

Answer

VSD compressors match motor speed to actual demand, consuming significantly less energy at part-load than modulated fixed-speed units.

Certified Energy Auditor Certification Practice Test