CARO Compressor Operations and Maintenance 3 — Questions and Answers
Question 1: In an oil-flooded rotary screw compressor, what is one of the main functions of the injected oil besides lubrication?
- Raising the discharge temperature for better condensing
- Preventing ammonia from mixing with water
- Increasing the refrigerant's latent heat
- Sealing clearances between rotors and removing heat of compression (Correct answer)
Correct answer: Sealing clearances between rotors and removing heat of compression
Injected oil seals rotor clearances, cools the compressed gas, and lubricates bearings in a flooded screw compressor.
Question 2: What component typically controls capacity on most industrial rotary screw compressors?
- A slide valve that changes the effective rotor length used for compression (Correct answer)
- Variable clearance pockets on the cylinder head
- A hot-gas bypass valve only
- Cylinder unloaders
Correct answer: A slide valve that changes the effective rotor length used for compression
The slide valve moves to return part of the suction gas before compression, reducing capacity on a screw compressor.
Question 3: Why is a screw compressor normally started with the slide valve at the minimum-load position?
- To ensure the oil separator fills with refrigerant
- To reduce starting torque and motor inrush load (Correct answer)
- To prevent the volume ratio from changing
- To maximize discharge pressure quickly
Correct answer: To reduce starting torque and motor inrush load
Starting unloaded reduces the torque the motor must develop, limiting starting current and mechanical stress.
Question 4: On a screw compressor, the volume index (Vi) should ideally be matched to what?
- The motor's horsepower rating
- The evaporator's surface area
- The system's operating compression ratio (Correct answer)
- The oil viscosity
Correct answer: The system's operating compression ratio
A Vi matched to the actual pressure ratio avoids over- or under-compression losses and improves efficiency.
Question 5: An operator sees the oil separator level on a screw package steadily dropping while the compressor runs. What is the most likely cause?
- Excessively low discharge temperature caused by high superheat
- Oil filter too clean
- Slide valve stuck at full load
- Oil carryover into the system due to a faulty coalescing element or high liquid level (Correct answer)
Correct answer: Oil carryover into the system due to a faulty coalescing element or high liquid level
Steady oil loss usually means oil is passing out with discharge gas because of failed coalescers or separator problems.
Question 6: What is the purpose of the oil filter differential pressure reading on a screw compressor?
- To indicate when the filter is clogged and needs replacement (Correct answer)
- To measure refrigerant flow rate
- To set the slide valve position
- To show oil temperature rise across the cooler
Correct answer: To indicate when the filter is clogged and needs replacement
A rising pressure drop across the oil filter shows the element is loading with contaminants and should be changed.
Question 7: A high discharge temperature alarm on an oil-injected screw compressor is most likely caused by which condition?
- Low compression ratio
- Insufficient oil cooling or reduced oil flow (Correct answer)
- Excessive liquid in the suction gas
- Too much oil injection
Correct answer: Insufficient oil cooling or reduced oil flow
Oil removes heat of compression, so inadequate oil cooling or flow allows discharge temperature to climb.
CARO Compressor Operations and Maintenance 3
In an oil-flooded rotary screw compressor, what is one of the main functions of the injected oil besides lubrication?