CARO Compressor Operations and Maintenance 3

Question 1

In an oil-flooded rotary screw compressor, what is one of the main functions of the injected oil besides lubrication?

Accepted Answer

Sealing clearances between rotors and removing heat of compression

Answer

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?

Accepted Answer

A slide valve that changes the effective rotor length used for compression

Answer

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?

Accepted Answer

To reduce starting torque and motor inrush load

Answer

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?

Accepted Answer

The system's operating compression ratio

Answer

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?

Accepted Answer

Oil carryover into the system due to a faulty coalescing element or high liquid level

Answer

Steady oil loss usually means oil is passing out with discharge gas because of failed coalescers or separator problems.

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