Compressor Operation and Maintenance Flashcards

On an oil-injected rotary screw compressor, the slide valve is primarily used for:

Answer: Capacity control

Moving the slide valve shortens the effective rotor length so less gas is compressed, which reduces capacity.

What does the volume index (Vi) of a screw compressor describe?

Answer: The built-in ratio of suction trapped volume to discharge volume

Vi is the built-in volume ratio, and a Vi mismatched to operating pressures causes over- or under-compression losses.

Which of the following is NOT a purpose of oil injection in a rotary screw compressor?

Answer: Increasing the refrigerant's latent heat

Injected oil seals, cools and lubricates, but it cannot change the latent heat of the refrigerant.

Where is the oil separator located on a screw compressor package?

Answer: In the discharge line after the compressor

Oil leaves the compressor mixed with hot discharge gas, so the separator is on the discharge side to recover it.

Before starting a screw compressor, the slide valve should normally be positioned at:

Answer: Minimum load (fully unloaded)

Starting unloaded reduces starting torque and current draw on the motor.

Which of the following is NOT a common oil cooling method for screw compressors?

Answer: Suction gas hot bypass

Thermosyphon, liquid injection and water/glycol coolers are standard oil cooling methods; hot gas bypass adds heat rather than removing it.

Why must a screw compressor have a discharge check valve?

Answer: To prevent backflow of high-pressure gas that could spin the rotors in reverse at shutdown

Without a check valve, discharge gas flowing back through the compressor at shutdown can drive the rotors backward and damage bearings.