Screw Compressors

Question 1

On a screw compressor housing where is the inlet located and where is the outlet located?

Answer 1

The inlet is on the top and the outlet is on the bottom

Question 2

What is the typical clearance on the discharge end between the screws and the housing?

Answer 2

2-3 thou

Question 3

Where are the fixed bearings located? And what kind of bearing are they?

Answer 3

On the discharge end

Tapered roller bearings in North America

And

Angular contact bearings in Europe

Question 4

Why do you want floating bearings on the suction end?

Answer 4

To allow for any thermal growth.

Question 5

Does a wet screw drive using timing gears or speed increasing gears?

Answer 5

Speed increasing gears, the male lobe screw then drives the female flute.

Question 6

What rotor profile is more commonly used asymmetrical or symmetrical and what is the difference?

Answer 6

Asymmetrical is more commonly used because it allows the rotors to mesh with tighter clearances which translates into less leakage and improved efficiency.

Question 7

Name 4 types of screw compressors

Answer 7

1. Single rotor
2. Dual rotor wet or oil flooded
3. Dual rotor dry or oil free
4. Dual rotor dry water sprayed

Question 8

What is a positive displacement compressor?

Answer 8

A positive displacement compressor captures a volume of air and compresses that said volume and continues to do so until it discharges

Question 9

What is a major advantage of the single rotor screw compressor design?

Answer 9

Since there are 2 star rotors on opposite sides, 2 compressions happen simultaneously.

Question 10

List three functions of the oil injected into a wet screw compressor

Answer 10

1. Lubricate the male lobe and female flute to reduce friction and allow a thin film to create.
2. Carry away heat
3. Reduce the noise

Question 11

Screw compressors have a “built in pressure ratio”. What is meant by this term?

Answer 11

That the pressure is a constant ratio between the inlet and outlet.

Question 12

What is the gear set used for in an oil free screw compressor?

Answer 12

Timing gears because there is no lubrication for the male rotor to drive the female so they must have timing gears to stay in sinc and mesh properly

Question 13

Name three methods of adjusting capacity of a screw compressor And which capacity control method is the most efficient?

Answer 13

1. Suction throttling
2. Rotating speed
3. Displacement control

Displacement control is the most efficient.

Question 14

How does suction throttling work? And what are some key points of it?

Answer 14

There is 2 modes 
Fully loaded (open inlet)
Fully unloaded (closed inlet) 

When the system reaches the desired maximum pressure a signal is sent through a control air line that will close a butterfly valve in the inlet. When there is a demand for pressure the butterfly valve will once again open allowing air to enter back into the inlet and allow the compressor to compress said air.

There is small orifices in the valve plate that allows air in the inlet so a large negative pressure isn’t present.

Question 15

What is the pressure on the inlet side of the compressor before being compressed?

Answer 15

Just below 14.7 psi at sea level.