screw compressor

Question 1

What dual rotor screw compressor requires timing gear to prevent rotor contact

Answer 1

dry screw or oil free

Question 2

dual rotor screw compressor uses injected oil to lubricate, the male rotor is usually driven and rotors are seperated by oil film

Answer 2

wet screw or oil flooded

Question 3

What is helical lobe

Answer 3

Crossover between lobe blower and compressor

Question 4

which is the drive rotor on a screw compressor

Answer 4

male rotor

Question 5

the main factor that reduces volumetric efficiency

Answer 5

back leakage

Question 6

how to decrease back leakage and increase volumetric efficiency

Answer 6

increase compressor speed

Question 7

4 things that determine compressor capacity

Answer 7

rotor length
diameter
lobe flute design
compressor speed

Question 8

how much more efficient are asymmetrical lobe profiles to symetrical lobe profiles

Answer 8

10-15%

Question 9

as the number of flutes increase

what happens to volume of air

Answer 9

decreases

Question 10

valve used to get minimum pressure on start up

Answer 10

minimum pressure valve

Question 11

emergency shutdown instrument , normal readings of 180-205 fahrenheit

Answer 11

discharge temperature probe

Question 12

valve acts as radiator for the purpose of keeping oil at required temperature

Answer 12

thermal mixing valve

Question 13

valve used to stop oil flow after compressor stops

Answer 13

oil stop valve

Question 14

valve that vents pressure from the separator on a shutdown to prevent oil from being pushed out of the compressor filter

Answer 14

blowdown valve

Question 15

3 ways to control capacity in a screw compressor

Answer 15

suction throttling
rotating speed (modulation control)
displacement control (internal bypass)

Question 16

capacity control that controls suction flow or discharge pressure

Answer 16

suction throttling

Question 17

most common type of suction throttling

Answer 17

load/no load

Question 18

capacity control best used when load fluctuates between 60-100%

Answer 18

modulation control

Question 19

3 methods of displacement control

Answer 19

turn valve
poppet valve
slide valve

Question 20

What are the two main categories of screw compressors

Answer 20

Oil flooded (wet)
Oil free (dry) 
May be either single stage or multi stage

Question 21

What function does the oil serve in an oil flooded screw compressor

Answer 21

Dissipate some of the heat
Seal gaps between rotors to prevent leakage
Lubricate bearings and gear box
Reduce noise

Question 22

What is the simplest type of control system in a screw compressor

Answer 22

Online/offline control

Question 23

Which control system allows the compressor to produce only the amount of air required by the system

Answer 23

Modulation control

Question 24

What must be included to prevent rotor contact in dry screw compressor

Answer 24

Timing gear

Question 25

Where are thrust bearings located on a screw compressor

Answer 25

Discharge end

Question 26

Air filters should be able to move air particals down to what size

Answer 26

5 microns

Question 27

What 2 functions does oil separators preform

Answer 27

Separate oil from air | Act as oil reservoir and air reciver

Question 28

What is function of thermal mixing valve

Answer 28

Maintain oil injection temperature

Question 29

What should first be done before putting compressor in service after overhual

Answer 29

Check for correct rotation

Question 30

What is a screw compressor?

Answer 30

Rotary positive displacement compressor designed to deliver | constant volume with a specific built in pressure ratio.

Question 31

What are the 2 categories of dual rotor compressors?

Answer 31

Wet and Dry. (Oil flooded and Oil free)

Question 32

What does a screw compressors overall efficiency usually | depend on?

Answer 32

Pressure ration and volumetric efficiency.

Question 33

What is the main factor that reduces volumetric efficiency?

Answer 33

Back leakage.

Question 34

What does and increase in compressor speed do to volumetric | efficiency? why?

Answer 34

Increases because there is less time for air to escape.

Question 35

What are screw compressors 2 lobe profiles? | What profile is more efficient and delivers more volume?

Answer 35

Symmetrical and Asymmetrical | Asymmetrical

Question 36

How is discharge pressure determined?

Answer 36

Somewhat by the length of the compression path along the lobes and fluted but mostly by the location of the inlet/discharge ports.

Question 37

Where must the fixed bearing be located?

Answer 37

On the discharge end.

Question 38

What does a dual rotor wet screw overhaul usually consist of?

Answer 38

Change bearings and drive gear.

Question 39

What loads must screw compressors be able to handle?

Answer 39

Radial and Axial

Question 40

The quality of incoming air has a direct effect on what?

Answer 40

Compressor unit and the lubricating oil.

Question 41

What is the purpose of a minimum pressure valve?

Answer 41

Build a minimum pressure inside the separator to get the oil | flowing on start up.

Question 42

Where is the discharge air pressure gauge/probe found?

Answer 42

On the separator and in the discharge air line.

Question 43

What are some capacity control methods?

Answer 43

``` Suction throttling (most efficient), Displacement control (internal bypass) and rotation speed (least efficient) ```

Question 44

With the suction throttling method, what are the 2 modes and | valve positions?

Answer 44

``` Fully loaded (open inlet valve) and Fully Unloaded(closed inlet valve) ```

Question 45

What 3 common ways can you achieve displacement control?

Answer 45

Turn valves, poppet valves and slide valves

Question 46

What displacement control is most efficient and used in | refrigeration systems?

Answer 46

Slide valve.

Question 47

When replacing the separator element filter, what must you be careful with in regards to the gaskets?

Answer 47

Leave staples installed in the gasket or a fire/explosion hazard may occur.

Question 48

What is normal running temperature for wet screw | compressors?

Answer 48

180f to 200f

Question 49

What is the difference between a compressor and a lobe blower?

Answer 49

Compressor compress or reduce volume of air. Lobe blowers | move a fixed volume of air from one side to the other as lobe rotates

Question 50

What is slip?

Answer 50

Leakage of higher pressure discharge air back through the | blower toward the inlet.

Question 51

Which rotor has lobes?

Answer 51

Male

Question 52

What determines the discharge pressure of the screw | compressor?

Answer 52

Location of the inlet port along the length of the rotor, the length of path along the lobes and flutes and the location of the discharge port.