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Millwright 3rd year 2021 > screw compressor > Flashcards

screw compressor Flashcards

1
Q

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

A

dry screw or oil free

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2
Q

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

A

wet screw or oil flooded

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3
Q

What is helical lobe

A

Crossover between lobe blower and compressor

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4
Q

which is the drive rotor on a screw compressor

A

male rotor

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5
Q

the main factor that reduces volumetric efficiency

A

back leakage

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6
Q

how to decrease back leakage and increase volumetric efficiency

A

increase compressor speed

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7
Q

4 things that determine compressor capacity

A

rotor length
diameter
lobe flute design
compressor speed

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8
Q

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

A

10-15%

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9
Q

as the number of flutes increase

what happens to volume of air

A

decreases

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10
Q

valve used to get minimum pressure on start up

A

minimum pressure valve

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11
Q

emergency shutdown instrument , normal readings of 180-205 fahrenheit

A

discharge temperature probe

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12
Q

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

A

thermal mixing valve

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13
Q

valve used to stop oil flow after compressor stops

A

oil stop valve

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14
Q

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

A

blowdown valve

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15
Q

3 ways to control capacity in a screw compressor

A 
suction throttling
rotating speed (modulation control)
displacement control (internal bypass)
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16
Q

capacity control that controls suction flow or discharge pressure

A

suction throttling

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17
Q

most common type of suction throttling

A

load/no load

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18
Q

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

A

modulation control

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19
Q

3 methods of displacement control

A

turn valve
poppet valve
slide valve

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20
Q

What are the two main categories of screw compressors

A 
Oil flooded (wet)
Oil free (dry) 
May be either single stage or multi stage
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21
Q

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

A

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

22
Q

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

A

Online/offline control

23
Q

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

A

Modulation control

24
Q

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

A

Timing gear

25
Q

Where are thrust bearings located on a screw compressor

A

Discharge end

26
Q

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

A

5 microns

27
Q

What 2 functions does oil separators preform

A

Separate oil from air

Act as oil reservoir and air reciver

28
Q

What is function of thermal mixing valve

A

Maintain oil injection temperature

29
Q

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

A

Check for correct rotation

30
Q

What is a screw compressor?

A

Rotary positive displacement compressor designed to deliver

constant volume with a specific built in pressure ratio.

31
Q

What are the 2 categories of dual rotor compressors?

A

Wet and Dry. (Oil flooded and Oil free)

32
Q

What does a screw compressors overall efficiency usually

depend on?

A

Pressure ration and volumetric efficiency.

33
Q

What is the main factor that reduces volumetric efficiency?

A

Back leakage.

34
Q

What does and increase in compressor speed do to volumetric

efficiency? why?

A

Increases because there is less time for air to escape.

35
Q

What are screw compressors 2 lobe profiles?

What profile is more efficient and delivers more volume?

A

Symmetrical and Asymmetrical

Asymmetrical

36
Q

How is discharge pressure determined?

A

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

37
Q

Where must the fixed bearing be located?

A

On the discharge end.

38
Q

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

A

Change bearings and drive gear.

39
Q

What loads must screw compressors be able to handle?

A

Radial and Axial

40
Q

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

A

Compressor unit and the lubricating oil.

41
Q

What is the purpose of a minimum pressure valve?

A

Build a minimum pressure inside the separator to get the oil

flowing on start up.

42
Q

Where is the discharge air pressure gauge/probe found?

A

On the separator and in the discharge air line.

43
Q

What are some capacity control methods?

A 
Suction throttling (most efficient), Displacement control (internal
bypass) and rotation speed (least efficient)
44
Q

With the suction throttling method, what are the 2 modes and

valve positions?

A 
Fully loaded (open inlet valve) and Fully Unloaded(closed inlet
valve)
45
Q

What 3 common ways can you achieve displacement control?

A

Turn valves, poppet valves and slide valves

46
Q

What displacement control is most efficient and used in

refrigeration systems?

A

Slide valve.

47
Q

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

A

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

48
Q

What is normal running temperature for wet screw

compressors?

A

180f to 200f

49
Q

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

A

Compressor compress or reduce volume of air. Lobe blowers

move a fixed volume of air from one side to the other as lobe rotates

50
Q

What is slip?

A

Leakage of higher pressure discharge air back through the

blower toward the inlet.

51
Q

Which rotor has lobes?

A

Male

52
Q

What determines the discharge pressure of the screw

compressor?

A

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.

Millwright 3rd year 2021 (21 decks)

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