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Millwright Third Year 2018 > Screw Compresors > Flashcards

Screw Compresors Flashcards

1
Q

What are the two basic types of screw compressors?

A
  • Single Rotor

* Dual Rotor

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2
Q

Which design is more common, Dual or Single rotor?

A
  • Dual Rotor is by far the more common
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3
Q

What are the two category of dual rotor of screw compressors?

A
  • Wet Screw/Oil-flooded

* Dry Screw/Oil-Free

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4
Q

What do some oil-free screw compressors incorporate for cooling and sealing?

A

A water spray

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5
Q

What do dry screw compressors require to prevent rotor contact?

A
  • Timing Gears
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6
Q

Describe built-in pressure ratio?

A

The compressor is designed to reach its built-in pressure ratio just before it discharges into the discharge port. (This pressure cannot be altered) Once the pockets of air are exposed to the discharge port the pressure becomes the same as the compressed air line or receiver.

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7
Q

What are the two lobe profiles?

A
  • Symmetrical

* Asymmetrical

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8
Q

Which profile is the most commonly used?

A

The Asymmetrical profile.

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9
Q

Why is the asymmetrical profile more common?

A

The asymmetrical profile is the most commonly due to it’s improved efficiency and increased delivery volume.

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10
Q

What capacities do screw compressors delivers?

A

They deliver air at lower pressure Usually at 100 to 175 PSI, whit volumes ranging from 25 to 8500 CFM

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11
Q

What determines the pressure ratio of the system?

A

*The location and size of the inlet port.

Note: A wider discharge port will allow air/gas to exit sooner, resulting in pressure drop.

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12
Q

Where must the fixed bearing be install to keep critical clearance distances to prevent leakage past the flutes?

A

The fixed bearings must be located on the discharge end.

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13
Q

Compressors manufactured in North America uses what type of bearings on the discharge end?

A

*Tapered Roller Bearings.

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14
Q

Compressors manufactured in Europe uses what type of bearings on the discharge end?

A

*Angular Contact Bearings.

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15
Q

Describe a single rotor screw (Mono-Screw)

A

The single rotor screw has only one main rotor with two meshing star shaped rotors.

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16
Q

What is the purpose of the two star rotors on opposite sides of the main rotors?

A

They reduce the side loading on the bearings while producing two compressions simultaneously.

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17
Q

What are the operating pressures for an Oil-Flooded (Wet) Screw Compressor?

A

100-125 PSIG and is common for them to run 40 000 to 100 000 hours?

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18
Q

What is a benefit of a wet screw compressor?

A

They are very simple machines with few moving parts. Overhaul involves changing out the bearings and the drive gears.

19
Q

What is a micron?

A

*One million of a meter.

20
Q

What are the three (3) means of capacity controls on a screw compressor?

A
  • Suction Throttling
  • Rotating Speed (Modulation Control)
  • Displacement Control (Internal Bypass)
21
Q

What is Suction Throttling

A

Suction throttling is used to control the suction flow or discharge pressure using the most common method load/no load compressor control.

22
Q

Suction throttling runs in one (1) of two (2) modes?

A
  • Fully Loaded (Open Inlet Valve)

* Fully Unloaded (Closed Inlet Valve)

23
Q

Describe modulation control and when it’s best used?

A

Modulation is used where the load fluctuates regularly, and is normally found in compressors where the demand varies from 60% to 100%.

24
Q

What is the purpose of displacement control?

A

The displacement control method physically alters the displacement of the compressor.

25
Q

What is displacement control sometimes referred to as?

A

Displacement control is referred to as ‘Geometry Control’ or ‘Internal Bypass Control’.

26
Q

What are the three ways of displacement control/geometry control?

A
  • Turn Valves
  • Poppet Valves
  • Slide Valves
27
Q

How do the three (3) ways of displacement control operate?

A

All three (3) systems work on the same basic principal which is to let some air in the compression chamber escape back to the suction without being compressed.

28
Q

What is the most efficient Geometry control?

A

*The Slide Valve.

The slide valve provides better part load efficiency, while at the same time delivering infinitely variable flow from 100% down to as low as 10%

29
Q

In order to increase the discharge pressure on the screw compressor the radial part/component on the discharge will be what?

A

It will be closing the port towards the discharge end.

30
Q

During Operation which of the following is least efficient?

(a) Under Compressing
(b) Over Compressing
(c) Static Compressing
(d) Variable Compressing

A

*Over Compressing

31
Q

What is the main factor that reduces volumetric efficiency?

A

Back leakage and Over compressing

32
Q

The air intake filter should filter air particles down to how many microns?

A

*5µ or 5 microns

33
Q

Where is the scavenge line located and what is its purpose?

A

Is placed about 1/8 of an inch off the bottom of the air/oil separator and which it feeds the oil back to the compressors through the injection ports.

34
Q

What is the purpose of the minimum pressure valve and where is it located?

A

The purpose of this valve is to built a minimum pressure inside the separator to get the oil flowing on start-up. It’s located immediately after the Ai/oil separator.

35
Q

What is the low oil pressure shutdown for?

A

It senses oil pressure at the injection port and shuts the unit down if the pressure falls below a preset limit during operation.

36
Q

Thermal mixing valves purposes?

A

It keeps oil at a minimum temperature and it prevents oil moisture from condensing out of the air inside the compressor.

37
Q

Blowdown valve purposes?

A

Is to vent the pressure from the separator on a shutdown to prevent oil from being pushed out of the compressor filter. And to prevent the separator from building pressure on start up.

38
Q

Where are the oil injection nozzles located?

A

These nozzles port are located at an area of less pressure to allow the pressure differential to push the oil into the compressor.

39
Q

Oil stop valve purposes?

A

Solenoid or air operated programed into the shut-down sequence to stop the flow of oil just as the compressor shuts down.

40
Q

Where is the turn valve located?

A

The valve sits beneath a series of ports that are located towards the inlet side of the floor of the compression side.

41
Q

Why would water be injected into a dry screw compressor?

A

To promote better sealing.

42
Q

What effect would have if you increase the rpm on a screw compressor?

A

It would put out a larger volume of air

43
Q

If the intercooler on a multi-stage was to scale up, what would the affect be?

A

A high discharge air temperature

44
Q

What affect with shortening the slide have on the variable volume ratio control?

A

A low Vi

Millwright Third Year 2018 (15 decks)

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