Screw Compressors/lobe blowers/liquid ring Flashcards
How is this compressor typically installed?
Almost without exception it is installed as the package unit with all support equipment mounted on a rigid base
What are some advantages of the screw compressor?
Their small lightweight and vibration free. They need no special foundation for installation. They can easily be mounted on the small trailer and transported to wherever it needs to be used
How was the delivery of air superior to that of a reciprocating compressor?
Rotary screw compressor’s deliver a smooth and pulsation free supply of compressed air
What are the two basic types of screw compressors?
- single rotor
- dual rotor
Which design is more common, single or dual rotor?
Dual rotor is by far the more common design
What are the two categories of dual rotor screw compressors?
- oil flooded wet screw,
- oil free dry screw
What do some oil free screw compressors incorporate for cooling and sealing?
A water spray
What do dry screw compressors require to prevent rotor contact?
They require timing gears
What determines the pressure ratio of the system?
The locations and size of the discharge port. A wider discharge port will allow air to exit sooner, resulting in lower pressure.
What are screw compressors commonly used to deliver?
They deliver air at lower pressure is usually 100 to 175 PSI with volumes ranging from 25 to 8500 CFM
Describe built-in pressure ratio
The compressor is designed to reach its built-in pressure ratio just before it discharges into the discharge port. Once the pockets of air are exposed to the discharge port the pressure becomes the same as that in the compressed air line or receiver
What is the main factor that reduces volumetric efficiency?
Back leakage where compressed air tries to get back through the compressor to the lower suction pressure
What helps determine the compressor capacity?
The rotor length, diameter, lobe/ flute design, and speed
What are the two lobe profiles?
Symmetrical and asymmetrical. The asymmetrical profile is the most commonly used due to its improved efficiency and increased delivery volume
What are 3+4 asymmetric profile rotor profiles used for in the screw compressor?
High-volume flow but low pressure
What is the 6+8 asymmetrical profile lobe used for?
Used for higher pressure lower volume flow applications
Where must the fixed bearings be to keep the critical clearance distances to prevent leakage past the flutes?
The fixed bearings must be located on the discharge end
Compressors manufactured in North America is what type of bearings on the discharge end? Compressors manufactured in Europe use what kind of bearings on the discharge end
North American compressors use tapered roller bearings, and European manufacturers use angular contact bearings
Describe a single rotor screw
It only has one main rotor with two meshing star shaped rotors
What happens because there are two star rotors on opposite sides of the main rotor?
To compressions happen simultaneously this reduces sideloading on the bearings
What pressures are retained with an oil flooded single stage wet screw compressor? And why?
They can make 100 to 125 psi in a single stage due to the increased sealing effect of the oil
What is a benefit of a wet screw compressor?
They are very simple with very few moving parts it is common for them to run 40,000 -100,000 hours between major overhauls
What is a micron?
One millionth of a meter
What has a direct effect on the life of the compressor unit and the lubricating oil?
The quality of the incoming air
What does the air filter trap particles to in a wet screw compressor?
It traps particles down to 5 µm
What is the purpose of the minimal pressure valve?
The purpose is to build a minimum pressure inside the separator to get the oil flowing on start up
What is the purpose of the thermal mixing valve?
The purpose of the mixing valve is to keep the oil at the required temperature
Which screw compressor is best used to handle dirty dust filled gases?
The dry screw
How is a dry screw compressor cooled?
These air cooled casings unless the heat is too high in that case do use water jackets around the compressor
What increases the volume metric efficiency of a drive screw compressor?
Ones that use a small amount of water injected into the compression chamber. This increases the volumetric efficiency through improved ceiling reduce temperature and reduced horsepower requirements
What are the three means of capacity control on a screw compressor?
Suction throttling, modulation control, displacement control. Modulation control is also known as changing the rotating speed, and displacement control is known as internal bypass
Describe section throttling.
Suction throttling is run in one of two modes fully loaded or fully unloaded
Describe modulation control and when it’s best used
It is used with the load fluctuates regularly, and it’s usually found in compressors where the demand varies between 60 and 100% of capacity
What are the three ways to use displacement control, also referred to as geometry control or internal bypass
Turn valves, poppet valve and slide valve
How does internal bypass work?
All three ways work on the same basic principle which is to let some of the air in the compression chamber escape back to the suction without being compressed
Which is the most efficient form of geometry control?
The slide valve provides the best part though deficiency. It also delivers infinitely variable flow from 100% down to 10%
What is the major difference between a wet and dry screw compressor? The dry screws have what
They have timing gears
In order to increase the discharge pressure on the screw compressor the radio component on the discharge port will be what?
Closing the port towards the discharge end
Which of the following is better when the screw compressor is operating? Under compressing Over compressing Static compressing Variable compressing
Under compressing
In regards to the rotor length diameter material and design what will not factor in determining the capacity?
Rotor material
Where was the fixed bearings be on the screw compressor?
Discharge end
The air intake filter should filter particles down to what?
5 µm
On the screw compressor wear with the minimum pressure valve be located?
Immediately after the air oil separator
Why would water be injected into a dry screw compressor?
To promote better sealing
What effect would increasing the driver rpm have on the screw compressor?
The compressor would put out a larger volume of air
Geometry control is also referred to as what
Internal bypass
If an intercooler on a multistage dry screw compressor was scaled up what with the effect be?
A high discharge air temperature
What effect with shortening the slide have on the variable volume ratio control?
A lower Vi
What is also referred to as a hybrid between a blower and a compressor?
Cycloidal blower
What is not a function of a love blower
Pneumatic tooling
When referencing a lobe blower what is slip?
The discharge air leaking back towards the inlet
The size of a blower is given as 7.15 what do the numbers refer to?
The seven is the diameter of the timing gear, 15 is the length of the lobe
When performing a typical set of impeller to impeller clearance checks the preferred angle is?
45° from vertical
Even though lobe blower does not produce substantial axial thrust, thrust bearings are required why?
To maintain rotor position relative to the end plates
What is one critical criteria about timing gears?
They cannot have excessive backlash
What to features work together on a sliding vein compressor that allows it to only deliver volume of air required by the system
The servo valve and the intake valve
What are the key things oil system is used for in the sliding vane compressor
It is used for sealing, heat transfer , lubrication, and controlling the air inlet valve
Where is the oil release valve found on a sliding vane compressor, and what is its purpose
It is located at the bottom of the stator housing and it drains oil out of the stator into the sump
What is the function of the oil return valve on a sliding vane compressor
Drains the oil from the separator housing back to the sump
What is the purpose of machining an extra cut into the stator between the discharge and intake ports on a sliding vane compressor
To help reduce the volume of compressed air trapped between the vanes
What psi range are dry type sliding vane compressors used for
Approximately 50 psi
What do sliding vane compressors rely on to seal between the vane tips and the stator
Centrifugal force
What main factor determines the built-in pressure ratio on a liquid ring compressor
Location of the discharge port
Why are liquid ring compressors able to run for long periods of time under adverse conditions?
Because there are no rubbing parts, And no metal to metal contact
What are liquid ring compressor often used for
Vacuum service.
If the liquid ring compressor is used for vacuum service where would you position the vacuum gauge?
Immediately before the air intake
What is the ratio between the cones having some machined off it to the gasket size?
For everyone.001 inch machine off of it you need to add .007 gasket
If an increase in oil was noticed in the discharge air on a sliding vane compressor it could be an indication of a problem with what valve?
The oil return valve
If you took a cut off the stater on the sliding vane compressor what would the result be
An increase in volume and decrease in pressure
What is the purpose of the scavenger line
As oil passes through the separator some will carry through and pool in the bottom of the separator element. Scavenge line is placed 1/8 of an inch off the bottom of this and oils fed back into the compressor through injection ports
What is the purpose of the staple through the gasket on the screw compressor separator element
It’s purpose is to ground the lid of the separator and prevent static buildup
What is the purpose of the oil stop valve on a screw compressor and where is it found?
It is programmed to stop the flow of oil just as the compressor shuts down. It is found immediately before the oil injection ports
List three differences between a wet and dry screw compressor
Dry screws have timing gears, dry screws compress dirty gases, the oil used in a wet screw is also use for lubricating bearings and reducing gears
List three methods of capacity control on a screw compressor
Speed control using a VFD, geometry control, suction throttling
How are the rotors prevented from coming into contact with each other in a lobe blower
By using helical cut timing gears
What are two advantages of a three lobe design
Quieter, less pulsation so, more efficient because of less slip or internal leakage
