screw compressor Flashcards
What dual rotor screw compressor requires timing gear to prevent rotor contact
dry screw or oil free
dual rotor screw compressor uses injected oil to lubricate, the male rotor is usually driven and rotors are seperated by oil film
wet screw or oil flooded
What is helical lobe
Crossover between lobe blower and compressor
which is the drive rotor on a screw compressor
male rotor
the main factor that reduces volumetric efficiency
back leakage
how to decrease back leakage and increase volumetric efficiency
increase compressor speed
4 things that determine compressor capacity
rotor length
diameter
lobe flute design
compressor speed
how much more efficient are asymmetrical lobe profiles to symetrical lobe profiles
10-15%
as the number of flutes increase
what happens to volume of air
decreases
valve used to get minimum pressure on start up
minimum pressure valve
emergency shutdown instrument , normal readings of 180-205 fahrenheit
discharge temperature probe
valve acts as radiator for the purpose of keeping oil at required temperature
thermal mixing valve
valve used to stop oil flow after compressor stops
oil stop valve
valve that vents pressure from the separator on a shutdown to prevent oil from being pushed out of the compressor filter
blowdown valve
3 ways to control capacity in a screw compressor
suction throttling rotating speed (modulation control) displacement control (internal bypass)
capacity control that controls suction flow or discharge pressure
suction throttling
most common type of suction throttling
load/no load
capacity control best used when load fluctuates between 60-100%
modulation control
3 methods of displacement control
turn valve
poppet valve
slide valve
What are the two main categories of screw compressors
Oil flooded (wet) Oil free (dry) May be either single stage or multi stage
What function does the oil serve in an oil flooded screw compressor
Dissipate some of the heat
Seal gaps between rotors to prevent leakage
Lubricate bearings and gear box
Reduce noise
What is the simplest type of control system in a screw compressor
Online/offline control
Which control system allows the compressor to produce only the amount of air required by the system
Modulation control
What must be included to prevent rotor contact in dry screw compressor
Timing gear
Where are thrust bearings located on a screw compressor
Discharge end
Air filters should be able to move air particals down to what size
5 microns
What 2 functions does oil separators preform
Separate oil from air
Act as oil reservoir and air reciver
What is function of thermal mixing valve
Maintain oil injection temperature
What should first be done before putting compressor in service after overhual
Check for correct rotation
What is a screw compressor?
Rotary positive displacement compressor designed to deliver
constant volume with a specific built in pressure ratio.
What are the 2 categories of dual rotor compressors?
Wet and Dry. (Oil flooded and Oil free)
What does a screw compressors overall efficiency usually
depend on?
Pressure ration and volumetric efficiency.
What is the main factor that reduces volumetric efficiency?
Back leakage.
What does and increase in compressor speed do to volumetric
efficiency? why?
Increases because there is less time for air to escape.
What are screw compressors 2 lobe profiles?
What profile is more efficient and delivers more volume?
Symmetrical and Asymmetrical
Asymmetrical
How is discharge pressure determined?
Somewhat by the length of the compression path along the
lobes and fluted but mostly by the location of the inlet/discharge
ports.
Where must the fixed bearing be located?
On the discharge end.
What does a dual rotor wet screw overhaul usually consist of?
Change bearings and drive gear.
What loads must screw compressors be able to handle?
Radial and Axial
The quality of incoming air has a direct effect on what?
Compressor unit and the lubricating oil.
What is the purpose of a minimum pressure valve?
Build a minimum pressure inside the separator to get the oil
flowing on start up.
Where is the discharge air pressure gauge/probe found?
On the separator and in the discharge air line.
What are some capacity control methods?
Suction throttling (most efficient), Displacement control (internal bypass) and rotation speed (least efficient)
With the suction throttling method, what are the 2 modes and
valve positions?
Fully loaded (open inlet valve) and Fully Unloaded(closed inlet valve)
What 3 common ways can you achieve displacement control?
Turn valves, poppet valves and slide valves
What displacement control is most efficient and used in
refrigeration systems?
Slide valve.
When replacing the separator element filter, what must you be
careful with in regards to the gaskets?
Leave staples installed in the gasket or a fire/explosion hazard
may occur.
What is normal running temperature for wet screw
compressors?
180f to 200f
What is the difference between a compressor and a lobe blower?
Compressor compress or reduce volume of air. Lobe blowers
move a fixed volume of air from one side to the other as lobe rotates
What is slip?
Leakage of higher pressure discharge air back through the
blower toward the inlet.
Which rotor has lobes?
Male
What determines the discharge pressure of the screw
compressor?
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.
