Compressors Flashcards
what is a positive displacement compressor?
Where the inlet and the discharge are positively sealed from each other
what is a nonpositive displacement compressor?
The rely on airflow through the compressor to separate high-pressure from low pressure
What is gauge pressure?
Absolute pressure -14.7 psi
what is absolute pressure?
Gauge pressure +14.7 psi
what is atmospheric pressure?
14.7 psi
What are the four main properties of gas under pressure?
-gas can be compressed
-Gas will take the shape of a container
-Gas will flow from high-pressure to low pressure
-Gas will fill the complete container
what two ways can the pressure of a gas be increased?
Reducing the size of the container
Heating the gas
what are two examples of rotary compressors that are considered as being nonpositive displacement?
Radial flow
Axial flow
how is a compressor capacity specified?
Volume/CFM
what are four examples of rotary compressors that are considered positive displacement?
sliding vein
Screw (helical)
Liquid ring (piston)
Straight lobe
what is a single stage compressor?
It compresses the air from the inlet to the final discharge pressure in one stroke
What is a multi stage compressor?
More than one stroke or cycle of compression is needed to go from inlet pressure to final discharge pressure
List five examples of a compressors prime mover
Motors
Engines
Turbines
Gas
Steam
What is the purpose of using intercoolers between stages?
It removes the heat generated by compression between stages
Where is the aftercooler located, and what is the purpose of using after coolers?
It’s located after the discharge, and it removes heat from the compressed, gas after the final stage, and condenses out the moisture
what are the manufacturers of air receiver supposed to incorporate into the design of an air receiver?
Pressure safety valves
Gauge
Drain
What styles of air filters may you find on a positive displacement compressor?
dry filters
Labyrinth
Oil bath
In a sliding vein, compressor, explain the position of the rotor, which enables it to compress air
The rotor is offset, the veins go around the crescent shape, which compresses the gas, and allows it to escape through a small opening at the discharge port
what keeps the veins in contact with the housing?
Centrifugal force
is lubrication critical in the operation of a sliding vein compressor?
The metal veins must be lubricated, the synthetic ones must not be
Why must the rotor slot clearance be checked periodically?
Slot where it can cause the veins to wear or break
what two methods are commonly used to cool vein compressors?
Water jacket
Air cooled
what four methods may be used for unloading vein compressors
Start stop
Vent the external bypass
Block the inlet
Very the speed
what family of compressors does a liquid ring compressor fall into?
Positive displacement – rotary
what is the purpose of the liquid in a liquid ring compressor?
To provide oil free air
what are the two main designs of the liquid ring compressor?
Concentric– balanced
Eccentric – unbalanced
is the liquid ring compressor oil free?
Yes
What two functions can the liquid ring compressor perform?
supply oil free air
Cooling of the air
explain the difference between the dry screw (timed) and the wet screw
The dry screw does not need lubrication because it has timing gears and the screw uses injected oil to seal and cool the gas during compression
which type of screw compressor requires timing gears
Dry screw
which combination of male/female lobe construction is most common?
6 flute female, 4 lobe male
what three purposes does the lube, oil and a wet screw compressor have?
seals internal clearances
Cools gas during compression
Lubricates the rotors
What methods are used to unload a screw compressor?
intake throttling
Slide valve
Variable speed
Stop/start
What family of compressors does the lobes style compressor fall into?
Positive displacement – rotary
what two uses does the lobes style compressor have?
It can create pressure and vacuum
does the lobes style compressor have a built-in pressure ratio?
No, because there’s no reduction in volume
T or F
A lob style compressor does not require timing gears
False
what are the two types of dynamic compressors?
Radial/centrifugal
Axial flow
on a centrifugal compressor, what stationary device is used to redirect flow and slow the velocity of the air or gas
Stationary diffuser
what are the shaft speeds for dynamic compressors as compared to reciprocating compressors?
Very high 20,000 to 100,000 RPM
can centrifugal compressors be designed as single or multi stage?
Both, but they’re usually single stage
can axial flow compressors be designed as single stage or multi stage?
They are multi stage only
what two designs of casting are used on centrifugal compressors?
horizontal split – multi stage
Vertical split – higher pressures, better sealing
What styles of bearings are used on centrifugal compressors?
Plain bearings
what direction is the gas flow in axial flow compressors?
Parallel to the shaft axis
explain the configuration of the rotor in an axial flow compressor
tapered drum:
Small end long blades at the inlet, and large end short blades at the discharge
Which direction is the thrust in an axial flow compressor, and what method is used to control it?
Discharge to inlet, use a thrust, bearing and a balance piston
what methods of adjustment are used on axial flow compressors, to vary the output flow?
Adjustable stater blades
Restrict the inlet
what is compressor surge?
Too low of a pressure
what do the term stonewalling or choking refer to?
Airflow is too high
What is the effect of blow by on compressor performance?
Reduced efficiency
what is the purpose of wear bands?
To support the piston so there is no contact on the cylinder
if all four-cylinder compressor, discharges into a common line, how many stages does it have?
1
what is the most common way to control the output volume of a large reciprocating compressor?
-Opening or closing clearance pockets
-Control suction valves from discharge pressure
How does piston and clearance affect, output pressure or output volume?
More end clearance = less output volume
It does not affect pressure
Why is it important to install dry liners to the specified fit?
For proper heat transfer
what are three ways to set piston end clearance?
Count the threads in the crosshead
Trammel
Lead wire crush or feeler gauges
what is the quickest way to check rod bearing clearances?
Bump or lift check
which is the greatest force on the rod of a double acting compressor the tension or the compression?
Compression
is rod load affected by the inlet or the discharge pressure?
Both
on a single stage compressor is the discharge pressure achieved on one stroke or more than one
one stroke
what component in a compressor requires the most maintenance?
Valves
what are two of the best methods for unloading a large compressor?
Clearance pockets
Inlet unloading
what is the purpose of a crosshead?
keeps the piston in line with the centreline of the cylinder
what are two reasons for an intercooler
Increases efficiency
Reduces temperature between stages
How was piston and clearance calculated
2/3 at the head end and 1/3 at the rod end
does an exhaust valve that is bypassing run hotter or colder?
Hotter
What effect does valve lift have on the performance of a compressor?
increased lift = increase flow rate
do packing rings match marks, face towards the pressure or away from the pressure?
Towards
which type of rings require the deepest cup a metallic ring or a Teflon ring?
Teflon
where is the wiper packing located?
In the frame
Where is the aftercooler located?
Between the last stage and the receiver
can absorption material be reused?
No
What are three advantages of a refrigerant dryer?
no chemicals
Continuous operation
Automatic
what type of material can be regenerated?
adsorption
what are two definitions of dewpoint?
-Temperature at which water vapour saturates a gas
-Temperature at which water vapour condenses from a gas
is dewpoint affected by temperature?
No, it is a temperature change
is dewpoint affected by pressure?
yes, increase pressure raises dewpoint
if the pressure of a gas drops, will the dewpoint increase or decrease?
Decrease
which type of dryer can achieve the lowest dewpoint
adsorption
can receiver, air pressure be used to operate intake unloaders?
yes
what type of compressor is used mainly for high-volume but low pressure?
Axial flow
Lobe compressor
What does Rod runout indicate?
Worn wear bands and cylinder
Worn crosshead shoes
why is airline diameter important?
it should be large enough to minimize pressure drop
do dynamic compressors need to receiver?
no
are dynamic compressors mainly used for high volume or low volume?
High volume
What type of screw compressors would be the best for oil free air?
Dry screw
What are the two different rod loads?
compression
Tension
What is the purpose of a dummy piston on a compressor?
Dynamic balance
What happens when a ruptured disc ruptures on a distribution block?
It can shut down the compressor automatically
What does a distribution block system do?
Make sure a set amount of oil gets into each lube point regardless of pressure
