Compressors Flashcards
What is the function of rider bands?
Support the weight of the piston and half the weight of the rod
List 4 ways to test compressor valves
Lift check, leak test, pressure drop test. Flow meter test
What elements must all pressure vessels have?
Guage, drain, safety relief valves
What is the location and function of aftercoolers?
After the last stage of compression. Removes moisture after compression is complete.
What is the function and location of intercooler?
Cool compressed gas between stages.
*cooling reduces volume of the gas, reducing power requirements by 15%
Why is rod reversal an integral function?
Allows oil to flood crosshead pin and connecting rod pin bushings
What is combined gas law?
Combo of Boyle, Charles, and Gay-Lussac
Relationship between absolute pressure, volume, and temperature
What is Gay-Lussacs law?
Volume of gas is constant. The pressure exerted by the gas is directly proportional to the absolute temperature
What is Charles Law?
If pressure on a given volume of gas is held constant, the volume changes in direct proportion to the absolute temperature
What is Boyles Law?
If temperature remains constant, the volume of gas changes with pressure
What is Bernoullis principle?
As velocity increases, pressure decreases.
And vice versa
What is Pascals Law?
F= P/A. Pressure acting on any fluid at rest in a container is transmitted undiminished and equal in all directions
Define pressure
Restrictions in flows path.
A load containing fluid
What is a barometer?
Measures vacuum and scale in inches or millimeters of mercury
Predicts weather
What is a manometer?
Measures minute variations in pressure above and below atmospheric pressure.
What are Bourdon tube’s?
Hollow tube of brass. Tube curves or straightens depending on pressure rating. Not accurate.
What are standard conditions at sea level?
Pressure: 14.7 psi
Temp: 60-68°F
Humidity: 36%
What is clearance volume?
Space between piston and cylinder head at TDC
What is swept volume?
Volume of air that is swept or pushed out of the cylinder on the compression stroke.
=piston area x length of stroke
*does not include clearance volume
How do you calculate Volumetric efficiency?
Actual compressor capacity /
Calculated displacement
- expressed as a %
What is a pressure ratio?
Absolute discharge:absolute intake
* limited to 4:1, if higher must be multi-staged
What is choked flow?
Flow is too great for size of tubing used causing turbulence which further restricts flow
When are bias cut rods used?
When the rod must exit through the cylinder bore
What is the most common cut connecting rod
Straight cut
In what compressor would you find an articulating connecting rod?
Large displacement integral compressors
When checking clearance with a feeler gauge on a main bearing where would you measure?
Above and below the splits on both sides, as well as top and bottom
What lube system uses dippers slingers and at time the crankshaft?
Splash lubrication
Up to what discharge temperature would you use air cooling for a cylinder?
88°C (190°F)
Up to what discharge temperature would you use thermo-syphon cooling for a cylinder?
88°C (190°F)
*still less than 121°C (250°F)
At what discharge temperature would you use circulated cooling for a cylinder?
121°C (250°F) and above
When liquid cooling a cylinder, what temperature should the cooling water be?
Keep the temperature of the cooling water entering the cylinder higher than that of the incoming gas
prevents condensation
What type of cylinder liner relies on good contact with the housing for efficient heat transfer?
Dry liner
What type of cylinder liner comes into contact with the coolant?
Wet liner
What system pressure is required for oil-lubricated cylinders?
Oil system must be at a slightly higher pressure than that of the compressed gas to allow it to enter the cylinder
Honing creates what pattern inside a cylinder?
Crosshatch from 40-60°
What do you set crank end and head end clearances at?
1/3 and 2/3 respectively for thermal expansion
What do oil wipers seal? What component do they need to seal?
1) Keeps oil in crankcase
2) prevents condensate and other system lubricants out of the crankcase
They rely on a garter spring on their OD
*normally used in pairs
Where does the lubrication enter in a lubricated packing case?
At the top
Where does the coolant enter in a liquid cooled packing case?
The bottom
What is the purpose of a distance piece?
1) prevents cylinders leaking compression cases from pressurizing the crankcase or main frame
2) prevents crankcase oil from entering the cylinder
3) allows access for maintenance
When might you use a longer style distance piece with oil baffles on the piston rod?
In an oil free application, when you can’t have contaminated gases in the cylinder
When would you use a two compartment distance piece?
Applications where you need to completely seal an explosive has from the compression chamber
*uses two sets of piston rod packing
What might you use on smaller compressors to reduce noise and vibration transfer?
An isolator
What machining process allows the correction of main bearing bores in a main frame that are out of alignment?
Line boring
What are the advantages of using polymeric grout when installing a sole plate?
Polymeric groups are non-shrinking and able to resist moisture and chemical attack
What are two different ways crankshafts balance reciprocating forces?
Opposing throws and counterweights
even number of cylinders required for opposing throws
You suspect a crankshaft is bent. What tool will you use to to measure? And where on the crankshaft will you measure?
Deflection Guage, between the webs while rotating the shaft
*on small crankshafts, may set up on v blocks and use dial indicator
What component protects a lubrication system in the event of over pressure?
A rupture disc
Which device always follows an aftercooler?
Moisture separator
What are you measuring when using plastiguage?
a) contact
b) pressure
c) clearance
d) temperature
C) clearance
Which compressor design has a piston but no piston rings?
Labyrinth
Which compressor design has a piston but no piston rings?
Labyrinth
Which type of compressor uses both single and double acting pistons?
Integral
What is the ratios of clearance for TDC and BDC?
2/3 and 1/3
What are 3 types of rider bands? What are they most commonly made of?
Uncut, angle cut, double twin. Teflon.
What is the absolute unit of measurement is the equivalent to Celsius?
Kelvin
What is the absolute unit of measurement is the equivalent to Fahrenheit?
Rankine
What are the basic components in the MAIN lubrication system?
Pumps, piping, galleries, cooling and filtration systems
Large double acting compressors commonly have 2 types of lubrication systems. What are they?
One internal to the main frame, and one to lube the cylinder and packing
What is the most common oil pump on a compressor?
External gear
What is the main cause of main bearing wear?
Lack of lubrication
What is the function of the oil cooler?
Cool the oil, which reduces oxidation, retains viscosity
What is the main indicator of bearing wear
Lower trending oil pressures, heat and vibration
Why do compressors have an external lube system?
To supply precisely metered oil to cylinders, and other components
What cylinder lube system uses only 1 pump for the system
Distribution block system
What is valve lift?
The distance the sealing element lifts or moves off the seat plate
What does excess valve lift impact?
Life of the valve, causes valve flutter, late closing
What does product gas in the distance piece indicate?
Leaking packing case or packing failure
Describe single point ( pump to point) lube system
Multiple adjustable low volume pumps to deliver lube to many single points
What can routine oil samples and analysis help indicate and track?
Bearing or metallic part wear rates, oil condition (viscosity, oxidation, additive levels), oil contamination, and filter effectiveness
Describe a distribution block (divider valve) lube system
A single pump delivers lube to a manifold
What does piston-drop indicate?
Wear in the rider bands or the crosshead
Your rod-drop sensor shows run-out. After investigation, neither the piston nor crosshead are showing wear. What is potentially the issue?
Bent piston rod
What is the purpose of a bump test on a motor?
To ensure correct rotational direction
*running the pump in reverse causes a loss of lubrication
What should the oil pressure read at normal operating temperatures?
45 psig
What type of graph provides a good indication of compressor performance?
Pressure/volume
True or false. You can use hydraulic jacks for blocking.
False
True or false. Gas and diesel are excellent for cleaning parts.
False. They are flammable solvents.
Pros/ cons of cast iron piston rings
Pro: low cost, conform quickly
Cons: brittle, wear quickly
Pros/cons of lead-bronze piston rings
No cons listed
Pros: ability to withstand deformation in high pressure (>2500psi)
Pros/cons of Tin-bronze piston rings
Pros: can stretch to fit over the piston and provide outward push to hold against cylinder wall
No cons listed
Pros/cons of synthetic piston rings
Pros: outlast metallic rings (ex. abrasion, corrosion, acidic gases), long life, don’t score cylinder, conform for exceptional seal, no break in period
No cons listed
What does the wrist pin do?
Connects the piston to the connecting rod in a single acting compressor
List 5 ways of compressor unloading
Start/stop, inlet valves unloading, variable speed control, bypass unloading, variable volume clearance pocket (VVCP)
Absorption vs adsorption
Absorption- absorbs, soaks up
Adsorption- adheres, sticks to
In order, list the components required to condition air after compression
Filter, regulator, lubricator (FRL) units
Purpose of the crosshead
Keep the piston rod centered through the packing to minimize leakage
Elements of a valve
A sealing element, a valve seat, a valve guard
List 5 types of valve designs
Concentric ring and plate, dampened disc, poppet, feather, channel
What is a major drawback in a pump to point system?
Every pump needs it’s own alarm system, which is also costly
In a divider valve lubrication system, the valves are labeled with either a “S” or a “T”
What do the letters signify?
Single (S) output and Twin (T) output.
Ex. 24T is 0.024 cubic inches per stroke to two locations
What type of compressor is a screw compressor?
A rotary positive displacement compressor
Screw compressors are designed to deliver a constant volume with a specific built-in pressure ratio.
What does pressure ratio (PR) represent in screw compressors?
The ratio of the absolute incoming air pressure to the absolute compressor discharge pressure
For example, a PR of 4 means air at 14.7 psia is compressed to 58.8 psia.
What is the typical pressure range for screw compressors in a single stage?
100 to 175 psig
This range is common for screw compressors designed for lower pressures.
What are the typical volume ranges for screw compressors?
25 to 8500 cubic feet per minute (cfm)
Higher volumes, up to 30,000 cfm, may be achieved in specific applications.
What is a characteristic feature of screw compressors regarding installation?
They can be installed directly on the floor without special foundations
This is due to their small size, lightweight, and vibration-free operation.
What are the two basic types of screw compressors?
- Single rotor
- Dual rotor
The dual rotor design is the more common type used in industry.
What is a benefit of the limited number of moving parts in screw compressors?
They are very economical, reliable, and easy to maintain
This design feature contributes to their popularity in various applications.
True or False: Screw compressors can operate effectively without an air receiver.
True
However, operating without a receiver can cause excessive cycling of the compressor.
What type of air supply do rotary screw compressors deliver?
A smooth, pulsation-free supply of compressed air
This characteristic allows them to match air output to plant air demand effectively.
Fill in the blank: Screw compressors are commonly used to deliver air at lower pressures, commonly _______.
100 to 175 psig
What enables screw compressors to be leaders in mobile compression?
The entire skid package can be easily mounted on a small trailer
This allows for transport to various locations for use.
What is a disadvantage of operating a screw compressor without a receiver?
It can cause the compressor to cycle excessively
A receiver can help the compressor’s control system work more effectively.
What is the primary function of injected oil in an oil-flooded (wet screw) compressor?
To lubricate, seal, and cool the air end
The air end is the actual screw compressor in the compressor package.
What is the common name for the actual screw compressor in a compressor package?
Air end
The air end is crucial for the operation of the compressor.
In an oil-flooded (wet screw) compressor, which rotor is usually driven?
Male rotor
What separates the two rotors in an oil-flooded (wet screw) compressor?
Oil film
Do oil-flooded (wet screw) compressors require timing gears?
No
What is the primary characteristic of an oil-free (dry screw) compressor?
Absence of any lubrication in the compression chamber
What is a significant advantage of oil-free (dry screw) compressors?
They deliver oil-free air
How do some oil-free screw compressors provide cooling within the compression chamber?
Incorporate a water spray
What is required in oil-free (dry screw) compressors to prevent rotor contact?
Timing gears
What type of system do oil-free (dry screw) compressors need to lubricate the bearings?
Separate oil system
What is necessary to prevent gas leakage in oil-free (dry screw) compressors?
Sealing system
Fill in the blank: An oil-flooded (wet screw) compressor uses _______ to lubricate and seal.
Injected oil
True or False: Oil-free (dry screw) compressors do not need any lubrication.
True
What is more efficient when running a screw compressor over compression or under compression
Under compression; less wasted energy
What factors mainly determine a screw compressor’s overall efficiency?
Optimizing the built-in pressure ratio and the volumetric efficiency
Volumetric efficiency is crucial for performance.
How is volumetric efficiency defined?
Comparison of the amount of air brought into the suction port to the amount delivered to the discharge port
It quantifies the effectiveness of air intake and delivery.
What is the main factor that reduces volumetric efficiency?
Back leakage
Back leakage occurs when compressed air moves back through the compressor.
Where does leakage occur in a screw compressor?
Between the two rotor surfaces, between the rotors and housing bore, and between rotor ends and housing bore ends
These locations are critical for maintaining efficiency.
How is leakage minimized in screw compressors?
By reducing clearances to a minimum
Modern machining tolerances allow for very small clearances.
What advancements have been made with wet screws in screw compressors?
Significant gains in efficiency by using injected oil to seal clearances and prevent back leakage
This innovation improves the compressor’s performance.
How does compressor speed affect volumetric efficiency?
Volumetric efficiency tends to increase with compressor speed
Faster rotor speeds reduce the time for air to leak back.
What happens to volumetric efficiency at extreme rotor speeds?
Friction and turbulence cause a restriction in the amount of air that can enter the suction porting
This can negate the benefits of increased speed.
What profile is now the most used in screw compressors?
Asymmetrical, as it provide higher efficiency and increased delivery volume, this is due to smaller clearances
