Dynamic Compressors/fans Flashcards
How does a centrifugal compressor work?
They convert velocity into pressure using diffusers, a volute casing or a combination
Describe a single stage centrifugal compressor
It would consist of a single impeller that attaches directly to an extended motor shaft. This is known as single stage it is common in the industry and delivers a large amount of air at low-pressure
What is used to increase the efficiency and capacity of a single stage compressor?
Diffuser vanes fit to the inside of the housing. Increasing the speed, increases the volume
How many stages (impellers) can you put on a single rotor?
As many as 12
What is an alternate design to multistaging?
Having 4 or 5 individual impellers with their volute casing around a central bull gear. This arrangement allows for intercooling
How does the operation work for multistage centrifugal compressor
Gas draws into the eyes of the impeller where it accelerates through centrifugal force and velocity energy imparts to the gas. If then flows into the diffuser or volute decelerates. The velocity transforms into pressure energy in the process repeats through each stage resulting in a higher pressure ratio
What are the three different designed casings for centrifugal compressors
Sectional casing, horizontally split, and vertically split casing
Describe a sectional casing and its use
Sectional casings consists of a number of sections each bolted to another with tie rods. It is usually used a small units with lower pressure ratios and small volumes
Describe horizontally split casings for centrifugal compressors
The casing splits along the centerline of the housing bolts together on a flange. This enables for easy removal of the upper section which allows you to inspect the rotor, impellers, bearings and seals. These are usually limited to less than 1000 PSI, because the metal to metal seal
What is a dynamic compressor?
It is also known as a non-positive displacement compressor. There is no positive seal between inlet and discharge
Describe vertically split casings
They are a cylindrical or barrel casing with a removable front cover. They are used in high pressure applications, up to 10000 psi
What provides the acceleration velocity for conversion to gas pressure?
The impellers. During assembly each impeller is fitted on the shaft, and the rotor shaft assembly is dynamically balanced
What are the three basic styles of impellers
Open, semi closed, closed.
Describe an open impeller, and it’s purpose
Simple design, used in single stage applications and can be used where dirty gasses need to be moved. The vanes radiate from a central hub, and have no front or back
What is a closed impeller?
Has a front and back, the enclosure of the vane allows it to achieve higher pressure ratios. The close tolerances result in higher efficiencies
What are guide elements?
They control and direct gas flow. They may be stationary or moveable guide vanes, or diffusers.
What do inlet guide vanes do?
Control compressor output, and increase efficiency. Output is controlled by throttling the inlet gas flow
What is the purpose of the diaphragm ?
It directs flow through the compressor. It separates the impellers in the compressor casing
What are the methods of thrust control?
Balance Pistons or drums, thrust bearings, and impeller alignment
Where are thrust collars usually installed?
At the suction end of the compressor.
What type of thrust bearings are usually used?
The tilting pad, also known as Kingsbury bearing
How does impeller arrangement control thrust?
If they are arranged in back to back arrangement it creates opposing flows which reduce axial thrust
What do seals prevent leakage from?
Stage to stage, compressor to atmosphere, atmosphere to compressor, oil to compressor
What are the main types of seals?
Labyrinth seals, carbon ring seals, mechanical seals, dry gas seals
Which are the most common types of seals?
The labyrinth seal and it is found throughout the compressor
What type of labyrinth seal is used for higher pressures?
Interlocking labyrinth seals. They consist of a fixed labyrinth ring onto the casing and a matched set evenly spaced fixed to the shaft
What determines the length of the labyrinth seal
The higher the pressure drop across the seal the longer the seal must be
Where are carbon ring seals used?
They’re located where the shaft emerges from the casing before going into the bearing housing. They are used where there are low-pressure differentials.
What is a problem with carbon ring seals?
Their self lubricating, but have a tendency to corrode the shaft with the compressor shuts down there also limited to smaller shaft diameter’s
Why are mechanical seals limited to slower shaft speeds?
Because the contact between the seal faces, their is friction and heat generated from the friction limit the rotational shaft speed
Describe a dry gas seal
Consists of a stationary ring, held against a rotating ring with spring pressure.
What kind of bearings are used on a large multi stage compressor?
Plain journal bearings. You can also use magnetic with dry gas seals
How is the prime mover selected
selected for their speed range, available power, horsepower requirements,
Describe an axial compressor
Gas flows in an axial direction through a series of rotating blades and stationary vanes. They generate a small pressure difference across the stage. Because the pressure rise across each stage is small it required multi staging
What is the best way to control the axial compressor?
By varying the speed
What are the main components in an axial compressor?
Rotor, stator, and housing
What are the two types of rotors?
Drum type or solid disk type
Describe a drum rotor
It consists of disks that are flanged to fit each other, and held together with tie bolts
Describe disc rotors
They consist of individual rotor discs that mount onto the rotor shaft
What is the benefit of discs bolted together?
They are lighter weight, stronger, less likely to bend
Describe the stator
Consists of a series of blades that attach to the casing wall. They can be fixed or adjustable, solid or hollow
Describe continuously adjustable stator blades
They are adjusted to suit flow rates within the compressor while It is operating
What is a combination flow compressor?
An axial flow compressor that includes a centrifugal impeller or impellers as the final stage to increase the compression ratio
Describe an axial compressor
Gas flows in an axial direction through a series of rotating blades and stationary vanes. They generate a small pressure difference across the stage. Because the pressure rise across each stage is small it required multi staging
What is the best way to control the axial compressor?
By varying the speed
What are the main components in an axial compressor?
Rotor, stator, and housing
When controlling thrust in an axial compressor explain opposing gas flows
Gas flows in from opposite directions at both ends and flows to the middle producing opposing thrust forces
What are some common things that maintenance needs to be done on on an axial compressor
Bearings, seals, oil pumps, air filters, oil filters, intercoolers, after coolers. The vanes themselves need the most maintenance
What is blade fouling?
When the stator blades get debris stuck to them mainly from dirty gasses
What are some symptoms of blade fouling?
You will get a slight reduction in gas flow
What is a borescope?
An optical device you can insert through a small orfice
What is the most common method of cleaning fouled blades?
Inserting crushed walnut shells through the compressor while it is running. Newer blades use a special non stick coating to prevent deposits from sticking
What are some things to monitor in dynamic compressors, due to their high rotational speeds
Bearing temperature, vibration levels, rotor position, the temperature and pressure of gas inlet, gas outlet, and lubrication oil
At what points does a compressor run in a stable condition?
Between the surge point, and the stonewall point