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
Describe surge point
The point of highest possible pressure on the outlet. More pressure and it will begin to work backwards through the compressor. The low flow causes flow reversal
Describe stonewall point
The point when the compressor is producing maximum volume (lower pressure). Any more volume, and the speed of it will break sonic barrier. It is controlled by compressor speed
What is used to control surge?
A recycle valve, bypass valve, or a variable speed unit
What is one inch of water in psi?
0.036 psi
What three types of pressures are developed by fans?
Static pressure, velocity pressure, and total pressure, which is the sum of the static and velocity pressures
How are fan pressures measured?
On a manometer which uses inches of water as a gauge
Describe static pressure, and velocity pressure
Static pressure would be the pressure pushing against the ductwork(higher if flow is restricted)
Velocity pressure is the air flowing through ductwork
What happens to pressures when ductwork converges?
The static pressure reduces and the velocity pressure increases
How is static electricity generated in ductwork? How is it controlled?
It strips electrons off the ductwork. It is controlled by putting grounding strips that contact the shaft and go to ground
What is the behavioural result know as when a fan is in a region of instability?
Hunting, or surge, it sounds like the fan is breathing
What are the two types of fans?
Centrifugal, and axial flow
What is another name for a centrifugal fan?
Squirrel cage. The blades can be straight radial’ forward curved, or backward inclined with airfoil inlet blades
Describe an axial fan
They have blades that look like an aircraft or boat propeller
What are the three types of axial fans?
Tube axial, vane axial, and propeller
What are the components in a centrifugal fan
Rotating element, stationary scroll (casing) a pedestal to mount components, inlet screens, dampers or guide vanes
How are large single suction fans generally mounted?
Double row spherical roller bearings
How is the high velocity pressure energy of the air converted to static pressure?
Within the scroll
What are series and parallel fans used for?
Series fans increase outlet pressure and parallel in where a spare or backup for emergency is required also for more volume
Fans can be used to produce an artificial draft what are the three types?
Induced draft, forced draft,balanced draft
Describe induced draft and describe forced draft
Induced draft indicates that the pressure in the furnace is below atmospheric pressure the fan is located in between the furnace and the chimney creating a draw in the furnace. Induced draft is putting a fan at the inlet and blowing air through it. Balanced is having one of each
What are some advantages of mechanical draft
They are independent of atmospheric air temperatures, independent of the chimney temperature, they can be regulated more accurately, they have better draft regulation which increases the efficiency of combustion
Centrifugal fanblades can have three different backward inclined shapes. What are they
Flat, curved, and airfoil
What centrifugal fan blade is the most efficient
Airfoil
Describe forward blade centrifugal fans
A forward curved squirrel cage, more desirable for small fans. Normally limited to clean service and not constructed for high-pressure or harsh service
Which of the following methods doesn't control flow in the centrifugal fan A inlet dampers B vary speed C vary pitch D inlet guides
Vary pitch, you cannot change the pitch of them.
What are radial fan blades better for?
Ideal for high pressure service
What are the two types of radial fan blades?
Open wheel, radial back plate wheel
What does the back plate radial wheel prevent?
It prevents long stringy fibrous material from wrapping around the wheel
What can be done to control air swirl on inlet?
Ductwork can be increased or decreased
What is the purpose of the centrifugal fan casing?
Collect the air leaving the impeller wheel and change the velocity pressure into static pressure
What are the three types axial flow fans?
Propeller, tube axis fan, vane axis fan
What are the advantages of axial flow fans?
Low cost, compactness. Common applications are in exhaust conditions with small airborne particle size
How much power is used when a prop fan is at maximum delivery?
Lowest power
Describe a tube axial fan
Heavier duty prop fan that can be mounted as a unit within the ducting.
What is often used with a tube axial fan, and what is its purpose?
A baffle to cut down the air swirl
What is a vane axial fan?
It is a tube axial fan with outlet end guide vanes behind the impeller to prevent swirl
What is the combination of centrifugal and axial fan called?
Centrifugal fan, axial tube or axial and tubular centrifugal fan
What are the two ways the fan shaft is supported?
Either overhung, where both bearings are behind the fan wheel, or straddled
When are outlet damper controls used?
On fans with constant speed motors
Describe variable inlet vane controls
It is a form of suction throttling, the vanes are all interconnected, so they rotate open and closed in unison
What do spring type isolators do?
They absorb vibration, and allow For thermal expansion
Describe rubber disk isolators
There more compact than spring type isolators less flexible and become hard with age
What is an oil type vibration isolator
It will absorb more capacity of vibrations
How long should the outlet duct be?
It needs to be long enough to allow the turbulent flow from fan discharge charge to change into uniform velocity profile
What is the formula to figure out the duct diameter if it is rectangular
Square root of 4AB/3.1415927
What contributes to leakage?
Leaks tend to increase with the age of the system. Gaskets dry out, sealed to become brittle, joints listening from vibration and sagging from in adequate supports
What can be done for predictive maintenance on fans?
Online vibration monitoring.
What is a good rule of thumb when it comes to measuring shafts for undersize
A maximum of .0005”where bearing hub and sheave coupling locations
What should blade to housing ring clearances be on the cooling tower induced draft fan?
One half a percent fan diameter
Explain how a dynamic compressor increases pressure
The convert velocity into pressure using diffusers of volute casing or combination the slowdown of the air creates pressure
How is multi staging achieved in centrifugal compressors
It will have several individual impellers the rotor shaft, one is directed into that next
What are the three different designs for centrifugal compressor casings
Sectional, horizontal split, vertical split. Sectional is built for the lowest pressure and is often used for vacuum service
What is dynamic balancing
It means it is balanced well if the part is being rotated
What are the three types of centrifugal compressor impellers
Open impeller, semiclosed impeller, closed impeller
What is the primary function of the diaphragm
It separates the impellers in the compressor casing. The direct flow through the compressor from discharge to inlet of the next impeller
What is the problem with carbon ring shaft seals
They have a tendency to corrode the shaft when compressor is shut down
How can compressor surge be controlled
Recycle valve, surge valve, bypass valve
What is the blade fouling and how can it be minimized
It can be minimized by using a special nonstick coating on the blades. If they need to be cleaned crushed walnut shells can be run through the system
1 inch of water is equal to what in PSI?
.036 psi
What is the three main groups of axial flow fans
Propeller fan, tube axis fan, vane axis fan
Backward inclined fans with airfoil please utilize what law
Bernoulli’s principle
How is pressure measured in a fan system?
Manometer
In order to measure static pressure in a duct the manometer opening it has to what
Be at 90° to the airflow
Under what condition with the fan have the greatest velocity pressure
Baffles wide open
Which fan system would have the greatest amount of wear
Induced draft
Which backward inclined fan blade has the greatest efficiency
Airfoil blade
What atmosphere at sea level is equivalent to what
14.7 psi a
Defenseless mounted at the base of the cooling tower and exhausted the air out the top it would be what
Forced draft
What is used in the to the axial fan to stabilize the discharge flow
Discharge channel or discharge baffle
Fans mounted in series are used to?
Increased pressure
Dynamic compressors work on what principal
Bernoulli’s principle
Axial compressor operating at a higher altitude will what
Take longer to fill a compressed air system
What is necessary in order for gas to flow from one area to another
There must be a pressure differential
Stonewalling describe the condition where?
Maximum flow and velocity has been achieved
Surge occurs when,
Flow reverses through an axial compressor
In a radial compressor what may be used to convert velocity into pressure
Diffuser
Controlling compressor output by restricting the inlet flow is called what
Suction throttling
Dynamic compressors use impellers to impart what energy to the gas stream
Kinetic energy
What are single stage compressor is commonly used for in industry
Deliver high-volume low-pressure gas
Which compressor is commonly used in the vacuum service
Sectional split
What is the advantage gained by using a multi flow or combined flow compressor?
It can be made smaller because it uses centrifugal and radial
