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ROTARY COMPRESSORS
Ejector Dynamic
Intermittent flow
COMPRESSORS
Continuous flow
Positive displacement
Radial flow
Mixed flow
Axial flow
Rotary Reciprocating
•Helical lobe
•Straight lobe
•Sliding vans
• Liquid piston
Mechanical piston
ROTARY COMPRESSORS
Ejector Dynamic
Intermittent flow
COMPRESSORS
Continuous flow
Positive displacement
Radial flow
Mixed flow
Axial flow
Rotary Reciprocating
•Helical lobe
•Straight lobe
•Sliding vans
• Liquid piston
Mechanical piston
ROTARY COMPRESSORS
ROTARY COMPRESSORS
Rotary compressors as a group make up the balance of the positive displacement machine.
This group of compressors has several features in common despite differences in construction.
Probably the most important feature is lack of valves as used on the reciprocating compressor.
The rotary is lighter in weight than the reciprocator and does not exhibit the shaking forces of the reciprocating
compressor making foundation requirements less rigorous.
ROTARY COMPRESSORS
Even though rotary compressors are relatively simple and single rotor construction is found. Rotor design is one of
the main items that distinguish the different types. Size and operating range is another area unique to each type of
rotary.
Ejector Dynamic
Intermittent flow
COMPRESSORS
Continuous flow
Positive displacement
Radial flow
Mixed flow
Axial flow
Rotary Reciprocating
•Helical lobe
•Straight lobe
•Sliding vans
• Liquid piston
Mechanical piston
ROTARY COMPRESSORS
ROTARY COMPRESSORS
Helical lobe
ROTARY COMPRESSORS
Helical lobe
Another name for the helical lobe compressor is the Screw compressor.
The rotary screw compressor is a positive displacement machine that operates without the need for suction or
discharge valves.
It has the ability to vary suction volume internally while reducing part load power consumption.
Features of the Screw Compressor
ROTARY COMPRESSORS
Helical lobe
The machines are much smaller and create much lower vibration levels than piston machines as well.
Screws provide a much wider operating range and lower maintenance costs than conventional reciprocating
machines.
Features of the Screw Compressor
ROTARY COMPRESSORS
Helical lobe
There are no valves, pistons, rings, or connecting rods that require regular
maintenance.
Features of the Screw Compressor
Reduced Maintenance:
Screw compressors offer turn down capabilities up to 90% of full load with very
good part load power requirements.
Turn Down:
This turn down capability occurs within the machine and is independent of engine speed
or bypass.
ROTARY COMPRESSORS
Helical lobe
This makes the machine an attractive alternative for areas where flow rates and
operating conditions are not constant.
Features of the Screw Compressor
Turn Down:
The capacity control can typically be handled manually or automatically within the machine
to meet the exact demands of the overall system.
The screw compressor can vary capacity automatically where the reciprocating machine
must have the variable volume pockets adjusted manually.
ROTARY COMPRESSORS
Helical lobe
Screw compressors can operate from roughly 2 to 20 ratios of compression on a single
stage while maintaining high volumetric efficiencies.
Features of the Screw Compressor
High Compression Ratios:
These efficiencies are achieved by injecting large quantities of lube oil into the machine
during the compression process.
Oil will work as a liquid piston Why?
ROTARY COMPRESSORS
Helical lobe
The screw compressor itself can operate over a very wide range with little or no changes
required to the machine.
Features of the Screw Compressor
In contrast, screws are designed to operate over the entire range with no changes to the
machine.
Accommodates Wide Operating Ranges:
ROTARY COMPRESSORS
Helical lobe
Rotary screws provide high capacities with minimal installation space compared to piston
machines.
Features of the Screw Compressor
Based on a full speed design of 3,600 rpm, a large screw machine can provide over 50 MMSCFD of
gas based on a 100 psig suction pressure.
Smaller Package Sizes:
The physical size of the compressor is much smaller than a comparable piston type
machine.
ROTARY COMPRESSORS
Helical lobe
With only two major moving parts operating in a circular motion, screws create much lower
vibration levels than reciprocating machines.
Features of the Screw Compressor
Although the slide valve assembly also moves to control capacity, it happens at such a slow rate that we do not consider it a maintenance
concern.
Lower Vibration :
In general, screw compressors are considered to provide very high reliability, resulting in lower maintenance costs and
reduced down time compared to reciprocating machines.
ROTARY COMPRESSORS
Helical lobe
Description
ROTARY COMPRESSORS
Helical lobe
Suction processBasic Operation Principle
ROTARY COMPRESSORS
Helical lobe
Suction processBasic Operation Principle
ROTARY COMPRESSORS
Helical lobe
CompressionBasic Operation Principle
ROTARY COMPRESSORS
Helical lobe
CompressionBasic Operation Principle
ROTARY COMPRESSORS
Helical lobe
Discharge ProcessBasic Operation Principle
ROTARY COMPRESSORS
Helical lobe
Discharge ProcessBasic Operation Principle
ROTARY COMPRESSORS
Helical lobe
Basic Operation Principle Discharge Process
ROTARY COMPRESSORS
Helical lobe
The rotary screw compressor is designed for low pressure applications with inlet pressures ranging from vacuum
pressure up to 100 psig and discharge pressures up to 350 psig.
These pressure ranges are typical for most process style machines and can vary depending on manufacturer, frame
size and operating speed.
Applications
ROTARY COMPRESSORS
Helical lobe
There are some screw machines available capable of operating at higher pressures by using cast steel casings
but these are not yet commonly used in the natural gas industry due to capital cost and availability.
Applications
ROTARY COMPRESSORS
Helical lobe
Screw compressors are commonly used in:
Applications
A variety of air
Process gas
Process refrigeration
Natural gas applications
Although most natural gas applications are based on a specific gravity of 0.57 – 0.65, screw compressors can be
used on very light gases such as hydrogen and very heavy mole weight gases where specific gravities exceed 2.0.
ROTARY COMPRESSORS
Helical lobe
The most common applications for screw compressors in natural gas service range in horsepower from roughly 90 to
1,500 and are available in both engine and electric drive.
Applications
Screws were originally developed to operate with electric drive two pole motors at 3,550 rpm. As they have become
more popular in the natural gas industry, engine drive applications have become much more common.
On most of these applications, the screw is operating direct drive at 1800 rpm, or half the rated speed.
ROTARY COMPRESSORS
Helical lobe
In a reciprocating compressor, the discharge valves open when the pressure in the cylinder exceeds the pressure in
the discharge manifold.
Volume Ratio
Because a screw compressor does not have valves, the location of the discharge ports determine the maximum
discharge pressure level that will be achieved in the screw threads before the compressed gas is pushed into the
discharge pipe.
ROTARY COMPRESSORS
Helical lobe
Volume ratio is a fundamental design characteristic of all screw compressors.
Volume Ratio
The internal volume reduction ratio of the compressor (Vi) is defined as :
The comparison of the volume of trapped gas at suction, (Vs) to the volume of trapped gas remaining in the
compression chamber when it opens to discharge, (Vd).
ROTARY COMPRESSORS
Helical lobe
"Vi" determines the internal pressure ratio of the compressor and the relationship between them can be
approximated as follows:
Volume Ratio
d
si V
VV
Where:
Vi =Volume ratio or index.Vs= Volume at suction.
Vd= Volume at discharge.
kii VP
Where:
Pi = Internal pressure ratio.k = specific heat ratio of the
gas being compressed.
ROTARY COMPRESSORS
Helical lobe
If the internal volume ratio (Vi) of the compressor is too high
for a given set of operating conditions the discharge gas will be kept trapped too long
and be raised above the discharge pressure in the
piping.
Volume Ratio
This is called overcompression.VsVd
Pd
Pd
System
Internal
Lost work
Ps
ROTARY COMPRESSORS
Helical lobe
If the discharge port opening occurs before the internal pressure in the compressor trapped pocket has reached
the system discharge pressure level, the higher pressure gas outside the compressor flows back into the lower
pressure pocket, raising the thread pressure immediately to the discharge pressure level.
Volume Ratio
This is called undercompression.
What happens then?
ROTARY COMPRESSORS
Helical lobe
The compressor then has to pump against this higher pressure level, rather than pump against a gradual build up
to discharge pressure level if the volume ratio had been higher, keeping the trapped pocket closed longer.
Volume Ratio
ROTARY COMPRESSORS
Helical lobe
The compressor then has to pump against this higher
pressure level, rather than pump against a gradual build
up to discharge pressure level if the volume ratio had been higher, keeping the trapped
pocket closed longer.
Volume Ratio
VsVd
Pd
Pd
Internal
System
Lost work
Ps
ROTARY COMPRESSORS
Helical lobe
Volume Ratio
VsVd
Pd
Pd
Internal
System
Lost work
Ps
Vd
Pd
Pd
System
Internal
Lost work
Ps
Vs
ROTARY COMPRESSORS
Helical lobe
Volume Ratio
In both cases the compressor will still function, and the same volume of gas will be moved, but more power will be required than if the discharge ports are correctly located to
match the compressor volume ratio to what the system needs.
ROTARY COMPRESSORS
Helical lobe
Capacity Control
Capacity control is used in screw compressors to vary the amount of gas drawn into the compressor.
Common capacity control methods are:•Slide valve controlling discharge port.
•Slide valve controlling discharge port and volume ratio.
•Slide valve not controlling discharge port.
•Plug valves.
•Variable speed.
ROTARY COMPRESSORS
Helical lobe
Capacity Control
Slide valves controlling the discharge port are a very common type of capacity control device used in screw
compressors.
They are popular because they can give infinitely adjustable control of capacity, often from 10 to 100%.
This type of slide valve works by opening a recirculation passage in the high pressure cusp which allows a portion of the trapped gas in the "V" shaped compression chamber to
be recirculated back to the suction cavity before it begins compression.
ROTARY COMPRESSORS
Helical lobe
Capacity Control
ROTARY COMPRESSORS
Helical lobe
Capacity Control Slide valve controlling discharge port.
ROTARY COMPRESSORS
Helical lobe
Capacity Control Slide valve controlling discharge port and volume ratio.
ROTARY COMPRESSORS
Helical lobe
Capacity Control Slide valve not controlling discharge port.
ROTARY COMPRESSORS
Helical lobe
Capacity Control Plug valves.
ROTARY COMPRESSORS
Helical lobe
Capacity Control
ROTARY COMPRESSORS
Helical lobe
Capacity Control
ROTARY COMPRESSORS
Helical lobe
Capacity Control
ROTARY COMPRESSORS
Helical lobe
Dry compressors Application note
Screw compressors of the dry type generate high frequency pulsations, which move into the system piping
and can cause acoustic vibration problems.
These would be similar to the type of problems experienced in reciprocating compressor applications,
except that the frequency is higher. While volume bottles will work with the reciprocator, the dry type screw
compressor would require a manufacturer-supplied proprietary silencer, which would take car of the problem
rather nicely.
ROTARY COMPRESSORS
Helical lobe
While on considerations of the dry compressor there is one problem, the compressor can handle quite well. Unlike
most other compressors this one will tolerate a moderate amount of liquid.
The compressor also takes reasonably well to fouling service, if material is not abrasive. The foulant tends to
help seal the compressor and in time, may improve performance.
Dry compressors Application note
ROTARY COMPRESSORS
Helical lobe
One other application for which the dry machine is particularly well suited, is for hydrogen rich service, where the molecular weight is low, with a resulting high adiabatic
head.
For larger flow streams, within the centrifugal compressor's flow range, the screw compressor is a good alternative.
While the high adiabatic head requires expensive, multiple centrifugal casings, the positive displacement characteristic
of the screw compressor is not compromised by the low molecular weight.
Dry compressors Application note
An Introduction to Pumping Equipment
2
20
200
102 103 104 105 106
Multistage Reciprocating
Single stage Recip.
Flow rate (CFM)
Pre
ssu
re r
atio
Rotary compressors
Multistage Centrifugal
Single S Cen.
Multistage Axial
ROTARY COMPRESSORS
Helical lobe
For very low molecular weight gas, such as pure hydrogen or helium, a good seal is important to keep the slip in
control. This can be tedious and in extreme cases a liquid injection is used for leakage control to maintain
performance.
Dry compressors Application note
ROTARY COMPRESSORS
Helical lobe
Dry compressors Flooded compressors (lubrication)
Oil pressure regulating valve
Fine filter
Oil pump
Oil strainer Oil cooler
Oil separator
Oil level sight glasses
Oil heater and thermostat
Water supply
Manifold
B & CC
Injection ports
ROTARY COMPRESSORS
Helical lobe
Dry compressors Flooded compressors (separation)
Strainer
P
Oil separator
Discharge
TDischarge
valve
1st Stage
3rd Stage
2rd StageP
Suction
Suction valve
Relief valve
ROTARY COMPRESSORS
Helical lobe
Screw Compressor Package Components
ROTARY COMPRESSORS
Helical lobe
Casings
Most casing on both flooded and dry compressors are cast, normally of grey cast iron. API 619 limits the use of cast
iron by specifying steel for services in excess of 400 psig, discharge temperatures in excess of 500 oF and for
flammable or toxic gases.
While extremely rare, austenitic and high nickel casings have been furnished.
ROTARY COMPRESSORS
Helical lobe
Casings
On dry compressors the casing normally includes a water jacket. While referred to as a cooling jacket, the cooling
water or alternative fluid is used as a heat sink or casing stabilizer to help control distortions and clearance changes.
While casting is used for the iron casings, steel casing may be fabricated or cast.
Most casings are vertically split, using end closures and withdrawing the rotors axially for maintenance.
ROTARY COMPRESSORS
Helical lobe
Casings
On the larger dry machines, the casing is horizontally split, to facilitate the removal of the heavier rotors.
ROTARY COMPRESSORS
Helical lobe
Rotors
Some dry compressors are furnished with hollow rotors through which cooling fluid is circulated.
Materials of construction are steel in most applications.
The material may be either a forging or bar stock, based on size availability of the bar stock in the quality needed.
Other materials are used whenever carbon steel is not compatible with the gas being compressed. These range
from stainless, either of the austenitic or 12 chrome type, to more exotic nickel alloys.
ROTARY COMPRESSORS
Helical lobe
Rotors
Some vendors furnish coatings for the rotors in order to keep the rotor from wearing and losing seal clearance.
ROTARY COMPRESSORS
Helical lobe
Bearing and seals
In the larger, dry process compressors, the radial bearings are of the sleeve or tilting pad type.
API 619 requires the bearings to be removable without removing the rotors or the upper half on the horizontally
split machine.
Thrust bearings are generally tilt pad type, though not necessarily symmetric.
On standardized compressors for air or refrigeration, the bearings are normally the antifriction type.
ROTARY COMPRESSORS
Helical lobe
Bearing and seals
Some standardized dry compressors use a tapered land thrust bearing.
Most of the flooded dry compressors and some of the standardized dry compressors use antifriction thrust
bearings.
In all cases the bearings are pressure-lubricated with some compressors using the gas differential pressure to circulate
the lubricant and thus pressurize the bearings.
ROTARY COMPRESSORS
Helical lobe
Bearing and seals
For difficult services mentioned previously, unusual bearings may be used, such as graphite with a sulfuric acid
flooding medium. In dry compressors, shaft end seals are generally one of
four types:
Labyrinth
Restrictive ring Mechanical contact
Liquid film
ROTARY COMPRESSORS
Helical lobe
Bearing and seals
The labyrinth seal is generally ported at an axial point between the seals in order to use an educator or ejector to
control leakage and direct it to the suction or a suitable disposal area. Alternatively a buffer gas is used to prevent
the loss of process gas.
Probably the most common seal is the restrictive ring type, normally used in the form of carbon rings. This seal
controls leakage better than the non-floating labyrinth type, although it wears faster.
ROTARY COMPRESSORS
Helical lobe
Bearing and seals
The carbon ring seal does not tolerate dirt as well as the labyrinth seal.
The mechanical contact seal is a very positive seal. The seal may be oil or gas buffered.
The carbon ring seal and the labyrinth seal may be ported for clean gas injection, ejection or a combination of both.
The mechanical seal, which is the most complex and expensive, is used where gas leakage cannot be tolerated.
ROTARY COMPRESSORS
Helical lobe
Bearing and seals
This may be due to the cost of the gas, as in closed loop refrigeration or where the process gas is toxic or
flammable.
The mechanical contact seal requires more power than the other seals, which is a deterrent to its use on lower power
compressors.
The liquid film seal uses metallic sealing rings and is liquid buffered to maintain a fluid film in the clearance area and
thereby preclude gas leakage.
ROTARY COMPRESSORS
Helical lobe
Bearing and seals
It is not unusual in the screw compressor to find the radial bearing and seal combined.
ROTARY COMPRESSORS
Helical lobe
Timing gears
In screw compressors of the dry type, the rotors are synchronized by timing gears.
Because the male rotor, with a conventional profile, absorbs about 90% of power transmitted to the
compressor, only 10% of the power is transmitted through the gears.
ROTARY COMPRESSORS
Helical lobe
Timing gears
Timing gears are machined from low alloy steel, normally a Chrome, Nickel and Molybdenum chemistry. The gears are
helical type which also helps control noise. The pitch line run out must be minimized to control torsional excitation.
The gears are housed in a chamber outboard from the drive end and are isolated from the gas being compressed.
ROTARY COMPRESSORS
Helical lobe
Troubleshooting
Low Discharge Temperature
1. Discharge temperature could be too low because liquid refrigerant is being carried into the suction or economizer lines. Excessive foaming in the separator or frost appearing far down the compressor suction housing can also indicate this.
ROTARY COMPRESSORS
Helical lobe
Troubleshooting
Low Discharge Temperature
2. Low discharge temperature may indicate condensing in the discharge line during off cycle, running back into the oil separator, and feeding excess liquid to the compressor until the package warms up.
ROTARY COMPRESSORS
Helical lobe
Troubleshooting
Low Discharge Temperature
3. Low discharge temperature could indicate oil flow above design level. Is main injection adjusted properly or could balance piston wear be increasing the flow of oil.
ROTARY COMPRESSORS
Helical lobe
Troubleshooting
High Discharge Temperature
1. High discharge temperature can be caused by suction or economizer superheat running above design level.
2. Restricted oil flow to the compressor will cause high discharge temperature. Check for main injection valve, oil orifice, or filter blockage.
ROTARY COMPRESSORS
Helical lobe
Troubleshooting
High Discharge Temperature
3. Is the volume ratio and slide valve correctly calibrated and working properly. If the compressor is running at the incorrect volume ratio for the application, excessive horsepower will be consumed in the compression. Excess power consumption always shows up as too high a discharge temperature.
ROTARY COMPRESSORS
Helical lobe
Troubleshooting
High Discharge Temperature
4. Is the compressor in the early stages of failure or loosing axial rotor position due to thrust bearing wear? Any condition that causes a loss of rotor position will lead to excessive thread to thread leakage and higher discharge temperatures. If this is suspected, vibration analysis is the best way to assess compressor condition. If vibration analysis is not possible, teardown inspection may be called for if all other possibilities have been investigated.
ROTARY COMPRESSORS
Helical lobe
Troubleshooting
High Discharge Temperature
5. Is the oil cooling working correctly? This is easy to sort out as a possibility with rating software for the compressors if the actual operating oil temperature is input to the rating program. If the measured discharge temperature at an elevated oil temperature is still in agreement with predictions, it is fairly certain the compressor is healthy but perhaps the oil cooler should be investigated.
Ejector Dynamic
Intermittent flow
COMPRESSORS
Continuous flow
Positive displacement
Radial flow
Mixed flow
Axial flow
Rotary Reciprocating
•Helical lobe
•Straight lobe
•Sliding vans
• Liquid piston
Mechanical piston
ROTARY COMPRESSORS
ROTARY COMPRESSORS
Straight lobe
ROTARY COMPRESSORS
Straight lobe
Compression cycle
Straight lobe compressors or blowers, as they are commonly called, are low-pressure machines.
The features rotary compressors have in common are:• They impart energy to the gas being compressed by way of an input
shaft moving a single or multiple rotating elements.
• They perform the compression in an intermittent mode.
• They do not use inlet and discharge valves.
• The rotors are timed by a set of gears.
ROTARY COMPRESSORS
Straight lobe
Compression cycle
• As the rotors turn and pass the inlet port, a volume of gas is trapped and carried between the lobes and the outer cylinder
wall.
• When the lobe pushes the gas toward the exit, the gas is compressed by the backpressure of the gas in the discharge
line.
ROTARY COMPRESSORS
Straight lobe
Compression cycle
Volumetric efficiency is determined by the tip leakage past the rotors, not unlike the rotary screw compressor.
The leakage is referred to as slip.
Slippage is a function of the rotor diameter and differential pressure, for a given gas. Slippage is determined by teat.
ROTARY COMPRESSORS
Straight lobe
Compression cycle
For the test, the differential pressure is imposed on the blower and the speed gradually increased until the point is
reached where the output just matches the slip leakage. This point is detected by watching for the machine to just
begin to give a positive output.
The speed at which this occurs is called the slip speed. A slip speed is determined for each of several pressure
differentials.
ROTARY COMPRESSORS
Straight lobe
Sizing
Sizing for the straight lob compressor is normally done using catalog data. Rotor lengths range from
approximately one to two times the rotor diameter. Individual frame sizes within a given vendor's line may
exceed these limits. Maximum tip speeds are in the 125 fps range with some unit approaching 140 fps.
ROTARY COMPRESSORS
Straight lobe
Applications
The straight lobe blowers are used in both pressure and vacuum service.
Larger units are direct connected to their drivers and the smaller units are belt driven. The drivers are normally
electric motors.
Some of the larger models offer an internal gear arrangement to permit the direct connection of a two or
four pole electric motor.
ROTARY COMPRESSORS
Straight lobe
Applications
The main limitation to this rotary compressor is the differential pressure with the longer rotors where deflection
is large.
For a two lobe machine, caution should be used when the rotor is more than 1.5 times the rotor diameter at
pressures in excess of 8 psi differential.
The three lobe compressors inherently have a stiffer rotor and can sustain a higher differential with less difficulty.
ROTARY COMPRESSORS
Straight lobe
Mechanical construction
Straight lobe compressor casing, also called housings or cylinders by different manufacturers, are furnished in cast
iron by all vendors. There is an optional aluminum construction available for special applications.
Rotors are cast from ductile iron. Again, the exception is the aluminum construction. Shafts are steel and are cast
into the rotors or are pinned to the rotor in a stub shaft construction method.
ROTARY COMPRESSORS
Straight lobe
Mechanical construction
Straight lobe compressor casing, also called housings or cylinders by different manufacturers, are furnished in cast
iron by all vendors. There is an optional aluminum construction available for special applications.
Rotors are cast from ductile iron. Shafts are steel and are cast into the rotors or are pinned to the rotor in a stub shaft
construction method.
An alternate design has the rotors drilled for through shafts.
ROTARY COMPRESSORS
Straight lobe
Mechanical construction
Rotors are supported by a set of antifriction bearings on the outboard end of each rotor.
Lubrication is splash type. There are variations available with internal pressure lubrication systems. Some models
can be equipped with an external lube system and for rare cases, API 614 lubrication systems have been proposed.
Ejector Dynamic
Intermittent flow
COMPRESSORS
Continuous flow
Positive displacement
Radial flow
Mixed flow
Axial flow
Rotary Reciprocating
•Helical lobe
•Straight lobe
•Sliding vans
• Liquid piston
Mechanical piston
ROTARY COMPRESSORS
ROTARY COMPRESSORS
Sliding vane
ROTARY COMPRESSORS
Compression cycle
A sliding (rotary) vane compressor has a solid rotor mounted inside a water jacketed cylinder, similar to that of
a jacketed water section of a reciprocating cylinder.
Sliding vane
The water jacket around the cylinder is used for cooling. The rotor is filled with blades that are free to move in and
out of the longitudinal slots in the rotor.
Blade configurations range from 8 to 12 blades, depending upon manufacturer and pressure differentials.
ROTARY COMPRESSORS
Compression cycle
A sliding (rotary) vane compressor has a solid rotor mounted inside a water jacketed cylinder, similar to that of
a jacketed water section of a reciprocating cylinder.
Sliding vane
Vanes which are free to move in slots in the rotor are kept in contact with the cylinder wall by centrifugal force,
sometimes augmented by springs or oil pressure.
ROTARY COMPRESSORS
Applications
Rotary vane positive displacement machines are typically used either as compressors or as expanders.
Sliding vane
When used as compressors the gas or vapour fills the compressor chambers during the period that these
chambers are increasing in volume.
No more fluid is admitted through the intake port when the chamber volume has reached its maximum volume.
ROTARY COMPRESSORS
Applications
The chamber volume reduces during further rotation of the compressor causing the pressure to rise.
Sliding vane
When the required pressure has been reached the compressor empties the high pressure vapour or gas
through a controllable discharge port while the displacement volume is reduced to zero.
ROTARY COMPRESSORS
Applications
When the rotary sliding vane machine is used as an expander, this process is reversed. The higher density gas
or vapour fills a chamber of relatively small volume.
Sliding vane
The volume of this chamber increases with rotation thus reducing the fluid pressure.
When the required lower exit pressure has been reached the gas
or vapour leaves the expander.
ROTARY COMPRESSORS
Applications
The slide vane compressor is used in gas gathering and gas boosting applications in direct competition with the
reciprocating compressor.
Sliding vane
Sliding (rotary) vane compressors are designed to be utilized in very harsh environments.
When it comes to vapor recovery, landfill gas, and other low ratio and discharge pressure applications, the sliding
vane compressor is typically the most commonly used due to the lubrication system.
ROTARY COMPRESSORS
Mechanical construction:
The cylinder is generally constructed of cast iron and includes the water jacket.
Sliding vane
The bore is machined and brought to a good finish to reduce the van sliding friction.
The inlet and outlet connections are flanged. The heads, which also house the bearings and stuffing box, are also
made of cast iron.
ROTARY COMPRESSORS
Mechanical construction:
The rotor and shaft extension are machined from a single piece or bar stock or from a forging in all but the largest
sizes where the rotor and shaft may be made as two separate parts.
Sliding vane
The rotor body is attached to the shaft using a press fit. Keys are used to lock the rotor body to the shaft. Vanes
attach to the rotor body by means of milled slots.
ROTARY COMPRESSORS
Mechanical construction:
For the lubricated machines vanes are made of a laminated asbestos impregnated with phenolic resin. For a
non-lubricated design, carbon is used. The vane number influences the differential pressure between adjacent vane cells. This influence becomes less as the number of vanes
increases.
Sliding vane
Roller bearings are widely used, generally the antifriction type.
ROTARY COMPRESSORS
Mechanical construction:
Seals are either a packing or mechanical contact type. Packing and bearings are lubricated by a pressurized
system.
Sliding vane
For the non-flooded, lubricated compressor a multiplunger pump, similar to the one used with reciprocating
compressors, is used.
Lubrication is directed from the lubricator to drilled passages in compressor cylinder and heads. One feed is
directed to each of the bearings.
ROTARY COMPRESSORS
Mechanical construction:
Other feeds meter lubrication onto the cylinder wall.
Sliding vane
As the vanes pass the oil injection openings, lubricant is spread around the cylinder walls to lubricate the vane tips
and eventually the vanes themselves.
The oil entering the gas stream is separated in the discharge line. Because of the high local heat, the
lubricant may have broken down and therefore, is not suitable for recycling.
ROTARY COMPRESSORS
Mechanical construction:
Flooded compressors pressure feed a large amount of lubricant into the compressor where it both cools the gas
and lubricates the compressor. It is separated from the gas at the discharge line and recycled.
Sliding vane
Ejector Dynamic
Intermittent flow
COMPRESSORS
Continuous flow
Positive displacement
Radial flow
Mixed flow
Axial flow
Rotary Reciprocating
•Helical lobe
•Straight lobe
•Sliding vans
• Liquid piston
Mechanical piston
ROTARY COMPRESSORS
ROTARY COMPRESSORS
Liquid piston
ROTARY COMPRESSORS
Liquid piston
Operation :
The liquid piston compressor is a unique type of rotary compressor in that it performs its compression by use of a
liquid ring acting as a piston.
As with the sliding vane compressor, the single rotor is located eccentrically inside a cylinder or stator. The rotor
has, extending from it, a series of vanes in a purely radial or radial with forward curved tips orientation.
ROTARY COMPRESSORS
Liquid piston
Operation :
Gas inlet and outlet passages are located on the rotor. A liquid compressant partially fills the rotor and cylinder and
orients itself in a ring moves in an oscillatory manner.
The center of the ring communicates with the inlet and outlet ports and forms the gas pocket. As the rotor turns
and the pocket is moving away from the rotor, the gas enters through the inlet and fills the pocket.
ROTARY COMPRESSORS
Liquid piston
Operation :
As the rotor turns it carries the gas pocket with it. Further turning takes the liquid ring from the maximum clearance
area toward the minimum side.
The ring seals off the inlet port and traps the pocket of gas. As liquid ring is taken into the minimum clearance area the
pocket is compressed. When the ring uncovers the discharge port the compressed pocket of gas is
discharged.
ROTARY COMPRESSORS
Liquid piston
Operation :
The cooling of liquid-ring compressors is direct rather than through the walls of a casing. The required additional
cooling liquid is fed into the casing where it comes into direct contact with the gas being compressed. The excess
liquid is discharged with gas.
The discharged mixture is passed through a conventional baffle or centrifugal type separator to remove the free
liquid. Because of the intimate contact of gas and liquid, the final discharge temperature can be held close to the
temperature of the inlet cooling water.
ROTARY COMPRESSORS
Liquid piston
Operation :
However, the discharge gas is saturated at the discharge temperature of the compressing liquid.
The amount of liquid that may be passed through the compressor is not critical and can be varied to obtain the
desired results. The unit can handle saturated vapors, entrained liquid and occasional foreign matter.
The unit will not be damaged if a large quantity of liquid inadvertently or by intent, enters its suction.
ROTARY COMPRESSORS
Liquid piston
Operation :
Lubrication is required only in the bearings, which are generally located external to the casing. The gas or air
being compressed is therefore oil free. The liquid itself acts a lubricant, sealing medium and coolant for the stuffing
boxes.
Two-staging is possible by putting two machines in series.
ROTARY COMPRESSORS
Liquid piston
Performance :
Efficiency of the liquid piston is about 50%, which is not very good compared to the other rotary compressors.
Because of significant differences in the construction of the various competitive makes of this compressor, no
universal sizing data are available.
The mechanical or process engineer will therefore have to rely on catalog data for sizing estimates.
ROTARY COMPRESSORS
Liquid piston
Performance :
The liquid ring compressor is most often used in vacuum service although it can also act as a positive pressure
compressor.
The liquid piston machine can be staged when the application requires more differential pressure than can be
generated by a single stage.
The liquid piston compressor can be used to compress air to 100 Psig. Vacuums of 26 inhg are possible. Flow
capacity ranges from 2 cfm to 16,000 cfm.
ROTARY COMPRESSORS
Liquid piston
Mechanical Construction :
Standard materials for the compressor are cast iron for the cylinder and carbon steel for the shaft.
The rotor parts are steel.
The liquid piston compressor has another feature that compensates for low efficiency. By using special materials
of construction and compatible liquid compressant, unusual or difficult gases may be compressed.
ROTARY COMPRESSORS
Liquid piston
Mechanical Construction :
Both antifriction and split sleeve bearings are used. Normally, packing is used for shaft sealing or for special
services, mechanical contact seals can be used.