Rotary Compressors

ROTARY COMPRESSORS

ROTARY COMPRESSORS

COMPRESSORS Intermittent flow

Continuous flow

Positive displacement Reciprocating Mechanical piston

Dynamic

Rotary •Helical lobe •Straight lobe •Sliding vans • Liquid piston

Radial flow

Mixed flow

Ejector Axial flow

ROTARY COMPRESSORS

COMPRESSORS Intermittent flow

Continuous flow

Positive displacement Reciprocating Mechanical piston

Dynamic

Rotary •Helical lobe •Straight lobe •Sliding vans • Liquid piston

Radial flow

Mixed flow

Ejector Axial flow

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.

ROTARY COMPRESSORS

COMPRESSORS Intermittent flow

Continuous flow

Positive displacement Reciprocating Mechanical piston

Dynamic

Rotary •Helical lobe •Straight lobe •Sliding vans • Liquid piston

Radial flow

Mixed flow

Ejector Axial flow

ROTARY COMPRESSORS

Helical lobe

ROTARY COMPRESSORS

Helical lobe Features of the Screw Compressor 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.

ROTARY COMPRESSORS

Helical lobe Features of the Screw Compressor Screws provide a much wider operating range and lower maintenance costs than conventional reciprocating machines. The machines are much smaller and create much lower vibration levels than piston machines as well.

ROTARY COMPRESSORS

Helical lobe Features of the Screw Compressor Reduced Maintenance:

There are no valves, pistons, rings, or connecting rods that require regular maintenance.

Turn Down:

Screw compressors offer turn down capabilities up to 90% of full load with very good part load power requirements. This turn down capability occurs within the machine and is independent of engine speed or bypass.

ROTARY COMPRESSORS

Helical lobe Features of the Screw Compressor Turn Down:

This makes the machine an attractive alternative for areas where flow rates and operating conditions are not constant. 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 Features of the Screw Compressor High Compression Ratios: Screw compressors can operate from roughly

2 to 20 ratios of compression on a single stage while maintaining high volumetric efficiencies. These efficiencies are achieved by injecting large quantities of lube oil into the machine during the compression process.

Why?

Oil will work as a liquid piston

ROTARY COMPRESSORS

Helical lobe Features of the Screw Compressor Accommodates Wide The screw compressor itself can operate over a very wide range with little or no changes Operating Ranges: required to the machine. In contrast, screws are designed to operate over the entire range with no changes to the machine.

ROTARY COMPRESSORS

Helical lobe Features of the Screw Compressor Smaller Package Sizes:

Rotary screws provide high capacities with minimal installation space compared to piston machines. 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.

The physical size of the compressor is much smaller than a comparable piston type machine.

ROTARY COMPRESSORS

Helical lobe Features of the Screw Compressor Lower Vibration :

With only two major moving parts operating in a circular motion, screws create much lower vibration levels than reciprocating machines. 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. 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 Basic Operation Principle

Suction process

ROTARY COMPRESSORS

Helical lobe Basic Operation Principle

Suction process

ROTARY COMPRESSORS

Helical lobe Basic Operation Principle

Compression

ROTARY COMPRESSORS

Helical lobe Basic Operation Principle

Compression

ROTARY COMPRESSORS

Helical lobe Basic Operation Principle Discharge Process

ROTARY COMPRESSORS

Helical lobe Basic Operation Principle Discharge Process

ROTARY COMPRESSORS

Helical lobe Basic Operation Principle Discharge Process

ROTARY COMPRESSORS

Helical lobe Applications 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.

ROTARY COMPRESSORS

Helical lobe Applications 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.

ROTARY COMPRESSORS

Helical lobe Applications Screw compressors are commonly used in: 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 Applications 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. 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 Volume Ratio In a reciprocating compressor, the discharge valves open when the pressure in the cylinder exceeds the pressure in the discharge manifold. 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 Volume ratio is a fundamental design characteristic of all screw compressors. 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 Volume Ratio "Vi" determines the internal pressure ratio of the compressor and the relationship between them can be approximated as follows: Vi =

Vs Vd

:Where Vi =Volume ratio or index. Vs= Volume at suction. Vd= Volume at discharge.

Pi = Vik

:Where Pi = Internal pressure ratio. k = specific heat ratio of the gas being compressed.

ROTARY COMPRESSORS

Helical lobe Volume Ratio 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. This is called overcompression.

Pd

Pd

Internal Lost work System

Ps Vd

Vs

ROTARY COMPRESSORS

Helical lobe Volume Ratio 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. This is called undercompression.

What happens then?

ROTARY COMPRESSORS

Helical lobe Volume Ratio 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.

ROTARY COMPRESSORS

Helical lobe Volume Ratio 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.

Pd

Pd

System Lost work Internal

Ps Vd

Vs

ROTARY COMPRESSORS

Helical lobe Volume Ratio Pd

Pd

Internal Pd

Lost work System

Pd

System Lost work Internal

Ps

Ps Vd

Vs

Vd

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 Application note Dry compressors 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 Application note Dry compressors 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.

ROTARY COMPRESSORS

Helical lobe Application note Dry compressors 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.

An Introduction to Pumping Equipment

Pressure ratio

200

Multistage Reciprocating 20

Single stage Recip.

Multistage Centrifugal

Rotary compressors 2

Multistage Axial

.Single S Cen 102

103

104 )Flow rate )CFM

105

106

ROTARY COMPRESSORS

Helical lobe Application note Dry compressors 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.

ROTARY COMPRESSORS

Helical lobe Dry compressors

Flooded compressors )lubrication)

Fine filter Oil pressure regulating valve

Oil separator

Manifold

Oil level sight glasses

Oil pump B & CC

Oil heater and thermostat Injection ports

Oil strainer

Oil cooler

Water supply

ROTARY COMPRESSORS

Helical lobe Dry compressors

Flooded compressors )separation)

Suction Discharge

Suction valve

Strainer Relief valve

P

3rd Stage

T

2rd Stage

P

Oil separator

1st Stage

Discharge 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 carbon ring seal and the labyrinth seal may be ported for clean gas injection, ejection or a combination of both. The mechanical contact seal is a very positive seal. The seal may be oil or gas buffered. 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 1. 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 1. 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. 1. 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 1. 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 1. 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 1. 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.

ROTARY COMPRESSORS

COMPRESSORS Intermittent flow

Continuous flow

Positive displacement Reciprocating Mechanical piston

Dynamic

Rotary •Helical lobe •Straight lobe •Sliding vans • Liquid piston

Radial flow

Mixed flow

Ejector Axial flow

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.

ROTARY COMPRESSORS

COMPRESSORS Intermittent flow

Continuous flow

Positive displacement Reciprocating Mechanical piston

Dynamic

Rotary •Helical lobe •Straight lobe •Sliding vans • Liquid piston

Radial flow

Mixed flow

Ejector Axial flow

ROTARY COMPRESSORS

Sliding vane

ROTARY COMPRESSORS

Sliding vane 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. 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

Sliding vane 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. 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

Sliding vane Applications Rotary vane positive displacement machines are typically used either as compressors or as expanders. 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

Sliding vane Applications The chamber volume reduces during further rotation of the compressor causing the pressure to rise. 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

Sliding vane 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. 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

Sliding vane Applications The slide vane compressor is used in gas gathering and gas boosting applications in direct competition with the reciprocating compressor. 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

Sliding vane Mechanical construction: The cylinder is generally constructed of cast iron and includes the water jacket. 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

Sliding vane 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. 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

Sliding vane 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. Roller bearings are widely used, generally the antifriction type.

ROTARY COMPRESSORS

Sliding vane Mechanical construction: Seals are either a packing or mechanical contact type. Packing and bearings are lubricated by a pressurized system. 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

Sliding vane Mechanical construction: Other feeds meter lubrication onto the cylinder wall. 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

Sliding vane 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.

ROTARY COMPRESSORS

COMPRESSORS Intermittent flow

Continuous flow

Positive displacement Reciprocating Mechanical piston

Dynamic

Rotary •Helical lobe •Straight lobe •Sliding vans • Liquid piston

Radial flow

Mixed flow

Ejector Axial flow

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.