selecting a compresor for process & speciality gas applications

8/10/2019 Selecting a Compresor for Process & Speciality Gas Applications

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TECHNICAL BULLETIN

SELECTING A COMPRESSOR FOR PROCESS & SPECIALTYGAS APPLICATIONS

INTRODUCTION

The question often asked of us is which type of gas compressor is the BEST? The answeris not which type is best, but which is best suited for a specific application.

To intelligently select a gas compressor many factors must be considered:

What is the composition of the gas being compressed?

What is the specific gravity or molecular weight of the gas?

What is specific heat ratio of the gas?

Is the gas explosive?

Is the gas corrosive? Is the gas toxic?

Is the gas reactive?

Is it a dry gas or a wet gas?

What is the available suction pressure?

What discharge pressure is required?

What volume of gas must be compressed?

What will the duty cycle be?

What materials are compatible with the gas?

What is a permissible degree of impurities (lubricating oil carry over etc.)?

What ambient temperature will the compressor operate on? What will be the suction temperature of the gas itself?

Is there an inexpensive source of cooling water or is air-cooling required?

What elevation will the compressor operate at?

To understand which type of compressor suits an application, we must first understand theoperation of each type of compressor.


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Three methods are used to compress a gas:

1. Trap consecutive volumes of gas in a chamber and reduce the volume of the chamberto increase pressure, and then open the chamber to allow the gas out (reciprocating &rotary screw, guided rotor, vane compressors etc.)

2. Trap consecutive volumes of gas in a chamber; carry it without a change in volume tothe discharge and build-up pressure against backflow in the piping (lobe type blowers).

3. Accelerate the velocity of the gas using high-speed impellers and then reduce thevelocity suddenly, to create pressure (centrifugal, axial compressors).

COMPRESSOR CLASSIFICATIONS

From these compression methods compressors and blowers are classified into twocategories these are POSITIVE DISPLACEMENT units which use the first two methods andROTARY DYNAMIC units (sometimes simply called dynamic) which use the third method.

Each has its own individual features, benefits and drawbacks.

Positive Displacement Compressors

These Compressors or blowers compress by trapping consecutive volumes of gas in achamber and then increase the pressure by decreasing the size of the chamber.

Positive displacement units generally consist of the following types:

1. Reciprocating Compressorsare available in either lubricated or oil-free versions andcompress through the reciprocating motion of a piston.

Their ability to be multi-staged for high pressures make reciprocating units one of the

most common types of compressors.

Because of their reciprocating motion some attention must be given to piping pulsationand unit vibration, particularly in larger frame machines.

Pressures as high as 6000 PSIG are attainable.

Reciprocating Compressors


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Single Acting Reciprocating Compressors

With a SINGLE-ACTING reciprocating compressor, compression takes place at one end ofthe cylinder only. The other end of the cylinder is normally open to the crankcase or adistance piece. In other words, there is only one compression stroke for every revolution ofthe crankshaft.

A SINGLE-ACTING compressor brings gas in through intake valve A during the suctionstroke, as the piston moves to the right. Gas is compressed during the second half of thecrankshaft revolution when the piston moves to the left. Compressed gas goes out dischargevalve B.

Single Acting Compressor

Double Acting Reciprocating Compressors

DOUBLE-ACTING compressors provide TWO complete compression cycles per revolution ofthe crankshaft. Compression stroke for intake A acts as the suction stroke for intake C andvice versa.

Double Acting Compressor

Both ends of the cylinder are enclosed and the piston rod is fitted with a packing glandwhere it enters the cylinder to prevent a loss of efficiency from blow-by.


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Double Acting Compressor

Cylinder Configurations

The earliest reciprocating compressors were of vertical design and required heavy concretefoundations to absorb the inertia forces and movements caused by the reciprocatingmovement.

Many modern compressors now use a 90o

V-type cylinder formation in which the inertiaforces of one cylinder is compensated by the corresponding component of the other cylinder.

As a result, these compressors can be operated at up to about 1800 RPM, which was notpossible with the original one cylinder vertical design. For the most part however, piston

compressors operate in the 400 900 RPM range.

Besides inertia compensation, the compact design is another advantage of the V-typecompressor described above, in particular in the case of the two-stage compressors,because there is space between the cylinders for the accommodation of the intercooler.

This space can also be used for an additional cylinder which results in an even more spacesaving and compact W-type unit, making foundationless installation possible with a V-angleof 60o between the cylinders.


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Twin Screw Compressor

Speed increasing gears are generally used to turn the drive rotor at the correct speed.

In conventional oil lubricated screws, the female rotor is driven by the male rotor, with themale rotor absorbing approximately 85% of the input power. A thin film of oil, which is

injected into the compressor, prevents metal-to-metal contact.

The use of one rotor driving the other is known as a pitch line drive system. The strongermale rotor being used as the drive rotor, while the female rotor acts as an idler.

A clearance of 3 to 5 thousandths of an inch is maintained between the rotors, by an injectedfluid film, therefore, there is no metal-to-metal contact.

Sealing strips consisting of a raised lip and a groove on the male rotor, and a raised lip on thetip of the female rotor, reduces the amount of gas that leaks back into the previous cavity.

In most units the inlet is located at the top of the compression element and at the drive end,while the discharge port is located on the bottom on the opposite end.


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GAS INLET

DRIVE COUPLING GAS DISCHARGE

Gas is compressed in much the same way that it would be done if a bowling ball were rolledthrough a pipe with very tight tolerances. The ball would advance like a piston, compressingthe gas that is trapped in front of it as it moves forward.

As the two rotors revolve in opposite directions, gas is trapped in the pockets between therotors and compression is accomplished by moving the trapped volume of gas away from theinlet and towards the discharge.

As the position of the lobes completes the discharge phase, the voids at the opposite (inlet)end began to fill with gas through the inlet port. When the female lobe is filled with gas alongits entire length, the intake phase is completed.

Further rotation causes the male lobe to mesh with the female rotor, trapping the gas that hasbeen taken in.

The male rotor then begins to squeeze the trapped gas toward the discharge end of thecompressor. As the male rotor progressively reduces the trapped gas volume, oil is injectedinto the compression chamber.

Upon reaching its maximum discharge pressure the rotors pass over the discharge port andthe gas is discharged.


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Intake

Compression

Discharge

Twin Screw Compression Cycle


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In oil-free rotary screw compressors the compression cycle is identical to a lubricated unithowever, the rotating screws are synchronized by means of external timing gears. Since therotors do not touch each other, lubrication is not required within the compression chamberand the gas is therefore oil-free.

For oil-injected compressors discharge pressures of up to 350 PSIG can be achieved insingle stage versions and up to 500 PSIG on two-stage units.

Oil-free models are limited to approximately 150 PSIG.

Twin Rotor Screw Compressor

3. Single Screw Compressors(often called a monoscrew)

Although similar in operation to a twin-screw, the more recently developed monoscrewdesign consists of a single female rotor and two intermeshing male gate-rotorsmounted on opposite sides of the main rotor.

The compression cycle begins after inlet gas fills the top and bottom grooves of themain screw at the suction end of the casing. Because the screw compressor has twogaterotors, the compression process occurs simultaneously on opposite sides of thescrew. As the main screw rotates, it drives the gaterotors. The meshing of thegaterotor with a screw groove traps the gas and begins the compression process.

As the screw rotates, the engagement of the gaterotor continues, reducing the initialvolume of the groove and increasing the pressure. This occurs simultaneously onopposite sides of the screw. As the main screw rotates towards completion of thecompression cycle, the groove aligns with a port in the housing at the discharge end ofthe casing. The gas in the groove is discharged radically through the discharge port.

One advantage of the single screw compressor is that there are no radial or axialforces exerted on the main screw. Because the compression process occurssimultaneously on opposite sides of the screw, the forces caused by compression arecancelled out.


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Also, because the monoscrew compressor has six compression strokes per revolutionand generally operates at 3600 rpm, the amount of gas compressed with each grooveis a very small percentage of the total flow. The number of strokes and the speed ofrotation result in very low pulsations at the compressor outlet and negligible pulsationsdownstream of the compressor.

Despite these advantages, the single screw compressor is just beginning to be widelyused because until recently the machining of the rotors for single screw compressorsrequired the use of specially designed machine tools and the utilization of expensivemanufacturing processes.

With the exception of the compression element, the flow and downstream componentsused on both the twin-screw and mono-screw compressors are identical.

Single Rotor Screw Compressor

Rotary Sliding Vane Compressors Rotating within a cylindrical compression chamber orstator is an eccentrically mounted slotted rotor, which is fitted with sliding vanes. As the rotorspins, the vanes are held in contact with the stator wall by centrifugal force, or in some casesby springs. They are generally of single stage design and suitable for pressures up to 150

PSIG, but are most widely used for pressures in the 50 100 PSIG range.

The gas is compressed by the decreasing volume of the compartments formed by the rotor,stator and vanes, until the delivery port is uncovered by the vanes.


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As one of the blades passes a sealing point between the inlet and discharge ports, it preventsany of the compressed gas from leaking back to the suction.

Rotary Vane Compressor

4. Guided Rotor Compressors. In a Guided Rotor Compressor the compressionvolume is defined by a trochodally rotating rotor mounted on an eccentric shaft.

A single rotor GRC compressor assembly is made-up of a housing, a rotor, rollerseals, suction side plate, discharge side plate, shaft bearings, end covers and aceramic face seal. The compressor is modular in construction and can be configuredas a multi-rotor and multi-stage assembly.

There are no compressor valves in the GRC, which instead uses simple inlet, anddischarge parts, which are opened and closed as the rotor passes over them.

A wide range of construction materials (including stainless steel), make them wellsuited to a variety of gases including: natural gas, hydrogen, carbon dioxide, biogas,

syngas etc.

These compressors are more tolerant of contaminants such as water in the gas, theyare compact in size and have a low parts count (no compressor valves, no pistonrings, no linear strip seals).

Because liquid lubricant is introduced and is present within the compression volume,the GRC experiences a degree of isothermal cooling during compression. As a result,discharge temperatures experienced with the GRC are significantly lower than forcomparable reciprocating compressors.

Discharge pressures are generally limited to around 800 to 1000 PSIG.


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5. Hydraulic Driven Gas Pressure Boosters

Most of these compressors are designed using a double-acting hydraulic cylinder witha high-pressure barrel attached to each end. As the hydraulic cylinder is cycled, thehigh-pressure pistons alternately compress and eject the gas from one barrel while

simultaneously filling the opposing one. In the case of a two-stage compressor, gasfrom the first stage is used to fill the second and then further compressed by thesecond stage piston.

The direction of the flow of the gas is controlled by inlet and discharge check valves.The hydraulic fluid is isolated from the gas being compressed and the section betweenthe two is vented to the atmosphere.

These compressors are particularly well suited for booster applications and arecapable of discharge pressures as high as 60,000 PSIG.

Interstage Cooler

LP Gas Inlet HP Gas Outlet


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6. Diaphragm-type Compressors In a diaphragm compressor, oscillating hydraulicpressure on one side of the diaphragm causes it to move inside a chamber,compressing the gas on its other side. Inlet and discharge valves allow low pressuregas to enter the compressor, and the pressurized gas to be discharged to the system.The oscillating pressure is generated by a piston moving in a cylinder.

The crankcase of the compressor is essentially a hydraulic pump. The piston movesin the cylinder and pulses the hydraulic fluid in the head producing an oscillatingmovement of the diaphragm.

The diaphragm group consists of three diaphragms clamped and sealed at theperiphery between the gas plate and orifice plate.

The orifice or channel plate has the role of distributing the hydraulic fluid uniformlyunder the diaphragm and the gas plate. Contains the suction and discharge check-values. They are generally limited in output although some larger units are used inspecialized applications.

Pressures as high as 10,000 PSIG are attainable.

Diaphragm Compressor

7. Scroll Compressors

In a scroll compressor, two spiral-shaped members fit together, forming crescent-shaped gas pockets. One member remains stationary, while the other orbits aroundthe stationary one. The orbiting motion causes continuous crescent-shaped gaspockets to be formed and become smaller in volume as they near the center of the

form.

Air is drawn into the outer pocket created by the two members, sealing off the openpassage. As the spiral motion continues, the gas is forced toward the center of thescroll as the pocket continuously becomes smaller in volume, creating higher gaspressures. When the compressed gas reaches the center of the fixed scroll member,it is discharged.


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Several pockets of gas are compressed simultaneously. The result is a smooth,nearly continuous compression cycle.

There is space remaining between the scrolls at the completion of the discharge phasebut this is not a capacity inhibitor like clearance volume on a reciprocating compressor.This is because this volume is never opened to the suction.

At present, scroll compressors are limited to relatively low capacities and pressures ofapproximately 100 150 PSIG.

Scroll Compressor Cycle

Scroll Compressor


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Hook and Claw Compressors The hook and claw compressor is an OIL-FREE,positive displacement, rotary machine.

Steps on two meshing rotors form the equivalent of cylinders in a reciprocatingcompressors, however no inlet or discharge valves are used.

Instead, gas enters the casing through an inlet port located at the center, between therotors. The gas is drawn is into a recessed zone of FIVE interconnected pockets onthe rotor. This pocket is then sealed off as the rotor turns past the inlet.

Hook and Claw Compressor

Raised protrusions on the opposite rotor enter the recessed pockets and force the gasinto the last central pocket, compressing the gas.

By the time all of the gas is in the last pocket, this pocket has rotated to the dischargeport, located at the top of the casing and near the center of the rotors.

Four of these cycles per revolution result from two series of pockets on each rotor.

Because the stepped rotor has no oil injection for sealing, close machining tolerances

are necessary for efficient operation. The steps on the rotor are separate machinedblocks, bolted to a cylindrical roller.

At present the largest of these machines has a capacity of about approximately 500CFM and pressures are limited to approximately 50 PSIG for single stage units, butcan be multi-staged for higher pressures.


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Two-lobe and Three-Lobe Rotary Blowers (often called Roots principle blowers) usestraight mating lobed impellers to trap gas and carry it from intake to discharge. Thereis no compression or reduction of gas volume during the turning of the rotors.

Compression is by backflow into the casing from the discharge piping when thedischarge port is uncovered. Then the compressed gas is displaced into the dischargesystem. These are generally single-stage units used for pressures up to about 12 15PSIG. Three-lobe designs are also available, but operation is similar to two-lobedesigns. Sealing is by close clearances and lubrication is not required within thecompression chamber.

GAS DISCHARGE

GAS INTAKE

Rotary Lobe Blowers

8. Liquid Ring Compressors (or liquid piston) compressors use a rotor with multipleblades driving a captive ring of liquid (usually water) around the inside of an ellipticalcasing. The liquid acts as a piston to compress and displace the gas. Discharged gasis saturated at the discharge temperature. Excess liquid is removed by passing thegas-liquid mixture through a baffle or centrifugal separator to remove the free liquid.Final discharge temperature of the gas can be close to the temperature of the inlet

cooling water, providing a continuous flow of cool compressed gas.

Liquid Ring Compressor


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Rotary Dynamic Compressors

Dynamic compressors are centrifugal (radial) type or axial types and gain their namebecause they operate on the principle of DYNAMIC compression. Although this term soundscomplicated, their design is very simple and straightforward.

Dynamic compression takes place when the velocity of the gas is accelerated to a very highspeed, and then passed through a diffuser that decelerates (slows down) the high-speed gasresulting in a pressure increase.

This is different from positive displacement compressors because it does not use acompression chamber to trap consecutive volumes of gas.

As their name implies, flow through radial or centrifugal compressors is in a radial direction,while flow through an axial compressor is in an axial direction.

For the most part, dynamic compressors are not widely used for gas compression, except forlarge capacities.

Radial Flow Centrifugal Compressor

Axial Flow Compressor

CANADIAN PURCELL MACHINERY

400 Industrial Road ACranbrook, BC V1C 4Z3

Phone: (866) 517-3180Fax: (250) 417-3183

Email: [email protected] Site: www.canadianpurcell.com

Copyright by Canadian Purcell Machinery. All rights Reserved. No part may be reproduced, stored in a retrieval system or transcribed inany form without prior written permission of the Company.

selecting a compresor for process & speciality gas applications

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