Double trouble tonight
Double trouble, double, double trouble tonight
Double trouble, double, double trouble tonight
(I'm in) Double trouble tonight

("Double Trouble," written by R.Yacoub, A. Birgisson, and S. Kotecha for the 2020 movie "Eurovision," sung by Will Ferrell and Molly Sanden)

 

HOW MULMULTI-STAGE CENTRIFUGAL COMPRESSORS BUILD UP THE PV PRESSURE OF GASES

The number of stages a Multi-Stage Centrifugal Compressor has is defined by the number of Impellers the Compressor has.

The design of a Multi-Stage Centrifugal Compressor is structurally similar to the Multi-Stage Centrifugal Pump which was featured in  PTOA Segment #176.

The gas that flows into the Suction Line  of a Multi-Stage Centrifugal Compressor is directed to flow into the Eye of the first Impeller, typically via a Guide Vane.

The nearby graphic of a 3-Stage Centrifugal Compressor indicates the Suction Line is on the right side of the Compressor because the gas flow is guided into the Eye of the first  Impeller.  The Discharge Line is on the left side of this 3-Stage Centrifugal Compressor.

Look closely! There is a second hint that reveals which end of the Compressor is which:

Note the small line of flow drawn from the bottom of the discharge side of the Compressor that flows way over to the right side of the Compressor where the Axial Thrust Bearing is located.

A "gas balance line" uses the PV Discharge Pressure of the gas to help mitigate the axial thrust of the Compressor.  This  Multi-Stage Centrifugal Compressor design helps to stabilize the thrust tendencies of the Rotor.

Axial and Radial Thrust Bearings were introduced in PTOA Segment #182.

The nearby graphic of a naked 4-Stage Centrifugal Compressor Rotor indicates the Suction Line would be located on the left side and the Discharge Line would be located on the right  side … because that is what the labels infer.

In the 4-Stage Centrifugal Compressor Rotor graphic, the gas flow through the four stages is emphasized as outwardly flung gas that makes a U-turn and hence flows into the Eye of the next Impeller.

Note: Although just one line of flow is emphasized, the same flow path is happening  around all 360 degrees of shaft rotation!

Why does the gas make a U-turn and enter the next Impeller?  Why doesn't it leak into a different compressor area?

In a Multi-Stage Centrifugal Compressor, Diaphragms and Diffusers control the flow path and PV Pressure buildup as the gas flows through each successive stage of the Compressor (aka flows through each Impeller).

 

Diaphragms and Diffusers

Understanding Diaphragms and Diffusers takes a little brainwork.

The Diaphragm is the stationary hardware which creates the Diffuser passageway.

The Diffuser slows down the velocity of the gas that is being flung outward by the spinning Impeller. Ergo, the velocity of the gas is swapped into the PV Pressure within each interstage Diffuser passageway.

Then the Diffuser passageway forces the gas flow to make a U-Turn.

The nearby graphic once again uses the example of a  Single-Stage Centrifugal Compressor to clarify the the concept of a (Radial) Diffuser.

PTOA Readers and Students will recall that a Single-Stage Centrifugal Compressor does not have a Diaphragm but rather depends upon the Volute to convert the velocity of the compressed gas into the PV Discharge Pressure.

PTOA Readers and Students should note that the labelled (Radial) Diffuser is simply a passageway created by the Volute hardware which surrounds it.  Note that the Diffuser passageway surrounds the outer edge of the Impeller.

In a Multi-Stage Centrifugal Compressor, the job of the Diaphragm is to create separate chambers for each Impeller, thus guiding the outward flow from each spinning Impeller into the Diffuser passageway which hence guides the gas flow out of the Diffuser passageway into the Eye of the next Impeller.

In other words:

The Diaphragm is a stationary hardware element situated between stages of a Multi-Stage Centrifugal Compressor, typically with guide vanes to direct the flow into the next Impeller.

 

Adjacent Diaphragms form the Diffuser passageway which surrounds the Impeller. The velocity of the gas is swapped for PV Pressure within each Diffuser passageway.

The difference in design between Horizontally-Split Casings and Radially-Split Casings is explored in the next PTOA Segment.

A nearby schematic illustrates the Diaphragm of a Horizontally-Spit Multi--Stage Centrifugal Compressor Casing (top) and a Radially-Split Multi-Stage Centrifugal Compressor Casing (bottom).

In a Horizontally-Split Centrifugal Compressor Casing, the Diaphragm may be intricately fabricated into the top and bottom halves of the Casing.

In a Radially-Split Multi-Stage Centrifugal Compressor Casing (aka "Barrel Casing"), the Diaphragm will be made from circular ring structures. The schematic shows an (unlabeled) Impeller on the right, a Diaphragm on the left (arrow pointing to it), and a (unlabelled) Guide Vane sandwiched in the middle.

The gas discharged from the final stage of a Multi-Stage Centrifugal Compressor flows into a final, voluminous Diffuser passageway where the velocity of the gas is once more slowed down, swapping gas velocity for a final gain in PV Discharge Pressure.

Half of the PV Pressure gain over the Compressor occurs incrementally at each interstage Diffuser and half of the gain in PV Pressure occurs in the final Diffuser/Discharge Line.

 

TEMPERATURE LIMITATIONS OF MULTI-STAGE CENTRIFUGAL COMPRESSORS

As all PTOA Readers and Students can recite in their sleep …

Unlike liquids, gases are "Compressible." The gas particles grow increasingly hotter and agitated upon compression. Any increase in PV Pressure results in a corresponding increase in the PV Temperature.

The increased thermal activity of the gas particles results in increased frequency of gas particles banging into the interior surface of the Discharge Line. Thus ...

The diameter of the Discharge Line will be ill be significantly greater than the diameter of the Suction Line.

Furthermore, the number of compressor stages (aka Impellers) in a Multi-Stage Centrifugal Compressor is limited by the PV Temperature increase of the gas.

The work around to achieve higher PV Pressures is to install Multi-Stage Centrifugal Compressors in series with intermittent Fin-Fan Heat Exchangers to cool down the PV Temperature of a discharged gas before the gas is allowed to flow into the next Multi-Stage Centrifugal Compressor. Intercooling of compressed gas was featured in PTOA Segment #218.

 

MULTI-STAGE CENTRIFUGAL COMPRESSOR HARDWARE

PTOA Readers and Students should be able to deduce the following information from the nearby cutaway photo of a 5-Stage Centrifugal Compressor:

• The Suction Line is on the left, the gas is guided into the Eye of the First Impeller. The First Stage Impeller is on the left.
• The Diaphragm appears to be part of the Casing which is outlined in red. The Diaphragm creates individual chambers between each of the five Impellers.
• Each Diaphragm creates separate Diffuser passageways which surround each Impeller. The gas flung from the Impeller will make a U turn in each interstage Diffuser.
• The Diaphragm guides the gas flow into the Eye of the next Impeller (Admittedly, that statement cannot be deduced from the inactive drawing).
• The gas that exits the last Impeller enters a much larger final Diffuser volume prior to exiting via the Compressor's Discharge Line. The final Diffuser slows the gas down and is responsible for creating approximately half of the increase in PV Pressure as the gas flows through the Compressor.
• The Discharge Line has a significantly greater diameter compared to the Suction Line.

 

This PTOA Segment featured the form and function of Multi-Stage Centrifugal Compressors.  Of course, the spinning Rotor and its related hardware and auxiliary systems are crucial to generating the centrifugal force which dynamically increase the PV Pressure of the gas. The next PTOA Segment features maintenance considerations  of the Rotor and its associated hardware and the Operating Range of Multi-Stage Centrifugal Compressors.

 

TAKE HOME MESSAGES: The number of stages of a Multi-Stage Centrifugal Compressor is defined by the number of Impellers threaded on the Shaft of the Compressor. This PTOA Segment featured schematics of 3-Stage, 4-Stage, and 5-Stage Centrifugal Compressors  to illustrate the form and function of Multi-Stage Centrifugal Compressors.

Centrifugal Force created by a spinning Impeller increases the velocity of a compressed gas.  The Volute in a Single-Stage Centrifugal Compressor and the Diaphragm and Diffuser passageways in a Multi-Stage Centrifugal Compressor convert that gas velocity into the PV Pressure.  Without the Volute or Diaphragm and/Diffuser passageways, the PV Pressure of the gas would not increase.

The Diaphragm is a stationary hardware element situated between stages of a Multi-Stage Centrifugal Compressor, typically with guide vanes to direct the flow into the next Impeller. Adjacent Diaphragms form the Diffuser passageway which surrounds the Impeller.

The Diffuser passageway slows down the velocity of the gas after the gas has been flung outward from the Impeller and then forces that gas to make a U turn so that the gas flows within the Diaphragm toward the Eye of the next Impeller.

In a Horizontally-Split Centrifugal Compressor Casing, the Diaphragm may be incorporated into the interior of the Casing or be an insert that fits into the Casing. In an Axially-Split Centrifugal Compressor Casing, adjacent circular Impeller-Guide Vane-Diaphragm combinations create the Diffuser passageway.

Half of the PV Pressure gain is incremental through the stages of the Multi-Stage Centrifugal Compressor and half is developed in the final, voluminous Diffuser passageway which leads to the Compressor Discharge.

Hardware associated with the Multi-Stage Centrifugal Compressor include:

• Suction Line
• Guide Vane

• Impellers
• Eye of the Impeller
• Diaphragm
• Diffuser Passageway
• Final Diffuser (where half of the PV Pressure gain is developed)
• Discharge Line

 

Rotors are featured in the next PTOA Segment.

The Discharge Line will have a greater diameter than the Suction Line.

 

©2021 PTOA Segment 0220

PTOA PV PRESSURE FOCUS STUDY AREA
PTOA ROTATING EQUIPMENT AREA - DYNAMIC AND POSITIVE DISPLACEMENT COMPRESSOR

 

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