I'm a spy in the house of love
I know the dream that you're dreamin' of
I know the word that you long to hear
I know your deepest, secret fear

("The Spy," by The Doors, 1970)

 

FORM AND FUNCTION OF THE (DYNAMIC) SINGLE-STAGE CENTRIFUGAL COMPRESSOR

Brilliant PTOA Readers and Students just learned in PTOA Segment #218 that Centrifugal Compressors are the go-to Compressor when smooth delivery of gas and the ability to tolerate process fluctuations are required.

Centrifugal Compressors are also more reliable compared to Positive Displacement Compressors. "More reliable" means Centrifugal Compressors don't break down as often and thus require less attention and maintenance compared to Positive Displacement Compressors.

Brilliant PTOA Readers and Students … meaning those who are reading the PTOA Segments in the intended, sequential order …learned way  back in PTOA Segment #173 how Centrifugal Force creates the Centrifugal Action that dynamically builds up the PV Pressure in fluids.

Most of the below information about a Single-Stage Centrifugal Compressor should sound very familiar:

• Centrifugal Force is created because the Impeller is rotated by a Driver of some type.
• The gas which enters the Eye of the Impeller is captured between successive Vanes/Blades on the Impeller.
• Centrifugal Force flings the gas radially outward from the Eye of the Impeller to the outer diameter of the Impeller. The gas gains Velocity while it is flung outward.
• Diffuser Blades located toward the outer rim of the Impeller slow the Velocity of the gas down.
• The human-fabricated Volute … which is the interior side of the Compressor's Casing … is specifically fabricated to significantly decrease the Velocity of the gas that flows into it.
• The decreased Velocity of the gas is swapped for an increase in the PV Pressure. If that concept does not sound familiar, stop right here and reacquaint yourself with The PV Pressure ↔ Fluid Velocity Swap which was featured in PTOA Segment #159.
• The gas that is discharged via the Compressor Discharge Line will be at a significantly higher PV Discharge Pressure compared to the PV Suction Pressure.

 

PTOA Readers and Students … Be Aware!

Creating Centrifugal Force/Action alone would not build up the PV Pressure in a gas!

Without the Diffuser Blades located on the Impeller and especially the Volute, the gas that spins off the outer rim of the Impeller in a Single-Stage Centrifugal Compressor would just uselessly fly into space.

The Volute of the Compressor is intentionally fabricated to decrease the Velocity of the discharged gas. The decreased gas Velocity is swapped into an increased PV Discharge Pressure.

In other words …

The created Centrifugal Force PLUS the specially-fabricated Diffuser Blades and Volute work together to first increase the Velocity of the gas and then convert the Velocity into PV Pressure via the PV Pressure ↔ Fluid Velocity Swap.

 

 

Single-Stage Centrifugal Compressor Hardware 

The critical hardware components of a Single-Stage Centrifugal Compressor are labelled in the nearby schematic. In a brick and mortar educational facility, an UNLABELLED COPY of the nearby schematic would be given to students to fill in the blanks.

PTOA Readers and Students can refer to the labelled hardware in the drawing while reviewing the flow path of gas through a Single-Stage Centrifugal Compressor once again:

The Impeller can rotate because it is attached to a Shaft which is coupled to an (unseen) Driver.  The (Packing) Gland prevents leakage of the gas.

The gas is sucked into the Suction Line by the vacuum created by the rotation of the Shaft. The black arrow is supposed to be indicating that the gas is sucked into the Suction Eye of the Impeller.

The fast rotation of the Impeller flings the gas radially outward, hence the gas gains Velocity. The gas is slowed down by the Diffuser Plates situated near the outer edge of the Impeller.

The gas is significantly slowed down within the Volute, which is the internal side of the Casing.

The decreased Velocity is converted into an increase in PV Pressure.

The gas then exits the Discharge Line of the Compressor at a higher PV Pressure compared to the PV Pressure sensed at the Suction Line.

 

THE OPERATING RANGE OF A SINGLE-STAGE CENTRIFUGAL COMPRESSOR 

The nearby graphic illustrates the compared operating ranges of Compressors.

The X-axis of the graph indicates an increasing rate of Inlet Flow into the Compressor from the left side of the graph to the right side of the graph. The Inlet Flow is measured at the Suction Line in Actual  Feet per Minute (ACFM).

Don't stress about the descriptor "actual!"

The descriptor "actual" simply means that the flow rate measurement of the gas that has been sucked into the Compressor Suction Line has taken into account the real-world Temperature and real-world Pressure sensed within the Compressor Suction Line.

The Y--axis of the graph indicates increasing Compressor Discharge Pressure measured in psig. If the term "psig" is not familiar, review "Gauge Pressure" which was featured in PTOA Segment #150.

ALERT! Both the X-axis and Y-axis on the graph are not linear! The scales on both the X and Y axis increase rapidly! Each successive magnitude marker is ten times greater than the scale marker before it!

Find the box with the text "Single-Stage Centrifugal."  The "almost rectangle" that the label points to defines the operating range of a Single-Stage Centrifugal Compressor (aka a Centrifugal Compressor with a single Impeller). .

The graph indicates that the Inlet Flow to a Single-Stage Centrifugal Compressor is 80 to 1200 ACFM. This flow range is on the low-to-"just over mid range" of the graph.

The graph indicates that a Single-Stage Centrifugal Compressor can attain 1200 psig PV Discharge Pressure at 80 ACFM. The PV Discharge Pressure decreases below 1000 psig at the higher flow rate range of 1200 ACFM.

Guess what, Fred?

The Single-Stage Centrifugal Compressor depicted in the nearby graphic with labelled parts is a great teaching tool for explaining how a gas dynamically transforms its Fluid Velocity into the PV Pressure.

However, a Single-Stage Centrifugal Compressors would not be able to perform most Compressor job descriptions in a large processing facility. The operating range of Capacity and PV Discharge Pressure for a Single-Stage Centrifugal Compressor are both too low.

The nearby picture of a "Real-World" Single-Stage Centrifugal Compressor more accurately illustrates the magnitude of an industrial-sized machine compared to a human being. A utility use of a Single-Stage Centrifugal Compressor might be in an HVAC system.

In summary, most  industrial processing plants will require higher PV Discharge Pressure and more Capacity than a Single-Stage Centrifugal Compressor can create with its single Impeller.

Adding more Impellers (aka "adding more stages") increases the Capacity and PV Discharge Pressure of the Centrifugal Compressor.

The next PTOA Segment features Multi-Stage Centrifugal Compressors  A Multi-Stage Centrifugal Compressor is any Centrifugal Compressor with more than one Impeller.

 

TAKE HOME MESSAGES:  Centrifugal Compressors are selected when the process operation will be variable and continuous, non-pulsing flow is needed for the industrial process. Centrifugal Compressors are also more mechanically reliable than Positive Displacement Compressors.

This PTOA Segment details the dynamic operating theory which supports converting a gas's Velocity into the PV Pressure via:

• Creating Centrifugal Force/Centrifugal Action for the purpose of flinging a gas radially outward from the Eye of the Impeller to the outer rim of the Impeller.
• Incorporating Diffuser Blades at the Impeller's outer rim for the purpose of slowing the Velocity of the gas down.
• Fabricating a Volute to significantly slow the velocity of the gas further down which converts the gas Velocity in the PV Pressure. 

 

The critical hardware of a Single Stage Centrifugal Compressors which is needed to convert gas Velocity into the PV Pressure includes:

• Suction Line
• Eye of the Impeller
• Impeller with Diffuser Blades at the outer Rim
• Volute *the interior of the Casing) 
• Casing.
• Discharge Line

 

The Centrifugal Force is created by a Driver spinning the Shaft which rotates the Impeller. A (Packing) Gland may be used to prevent the gas from leaking out of the Single-Stage Centrifugal Compressor. 

One way to express the Capacity of a Compressor is Actual Cubic Feet per Minute (ACFM) measured at the Compressor Inlet. The descriptor "Actual" means the flow rate of the gas has been corrected for the real-world Temperature and Pressure sensed at the Compressor Suction.

Real-world Single-Stage Centrifugal Compressors are much larger than the small Single-Stage Centrifugal Compressors drawings used for instructional purposes. One real-world application of Single-Stage Centrifugal Compressors is in HVAC Systems.

The processing industries typically needs a compressed gas to have more Capacity and PV Discharge Pressure so the gas can perform a compressor job. For this reason most Centrifugal Compressors in a large processing facilities are Multi-Stage Centrifugal Compressors.

©2021 PTOA Segment 0219

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

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