What should I do?
Oh, oh
What should I do?
Oh, oh
What should I do?
Oh, oh
Feeling the pressure, pressure, pressure

("Feeling the Pressure," by the Dirty Skirts, 2013)

 

THE COMPESSOR FAMILY TREE 

OMG! Another family tree!

PTOA Readers and Students recently completed learning about every member shown in the nearby Pump Family Tree.

A quick glance over the Pump Family Tree and the Compressor Family Tree reveals many similarities.

Both the Pump Family Tree and the Compressor Family Tree have the same two main classifications … and the subsets of those lineage lines are likewise the same between Pumps and Compressors.

Specifically:

Both the Pump Family Tree and the Compressor Family Tree branch off into Positive Displacement and Dynamic lineage lines.

Ready for more similarities?

The two subsets of Positive Displacement Pumps are Reciprocating-Action PD Pumps and Rotary-Motion PD Pumps.

Likewise, the two subsets of the Positive Displacement Compressor family are Reciprocating-Action PD Compressors and Rotary-Motion PD Compressors.

PTOA Readers and Students already learned that two subsets of the Dynamic Centrifugal Pump family line are Radial-Flow Centrifugal Pumps and Axial-Flow Centrifugal Pumps.

So no PTOA Reader or Student is surprised by now to learn that the Dynamic Compressor Family lineage has subsets of Centrifugal Compressors and Axial-Flow Compressors.

PTOA Readers and Students will soon learn that the structural similarities of the Pump Family Tree and the Compressor Family Tree go beyond compared schematic drawings.

The operating theory of Dynamic Pumps is the same for Dynamic Compressors.  And the operating theory of Positive Displacement Compressors shares many similarities with the operating theory of Positive Displacement Pumps.

 

DEJA VU ALL OVER AGAIN LEARNING ABOUT COMPRESSORS

Truth be known, the brilliant PTOA Readers and Students … meaning those who are reading the PTOA Segments in the intended, sequential order … already possess a wealth of knowledge about the Rotating Equipment known as Compressors. Ergo, this PTOA Segment will serve more as a review rather than a true introduction to Compressors.

Compressors were described as the Rotating Equipment which increases the PV Pressure of gases in PTOA Segment #139.

The difference between Rotating Equipment and Stationary Equipment was clarified in PTOA Segment #161.

Back then PTOA Readers and Students learned that  …

Compressors compress gases and this is possible because  …unlike liquids … gases are "compressible."

Compressibility was defined in PTOA Segment #153. In a nutshell:

Compressibility means that the PV Pressure will increase when the Volume of a gas is decreased, and vice versa.

 

The relationship between the Volume, PV Temperature, and PV Pressure of a gas were explained via the "common sense gas laws" described in PTOA Segments #152, PTOA Segment #153, and PTOA Segment #154.

 

RECIPROCATING POSITIVE DISPLACEMENT COMPRESSORS

Reciprocating Positive Displacement Compressors increase the PV Pressure of a flowing gas by temporarily squishing the gas within the confines of a Cylinder.

 

 

The gas is thus compressed by a piston or plunger and these things happen:

• The gas molecules are shoved closer together in a smaller Volume.
• The gas molecules bang into each other more frequently, so their PV Temperature increases.
• The smaller the Volume, the higher the PV Pressure of the compressed gas and the higher the PV Temperature of the compressed gas … and vice versa …
• When the Volume of the gas increases, the PV Pressure decreases as does the PV Temperature of the gas.

 

The relationship between a changing PV Temperature and the  PV (Volumetric) Flowrate, Density, Specific Gravity, and Viscosity of a gas were featured in PTOA Segment #162.

As was mentioned above, increasing the PV Pressure of a gas will increase the PV Temperature of a gas and thus change the Density, Specific Gravity, Viscosity and Volumetric Flowrate of the gas that is being compressed.

 

 

DYNAMIC COMPRESSORS

The PV Pressure ↔ Fluid Velocity Swap featured in PTOA Segment #159 also applies to gases. The PV Pressure ↔ Fluid Velocity Swap was revisited in PTOA Segment #207 as "the Venturi Effect."

(Dynamic) Centrifugal Compressors increase the PV Pressure of gases by increasing the velocity of the gas as the gas is flung outward from the center of a spinning Impeller. Increasing the velocity of the flowing gas increases the PV Flowrate of the flowing gas and  thus decreases the PV Pressure of the gas.

Prior to exiting the Centrifugal Compressor  the PV Flowrate of the gas is decreased in the Volute of the Centrifugal Compressor's Casing, thus the gas that is discharged from the Dynamic Centrifugal Compressor at a higher PV Pressure.

 

The (Dynamic) Axial-Flow Compressor was one of the three crucial components in a Gas Turbine (the Combustor and Turbine are the other two crucial components). PTOA Segments #195 through #201 featured Axial-Flow Compressors within Gas Turbines. Gas Turbines are a crucially important Prime Mover/Driver that will be depended upon to bridge the gap to clean, non-carbon based energy sources.

The descriptor "Axial-Flow" infers that the flow of the compressed gas is more straight line flow down the axis of the Axial-Flow Compressor; the gas is not flung radially outward from an Impeller as occurs in a (Dynamic) Centrifugal Compressor.

The nearby Axial-Flow Compressor gif may appear like it also "flings gas radially outward." However, PTOA Readers and Students will soon learn that a series of rotating Rotor Blades and stationary Stator Blades guide the gas flow to more of an up-and-down, zig-zag motion as the gas flows down the axis of the Axial-Flow Compressor.

 

ISA SYMBOLS FOR DYNAMIC AND POSITIVE DISPLACEMENT COMPRESSORS

Dynamic Compressor ISA Symbols

PTOA Readers and Students can also identify the Dynamic Compressor ISA Symbol via extending what they learned about the Steam Turbine ISA Symbol in PTOA Segment #193. The Steam Turbine in the nearby graphic is driving the Air Compressor via a common shaft.

 

The gas entering any Compressor is going to get squished. Thus, the Volume of the gas is greater at Compressor Suction and much less at Compressor Discharge.

The vertical lines on the ISA Dynamic Compressor Symbol represent the Volume of the compressed gas at the Compressor Suction and the Compressor Discharge.

Thus the ISA Dynamic Compressor Symbol is cone-shaped with the taller vertical line at Compressor Suction and the smaller vertical line at Compressor Discharge.

The internal blades … or stages … also have successively smaller and smaller diameters as the flowing gas is compressed more and more with each stage.

Note that the below bird's-eye-view picture of GE's Harriet GT clearly shows the descending-radius cone shape of the Axial Compressor on the right and the increasing radius of a cone shape for the Turbine on the left.

 

Positive Displacement Compressor ISA Symbols

Below is a chart of ISA Symbols for Reciprocating-Action Compressors and Rotary-Motion Compressors. Your Mentor is familiar with Process Flow Diagrams (PFDs) and Piping and Instrumentation Diagrams (P&IDs) which represent Reciprocating-Action Compressors via the ISA symbols shown on the top left and bottom left of the chart.

 

The next PTOA Segment explains the duties and services of all Compressors and then explains which type of Compressor is best suited for each duty and service.

 

TAKE HOME MESSAGES: The two main types of Compressors are Positive-Displacement Compressors and Dynamic Compressors.

The ISA symbols for Positive-Displacement and Dynamic Compressors were revealed in this PTOA Segment. The Dynamic Compressor ISA Symbol is a cone with decreasing radius.  The vertical members on each side of the cone represent the Volume of the gas.  The longer vertical member is on the Suction Side of the Dynamic Compressor and the shorter vertical member is on the Discharge Side of the Dynamic Compressor.

The subsets of Positive Displacement Compressors are Reciprocating-Action Compressors and Rotary-Motion Compressors.

Positive Displacement Compressors build up the PV Pressure in a gas by compressing the gas. "Compressing a gas" means reducing the captured Volume of a gas because when the Volume of a gas is decreased, the PV Pressure of the gas increases. Additional outcomes of compressing a gas are a simultaneous increase in the PV Temperature, decrease in the PV Flowrate, and a change in fluid properties like Density, Specific Gravity, and Viscosity. 

The subsets of Dynamic Compressors are Centrifugal Compressors and Axial Flow Compressors. The operating theory of Dynamic Compressors is the PV Pressure ↔ Fluid Velocity Swap (also known as "The Venturi Effect").

Centrifugal Compressors swap PV Pressure and Fluid Velocity when the gas is flung radially outward from the Impeller, thus increasing the Velocity of the gas. The Velocity is then decreased in the Compressor's Volute, thus increasing the PV Pressure of the gas that is discharged from the Centrifugal Compressor.

Axial-Flow Compressors swap the PV Pressure and Fluid Velocity of a gas by the use of successive Stator and Rotor Blades which will be further explained in an upcoming PTOA Segment. The effect of the flow through a Axial-Flow Compressor is a more straight-line flow down the axis of the Axial Compressor shaft.

©2021 PTOA Segment 0216

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

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