HELLO AXIAL FLOW COMPRESSOR … MY OLD FRIEND!

Old friends, old friends
Sat on their park bench like bookends

("Old Friends," by Simon and Garfunkel, 1968)

DYNAMIC AXIAL FLOW COMPRESSORS

The handy-dandy nearby chart labelled "Compressor Types" is a reminder that the Dynamic Compressor Family branch on the right side of the chart includes two lineages, Centrifugal Compressors and Axial Flow Compressors.

By this point all PTOA Readers and Students should be able to prepare a short presentation explaining why the descriptor "Dynamic" infers that the PV Pressure of a fluid is increased by first increasing the velocity of the gas and then slowing the velocity of the gas down to convert the velocity into the PV Pressure. The PV Pressure↔Fluid Velocity Swap was explained in PTOA Segment #159.

The form and function of Single-Stage Centrifugal Compressors were featured in PTOA Segment #219.

The form and function of Multi-Stage Centrifugal Compressors were recently featured in PTOA Segments #220 and #221.

This PTOA Segment features the optimal process service applications of the Axial Flow Compressor and reviews the popular application of an Axial Flow Compressor as a component in the Prime Move/Driver known as a Gas Turbine. The next PTOA Segment features the form and function of Axial Flow Compressors.

Axial Flow Compressors do not have Impellers. Impellers are hardware that is associated with Centrifugal Compressors. .Impellers increase the velocity of a gas by throwing the gas radially outward.

Axial Flow Compressors have Rotor Blades that rotate with the Shaft. Each Rotor Blades\ is paired with a Stator Blade,

A Stator Blade is a stationary blade (aka non-moving) that is attached to the interior of the Compressor Casing..

Each Rotor Blade-Stator Blade combo makes a "stage" in the Axial Flow Compressor. The compressed gas is buffeted between the stages of Rotor Blade-Stator Blades which creates a zig-zag flow pattern down the axis of the Shaft.

The Rotor Blades, Stator Blades and internal components of an Axial Flow Compressor are susceptible to corrosion, pitting, and deposits. For this reason Axial Flow Compressors are used to compress clean gases, like filtered air and natural gas.

A QUICK BRAIN JOG DOWN MEMORY LANE

The Compressor in the Gas Turbine "squeezes" …aka compresses … the intake air; The compressed air flows into a Combustor and hence the Turbine sections of the GT.

Brilliant PTOA Readers and Students … meaning those who are reading the PTOA Segments in the intended, sequential order … already learned about the most popular industrial application of the Axial Flow Compressor.

Axial Flow Compressors are a crucial component of the Prime Mover/Driver known as a Gas Turbine (GT). Gas Turbines were featured in PTOA Segment #195 through PTOA Segment #201.

PTOA Readers and Students already learned that the Axial Flow Compressor "squeezes" ((aka "compresses") air that is sucked into the GT.

The compressed air is subsequently heated in the GT's Combustor and hence expanded in the GT's Turbine. While being rapidly expanded through the blades of the Turbine, the exhaust gas creates the rotary motion used to drive a Load.

When the Load is an iron core wrapped in copper wire, the Load is called an Electricity Generator because electricity is generated when an iron core wrapped in copper wire is rotated.

PTOA Readers and Students who do not remember how electricity is generated in this manner should review PTOA Segment #187 which featured the Prime Mover/Driver known as a Motor.

Gas Turbines are a "Bridge-Technology" to Green Power Production

Natural gas is mostly made up of Methane. Methane has one Carbon and "holds hands" with four Hydrogen atoms. When burned, methane produces the least amount of carbon dioxide compared to other hydrocarbons but still does contribute to global warming

Every PTOA Reader and Student already knows that GTs are called "Gas Turbines" because an atomized hydrocarbon gas is ignited in the Combustor of the GT.

Natural gas is the most simple and therefore most clean-burning of all the hydrocarbon fuels. Yet the exhausted gas from the GT will still include hot water vapor, hot and partially combusted byproducts like Carbon Monoxide(CO) and nitrogen oxides, and of most concern Carbon Dioxide gas (CO₂),

Carbon dioxide contributes to "greenhouse gases" which warm the atmosphere.The oxides of nitrogen contribute to smog and air pollution.

Therefore, GTs cannot be classified as a "green source" of power generation.

However, GTs will provide an important "bridge technology" as global economies move away from hydrocarbon based energies like coal and hydrocarbon oils to truly green energy sources like geothermal, solar, and wind energy. Fission and fusion nuclear energy will also play a larger role in the power industries.

The take-home message is that Axial Flow Compressors and GTs will be around for the career lifetimes of PTOA Readers ands Students and thus are well worth learning about.

NOT ALL GAS TURBINES HAVE AXIAL FLOW COMPRESSORS

The GT on the left is the Prime Mover/Driver for an Electrical Generator Load. The Single-Shaft GT will spin the Generator at a constant 1800 rpm. In this case the Compressor will be an Axial Compressor because of its superior efficiency. The GT on the right is spinning an unlabeled process load, like a Compressor or a Pump. The GT Shaft is split so the Load can be rotated … aka driven … at variable speeds. A Centrifugal Compressor might be a better fit for this process service.

Smaller GTs that are drivers for rotating equipment use Single-Stage Centrifugal Compressors instead of Axial Flow Compressors.

The Centrifugal Compressor is a better fit when the process application requires a variable-speed-driver that can change the rotation rate of the Load to respond to process fluctuations. Typically the Load will be sufficiently small to warrant use of a Single-Stage Centrifugal Compressor in the GT.

So … when will Axial Flow Compressors be used in a GT?

The Operating Efficiency of an Axial Compressor is 10% greater than the Operating Efficiency of a Centrifugal Compressor. Ergo, Axial Flow Compressors are less expensive to operate every day.

Generating electrical utility power requires rotating the Electrical Generator Load at a steady1800 rpms (or whatever steady rotation rate is required for local al power generation). In this application, operating efficiency is the name of the game!

The nearby graphic compares the efficiency of Centrifugal Compressors and Axial Flow Compressors as the intake flow rate Capacity increases.

The increased efficiency of Axial Flow Compressors translates into saving operating expenses every day.

This Compressor Operating Range Chart shows the limits of each type of Compressor in terms of inlet flow Capacity and Pressure Ratio.

The nearby graph compares the Operating Ranges of Positive Displacement Compressors, Centrifugal Compressors, and Axial Flow Compressors.

The X axis on this graph represents the gas flow rate into the Suction of the Compressor (aka, Capacity) measured in cfm (cubic feet per minute).

This Y-axis measures Pressure Ratio, a dimensionless number because::

(Compressor) Pressure Ratio =

The Discharge Pressure (psia)

÷ (Divided By)

The Suction Pressure i(psia)

The (Compressor) Pressure Ratio is often called "Compression Ratio."

Thus defined, this Operating Range Graph clearly shows that Axial Flow Compressors are used when:

The process application requires medium to very high Capacity AND
A Compression Ratio of less than 10 is warranted for the process application.

The Rotor Blades of GE's Harriet GT are in the foreground of the photograph. .

The Operating Range Graph also infers that an Axial Flow Compressor will have more stages (in the form of Stator Blade-Rotor Blade combos) to achieve the same Compression Ratio that a Single-Stage Centrifugal Compressor can attain.

For this reason, the Axial-Flow Compressor will have a smaller diameter than the Single-Stage Centrifugal Compressor that could be incorporated into a smaller GT. However, the Axial-Flow Compressor will always take up more foot-space in a Compressor House.

Even though the longer profile of the Axial Flow Compressor will take up more foot space, the capital investment made to accommodate the Compressor will be quickly paid out due to the increased operating efficiency of the Axial Flow Compressor.

TAKE HOME MESSAGES: Like the Centrifugal Compressor, the Axial Flow Compressor is a member of the Dynamic Compressor Family. Dynamic Compressors increase the velocity of the gas and then decrease the velocity of the gas, swapping gas velocity for the PV Pressure.

Axial Flow Compressors have stages made from pairs of Rotor Blades and Stator Blades. As the gas is buffeted between the Rotor Blades and Stator Blades it makes a zig-zag pattern down the axis of the spinning Shaft. Hence the name "Axial Flow Compressor."

Axial Flow Compressors can accommodate very high flow Capacity of the compressed gas. Otherwise stated, Acial Flow Compressors can move a much greater quantity of gas than either the Centrifugal or Positive Displacement Compressor. The operating efficiency of Axial Flow Compressors is also significantly greater than the Centrifugal Compressor.

The Compression Ratio of a Compressor is calculated:

The Discharge Pressure (psia)

÷ (Divided By)

The Suction Pressure i(psia)

Axial Flow Compressors can generate a sufficiently high Compression Ratio for most industrial process applications. However, when a very high Compression Ratio is needed for a process service, the Centrifugal Compressor and especially the Positive Displacement Compressor may be used.. .

The Axial Flow Compressor is meant to compress clean gases like filtered air and natural gas. The internal hardware of an Axial Flow Compressor would otherwise be corroded and pitted and hindered by deposits of a dirty gas.

The Axial Flow Compressor is lighter in weight and smaller in diameter than a Centrifugal Compressor. Since it takes several Rotor-Blade-Stator Blade stages to generate the Compression Ratio of even a Single-Stage Centrifugal Compressor, the Axial Flow Compressor takes up more floor space than a Centrifugal Compressor.

PTOA Readers and Students already "knowed-up" about the Gas Turbine. Axial Flow Compressors are a component in many Gas Turbines and especially the Single-Shaft GTs the are used to drive an Electricity Generator. Gas Turbines are an important bridge technology to the ultimate goal of achieving more green sources of power generation.

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

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