A knight’s tale about compressors and a plug on stoichiometry (or magic math as I call it)

A few weeks ago, I was trying to illustrate how centrifugal compressors actually increase the pressure of a gas by accelerating and then decelerating a gas through its impeller and diffuser.

It was extremely difficult to find a way to visualize what happens inside a compressor’s diffuser and correlate it to that exchange in velocity and pressure. And I appreciate all my friends who tried to help and encourage me to find even better ways to describe things.  This makes me appreciate engineering and science even more: the fact that there are still principles in nature that we cannot completely see, not even with our mind’s eye; and we must rely on equations and theoretical principles to describe them.

Then, a few days later I remembered reading a great description and comparison not only for how centrifugal compressors work, but also axial flow compressors.

If you love engineering, if you love airplanes, if you love turbines or engines, if you appreciate history and admire people that have curious and adventurous lives, then you should read the book Not Much of an Engineer, the autobiography of Sir Stanley Hooker.

AMAZON LINKL https://a.co/d/h4hgxMj

In this book you will learn about the life of this mathematician who, led by his curiosity, pretty much single-handedly took the Rolls Royce Merlin engine which was used in several WWII aircraft and tuned it and tripled its power output. He went on to perform other great feats for gas turbines and Rolls Royce. He was pals with the guys who invented and produced the very first gas turbine.

Yeah, he was drinking buddies with another Knight, Frank Whittle! Yes! THE Frank Whittle, the father of the Whittle Engine, the first practical turbojet engine that went into production to power an aircraft.

Anyhow, this is all mind-blowing stuff. And while jet designers were inventing gas turbines, they were simultaneously trying to figure out which type of compressor would allow them to harness as much energy from the combustion of gas to produce the most power and thrust. They were playing with both radial compressors (also known as centrifugal compressors) and axial flow compressors.

This brings us back to the explanation in Sir Stanley’s book, Chapter 5 which is titled “Axials”. Here Sir Stanley explains in very few but elegant and precise words how compressors work, along with more complex concepts like compressor surge, stall, and even resonant vibrations that affect axial compressor blades.

Sir Stanley basically says, and I paraphrase in non-knightly English:

Compression is like sweeping or driving water up a sloped ramp with a broom.

Imagine sweeping water up a wheelchair ramp with a broom.

Take the broom and try to push as much water as you can, through the entire ramp, all in one swoop! This is how centrifugal compressors work. In one “Stage” you can “push” a gas and achieve the desired compression of a gas.

Now imagine having several small brushes, and instead of making one big sweeping motion, you take these small brushes and in small but quick successive strokes, you push the water up the ramp. This is how axial flow compressors work.

Axial flow compressors are made up of multiple stages or rows of stator (non-rotating) and rotating blades, and each consecutively pushes/accelerates a gas and through the principle of dynamic compression, increases the pressure of the gas, stage by stage.

(There is so much more in this book that I can only insist that if you are an engineer and have to work with or study gas turbines or compressors, you should really read this book.)

I love this explanation because it helps make a distinction between centrifugal and axial flow compressors.

Centrifugal compressor impellers have the capability to achieve pressure increases (or what we call a pressure ratio) higher than a single axial flow compressor stage.

Yet axial flow compressor stages can achieve higher mass flows than a centrifugal stage.

This is why each design has its benefits depending on the application.

Today in petrochemical processes, high pressure ratios are desirable because they allow chemical reactions to occur more efficiently. So, if we are breaking down molecules or synthesizing others, high pressure is your friend.

On the other hand, in power generation gas turbines, what we want the most out of its compressors is high volumes of air to run lean combustions. When an engine runs lean, it means it has excess air. If an engine runs rich, it has excess fuel.

There is a very special ratio between how much air and fuel should be mixed to accomplish ideal complete combustion. This is called the Stoichiometric Ratio.

Since we want to be good to the environment, we want to ensure gas turbine engines run lean, so there is excess air to promote as much combustion of the fuel as possible. This is one way to reduce emissions.

Interesting fact: the term Stoichiometric is not someone’s last name. It has Greek roots, and it is the combination of two terms:

"Stoicheion" (στοιχεῖον) meaning "element" or "fundamental"

"Metron" (μέτρον) meaning "measure"

It was coined in 1792 by a German gentleman called Jeremias Benjamin Richter who had a passion and a good eye for balancing chemical reactions. If you studied chemistry in school, surely you had to balance chemical reactions like they were funny math equations or what I called magic math. Well, this is the guy we owe that to!

Back to compressors.I want to leave you with a basic side by side comparison of these two compressors used in both industrial applications and in aerospace applications.

When we look at side by side comparisons, we must keep in mind that we must create ranges or talk about typical or general applications or features. There will always be some very specific cases or applications that may not conform to a general comparison.

Another useful way to compare the qualities of these different types of compressors is by looking at a chart that plots pressure ratio vs. flow.

You can search for these on the internet and may run across dozens of charts. One prominent reference will be to a book called Centrifugal Compressors: A Basic Guide by Dr. Boyce. A second reference will be Gas Processors Suppliers Association: Engineering Data Book. I recommend getting a copy of Dr. Boyce’s book if understanding compressors is a part of your life.

These charts will plot Flow in the X axis and Pressure Ratio on the Y axis.

I am going to sanitize and non-dimensionalize my version of this chart, so I don’t get into any copyright issues. 

What I want you to see is that nothing beats a reciprocating piston when it comes to achieving high pressure ratios.

Axial flow compressors are built for flow and require several stages of blades to accomplish their work. To give you some perspective, a heavy industrial gas turbine may have 17 stages of axial flow compression resulting in a pressure ratio of 16:1 or 18:1.

In contrast, centrifugal compressors are very versatile and command the center of that chart. Making them ideal for chemical processing.

Finally, screw compressors, which I have not introduced yet. I often describe these as meat grinders for “air”. I look forward to explaining how these work one of these days.