Compressors, running towards elevators, and dreaming of curly hair

In my last posts, we spent some time learning about steam turbines, and how useful they are because they can produce mechanical work.

Today we will learn about compressors, what they are, and why or how they are used in industry. Particularly, the petrochemical industry.

Close your eyes and think back to those cartoons or tv shows where the Mad Scientist is at his lab. There are glass beakers, Bunsen burners, glass tubes, green or blue liquids boiling with thick white vapor coming out.

That is exactly what is happening inside a petrochemical facility. We are using heat, pressure, condensers, distillation columns. Instead of working with milliliters, we are working with thousands of gallons or measuring thousands of barrels. Instead of using glass, we are using stainless steel.

We are using heat and pressure to separate, purify, refine, and rejoin chemical compounds to make everything from fuels, to fibers, to plastics, to fertilizers.

In a large petrochemical complex, gases and liquids need to be moved from one place to another or need to be processed under heat and pressure. For this, we use pumps and compressors.

It turns out that the same guys who figured out the laws of thermodynamics and played with steam turbines for extracting energy from steam, also dabbled with compressors.

Before we get into the personalities, let’s talk about compression first.

What is compression?

I am going to narrow my explanation to focus on gases or compressible fluids. This is where my experience is. I know how pumps work, but I would not claim to be an expert in that area.

Being an engineer and knowing stuff is sometimes like what they say about owning a boat. You don’t have to know everything; just have a friend that knows what you don’t.

There are two main types of compression:

1. Positive displacement

2. Dynamic compression

I will attack positive displacement, because it is the one I understand most easily.

Positive Displacement

Imagine an empty syringe (without the needle please), and it is full of air.With the piston extended, put your thumb on the nozzle and press tightly to make a seal. Now press the plunger or piston.

You are compressing the air!! You are a compressor, a positive displacement compressor!

You took a specific amount of air and reduced its volume. You did not let the air escape.

When you began pressing on the plunger, you may have felt the air push back, almost bounce like a cushion. These are the air molecules reacting, pushing back on you pushing them together.

This was described eloquently by Robert Boyle, an astute Irishman that loved observing natural phenomena. In the year 1662, while playing around with glass tubes half filled with air and mercury, Boyle observed that the volume of air trapped in a syringe decreases in exact proportion to its increase in pressure!

Robert Boyle: Irishman and natural philosopher

This idea is now immortalized by humans and known as Boyle’s Law:

P1 V1 = P2 V2

P1 and V1 are the pressure and volume of the air before we push on the plunger.

P2 and V2 are the pressure and volume of the air once we have compressed the air.

(This is true as long as the air stays at the same temperature.)

So, this was the simple way to compress gases by the principle of positive displacement, where we change the volume of the container that holds the gas.

Now to the one that I always had a bit more trouble visualizing.

Dynamic Compression

This took me a while, but I think I found a good way to illustrate this.

Imagine watching a crowd of people running towards a closing elevator!

The moment they see their elevator starting to close they sprint, hoping to sneak a foot or a hand between the doors before they shut.

You watch them sprint, but as they get close to the entrance, since there are many of them, they don’t want to crash into each other or the other people already inside the elevator.

So, they decelerate and slow down at the last minute, with a classic two-footed slide move we’ve all done at least once in our lives.

They get packed and try to arrange themselves inside the elevator.

The more people get on that elevator, the higher the stress and tension of the passengers!

We all now how the stress and pressure rises in a situation like this.

That is how dynamic compression works!

An impeller or a set of blades in a compressor pushes, flings, accelerates the gas. It transfers kinetic energy into the gas.

The gas exits the rotating stage, moving fast, and then travels through a carefully designed maze or diffuser. This is designed to decelerate the gas, but we must respect the first law of thermodynamics and energy conservation. As the gas now decelerates and slows down, its pressure increases.

This magical exchange and conservation of energy all happens inside the diffuser!

And if you are familiar with centrifugal compressors, diffusers don’t look like much. They are literally just a small gap, where gas flows through. Diffusers must be the most underrated design component in turbomachinery.

In the next post, we will dive deeper into the magical world of a compressor’s geometry.

For now, let us contemplate these two methods of compressing gases and let me dream of having hair like Mr. Boyle.