The Rankine Cycle

In my last post, we met the guys that set the modern foundation for the steam cycle.

Today we are going to dive a little deeper into what happens with water and steam in a Rankine cycle.

I spent a lot of time this morning creating this graphic.

I call it “a steam turbine enthusiast’s Rankine cycle illustration” or “the Rankine cycle that makes engineering textbooks jealous”.

Let’s skip the pleasantries and dive right in.

In my last post, I literally said:

The cycle starts with water, water is pressurized, and then turned to steam. Steam will travel through the turbine converting/transferring power to the rotor, and then it will condense back to water so it can start the cycle all over again.    -me

1: it starts with water

This magnificent cycle starts with water. Not just plain water. Since we are going to be heating it up, and pushing it through pipes and things made of metal, we want to make sure it’s clean and not prone to causing corrosion or buildup.

We’ve all seen or experienced that nasty calcium buildup in showers, dish washers, and hot water heaters. Regular tap or potable water can contain a lot of minerals. The same way we install filters or water softening systems in our homes applies to industrial applications, especially steam power plants.

A lot of effort is put into keeping steam turbine water treated in the most perfect way. Some may even call it the diva of waters, and that is meant in the best way possible.

Therefore, the water that is used in a steam cycle is treated to the following qualities:

1. High purity and demineralized We need to make sure this water is free from any contaminants like chlorides and other solids to prevent corrosion.

   
   

   We need to make sure the water is free from dissolved minerals like calcium to prevent scaling and buildup. This is done using reverse osmosis filters like the ones we may install under our kitchen sink. Other methods include ion exchange or distillation.
   

2. De-aerated We need to make sure there is no air or other gases (oxygen, carbon dioxide) trapped in the water. These can cause corrosion and diminish optimal heat transfer in the boiler.

   
   

3. Chemically treated We must maintain a delicate pH balance within the steam cycle to prevent equipment and pipe corrosion.

   
   

So, back to the cycle, we start with water. This water needs to be pressurized as we go from steps 1 to 2.

To engineers W means work!

Another interesting thing about engineers is that a dot over a letter means “rate of transfer!”

Nerd fact: This correlates also to how we define a time derivative in calculus (my favorite subject in school!).

So, the dot over the W means “rate of work done,” which represents the fact that we are putting work into the water by increasing its pressure!

In the Rankine cycle, this is the only moment we add work; this is the only place where we increase the pressure of the fluid in the system.

To me this is important because, being a turbine enthusiast and always hanging around other turbine enthusiasts, it seems all the attention goes to the turbine. But we must recognize the importance and the work done, the heavy lifting (pun totally intended) by the boiler feedwater pump.

To the pump, the silent hero of the cycle, who increases water pressure ratio anywhere from 1:20 to 1:200 or even higher…we remember you!

In this step, we take water in liquid state and pressurize it to the same working pressure that we will expect the steam to get to when we inject it into the turbine.

2: the boiler

Now we have clean, demineralized, high-pressure water and we are going to turn it into steam.

Modern boilers are engineering marvels of their own. In my diagram I am oversimplifying it by illustrating it as a rectangle, where we burn fuel to make fire and heat the water travelling through a serpentine coil.

In the boiler we will turn the water to steam. The goal is to get it to pure steam, with no water content. This is called dry steam! This may be hard to visualize, since we know that, after all, steam is made of water. How can it be dry?

For instance, when we make coffee or tea, we see a tiny plume of steam coming from the kettle. This is wet steam, and the clouds we see rising from our teacups are tiny droplets of water.

Now, imagine we heat that cloud of steam even more until all the tiny droplets disappear, essentially turning it into ALL gas! THAT is what we want to go through our steam turbine!

This is the steam that has the most energy stored in it. Remember, the latent heat of vaporization of water allows it to store tons of energy in its steam!

To engineers Q means heat!

(The internet does not know the exact origins for this, but we know for sure the letter H was already taken by the guy that invented the concept of enthalpy. We also must consider that some of these discoveries and definitions go way back and were done in other languages, not English.)

That Q and the arrow pointing to the boiler, means this is where we ADD energy to the cycle. To heat up water we need a source of heat: it can be wood (biomass); it can be coal; it can be natural gas; it can be heat from something else (like a gas turbine). The fact is, we need energy to heat up the water. This is the only step in the cycle where we add energy to the system.

3: Superheated dry steam

Engineers often use the word super to emphasize that something is above and beyond the standard or expected levels.

For instance, supersonic, superconductor, superalloy, superheated.

At the exit of the boiler, we have the best, most energy-packed steam we can get.

Now we are finally ready to enter the turbine!

Before steam can enter the turbine, it must pass through some gates or valves.

The first valve is called the “trip valve”. The purpose of this valve is, in case of an emergency, to shut closed as quickly as possible to interrupt the flow of steam. These valves are built with massive springs, so that they fail close, like a giant mousetrap; they always close when necessary.

The next valve is a throttle valve. It’s really a system or assembly of valves that help “regulate” the amount of steam entering the turbine, the same way we put our foot on the accelerator of our car, and that in turn opens a tiny valve that lets in more fuel into our engine. The throttle valve lets in the steam that will do the work of converting the energy in the steam into mechanical energy.

Once the steam finally is allowed to enter the work envelope, it travels through a nozzle. The nozzle is basically like the showerhead in our shower. It helps accelerate steam, because it travels through a nozzle. It also points the steam to the rotating blades. When the steam passes through a set of rotating blades, the steam hits the blade airfoils, and this is where the magical energy transfer occurs. The steam changes direction and the blades on the rotor push the rotor and make it turn!

We will dive deeper into how this happens in my next post. For now, let’s keep it at this level so we can continue through the cycle.

Every time steam travels through a nozzle, a set of stationary blades, or a rotating blade, its pressure will decrease, and the steam will begin to expand. As the energy is extracted, the steam will begin to condense and cool down.

Once we have extracted as much energy as possible from the steam, it is ready to let it go out of the turbine. We do not want to condense the steam inside the turbine so that too much moisture or water droplets form. Water in a steam turbine can cause a lot of damage in the form of erosion or steam cutting.

Inside the steam turbine, the rotor and blades are moving fast, so if they hit water droplets, they can erode the metal, just as we can damage the paint in our car or cut through things if we get carried away with a pressure washer.

4: wet steam

We’ve now exited the steam turbine. The steam or vapor exiting the turbine is a mixture of water and gas.

Before we can do anything with this mixture, we must condense it all back to water so the cycle can start all over again.

The best way to condense steam is by cooling it in a device called a condenser. This is essentially a large cooler, where we run cold water through some pipes inside a large tank, so the steam condenses back into water— the same way a glass of cold water will form droplets on its outer surface.

And voila, we’ve made a full round trip through the Rankine or steam cycle.

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“But wait a minute!” you might say. “We put work in! We put heat in! to get work out! And then get heat out!”

Does that make sense? We put a lot of effort and energy in in order to get energy and effort out?

Well, that is the fundamental principle of Thermodynamics! It is all about energy conversion.

There is no free lunch, unfortunately. Remember, this whole journey is because we need machines that can produce energy or do work for us. If we don’t use machines, we must do work by hand or by using animals or windmills.

The thermodynamic principles behind the Rankine cycle and the fact that water has that capacity to store energy allow us to get more useful energy out of the system than the energy we had to put in.

In simple terms, the ratio of work and energy we put into the steam cycle over the work we get out is called… YES you guessed it, efficiency!

Despite our best efforts, the steam cycle is about 50% efficient. This means half of the energy we are capable of producing goes to the water pump, and it’s lost in inefficiencies. Inefficiencies everywhere!

Engineers are good people, trust me, but we are unable to create machines that are “ideal” or “perfect” that are free of losses. We must keep pushing, improving, always improving.