The steam cycle

I have been meaning to do a post on the steam cycle, otherwise known as the Rankine Cycle. It's pretty simple to describe the four-step process. The complicated part is designing, maintaining, and operating the machinery that makes it happen continuously, year after year.

The water/steam cycle is the most practical and commonly used means of converting heat energy into electrical energy.  The process works like this:  Fuel is burned for heat.  The heat vaporizes a liquid (typically very pure water), which is then expanded through a steam turbine.  Lastly it is condensed at very low pressure to extract all possible thermal energy and also to re-use the pure water. 

Very Basic Steam Turbine

Steam Turbines come in an infinite number of sizes and designs.  It's amazing how many ways a machine can be designed to extract energy from expanding steam.

The oldest known design dates from the Greeks about 10 yrs A. D., and is known as Hero's Aeolipile (How you pronounce that, I am not sure).  Here is a drawing of one from antiquity:



Apparently a few of these were made from bronze, and were simply curiousities.  Heat from the fire below boils water in the bronze reservior.  Steam from the boiling water is carried up in pipes to a hollow ball at the top.  The pipes that carry the steam up to the ball support it, but also allow the ball to rotate freely on the axis.  The steam is then allowed to escape from the ball via two nozzles, spinning the ball.

A couple of interesting things to note about this design: 

1. This design would not be useful as anything other than a curiousity because it has no output shaft.
2. The very first steam turbine design is a reaction turbine.

Item #1 probably prevented the steam age from happening for 1800 years.

Item #2 is interesting because although Issac Newton didn't explain the Third Law of Motion until 1687, the process was understood long before then!  Newton's third law is usually paraphrased as "For every action there is an equal and opposite reaction", thus the term "reaction" turbine.

Reaction turbines achieve movement (and actual work) by squirting steam through nozzles, like our Aeolipile above.  The steam squirts out, and the reaction force causes the ball to spin in the opposite direction from the steam jet.

The other type of turbine is called an impulse turbine.  In this design, steam is directed at a set of blades on a shaft and bounced off of them.  This provides an "impulse" to move.  Below is a simple diagram showing the difference between the two basic designs.

In other posts I will decribe a little of the evolution of piston steam engines, steam turbines, and show some intersting modern designs and cutaways.