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How does heating water in a steam engine boiler create steam pressure? Answered

I am thinking of doing an instructable on Model steam engines, how they work etcetera, and I am need of a Physicist to explain how the steam in the boiler builds up pressure. I know how the rest of the engines work, but unfortunately I am a bit lost for knowledge on pressure. I could just look at wikipedia or another such website but that defeats the object of the instructable.

The person who explains this best in a way a teenager could understand will get a collab on my Instructable, and subscribe.


When you heat water the molecules get energy from the heat source, this causes them to move around more and move further apart. Eventually they will be far enough apart to make the liquid into a gas - Water vapour or steam.

Steam takes up 1700 times more space than the original water this expansion creates pressure. A steam engine can run on as little as a few pounds per square inch, (PSI), or for larger engines 200 or 300 PSI.

Steam at 100 deg C (212 F) is a gas and invisible - What you see as white vapour is actually water droplets condensing out of the steam as the pressure drops.

Thank you very much. I just wanted to know why the pressure builds, I understand about the particles separating. So Pressure= Big Volume in small space?


7 years ago

btw, good luck with your project :p


7 years ago

The 3 general states of a substance: Solid, Liquid and gas.

Soldis state: The atoms are very close to eachother and sorted in a general structure which is repeated all through the material (think of something like this repeated in all 3 dimensions: http://www.delftintegraal.tudelft.nl/info/images/ACF1384.jpg )

Liquid state: The atos of the substance are still very close to eachother, but not as close as in the solid state (except for water, but that is not relevant for this explanation) They are however, somewhat free to move around and you can not define a general structure anymore

Gaseous state: In an ideal gas situation, the atoms are very far apart, so far in fact that they are not influenced by eachothers presence (besides the occasional collision) and move freely through the room.

I used the term "ideal gas" because a gas always strives to be ideal. IF you heat 1 mol (6.022 *10^23 atoms) of water to gas, it expands from 18 ml to 2,47 liters (you can calulate this using the ideal gas law: http://en.wikipedia.org/wiki/Ideal_gas_law )
If you heat water in a sealed chamber, this gas has nowhere to go, so the particles in the gas are pushed closer together to be able to fit all the gas in. (the water is also pushed together, but far less then the gas) This costs energy in the form of the heat that you added.
As I said before, gas always strives toward the ideal situation, and therefore wants to expand. This is pressure. When there is the option to expand, let's use the example of a piston-based steam engine, the gas will expand and push the piston for you, thereby using the internal energy (which you added in the form of heat) to give mechanical energy.

I seem to have rambled off a bit at the end, but I hope you understand what I said here, I took a few shortcuts in the explanation (like that I said gas wants to be in an ideal situation) which are not entirely correct, but would use way to much time to fully explain.

This is for the "Model Engines" guide isn't it? :-P Anyways, you almost finished with it? I know I still need to fill some out.

Sorry man. I'll do some later. I have been thinking about a wholy new type of gun...
I have done some of the instrutable, though I have been busy with exams etc.. So I haven't done much. It will just be an ongoing project You can add to from time to time.

Good choice of Best Answer! Rick gave you an excellent explanation, which you can and should use (properly cited :-) for your project. If you want to see the maths behind it, look up "ideal gas law." The basic expression is PV = nRT, where P=pressure, V=volume, n=number of molecules, R is a constant, and T=temperature (absolute, in kelvins).

When you're boiling water, the temperature is constant until all the water boils away. The volume of your sealed container is constant (until you explode or trip the relief valve :-). So you can rewrite the gas law as P = n(RT/V), where the parenthetical expression is constant. As you boil the water, the number of molecules in the vapor increases, and so P must increase.

Thanks, I have added him as a collaborator and will put his in the instructable

I must say you made a little failure: T is not a constant as the increased pressure increases the temperature water needs to boil. ;)

. http://en.wikipedia.org/wiki/Equation_of_state

IF you are looking at generating pressure by boiling water in a closed container PLEASE take a LOT of care - more people were killed in boiler explosions than through any other industrial accident when steam was in general use.

Building a boiler is not for an amateur and in most countries is regulated by various laws to make sure they are safe.

No of course not! I am too cautious. I have model engines that are safety-tested and are ready-made. The safety valves pop off before the pressure gets too high.

When water turns to steam it expands. Several hundred times it's original volume. All that expansion has to go somewhere or if confined as in a boiler it creates pressure.