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Below is a picture of one of the earliest designs of Watt's steam engine.

The basic principle of operation is this:

  1. The weight which is attached to the beam $E$ pulls the piston $P$ up, sucking steam from the boiler into the cylinder $B$.
  2. In the condensor $C$, the steam is cooled and condenses to water. This greatly lowers the pressure creating a pressure difference between the atmosphere and inside of the condenser and cylinder. So the piston is pushed down, doing work.

My question is, what makes the steam move from the cylinder to the condenser?

enter image description here

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The valve marked V' is opened. The process is something like the following.

To start the engine open V and V' and allow both chambers to fill with steam (this is just a bootstrap you heed to get air out of it). Now close both valves and follow the cycle below:

  1. spray water into the condenser which will cool the steam in it and form a partial vacuum (both valves are now closed, the cylinder is full of steam and the condenser is a partial vacuum);
  2. open V' (V is still closed) and steam from the cylinder rushes into the condenser, with the piston then being pushed down by atmospheric pressure until it almost reaches the bottom of the cylinder;
  3. close V' and open V, allowing steam into the cylinder, then close V (both valves are now closed);
  4. either after the last step, or while it is going on (they can be overlapped) go to (1).

The trick that is being done here is that the cylinder stays hot all the time: only the condenser is cooled. This means you don't have to repeatedly heat the walls of the cylinder after spraying them with cold water which a Newcomen engine did. This in turn means you don't waste a great lot of the steam you let in to the cylinder condensing on the walls to heat them.

You do have to repeatedly cool the condenser's walls, but remember that cooling water is essentially free: these things ran in environments where there was copious cold water, not least the water they were pumping. Cooling water is cheap, but steam isn't.

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  • $\begingroup$ Thanks for your answer! However, I have three issues with it: (1) If both chambers are filled with steam at the same time, why do we need two chambers in the first place? (2) Steam from the boiler enters the cylinder because the piston moves upward. I don't see why the upward motion of the piston should fill the condenser with steam as well. (3) Why is V' closed in your step 2? $\endgroup$ – Marc Nov 5 '17 at 0:12
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    $\begingroup$ #1, you only want to cool the condensor not the main cylinder - that was Watt's breakthrough. #2 the upward motion doesn't fill - the steam expands to fill both. #3 to stop the cold water going up into the cylinder - you want to keep the cylinder hot $\endgroup$ – Martin Beckett Nov 5 '17 at 0:21
  • $\begingroup$ @Marc You can indeed do it with one chamber -- the cylinder -- but if you do that then it gets cooled a lot when it is used as the condenser: Watt's trick was to separate the condenser from the cylinder which makes it more efficient as you have a hot side and a cold side of the engine. Both chambers get filled because otherwise there would be a different pressure in them, and since the valve between them is open, there can't be. Not sure why (or if in fact) V' is closed in step 2: I think it is from having observed them. $\endgroup$ – tfb Nov 5 '17 at 0:28
  • $\begingroup$ @MartinBeckett Thank you for explaining the third point! $\endgroup$ – tfb Nov 5 '17 at 0:30
  • $\begingroup$ Thanks for clarifying that the piston doesn't sucks in the steam. I still don't get why we have two chambers. In the cylinder, we have only hot steam. In the condenser, we first have hot steam and later cold steam and water. So what do we need the cylinder for? Isn't the condenser just equivalent to the cylinder of Newcomen's steam engine? $\endgroup$ – Marc Nov 5 '17 at 0:41
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tfb's answer correctly describes the working cycle - just wanted to capture some of the extra questions in the comments.

Ideally you want to fill the cylinder with steam to lift it and then have the steam disappear creating a vacuum to pull the cylinder down. In Newcomen's original engine this was done by spraying cold water into the cylinder, cooling the steam and creating a partial vacuum - air pressure on the top of the cylinder than pushes it down. (Incidentally this was the power stroke because they wanted the other end of the beam to go up to lift water out of the mine.)

The problem with this is that you cool the cylinder walls on each cycle, so the fresh hot steam for the next stroke immediately starts to condense on the cold walls and it isn't until the walls have heated above boiling point that any steam can start to lift the cylinder.

Watt's breakthrough was the idea that since steam is a fluid you can remove it from the cylinder by connecting it to a separate vacuum. After the steam has filled the second tank (the condensor) you close the valve and spray cold water into the condensor, condensing the steam and creating the vacuum ready for the next stroke. (You also have to occasionally pump out the water from the condensed steam.)

By doing this you keep the cylinder hot and the condensor cold so you don't waste energy and ultimately fuel.

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  • $\begingroup$ If the condenser is always cold, why doesn't the incoming steam condense right away? This would create the vacuum while the piston is on its way up and thus block the entire process. $\endgroup$ – Marc Nov 5 '17 at 15:39
  • $\begingroup$ You only open the valve to the condensor when the piston starts the down stroke $\endgroup$ – Martin Beckett Nov 5 '17 at 15:49
  • $\begingroup$ Thanks for pointing this out (in contradiction to tfb's answer)! This has been a major point of confusion for me. Now the only remaining question for me is why does the piston move down? In Newcomen's engine, it moved down because of the vacuum. But if the condensor hasn't created the vacuum yet, there is no pressure difference between the atmosphere and the inside of the cylinder. /edit: I think I get it... the gas moves into the condenser by diffusion and then, the condensation starts and creates the vacuum. Yes, I think I get it now! $\endgroup$ – Marc Nov 5 '17 at 15:58
  • $\begingroup$ Thanks a lot, tfb and Martin Beckett for engaging with me. Your answers have both been helpful. I also added a nice animation which I have just found as an extra answer. $\endgroup$ – Marc Nov 5 '17 at 16:29
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    $\begingroup$ You open the valve to the condensor to START the downstroke. The steam rushes out of the high pressure cylinder into the vacuum of the condensor and the cylinder falls. At the bottom the cylinder is empty of steam which is all in the condensor . Then you close the valve and spray water into the condensor turning the steam in it back into water (which you later pump out) and creating a vacuum ready for the next stroke $\endgroup$ – Martin Beckett Nov 5 '17 at 21:10
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Here is a good animation which shows the process in question: https://www.youtube.com/watch?v=9gWV9gLuslw&feature=youtu.be&t=355

In the same video, there are also good animations of Newcomen's engine (https://www.youtube.com/watch?v=9gWV9gLuslw&feature=youtu.be&t=236) and Watt's second innovation that the piston can do work while moving in both directions (https://www.youtube.com/watch?v=9gWV9gLuslw&feature=youtu.be&t=489).

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