Steam engine question. Not heating related, but steam and thermodynamics?

ChrisJ

I've been trying to wrap my head around something for months and can't get it but maybe someone here does.

Triple expansion steam engines from back in the day use 3 different size cylinders each feeding into the next at a lower pressure. The smallest being the first, high pressure, and the last being the largest low pressure cylinder.

What I don't understand is what causes the pressure drop between cylinders? If they are in series, shouldn't the pressure be identical from start to finish?

I think part of my problem, is I keep picturing this as an engine using compressed air and I realize steam doesn't behave like this. However, I'm not understanding why the pressure is dropping if no water is being formed anywhere.

ChrisJ

Hatterasguy said:

The pressure and temperature of the steam drop as it expands inside a cylinder.

If it is superheated to a significant degree, there is no condensation and the steam that is available from the first cylinder is still well above its saturation temperature so you can use it in the next cylinder.

You are thinking steam is at its saturation temperature like we have in the low pressure boilers. It cannot do any work to move a piston in that condition. You'd have a bucket of water in the cylinder.

Ok, why?

If I hook an air compressor and tank to a piston and cylinder like this and let 100 PSI in, it's going to push the piston down. The exhaust valve opens and the air comes out. That's all, nothing else changes.

What is going on that causes the temperature and pressure of the steam to drop? How is that energy being used?

It sounds like the heat stored in the steam is what's doing the work but I can't understand how we're pulling heat out of steam to push a piston down. And yet, on triple expansion and even quadruple expansion setups like Titanic had, that seems like what must be going on.

delta T

So steam goes into first cylinder at say 100 PSI, expands and cools driving the cylinder down, but at the bottom of the piston movement the steam is still too hot to condense, and still at say 60 PSI. Steam then moves to the next cylinder and again expands and cools pushing the cylinder down and ends up at say 20 PSI, then into the next cylinder where the last little bit of work that it can do before condensing is extracted out as it expands and cools again.

That is why the cylinders get sequentially larger as the pressure drops. more area needed for the lower pressure to do the same amount of work.

That is my understanding.

Hatt, any thing I'm missing?

BTW if anyone wants some pretty good steam engine and boiler porn watch this (there are a ton of episodes):

https://www.youtube.com/watch?v=xFQ-f1Ax_Pg&list=PLBwC0sil0vDlO828OYQfPiSBHivdePxNK

Almost all of the ones that I watched involved some kind of steam engine and they do a good job of actually showing how magnificent some of the old beasts really were and are. Its also a pretty well done and fairly interesting show, even the parts that don't involve steam engines . The host is the guy who plays Arthur Weasly in the Harry Potter movies.

delta T

Hmm didn't mean to embed the video, but if you search youtube for Industrial Revelations with Mark Williams you will find it, there are 40 some episodes. episodes 1 through 12 at least have tons of really cool steam engines and boilers, haven't gotten farther than that yet myself. Enjoy!

MikeSpeed6030

Some steam engine cycles send the steam exhausted from a stage back to the furnace to be reheated before admitting it to the next, lower pressure stage. Without reheat, saturated steam can be used to heat feedwater before it enters the boiler. Many options.

Harvey Ramer

Steam is a gas and pretty much all gasses have a pressure temperature relationship. Like refrigerant, but not as pronounced.

Harvey Ramer

In other words, If a gas expands in volume with the enthalpy remaining the same, it must drop in temperature.

ChrisJ

Perhaps that's my issue.
I'm picturing the steam acting as a hydraulic liquid, being constant from the inlet pipe, into the cylinder and out the exhaust, never changing.

Jamie Hall

Not to add to the confusion -- @Hatterasguy gives a pretty complete picture -- but the idea of a reservoir of steam at high pressure -- say your 100 psi -- was mentioned. Now... at the inlet to the engine there is a throttle which will reduce that pressure as steam flows through it, so you can work your engine on a lower pressure if you want to. And to further add, steam engines have variable valve timing (how cool is that!) which is referred to as "cutoff". The cutoff controls how far along the stroke the intake valve stays open. If it stays open for the full stroke, then you have your full pressure steam operating -- with very little expansion -- for the full stroke. As you bring the cutoff back towards the beginning of the stroke, you get more expansion as the piston moves, and more drop in pressure. This is a much more efficient use of steam, but at the expense of power produced. A good engineer will always be balancing the throttle and the cutoff to get the power he needs with the least use of steam. At the same time, of course, you don't want to have the pressure and cutoff in the high pressure cylinder so restricted that there is nothing left when you get to the end of the stroke in the low pressure cylinder! You need enough at that point to get to atmosphere or in ship work to your condenser...

Confused yet?

ratio

Remember that the inlet valve closes some time at the beginning of the power stroke, & the outlet valve doesn't open until some point at the end of the power stroke. (Valve timing dictates the precise points.) There is never an unrestricted path from the inlet to the outlet. Therfore, the pressure of the steam must be decreasing as it's volume is increasing during the stroke of the piston.

bob_46

FWIW automatic variable valve timed engines were called Corliss engines. Locomotive valve timing was manual controlled by the engineer with the Johnson bar .

ChrisJ

Ah,
I think I finally get it.

It's the same as when a water heater explodes. The steam continues to expand and release more and more energy as the pressure drops until it is a low enough temperature vs pressure that it condenses to water.

Forcing super super heated steam into an engine at full pressure and then exhausting it immediately would blow almost all of your energy out of the engine.


So, it's absolutely nothing like a hydraulic ram.

KC_Jones

@ChrisJ I can see if my grandfather can call you to talk about it. He was an engineer on Liberty ships all through WWII. He still knows a lot about those engines.

Jamie Hall

Hatterasguy said:

ChrisJ said:

Forcing super super heated steam into an engine at full pressure and then exhausting it immediately would blow almost all of your energy out of the engine.

Unless, of course, the load demanded that pressure, and then you utilize all of it to do the work.

If you don't need it all, you throttle it and reduce pressure as Jamie noted above.

And if you really want to get excited about really big locomotive steam engines (which are single expansion, so really a somewhat different critter) try and get a chance to see Norfolk & Southerns 611, or Union Pacific's 844 or Union Pacific 3985 starting a heavy train at full throttle and full cutoff... oh yeah.

CLamb

If you're anywhere in the area of Lancaster Co., PA this is a good place to see it happen next week. roughandtumble.org/

BillW

Knowing some of the people on this site, some would go to Lancaster, visit Rough & Tumble and never return home! Steam everything, even a lawn mower. Lots of "hit & miss" gas & kerosene engines , too. Strasburg RR has a couple operating steamers and offers shop tours and is in the general vicinity, across the street is the Railroad Museum of Pennsylvania, with lots of big steam on display, but not operating. I agree with Jamie, having had the car I was working on right behind C&O 614 working a 22 car train up the grade out of Port Jervis. WOW!

KC_Jones

I rode behind 614 out of Cumberland MD, I think it was 1981, I was 7. That excursion I will never forget.