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  • Steam power question

    I was watching Jay Leno's garage and while talking about steam-powered cars he touched on the fact that water is re-used in the system.

    My (probably flawed) understanding of steam power from sixth grade science class is this: Water in a contained vessel is heated which creates expanding steam. The expanding steam flows through a pipe and pushes against a piston to do work. Once the steam has moved the piston, it is expelled from the system and is lost to the atmosphere.

    This obviously can't be the entire story in practical use. This would mean that the steam that pushes the piston is also pushing against the walls of the boiler, that the water is a total loss, that power would decline as the supply of water is depleted, and that the boiler would have to be empty and depressurized prior to adding new water.

    Since I know that real-world steam engines condense the used steam back into water and use it again, how is this possible? How can liquid water be intoduced into a pressurized vessel without A) pressure forcing it back out or B) the system needing to apply greater force than it generates to push liquid water back to into the boiler?

  • #2
    A feed pump?

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    • #3
      Originally posted by oldstarfire View Post
      A feed pump?
      How would the pump defeat the pressure in the vessel without wasting a lot of energy?

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      • #4
        The water pump piston area is smaller than the steam piston area. The other way is a steam injector, more complicated to understand but has no moving parts other than valves.

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        • #5
          Okay, I think I understand that. Do you know of a resource I could use to read up on steam injectors?

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          • #6
            Here's a video of a reciprocating feed pump working: https://www.youtube.com/watch?v=a0nn0O4e71Y

            And a web page on injectors, with some references at the bottom: https://www.steamlocomotive.com/appliances/injector.php

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            • #7
              Originally posted by Commander_Chaos View Post
              Okay, I think I understand that. Do you know of a resource I could use to read up on steam injectors?
              This page has some nice cutaway drawings: https://en.wikipedia.org/wiki/Injector
              Location: Northern WI

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              • #8

                pumping liquid is less work than pumping compressible vapor. so expanding steam then compressing condensed liquid gives net work out.

                https://www.physicsforums.com/thread...0/post-4219410

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                • #9
                  Originally posted by Commander_Chaos View Post
                  I was watching Jay Leno's garage and while talking about steam-powered cars he touched on the fact that water is re-used in the system.

                  My (probably flawed) understanding of steam power from sixth grade science class is this: Water in a contained vessel is heated which creates expanding steam. The expanding steam flows through a pipe and pushes against a piston to do work. Once the steam has moved the piston, it is expelled from the system and is lost to the atmosphere.

                  This obviously can't be the entire story in practical use. This would mean that the steam that pushes the piston is also pushing against the walls of the boiler, that the water is a total loss, that power would decline as the supply of water is depleted, and that the boiler would have to be empty and depressurized prior to adding new water.

                  Since I know that real-world steam engines condense the used steam back into water and use it again, how is this possible? How can liquid water be intoduced into a pressurized vessel without A) pressure forcing it back out or B) the system needing to apply greater force than it generates to push liquid water back to into the boiler?
                  The water is a total loss in many engines, other reuse it. marine enginers the main reusers afaik as you need a cooling source to condense things (heat exchanger feed with sea water). They put it after the last cylinder in compound (steam goes through progressively larger cylinders to extract more energy from it) and it the lower pressure created ads to efficiency.

                  When you do reuse the water it has to be filtered as it will be full of oil (the hot well) - via a lubricator, oil is put into the steam on the way to the cylinder.

                  On locomotives and stationary engines I haven't seen a condenser (but I wouldn't hold myself out as the global expert). No idea on a car, but I'm doubtful as what is the cooling source? It wasn't done a marine engine afaik to save water, it was to get more efficiently as the cool waters free and available.

                  As for replenishing a boiler, its a must else disaster will strike. An locomotove would get a ways down the track and water level would get dangerously low. Mechanical pumps or injectors as mentioned replenish water into the boiler. Maintaining water height in the boiler is of paramount importance else high temps weaken steel and there's an explosion risk
                  Last edited by Mcgyver; 02-02-2020, 06:16 PM.
                  .

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                  • #10
                    What J Leno is talking about is called a "Closed System"
                    What you previously knew about atmospheric exhaust is called a open system
                    In a closed system you must have a "Condenser" to cool the steam back into water and then use a pump to pump the water back into the boiler
                    Condensers have several forms , but for Leno's car, it is a radiator. And instead of cooling hot water ( 180) to say 100 degrees , you cool steam ( 230 F ~) to 100 Degrees F
                    Now when steam drops below 212 F ( in a closed system !) , it condenses to 1/1700 of it's volume and actually forms a vacuum ..of about 26 inches or 13 PSI ( Vacuum )
                    and we know that that Atmospheric pressure that Barometers use in weather is about 30 Inches of Mercury.
                    That means in a vacuum there would be no pressure, but we do not live in a vacuum !. This means the barometer is really saying about 14-15 PSI ( = 30 inches Mercury ~)
                    SO the Engine has 30 PSI of steam pushing on the piston and the exhaust on the other side has a vacuum of 13 PSI so the Piston sees 43 pounds of force !
                    Because of the tremendous difference in Volume 1700 to 1 it is easy for the feed water pump to pump water.
                    So part of this equation is the loss of heat by using a condenser. free energy in a way as you only need airflow which is not a problem for a moving vehicle.

                    Rich

                    PS
                    I should add that once steam condenses on a surface (Radiator tubes ie) it attracts the other steam to it immediately . Condensing is like a wind storm. the steam particles rush to the cool surface. In other words, it's almost an instantaneous reaction or result/
                    Last edited by Rich Carlstedt; 02-02-2020, 05:32 PM.

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                    • #11
                      Ocean-going ships would have wanted fresh water in the boiler, and all that is out there is salt. So condensing is very practical.

                      Power stations would condense to get that last bit of power out... which could be almost as much as the engine produced without condensing. Steamships ditto. To get the most they might use a turbine for the low pressure / high volume going into the condenser.
                      1601

                      Keep eye on ball.
                      Hashim Khan

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                      • #12
                        I knew this was the right place to ask this question

                        On one of Leno's cars there is a "steam generator" rather than a boiler. Based on the answers above it now makes sense to me how this long tube could be expelling steam at one end but be taking on liquid water at the other. That was the source of my original question.

                        Thanks!

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                        • #13
                          Anyone interested in steam transportation will be well served reading the wealth of information left behind by Bill Lear, yup everything from 8 track stereo to small jet planes Bill, pertaining to his 70s era fascination with bringing steam back as a land transit propulsion system. Being Bill he had both a Chevy Montecarlo platform and a city bus platform going on together, and the bus did haul people in Frisco for a while.
                          Bill was no Marmiduke Surfaceblow, not by a long shot, but Bill did take steam a long way beforre hitting the brick wall.


                          By the time steam locomobiles and Stanleys hit the cobblestones and mud ruts the system had been well developed and refined by both traction engines and railroads. Thankfully Union Pacific dusted off a couple of their toys, including BigBoy and has them back on the rails teaching new generations. If you look at video of 4140 and 844 on the rails playing, notice the 2 and 3 car in the train. They're disguised water tanks tied to the boiler to provide water to the total loss system. In the days of Steam power they weren't necessary because of water & coal stops along the route and snorkel trouths. Many main line steamers pulled tenders with a drop down snorkel to pull water aboard at 30mph in a manner to a flying fire tanker drafting from a lake.

                          Water is far ore important than coal, you run out of coal the engine rolls to a stop, but running out of water can blow the boiler apart when the crown sheet above the fire lets go.

                          Early automotive steam had the same problems as RR steam, with a far less dependable source of water so builders tried to add condensers to reclaim the used water. That pretty much involved adding a vacuum pump tothe machine which was another set of problems that required another engine to drive the pump or leach power from the feed pump. Steamers did require an engineer to drive them though and internal combustion won the battle.

                          Another thing about boiler water is that it often requires compounding to protect the boiler from tube dissolved contamination in the water. Condensing and reuse protects boilers.

                          Condensation also comes into play on municipal steam systems such as Rochesters. The remaining downtown boiler plant sends out 1200° steam from now NG fired boilers to customers in the core city who squeeze as much heat as they can from it and then condense back to water that runs thru a water meter to record how much steam was used. Once a year an inspector wanders thru every user's facility to make sure no steamfitter accidentally routed a return to the sewer without passing thru the meter.

                          Sitting here writing this I'm reminded this town isn't yet 100 years from being primarily steam powered. Most industries 100 years ago burned coal and made their own electricity if they wanted electricity. American Laundry Machine built turbines to drive new Destroyers of World War 1 and used a pair that were cancelled orders at end of War to repower their power house from piston engines. One of the exciter meters hangs on my wall here. The AC generator Synchronizer sits awaiting reconditioning in the other room.

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                          • #14
                            Originally posted by Mcgyver View Post
                            and stationary engines I haven't seen a condenser
                            The original 18th century Newcomen engines were called 'atmospheric engines' because the cylinder itself was a condenser and the air pressure pushed the piston down into the vacuum created by the steam condensing in the cylinder. Boiler steam pressure was often below air pressure and in Cornish mines boilers were even made out of granite hollowed out.
                            The great advance came when James Watt patented the Separate Condenser which meant the cylinder did not have to be cooled down every stroke but pressures were still close or below atmosphere, He did not invent the steam engine as we know it. The next advance was by Trevithic introducing the dangerous concept of high pressure steam, even up to two atmospheres, which is what made transport possible.
                            Steam ships for non coastal operation would have an evaporator to produce fresh water for the engines but still used some sea water and measured the salinity daily. Serious accidents have occurred by letting the salinity rise and cause corrosion. This can also be a problem for locomotives where the groundwater has high dissolved salts such as South Africa where tannin was added to help suppress it's effects. Although railways have occasionally tried condensing and water recovery on land it is not economic. In cars it was sometimes introduced to prevent steam clouds scaring the horses.
                            The single long tube steam generator mentioned above is called a 'Monotube'.

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                            • #15
                              Some steam systems condensed the exhaust to save water but many use it to create draft up the stack to draw fresh air into the boiler firebox which makes the fire hotter. Feed pumps and injectors are a necessary evil on boilers. they replace the necessary water but use steam to do it so the overall efficiency of the boiler drops plus if there isn't any preheat (older boilers) the water enters the boiler cold which also lowers the efficiency. The water temp is lowered and more heat must be added to get it back to temp.

                              Ships that use boilers generally try and carry water picked on shore but most if not all will have evaporators or some newer tech to desalinate sea water to use as boiler feed water. Running them is costly in fuel but sometimes necessary.

                              There is no free lunch.

                              If you want books on steam, boilers, injector or pumps along with machining in general then go to the Internet Archive, https://archive.org/ and download them for free. Older books but good, better then anything written now days.
                              The shortest distance between two points is a circle of infinite diameter.

                              Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

                              Southwestern Ontario. Canada

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