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Triple expansion

  1. May 12, 2015 #1
    hi all am looking to do a project using triple expansion of gas like the old steam engines had. I've got 3 points of gas/compressed air at different pressures to run 3 turbines. am thinking of using a central shaft and sizing the turbines to produce the same rpm and torque down the drive shaft... can someone help me figure out the sizes of the turbines ?? am starting with 58bar @ 80c after that am lost .... help
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  3. May 12, 2015 #2


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    What do you mean by "triple expansion of gas" like old steam locomotives? There is only one expansion that I'm aware of in the old steam locomotive engine designs:


    And trying to balance 3 different turbines at the same time on the same shaft to produce power may be difficult. Can you just run 3 separate turbines with 3 generators and combine the electrical power?
  4. May 12, 2015 #3
    "Triple Expansion" is just that. In a triple expansion steam engine the steam immediately from the boiler enters cylinder #1 to produce a certain amount of work. Steam exits #1, enters #2 at lower pressure, producing a lesser amount of work. So on, for #2 to #3. I saw a large unit with three different cylinder sizes on an Early 20th-Century Great Lakes ore carrier / museum in Sault Sainte Marie, Michigan. A glorious piece of turn of the century engineering in an attempt to extract as much work as possible out of the Btu's pumped into the water to create steam.

    Got me interested in the thermodynamic aspects of this (I was NOT a good Thermo student), and found plans for a small model steam engine. It is the 4th entry on this page


    An explanation is in the more common name of "Compound Steam Engine"


    This type was used on the Titanic and this page has some interesting theory.


    As far as helping you figure out the size of the turbines...I suspect the Thermo cycle theory is where you would need to start. Thermo calculations of each phase of steam quality would lead to physical size of each phase. I suppose it would. As I said, I was not a good Thermo student.
  5. May 13, 2015 #4
    thanks for the replys I was going to use 3 generators and 3 turbines but am limited on space with in the unit, I think you may be right about balancing them down a shaft
  6. May 13, 2015 #5

    jim hardy

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    Mark's Handbooks from 1920's and 1930's have good chapters on reciprocating engines.

    Basically you ratio the volume of your stages to the specific volume of the working fluid in that stage. Hence the distinct look of a triple expansion recip. Turbine stages are likewise sized. ....
    steam enters in the middle and progresses toward ends through progressively bigger stages as it expands.
    Flow in opposite directions cancels axial thrust.

    Best explanation i ever encountered was in a hobbyist book
    "Model Steam Turbines and How to Build them " by Tubal Cain(i think a pen name)
    the copy i had showed a steam powered model destroyer on the cover. I think it's still available but under a different title. He is prolific in the hobby machinist genre. His book explained how to design the entry nozzles for optimum velocity , using (of all things) a Moliere chart..

    "Steam Turbines and their Cycles" by J K Salisbury is a serious classic , and is reprinted.

    I hope this helps
  7. May 13, 2015 #6


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    The turbine rotor picture above is for the low-pressure portion of the turbine unit. As you can see from the size of this rotor relative to the man standing in the middle, it is quite a large unit. The steam is admitted into the center of the rotor and flows outward. More importantly than canceling any axial thrust, the flow of the steam is split in order to keep the diameter of the last stage from being any larger than it is. The individual blades in the last few stages are quite long and slender, and any vibration induced in them could become catastrophic.

    For similar reasons, there are two low pressure cylinders in a large triple expansion steam engine. One large cylinder would be more difficult to balance, where the two LP cylinders can be located on opposite ends of the crank shaft to provide (relatively) smooth operation.


    The engine above was installed in the battleship USS Texas, which was commissioned in 1914 and presently moored in Houston as a war memorial. This engine was capable of developing about 14,000 horsepower at full load. The Texas had two such engines installed, and her top speed was about 21 knots.

  8. May 14, 2015 #7
    it looks like we are on the right track, Iam still stuck on the maths of sizing the turbines :( . am not going to use steam to produce the pressure, its compressed air heated up to min 10c @ 8bar max 65c @ 48bar with a compressor displacement/stroke of 10cc... I can build the unit but really have no clue on the maths side....
  9. May 14, 2015 #8

    jim hardy

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    I'd get a book on model engines....
    A good one will explain the thermodynamics.

    I'm not a genuine steam guy, would rather not risk steering you wrong.
  10. May 14, 2015 #9


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    Yeah, but he wants to used compressed air, not steam.

    Google "Air turbines" to see what these things look like in action.

    In any event, the OP must decide how much power he wants to generate from the 3 generators.
  11. May 14, 2015 #10

    jim hardy

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    okay that's the source...
    what flow rate do you plan to run through your turbine ?
    Are you starting at 8 bar or 65 bar, or in between ?

    10 cc per stroke.. is that at i bar ?
  12. May 15, 2015 #11
    hi lads I really appreciate any input and your time with this, I've always been the hammer and fettle type... but think its time to change lol...
    it will be starting at anywere between 8-48 bar but i can regulate its start pressure to a fixed pressure where ever is best. its 10cc @ any pressure as for flow rates I was thinking of setting that depending on what the ideal turbines needs.

    On a separate note you guys gave me an idea to use a cylinder set up like the compound engines in stead of turbines would this be a simpler set up to calculate? would it improve torque or would it loss to much energy in moving parts .... questions questions .... where's the hamme !!
  13. May 15, 2015 #12

    jim hardy

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    Turbine will be a higher speed machine hence smaller

    as i said i'm not expert, just learned enough to understand what it was that i was maintaining.

    You'll want to expand in equal ratios
    so from 9 bars(absolute) to 1 bar(absolute) in three stages, each stage would drop pressure by ratio cube root of 9, which isn't far from 2..

    So your stage to stage blade area(or cylinder area) would change by same proportion.

    Are you familiar with "isentropic expansion" ? It's a vertical line on the Molier chart....

    Here's an article in Wikipedia that introduces basic vocabulary for turbine design.
    There's two approaches - "impulse" and "reaction"
    (our turbine alternated impulse and reaction stages)

    Steam that's not near saturation behaves pretty much like any other gas so the principles in that wiki article will apply to your air.

    Surely there's hobbyists who have done this and written books & articles
    i'd explore those home machinist sites , were i building one. That Tubal Cain book i mentioned was surprisingly detailed.

    anyhow - for starters you'll pick a velocity less than sonic and size your blade area to pass your flow at that pressure and density.
    Our turbine's first stage was impulse so the nozzles were designed for substantial velocity.

    And you need more expert advice than i can give. That Wiki article should have plenty of search terms to help you get started up the learning curve.

    Are you anywhere near SE Florida? Contact "Florida Flywheelers" http://www.floridaflywheelers.org/
    their spring antique engine show is a genuine treat.
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