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Torque difference: steam turbine vs. piston?

  1. Jan 10, 2007 #1
    Hi,

    I was told that there was an inherent torque advantage to using a piston design over a turbine design (in this case for steam). I said I doubted there was any theoretical reason why this would be true, it would all depend on the design of the engine. My argument was that most turbines in current use are for electrical generation which is a low torque application, but turbines could just as easily be made high torque through appropriate blade configuration, etc.

    Any input on this question would be appreciated.

    Thanks,
    Jeff
     
  2. jcsd
  3. Jan 10, 2007 #2

    brewnog

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    The first thing which strikes me is that a piston design is inherently 'positive displacement'. I can't visualise how a turbine could be made so, but I don't know enough about turbines to know if this is a valid argument. The second thing is that turbines run much faster than reciprocating designs, and thus a turbine will be developing much less torque than a reciprocating engine of equivalent power.

    What makes you think electric power generation is necessarily a low torque application?
     
  4. Jan 10, 2007 #3
    Hi,

    I'm basically taking from what you are saying that turbines do generate less torque/horsepower than do reciprocating designs.

    re last comment: I assumed that because power generation does not involve frequent accelerations of the mass that this was a less torque intensive application.

    Thanks,

    Jeff
     
  5. Jan 10, 2007 #4

    brewnog

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    Well don't, because that isn't what I said.

    For EQUIVALENT power, any engine running at a faster speed will be developing less torque. However (for a number of reasons), turbines are chosen in preference to reciprocating engines for large power stations.

    A large power station (a few thousand MW) on the grid will obviously produce more power than a couple of Diesel generators on an oil rig. It'll also produce lots more torque. Please don't equate torque and power without taking speed into consideration.

    The rotating mass has very little to do with the torque required. It takes an absolutely tiny amount of torque to keep a turbine (or reciprocating engine) spinning. When you apply a few hundred megaWatts of electrical load to the alternators, that's when you need the torque.

    You don't need a change in speed to require torque.

    Here's some simple numbers to aid your understanding:

    To produce 1000MW of constant power at 50Hz:
    Let's say the turbine is running at 3,000 rpm (realistic assumption):

    Torque = Power / angular speed
    Angular speed = 3,000rpm / 60 seconds = 50 revolutions per second = 100pi rads per second,

    Torque = 1,000MW / 100pi = 3,200,000Nm

    Not quite low-torque is it, when your torquey Audi engine might develop 300Nm? :smile:
     
    Last edited: Jan 10, 2007
  6. Jan 10, 2007 #5
    Hi,

    I think this might be getting into symantics. My point: if a) turbines (normally) run faster than piston engines and b) faster engines produce less torque per unit power then, by combining statements a) and b) you are saying that turbines (normally) produce less torque per unit power. Again under the assumption that they are usually running at a higher speed.

    But this is useful because you MAY be saying (being careful not to misattribute here) that this is not an inherent/direct property of the engine per se but is true because of the normally high speeds at which it operates.

    This would in essence mean I'm half right, which I guess is better than nothing.

    Thanks,
    Jeff
     
  7. Jan 10, 2007 #6

    Danger

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    I think that the basic answer lies in what Brewnog said first, about the piston being 'positive-displacement'. You can stall a turbine much easier because steam can leak through it without necessarily making it turn. That can't happen with a piston, unless you count the very minor leak-down through the ring gap.
     
  8. Jan 11, 2007 #7

    FredGarvin

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    If you look at it solely from a standpoint of torque per unit of power produced, then naturally you should say that a turbine is a "lower torque" machine. You don't find many turbines (at least in my realm) that have shaft speeds below 10,000 rpm at idle whereas a recip at 10k is cooking. Perhaps the really big steam guys turn a bit slower but I can't say for sure. I would doubt it though.

    Your statement clouds an issue though, which is that for a recip to produce the same torque or power as a power turbine, that diesel has to be extremely large when compared to the turbine putting out the same power. That is where the real comparrisson comes in.
     
  9. Jan 11, 2007 #8

    AlephZero

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    Power generation steam turbines usually run at 3000 or 3600 RPM for 50hz or 60Hz electricity supplies) though some run at half that speed with a 4-pole generator.

    That's about the same max speed as the fan and LP turbine on a big jet engine - and they are easy to convert into 50MW power generators, just replace the fan at the front with a generator at the back.

    Of course you can design efficient turbines to run at lower speeds. Large wind turbine generators typically run at a constant 10 RPM with variable pitch blades to accommodate changing wind speed.
     
  10. Jan 11, 2007 #9

    brewnog

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    Precisely. Specific torque figures alone are meaningless when dealing with EPG.


    Fantastic example which had skipped my mind!

    Let's say 2MW at 10rpm

    Torque = 2MW / (10 / 60 * 2pi) = 2000000Nm give or take.

    Yep that does it for me.
     
    Last edited: Jan 11, 2007
  11. Jan 12, 2007 #10

    FredGarvin

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    That slow? Wow. I had no idea. I have seen articles where the discussion of high speed balancing of rotor groups for those big guys was in the 12,000 rpm range. Huh. Ya learn something new every day.
     
  12. Jan 12, 2007 #11

    AlephZero

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    The actual generator has to run at 3000/N or 3600/N RPM where N is the number of pole pairs (i.e. static magnets) to be in sync with the AC frequency.

    I don't think you would want to spin something with a rotor diameter of the order of 15 feet at 12,000 rpm :eek:
    http://www.powergeneration.siemens.com/en/products/steamturbinesitem/sst5_6000/index.cfm

    Gas turbine driven generators have do high speed rotors in the gas generator -that's the same general operating regime as aero engine technology.
     
  13. Jan 12, 2007 #12
    Hi,

    Thanks for all the responses.

    I admit, I'm still not entirely sure what the answer is. (If there is one.) I gather that larger steam turbines run more slowly because the larger diameters at high RPMs will produce excessive stress on the blade assemblies. So conversely I think this means that smaller steam turbines would run more quickly.

    To generalize, I think that smaller steam engines would tend to run faster, and therefore (normally) have lower torque/power ratios. However, going back to AdelphZero's comment about wind turbines, I was wondering if you COULD design a higher torque small engine by using different blade pitches or other changes to the typical design.

    Even if it isn't normally done, I could still "win the bar bet" (speaking figuratively) if it were theoretically possible.


    Thanks,
    Jeff
     
  14. Jan 12, 2007 #13

    brewnog

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    Fundamentally, all practical considerations out of the window, if you were to design an engine purely for torque, it would be a "positive displacement" piston type, rather than a turbine type.

    However, as you've probably gathered, this is a meaningless statement in real terms. If you're just trying to win a bar bet, then fine, but that shows a lack of understanding of the considerations involved with engine selection and the fundamental operating principles behind each one.
     
  15. Jan 12, 2007 #14
    Hi,

    This isn't literally a bar bet. It is a serious question, and you really have answered it by saying "if you were to design an engine purely for torque, it would be a 'positive displacement' piston type." This tells me there is an inherent reason - based on physical principles not merely accidents of current engineering norms - that suggest a piston type engine would be better if you were looking for torque. So, I'm wrong, end of story.

    Jeff
     
  16. Jan 12, 2007 #15

    brewnog

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    That's just the problem Jeff. It's not "end of story". Requiring merely torque is a meaningless objective. You could turn the fastest, most powerful, least torquey gas turbines into incredibly high-torque machines by using a simple gearbox.

    I could personally create more torque with my bare hands than the world's biggest Diesel engine, given a long enough lever. Does that mean I'd be better at powering a ship? Or generating electricity? Of course not.

    The fact is that engine types are NOT specified based on their torque outputs alone. To say "a piston type engine would be better if you were looking for torque" is misguided.

    Please stop quoting my statements without including the caveats that I imposed on them! :tongue:
     
    Last edited: Jan 12, 2007
  17. Jan 13, 2007 #16
    I always understood that the reason turbines are used where they are (mobile-wise that is) is the high power to weight ratio.
     
  18. Jan 14, 2007 #17

    brewnog

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    Power/weight ratio is often a reason why gas turbines are used for EPG in place of Diesel engines. Power/volume is even more impressive. However, initial cost, noise, and specific fuel consumption are all negatives of gas tubines which mean that for mobile generating sets, Diesel tends to have the upper hand.
     
  19. Jun 19, 2009 #18
    Torque dos not matter at all in applications with a relatively static load. Think cvts in cars or a jet at cruise. Power is all that matters in those situations.
     
  20. Jun 19, 2009 #19

    RonL

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    You might consider a vane motor in your positive displacement thoughts.:smile:

    P.S. Caught me not paying attention to thread date.:blushing:
     
    Last edited: Jun 19, 2009
  21. Sep 15, 2009 #20
    torque ALWAYS matters! torque is work. In IC engines, HP is a math function of torque. Carol shelby once said something like: HP sells cars, torque wins races
    I play with a lot of "old iron junk" and I am a firm believer, from practical experience, that torque thru long gear ratios is WAY more efficient than RPM and gearing down (over and over). Thats the same mind set that propagates "colder air charge = more hp". (that violates the laws of thermodynamics) and "electronic FI is better than anything mechanical" (electrical drag offsets the power gains)
    this is the law of diminishing returns! bigger, slower(rpm) and simpler is better...lol
    Example 1 4000 lb, 1974 car, 400 cu in BB V8, engine 3 speed autotrans. (cam grind from 69 high performance application w/ low overlap) average hiway milage 28 mpg @70+ (it got better the faster I went, estimated 140+ mph top speed, mpg there unknown, not cop friendly)
    Example 2. 3800 lb 1972 car 225 cu in /6, stone stock, 3 speed manual, 90k mile engine 30-36 mpg @ 2800 rpm
    Both transmissions have no overdrive
    Both engines build great torque in the low to mid power curve vs. modern engines
    these are basicly 1970's tech, with carbs, and electronic ignition, and NO modifications AT ALL.
    Example 2 (Dartman) can be seen every day currently going to work on a texas beltway, and Example 1 (Boltshaker) did 3-4 years of commute service before that. (Boltshaker can also weigh in on the sonic screwdriver discussion..aka name)
    high rpm's= wear!!!!!! critical balance!!!! the spectacular boom factor!!! large cam overlaps (intake and exhaust valve open at the same time) gear drag and wear, noise vs exhaust backpressure, and be a serial killer of fluids!!!!!!!!!,
    now if I went 20-1 Comp Ratio, tiny cam, tuned to 2500 rpm and a turbo/OD trans in example 2, I feel 50 mpg would be a disappointment.
    even though these are IC engines, all engines are just air pumps, its only flow, fluids, thermal efficience and density that really changes.

    dr
     
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