Torque difference: steam turbine vs. piston?

[jwebster]

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

 

[brewnog]

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?

 

[jwebster]

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

 

[brewnog]

Hi,

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

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.

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

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?

 

[jwebster]

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

 

[Danger]

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.

 

[FredGarvin]

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.

 

[AlephZero]

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.

 

[brewnog]

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.

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

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.

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.

 

[FredGarvin]

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 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.

 

[AlephZero]

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.

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
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.

 

[jwebster]

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

 

[brewnog]

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

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.

 

[jwebster]

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

 

[brewnog]

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.

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:

 

[Averagesupernova]

I always understood that the reason turbines are used where they are (mobile-wise that is) is the high power to weight ratio.

 

[brewnog]

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.

 

[Haha71687]

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.

 

[RonL]

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

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

P.S. Caught me not paying attention to thread date.

 

[dr dodge]

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.

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

 

[xxChrisxx]

Formatting is your friend.

It also wasn't really addressing the point of the discussion. As much as torque is important as that's what allows it to do work, for something like a turbofan/turbojet engine torque is a secondary concern. Thrust is directly relatied to speed of exhaust which is more reliant on RPM not torque. Edit, I've just reasised we're talking power generation here so no one cares about thrust. The point still stands though.

This is inherently the reason why people are haing troube making the comparison between IC engines and tubines.HaHa also stated that cars with CVT not standard transmissions. Therefore the only real consideration from the engine is how much power does it make and at what RPM, as the CVT geabox does all the torque multiplication for you whilst the engine sits at peak power or peak efficieny.

 

[brewnog]

torque ALWAYS matters! torque is work.

Not correct, sorry. As haha stated, the prime mover torque figure is irrelevant (providing power is produced) in a static load situation.

Also, torque isn't work. Work is the change in kinetic energy of an object. Torque is by how much a force produces torsion about an axis.

 

[dr dodge]

Not correct, sorry. As haha stated, the prime mover torque figure is irrelevant (providing power is produced) in a static load situation.

Also, torque isn't work. Work is the change in kinetic energy of an object. Torque is by how much a force produces torsion about an axis.

in transportation uses, the load is only static when not moving. a constant throttle position, and rpm can not be maintained without torque, or significant flywheel mass. in theory, if the road surface was perfect, level and flat, and you drove in a vacuum, the load would then be constant. In the turbine application, the losses in the turbine become combined with the gearbox losses, and the gear losses will be compounded with the more gear clusters needed to get a usable output speed. Diesel engines product more torque at low rpm, which is why for given work done fuel consumption is significantly reduced.

dr

 

[xxChrisxx]

Noone is saying that you don't need any torque what so ever, as its obvious you do. What they are saying is that torque is not the primary concern. So long as you have enough to overcome losses to keep the turbine at steady output, you don't care what the torque figure is. So long as the power figure is high.

Therre are two ways of making high power, lots of torque and low rpm or little torque and high rpm. Turbines are best suited to low torque high rpm. As its more difficult to get an axial flow to produce high torque.

Piston engines are more suited to making high torque and low rpm, because they rely on leverage, and the stresses of accelerating and decellerating the piston increase massively with engine speed.

And the reason why diesels do more for less fuel consumption is the high compression ratios over spark ignition engines and the fact they have no throttle plate. Turbines offer a varibale compression ratio besed on flow speed, higher rpm's give higher compression ratios. Gas turbines can have very high comression ratios at high speed, amkign then rather efficient.

You keep trying to apply piston engine reasoning to turbines. They are totally different beasts. The method they go about producing power is totally difference.

 

[brewnog]

in transportation uses, the load is only static when not moving. a constant throttle position, and rpm can not be maintained without torque, or significant flywheel mass. in theory, if the road surface was perfect, level and flat, and you drove in a vacuum, the load would then be constant. In the turbine application, the losses in the turbine become combined with the gearbox losses, and the gear losses will be compounded with the more gear clusters needed to get a usable output speed. Diesel engines product more torque at low rpm, which is why for given work done fuel consumption is significantly reduced.

dr

Bobbins. At a constant speed, on non-changing road conditions, load is constant. You don't need "theoretical conditions" to achieve that. It happens very, very frequently. For a constant load application, torque is irrelevant, providing the required power is available, and the gearing is designed to suit this.

My entire post

Correct.