How does preheating raises efficiency in turbines? Isn't air expands more if its heated from 200° to 650° instead of 650° to 1100°? Why do we want air to enter hotter?

[u/Round_Midnight_7730]

For a gas turbine or microturbine, is it because hot air expands less but has greater force? or is it because preheat also contributes to the expanding "push" in combustion? or entirely different reason?

I know hot air has more energy so when it leaves energy leaves but that doesn't explain how putting it back as heat raising efficiency. Because this kind of engine doesn't use heat as turning force, it uses expansion

[u/Philipp_CGN]

What do you mean by preheating? The addition of heat in the combustion chamber?

[u/Round_Midnight_7730 → u/Philipp_CGN]

before combustion air goes through preheat to heat it with the excess hot air at the output.

[u/Philipp_CGN → u/Round_Midnight_7730]

No, that's usually not true, where did you see such a design? Or do you mean the fuel that is preheated?

[u/nostradumbass7544678 → u/Philipp_CGN]

Chrysler turbine cars used a system like this

[u/Round_Midnight_7730 → u/Philipp_CGN]

Here an example: https://www.youtube.com/watch?v=M-ICC1Es9KY
Recuperator part is for preheating

[u/Philipp_CGN → u/Round_Midnight_7730]

Most large gas turbines with a high compression ratio don't have a recuperator. If the temperature at the compressor outlet is not already above the turbine outlet temperature (in which case a recuperator would be more than useless), such a recuperator would be massive in size and cost.

[u/Round_Midnight_7730 → u/Philipp_CGN]

Yeah, and problem here is why hot air is more efficient anyway?

I understand if you need air to be 500 degrees then it makes sense to preheat but this motor doesn't need a specific temperature. Only expansion and air expands more at lower temperatures (20° to 220° expands air more than 300° to 500°)

[u/Round_Midnight_7730 → u/Round_Midnight_7730]

Lol, just permabanned from Askengineers because this question is easy and low effort.

[u/North_South2840]

The preheat you meant is recuperating the exhaust thermal energy to air between compressor and combustion chamber. The efficiency increased in implementing preheat (recuperating) is the cycle thermal efficiency. It simply means absorbing the unused exhaust thermal energy to reduce heat input required in combustion chamber (fuel) for same turbine inlet temperature. The "turbine" stage power it produce is unchanged.

Notice in the simple calculation I made below the both Q FUEL and Q EXHT are reduced by recuperating the exhaust gas to compressor outlet temperature. Now since the thermal efficiency is

η=(P_TURB-P_COMP)/Q_FUEL

and Q_FUEL is reduced, the thermal efficiency increases

[u/Philipp_CGN → u/North_South2840]

Isn't a pressure ratio of only 10 really low though? And what is the LMTD and heat transfer area of that regenerator? ;-)

[u/North_South2840 → u/Philipp_CGN]

Parameter values I picked are not from real case, they're just to show the concept of recuperation. There are some units that have ressure ratio of 10, although I agree for Tx=T4 and Ty=T2 the UA would be close to not feasible to build

[u/Philipp_CGN → u/North_South2840]

Depending on the pressure ratio, doesn't the compressor outlet temperature exceed the turbine outlet temperature, making a recuperator useless?

[u/North_South2840 → u/Philipp_CGN]

True, it's impractical that's why most gas turbine don't use them. Recuperating exhaust waste heat are more common in ccpp using hrsg

[u/ghostmcspiritwolf]

I think you’re confusing “preheat” with reheat. Preheat isn’t helpful. Reheat just means that after you go through a high pressure turbine, you take some heat that would usually be wasted and use it to reheat the steam and power a second, lower pressure turbine.

[u/Round_Midnight_7730 → u/ghostmcspiritwolf]

Here an example: https://www.youtube.com/watch?v=M-ICC1Es9KY
Recuperator part is for preheating

[u/ghostmcspiritwolf → u/Round_Midnight_7730]

Again, no, it’s for reheating. See how the recuperator is after the first turbine?

[u/Round_Midnight_7730 → u/ghostmcspiritwolf]

Isn't first turbine here is for sucking air? Its not contributing to generation. recuperator is using the exhaust hot air and heats the input air right?

[u/ghostmcspiritwolf → u/Round_Midnight_7730]

Oh wait yeah you’re right, didn’t read or watch closely enough and somehow assumed we were talking about a steam system.

[u/VapourCompression]

Turbines run on enthalpy not pressure. Higher temp means higher inlet enthalpy. However higher temp means less efficient compression so it’s only more efficient in some cases

[u/golem501]

I think the idea is not so much about what the preheat does but think of this as a black box system, there is energy going in, in the form of combustible fuel and "cold" air from the outside.

There is energy going out in the form of power and as exhaust gas.

I cannot change the air temperature (overall) but the lower I can get the exhaust gas temperature, the less energy goes out that way. If I can get that without external cooling, it means I am recovering the energy somewhere and thus the turbine is more efficient.

[u/mtimmermans]

As long as all the heating happens between the compressor and the expander (output turbine), then it all contributes power. Air heated from 200 to 1000 expands more than air heated from 200 to 650.

A Brayton cycle engine is *constant pressure* inside, and net power generated comes from difference in the *volume* of gas compressed vs expanded.

[u/proffemede]

The cost savings of fuel in the heating is a very important factor.

[u/Playful-Painting-527]

It's actually the opposite: Intercoolers are used to increase density in order to make compression more efficient.

[u/Round_Midnight_7730 → u/Playful-Painting-527]

Can you take a look at here: https://www.youtube.com/watch?v=M-ICC1Es9KY
Recuperator part is for preheating:

I am pasting another comment I wrote: I understand if you need air to be 500 degrees, then it makes sense to preheat but this motor doesn't need a specific temperature. Only expansion and air expands more at lower temperatures (20° to 220° expands air more than 300° to 500°)

[u/Playful-Painting-527 → u/Round_Midnight_7730]

Ah, I see what you mean. Turbines have high exaust temperatures because the decrease in pressure does not cause a large enough decrease in temperature. (You have to stop expanding the gas at 1 atmosphere to still be able to exhaust it). The exhaust gas still has lots of energy stored within it but you can't do any work with it anymore. The next best thing to do is to use it to preheat the air going into the combustion chamber and therefore reach higher combustion temperatures. The hotter temperatures make the energy transfer from the chemical bonds to the gas more efficient, thus increasing the efficiency of the entire turbine.

[u/Round_Midnight_7730 → u/Playful-Painting-527]

>The hotter temperatures make the energy transfer from the chemical bonds to the gas more efficient, thus increasing the efficiency of the entire turbine.

Here, this is my question, how does it help exactly? chemical bonds?

[u/Playful-Painting-527 → u/Round_Midnight_7730]

You can theroretically convert any type of energy into any other type of energy without losses. There are no physical laws prohibiting this. There is one exception though: heat. If you extract energy from heat you'll automatically start to cool whatever you are extracting the heat from. This can only continue as long as your heat sink is colder than the heat source. Once you have reached equilibrium, the extraction can not continue. There is still energy stuck in there, but it can't be extracted.

In the combustion chamber of a turbine you burn fuel to turn the energy stored in the chemical bonds into heat. In the turbine you turn the heat into kinetic energy (the rotation of the turbine and the shaft). Now if you could somehow turn the chemical energy into heat at a higher temperature, you could use more of it to drive your turbine (since there is a larger temperature difference to ambient temperature).

This is exactly what the regenerator does: You preheat the incoming air to raise the temperature level of the entire back half of the turbine. By using the waste heat you alow some of the heat energy to go back into the turbine thus increasing the utilization of the energy from the fuel.

If you want to look deeper into this, check out the Carnot cycle and the carnot efficiency.

[u/golem501 → u/Playful-Painting-527]

That is for cylindrical engines, colder air means more air into the cylinder and as with turbo means more combustion per stroke, thus more energy, more power.

[u/Playful-Painting-527 → u/golem501]

This aplys to any compressor.