Expanding property of Compressed Air

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addiator
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Re: Expanding property of Compressed Air

Post by addiator » Wed May 24, 2017 10:38 am

cyberbadger wrote:I basically agree to disagree with everyone and I'm half sorry I brought it up. :)

-CB
I would like to ask you, what on earth are you actually asking about. I see every single answer just makes everyone more confused, because you have not defined your problem in clear thermodynamic terms.

In technical thermodynamics, one of the core concepts is a working fluid. A working fluid is a fluid, gas, liquid or vapor undergoing a phase change, that is being subjected to various processes to get a desired output. Using this nomenclature, we describe the working of engines in terms of cycles. For example, a saturated steam engine is described by the classic Rankine cycle. This concerns not processes inside the engine, but the whole plant. This is as follows:

Constant pressure (isobaric) heat addition in the boiler, turning water into steam
Isolated (adiabatic) steam expansion in the engine
Constant pressure (isobaric) heat extraction in the condenser, turning steam into water
Isolated (adiabatic) water pumping - increasing the pressure of it in the feedpump

That is to say in the ideal cycle the feedpump and condenser pumps are considered as one object. The amount of work supplied to those is minute compared to the expansion work of the steam. What may be confusing to a beginner is that the pressure in the boiler and condenser are constant. A modified Rankine cycle can also have superheating, which is also isobaric, but greatly increases the enthalpy (energy) of steam. This greatly increases the efficiency, because the energy given out in the condenser is still the same, and the useful work of the engine is a the difference between the two. The energy given out in the condenser is of a great quantity, but no quality, as the steam after expansion in the engine to a very low pressure will have a temperature close to room temperature.

A comparable cycle using air, would be the Brayton cycle:
Constant pressure (isobaric) heat addition
Isolated (adiabatic) air expansion
Constant pressure (isobaric) heat extraction
Isolated (adiabatic) air compression
Compression, because air does not change phase. Simply put, you compress a kilogram of air to lets say 10 bar, you heat it up under the same pressure, and you expand it back in a cylinder or turbine, where it will give out more work than you put in due to the added heat. This is how a gas turbine, or a jet engine works, where the heat addition happens in a combustion chamber in which the pressure does not increase (!). The heat extraction happens in the atmosphere which closes the cycle. Despite the fact that compression takes more work than pumping, this cycle can be made more efficient than the Rankine, as you can bring air to an even higher temperature than superheated steam.

Now, without the heat addition and in ideal conditions, your expansion will only give you the same amount of work you put into the compression. It would be the same with steam, if you compressed steam, or any other fluid for that matter.
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cyberbadger
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Re: Expanding property of Compressed Air

Post by cyberbadger » Wed May 24, 2017 12:36 pm

As indicated I am done talking about this thread.

-CB
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