Critical Temp & Pressure? 4

[altalab]

What exactly happens if you heat a given working fluid above its critical temp? Does the pressure stop rising? Is there any benefit in a closed heat engine system to do so? I am working with R134a for a working fluid
Thanks

 

[25362]

R134A is 1,1,1,2 tetrafluoroethane. Its critical point, CP, is at 101.2^oC and 4050 kPa. Above the critical temperature no pressure could manage to liquefy it.
Above the CP the gaseous and the liquid phases have identical characteristics, and there is no liquid-vapour transition.

 

[altalab]

Would the pressure continue to rise if you applied more heat after the critical point? Or, is there a cap on pressure generated?
Thanks

 

[TD2K]

Yes, the pressure continues to rise.

As 25362 says, it starts to behave more like a gas (it starts to behave more like an idea gas the farther you get away from the critical point).

If you heat a gas in an enclosed container, the pressure rises.

 

[altalab]

Ah ha,...I see. Do you know if the super critical working fluid/gas is usable in a closed Rankine cycle heat engine scheme? I just need to know if I can increase highside pressure for more power or is this not advised or dangerous?
Thanks for the response.
Bob

 

[GenB]

It is my believe that the pressure will rise just a little only because of expansion but the gastemp will rise according to the heat privided andexchanged with the gas and it will be very dangerous.
ER

 

[25362]

To altalab, in a Rankine cycle the liquid (generally water) is vaporized and the vapour is superheated at constant pressure prior to entering the turbine for its adiabatic expansion. The boiler and related facilities are designed to stand the relevant pressures. So, what danger are you referring to ?

 

[altalab]

25362
The danger, in my mind, is only the product of the unknown (to me) concerning super critical fluid/vapor state. I'm just being cautious.(see generalblr's response above)
Does superheat mean heat added above the critical point?
Is there a system advantage to add heat above the critical point?
In your explanation, when "superheated at contant pressure", is the pressure constant naturally or is there some device the ensure constant pressure?
Thanks

 

[25362]

It seems you are unacquainted with steam systems in general, and the Rankine cycle, in particular, or may be English is not you mother tongue and something is lost on translation.

Superheating has nothing to do with critical temperature. Superheating a vapour means heating it above its saturation temperature, after leaving the boiler.
Water boils at 100^oC at ordinary pressure, heating the vapours above this temperature means one is superheating them. Thus a vapor can be superheated and still be at a subcritical temperature.

Besides, there is nothing extraordinary in superheating steam above its critical temperature of 705.5^oF. In fact, many turbines are fed with steam at temperatures well above 750^oF. For example, take a power plant operating at 86 bar. Saturated steam would be at 300^oC. By superheating in the boiler house it is brought to 500^o, much higher than the CP of 374^oC.

We are speaking of a Rankine cycle. This ideal thermodynamic cycle consists -by definition- of heat addition at constant pressure, an isentropic expansion, heat rejection at constant pressure, and isentropic compression. It is used as an ideal standard for the performance of heat-engine and heat-pump installations operating with a condensable vapor as the working fluid, such as a steam power plant. Aka steam cycle.

A fluid may decompose, become reactive or corrosive or explosive at a certain temperature (a chemical effect), but this is unrelated to its critical temperature (a physical effect).

Pressure can be controlled within the design parameters with suitable instrumentation. Equipment is designed to stand the working pressures. Higher pressures and temperatures in a steam power cycle increase its thermal efficiency but also the capital investment. Thus, in practice, most power plants operate below 100 atm and 600^oC.

 

[altalab]

25362
Yes, you are correct about lack of experience with steam or rankine. My project requires some knowledge in this area, hence the post for basic answers. Thanks for taking the time to explain my query. It gave me some real information to ponder in my design.
Bob

 

[davefitz]

Supercritical steam cycles are used, and arenow the dominant form for the Rankine cycle for new , large utility units in japan , Korea, and Europe.

The original reason for using the supercritical cycle was, believe it or not , due to problems of scaling up units in sizes over 750 MWe. Above that unit size , it is difficult to arrange a steam drum and associated furnace circuitry components. Other benefits that were realized were better cycle efficiency and better fuel flexibility ( the final steam temperature is no longer related to the relative heat absorpiton characteristics of the superheater vs the steam generationg surface, which varies if a different slagging type fuel is burned).

You can use the supercritical cycle in a power cycle based on refrigerants, and R134A can be used. A variant of the Kalina cycle was proposed by ( Rosenfeld ?) circa 1990 which use supercritical ammonia to obviate the need for many of the distillation collumns required for the regular Kalina cycle.

 

[MortenA]

This started out as a question re. critical temperature and thats NOT related to superheating steam. Then somebody got rankine tangled into this - but its still not related to a "super critical" gas.

Superheated steam is steam thats hotter than the boiling point at a given temperature.

A gas in the dense phase/critical phase is a gas thats in the critical part of the phase map.

Best regards

Morten

 

[25362]

to MortenA, kindly note that it was not not "somebody" but altalab, the original questioner, that added (Jan 17) the query about Rankine and supercritical conditions.

 

[altalab]

To all respondants: Thank you for your valuable inputs. I now have a better understanding of a portion of an area of knowledge I must utilize in my project. Now that your here, I have another question I need to know about.
I am constructing a small electric generating solar/solid fuel system using R134a and a rotary vane compressor turned into a motor. The motor has a 1:3 expansion ratio. The question is:
Should I count on the pressure differential from boiler to condensor (about 500psi)to push/pull the motor efficiently or should I fit a larger (or faster of same displacement)secondary motor downstream to bump the ratio to 1:6 or 1:9? My experience with reciprocating steam engines indicates a higher ratio, but they were operating to the atmosphere and of ancient designs. What do typical large turbine installations work at concerning total expansion of the gas?

 

[davefitz]

modern turbines will expand the gas down to 10% liquid by weight in the exhaust. This 10% limit with steam/water is based on avoiding erosion of the L-1 stage . In combined cycle uits, the reheater steam is at 400 psig at 1050F expanding down to 2 in HgA condenser pressure , with exhasut gas UEEP at 10% liquid by weight. Thhis may be a 400:1 pressure ratio.

steam turbines on nuclear cycles will expand down to the point where liquid content is unaceptably high, then pass the steam thru a liquid separator, then pass the remaining saturated ( dry) steam thru the final stages of the turbine.

The actual pressure ratio needed for R134A to meet its erosion limit would have to be determined from testing.

 

[altalab]

davefitz
I wont nearly have those expansion ratios. I'm hoping that since I am using a positive displacement rotary expander, I'll be able to capture most of the pressure gradient energy between boiler and condenser (~500psi) As long as I dump the excess heat exiting the motor. One dump would be a feedstock preheat. I guess the other is fans on the condenser to throw away the heat (loss of eff?) If this doesn't produce desired results, I might have the option to run the exhaust through an identical expander running somewhat faster due to a mechanical connection with the primary expander. I will have to determine the amount of overdrive on the second to emulate a seemless expansion thru the system. Any thoughts?

 

[davefitz]

I do not have handy a T-S chart for R134a, so I cannot be sure, but it seems that if you have 500 psig available, you could expand the gas over a 5:1 ratio, send it thru a liquid separator, reheat it to ambient or higher temperaures, then expand the gas again ( from 85 psi to exhaust pressure) to get max output from the process.

 

[altalab]

Would this reheated gas go through another expander downstream of the first? Would it have to be a higher displacement unit as to match torque and/or speed of primary expander so they could be coupled together on the load?

 

[davefitz]

Nowadays there are solidstate devices that will convert the variable frequency power from a variable speed small expander to synchronized 60Hz. This means the 2 units can be separate and independent and non sychronized if each has such a solid state device. Similar devices are used on the microturbines from Capstone.

A similar concept can be applied to the use of Pelton wheel expanders - one can replace some pressure reducing valves in large processes ( ie refineries) with pelton wheels and generate electricity instead of simply losig a throttling loss.

 

[altalab]

Are these the same pelton wheels used in hydro power? If so, are they as efficient with a vapor as liquid?