Critical Temp & Pressure?
Critical Temp & Pressure?
(OP)
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
Thanks





RE: Critical Temp & Pressure?
Above the CP the gaseous and the liquid phases have identical characteristics, and there is no liquid-vapour transition.
RE: Critical Temp & Pressure?
Thanks
RE: Critical Temp & Pressure?
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.
RE: Critical Temp & Pressure?
Thanks for the response.
Bob
RE: Critical Temp & Pressure?
ER
RE: Critical Temp & Pressure?
RE: Critical Temp & Pressure?
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
RE: Critical Temp & Pressure?
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 100oC 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 705.5oF. In fact, many turbines are fed with steam at temperatures well above 750oF. For example, take a power plant operating at 86 bar. Saturated steam would be at 300oC. By superheating in the boiler house it is brought to 500o, much higher than the CP of 374oC.
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 600oC.
RE: Critical Temp & Pressure?
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
RE: Critical Temp & Pressure?
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.
RE: Critical Temp & Pressure?
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
RE: Critical Temp & Pressure?
RE: Critical Temp & Pressure?
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?
RE: Critical Temp & Pressure?
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.
RE: Critical Temp & Pressure?
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?
RE: Critical Temp & Pressure?
RE: Critical Temp & Pressure?
RE: Critical Temp & Pressure?
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.
RE: Critical Temp & Pressure?