Replacing working fluid in turbine

[yoel280206]

Hi all,
Please let me know your opinion.
I know a turbine with design working inlet pressure 150-200 psi and outlet pressure 50 psia.
So the pressure ratio is around 3-4.
The working fluid is R134a.
I want to replace fluid with methanol which has lower working pressure.
The design inlet pressure turbine is 1 bar = 14.5 psi.
And the outlet pressure turbine is 0.27 bar.
The pressure ratio is around 3-4 also.
Because of similar pressure ratio, you think, can i just replace the fluid of R134a with methanol?
Thank so much..

 

[ivymike]

are your seals and metals compatible w/methanol? any chance that moisture will get in?

 

[ivymike]

oh... and how big of a problem will it be if every little leak magically finds a way to catch fire?

 

[yoel280206]

The fluid is pure methanol and without contact with air.
The process is closed loop.
Because the pressure is low, i think the seal can prevent the leakage.
What do you think about the turbine?
Thank so much..

 

[jistre]

The seal is a mechanical object. Never assume it WILL do what it's supposed to. As Ivymike asked, WHEN the seals fail and you get leakage, what are the chance of an earth-shattering kaboom? Are those chances small enough to risk it?

You mentioned that the current working fluid is R-134A. Remember that a lot of the early refrigerants were poisonous/flammable, and safer operation, at least in part, drove development of the *FC's.

There's a big difference between 'can' do something and 'wise' to do something.

That's all I have. Just wanted to make sure you're considering all the risks.

 

[jistre]

Another thing I just though of is that since you're operating at above atmospheric pressure, are there any pressure relief devices in the system that may vent to areas that are incompatible with methanol, given that R-134a is currently in the system?

 

[MintJulep]

I'm not a turbine guy, but I'm pretty sure that mass flow comes into play in the power output.

In addition to all of the good advice provided above, you should also think about what methanol will do to the lube oil.

 

[yoel280206]

Thanks a lot for your advices.
I realize the consequency of using a flammable liquid.
Besides refrigerant, they have low boiling point and high heat latent.
Small flow rate of vapor with relative low pressure can produce high energy.
The pressure is the main problem because it is low.
I have searched turbine with inlet design pressure below 2 bar and outlet 0.27 bar, but i cannot find it in the market.
Theoretically, the system is possible, however in fact some equipments are unavailaible.
My system aims to work on temperature 100-120 Celcius (low grade heat).
About R134a, infinity turbine has done succesfully the experiment and go commercially.
I suppose to use their turbine but their working pressure
is much higher than my system as i mentioned above.
If i want to use methanol, my system should work on high temperature.
If anyone know the turbine which has design working pressure in below 2 bar, please let me know.

 

[rmw]

Turbines are all about nozzles, blades and flow velocities and pressure drops. I am not going to look up the properties of the two fluids, but unless the volume flow rate is similar so that the velocities are right for the nozzling and blading, I doubt it. Not, at least if you expect to get the same amount of work out.

The mass flow rate has to adequately fill the volume of the flow path for the turbine to work well.

rmw

 

[pmover]

yoel280206,

your question should be directed to the turbine mfg. the mfg should have the necessary means and resources to conduct the analysis for you . . .

you may want to conduct your own analysis using the enthalpy data from the NIST website (

http://webbook.nist.gov/chemistry/fluid/)

for both fluids (current and proposed). follow the thermo analysis written in the thermo textbook. since you have the the current fluid and operating/design parameters, you ought to be able to conduct a preliminary analysis (power, flowrate, etc.).

do not lightly take the fluid methanol . . . or casually think it is safe for the proposed use. remember, qualified personnel need to be trained in handling the fluid and in any maintenance practices. i would not want to be an operator or owner of a turbine using methanol . . .

from the website:

http://www.mgc-a.com/methanol/technical/msds.html

"Methanol is a clear, colorless, poisonous, flammable liquid. Excessive breathing of vapors can cause irritation of the respiratory tract and dizziness. Swallowing can cause blindness and death. Causes irritation to the skin, eyes and mucous membranes. Vapors are heavier than air and may travel along the ground to remote ignition sources. Vapors may then ignite explosively. Keep away from heat, sparks and open flames. Avoid breathing vapors. Avoid contact with skin, eyes and clothing. Wear protective goggles and gloves and impervious clothing when handling this product. Wash thoroughly after handling."

hope this helps and good luck.
-pmover

 

[davefitz]

Most vapor turbines will operate with a nearly t "flow factor" at the entrance to each stage; Fi= (Wi/P)*sqrt(sv/P)

where Fi = flow factor for stage i
Wi= mass flow thru stage i
P= abs press entering the stage
sv= spec vol at stage i

as I recall ,this yields a relative velocity of vapor against the rotating surface of 50% of soundspeed of the vapor,which maximizies the stage efficiency.

 

[gruntguru]

Because of similar pressure ratio, you think, can i just replace the fluid of R134a with methanol?
No. Pressure ratio is not the primary determinant of turbine power. Pressure difference times volume flow rate is a rough starting point (and on this score you are way down) but there are many factors that will need to be included in a proper analysis.