ORC, Freon instead of Water

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dusty
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ORC, Freon instead of Water

Post by dusty »

Does anyone have experience or knowledge concerning the use of Freon instead of water in an ORC (Organic Rankine Cycle) closed cycle system?
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Dick_Morris
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Re: ORC, Freon instead of Water

Post by Dick_Morris »

I believe William Lear (Lear Jet and stereo fame) experimented with its use in a bus about 40-50 years ago.
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dusty
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Re: ORC, Freon instead of Water

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Dick_Morris wrote: Thu Aug 26, 2021 12:33 am I believe William Lear (Lear Jet and stereo fame) experimented with its use in a bus about 40-50 years ago.
Yes, he called his mystery working fluid, "Learium". Since then, many thousands of ORC systems have been designed, built, and installed all around the world. Various Freon's & petroleum products have been used to replace water in all variety of expansion engines including piston, scroll, screw, and turbine. Different working fluids can have vastly different characteristics than water; for example, the critical point of water is 3191psi at 705F, compared to the critical point of R365mfc (a Freon used in ORC engines) of 467psi at 368F. If I want to build the most efficient steam turbine using water as the working fluid then I need to build it to use steam at 3200 psi at 705 degrees F; very expensive to do. But if I use R365mfc instead of water I only need to build it to withstand 467psi at 368 F; I could build the entire turbine using Aluminum and standard gaskets and O-Rings for sealing.

My problem, and the reason for my question; how to properly size a multi-tube boiler using a Freon to feed a 3 stage axial flow turbine if I know the total area of the "steam" nozzles is 0.23 square inches?
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ChipsAhoy
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Re: ORC, Freon instead of Water

Post by ChipsAhoy »

I spent some years working on propane powered generators that used those principals. I did not do any of the engineering. The company who designed and supplied the units was called Ormat. The generator was called an OEC.
If you go to their web site you may find some formulae to help. They do share a "schematic" of their generators, but I know first hand it looks only similiar to what's inside. They process is very efficient. Our need was to generate electricity for a battery bank in remote locations where we could only restock propane fuel during mid summer. Basically, in a closed sealed system, we drew a vacuum to bring the "freon" brew, close to a boiling point, add minimal heat, where it was allowed to expand across a "T" wheel. So, add a very few BTUs to make "Steam", then very little cooling to return , or "condense" the gas back to a liquid.... then recycle. All I can say, but try Ormat Energy Converter.
Scotty
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dusty
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Re: ORC, Freon instead of Water

Post by dusty »

ChipsAhoy wrote: Fri Aug 27, 2021 11:33 pm
The company who designed and supplied the units was called Ormat. The generator was called an OEC.
If you go to their web site you may find some formulae to help. ....., but try Ormat Energy Converter.
Scotty
Thanks for the info on Ormat. Looks like they're into Geothermal, Solar thermal, and waste heat technologies; all well suited for Freon use. Sadly I could not find any data that might be applicable to my project.
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Re: ORC, Freon instead of Water

Post by ChipsAhoy »

Digging thru my old training stuff, the long way around the mountain I got here.

https://www.nuclear-power.com/nuclear-e ... lculation/
I doubt this is what you need, but this may lead you to better resources.
I believe you need to look for "Enthalpy Change".
I did not work with this but these are the people supplied some of our training materials.
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John Hasler
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Re: ORC, Freon instead of Water

Post by John Hasler »

Googling

thermodynamic properties of R365mfc

gets los of hits.
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dusty
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Re: ORC, Freon instead of Water

Post by dusty »

ChipsAhoy wrote: Mon Aug 30, 2021 9:20 am
https://www.nuclear-power.com/nuclear-e ... lculation/

I believe you need to look for "Enthalpy Change".

Scotty
Thank you Scotty, for pointing my to a great resource,...it's exactly what I need to get started. I'm an old retired Electronics Engineer and have no formal training in Thermodynamics beyond a couple days in college learning about the basic properties of an ideal gas. I was hoping to find a table or graph that shows how many BTUs can be transferred across X square inches of metal tubing (Aluminum) per some quantity of time. The text you've pointed me to appears to be easy to understand and very much needed fundamental knowledge of thermodynamics.

I know I have much to learn,...its the primary reason I joined The Home Machinist.
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Re: ORC, Freon instead of Water

Post by dusty »

John Hasler wrote: Mon Aug 30, 2021 10:04 am Googling

thermodynamic properties of R365mfc

gets los of hits.
That's pretty much where I started, except I Googled, "ORC using R365mfc",...both searches result in very similar results. Problem is, I haven't found anything that tells me how many square inches of Aluminum tubing I need using my diesel fired forced air burner. Closest I've come so far is a formula from https://www.engineeringtoolbox.com/boil ... _1115.html

PBoHP = A / k

where

PBoHP = boiler horsepower

A = heat transfer area (ft2)

k = standard area (ft2) - in New Zealand it was common to use 17, in US it was common to use 10 before year 1900 - and more common to use 5 after

So at least for water/steam I can find the square foot area of water pipe I need in a boiler if I know how many Boiler Horsepower I want.

BUT, this simple formula doesn't seem to take into account the differences in heat transfer rates of Aluminum vs Stainless Steel vs Copper. So I think I need to keep looking and learning.
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Re: ORC, Freon instead of Water

Post by Wolfgang »

Dusty,
Without going into numbers detail, the metal of the tube or fire box wall (copper, steel, stainless steel, aluminum, etc.) has a very small effect on the overall heat transfer coefficient through that wall.

To get thermal energy from the products of combustion (fire side) to the working fluid (water, etc.) on the other side of the boiler wall, three distinct thermal resistances need to be overcome, of which the metal wall is only one.

Think of an electrical analogy to this problem, to wit, 3 electrical resistors in series.

Let's call them, from left to right, R1, R2, & R3.

R1 = 100 Ohms, R2 = 1 Ohm, and R3 = 10 Ohms.

The total series resistance is 100 + 1 +10 = 111 Ohms. Making the middle resistance 3, 4, or even 5 Ohms affects the total resistance very little.

This problem is exactly the same for trying to push BTUs or calories through a boiler wall.

The fire side resistance R1 may be 100 to 500 times the resistance of the metal wall R2, and the fluid side resistance R3 may be 10 to 50 times the resistance of the boiler wall.

While there may be numerous reasons for choosing one metal over another in a given heat transfer situation, corrosion being an important one, its conductivity is usually much less important.

This illustrates the reason why model locomotives, whether copper firebox and tubes, or steel firebox and tubes, perform very similarly in competent hands. w
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Re: ORC, Freon instead of Water

Post by John Hasler »

>The fire side resistance R1 may be 100 to 500 times the resistance of the metal wall R2, and the fluid side resistance R3 may be 10 to 50 times the resistance of the boiler wall.

Engineeringtoolbox has tables, equations, and calculators to help compute the heat transfer from the hot gas to the tube. As Wlofgang notes you can pretty much ignore the tube itself. The transfer rate to the fluid depends on the flow rate, the dimensions of the tube, and the properties of the fluid (easy to find for water, of course). The data you need for your coolant must be out there somewhere since transferring heat through the wall of a tube is pretty much how it's used.

I went through this calculation recently for a demand hot water heater. Worked out nicely (no boiling, though. That complicates it).
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dusty
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Re: ORC, Freon instead of Water

Post by dusty »

Wolfgang wrote: Tue Aug 31, 2021 10:21 am
The fire side resistance R1 may be 100 to 500 times the resistance of the metal wall R2, and the fluid side resistance R3 may be 10 to 50 times the resistance of the boiler wall.
Many Thanks to Wolfgang and John Hasler; all my instincts were telling me the exact opposite, that tube material would be the largest thermal resistance. Good to know that tube material is of minimal importance. :D
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