Scroll Expander vs Axial (turbo) Expander 1

[Dawsonh4]

I am trying to determine the advantages/disadvantages of different types of expanders for air expansion that will drive a generator. The two most promising seems to be turbo expanders and scroll expanders. Does anyone have an understanding of how one could be better than the other?

 

[BrianPetersen]

A scroll compresssor/expander is a (kinda) positive-displacement machine, albeit a leaky one. A turbine is a turbomachine.

You've given us NO indication of the scale of the equipment or what it's for or how it's going to be used. "more information = better answers". That in mind ...

Positive-displacement "stuff" (compressors, engines, expanders, etc) tends to be better at smaller sizes. Turbomachinery tends to be better at larger sizes. A lot of this has to do with a fluid-dynamic concept called the "Reynolds number". This is why your household refrigerator or air-conditioning unit compressor is a positive-displacement apparatus whereas the electrical generating station that supplies electricity to your city is a turbomachine.

Positive-displacement "stuff" (compressors, engines, expanders, etc) tends to work decently over a fairly wide range of operating speed and load without the efficiency becoming atrocious. Turbomachinery tends to like staying close to its design power and speed, and the efficiency drops into the toilet at low operating speed. This - in addition to the Reynolds-number-related scaling effects - is why the engine in your car (which probably functions adequately from 800rpm idle to 6000 rpm full power) is a positive-displacement device, and the engine in a transcontinental airliner (idle speed is probably half-ish maximum rated load) is a turbomachine.

 

[Dawsonh4]

Thanks for the input Brian. I am new to expanders and don't know how they would be sized. Maybe by volume? Looking for something that can handle 25-75 cubic feet per minute. It can be leaky - this is not a closed loop system like a refrigerator. I am just trying to get 0.25 kW worth of power out of a coupled generator.

The turbo expander I am looking at don't resemble a turbine I am used to. There are not multiple stages and is much more simple. Here is an example.

https://www.turbomachinerymag.com/view/expander-compressors-an-introduction

Thanks again for the help and patience!

 

[BrianPetersen]

25 - 75 cfm ... at what pressure and from what source??

At 250 W output power, this is going to be a small machine. This generator that you want to use ... are you prepared for the RPM at which a hypothetical turbine impeller of that small size is likely to be turning at?

The power that you're talking about, is probably ballpark the output of a pneumatic impact wrench or other common air tool. Those mostly (possibly all of them?) use vane motors. I don't know the efficiency of those, but it is surely terrible. My 2 hp air compressor won't keep up with the demand of my die grinder. I don't know how much power output the die grinder has, but it's surely nowhere near 2 horsepower.

Big picture: Back up. Back waaaaaaayyyyy up. Big picture. What are you actually trying to do.

 

[Dawsonh4]

Backing wayyyy up.

Traditionally natural gas wells have used pie charts that track things like temperature, pressure, and flow of a well. These charts require daily visits to remote well sites to gather information. Recently these charts have started to be replaced by digital meters and a modem to gather and broadcast the data to a central location once or twice a day. This equipment uses batteries to power them. I would like to evaluate the possibility of replacing batteries with the an expander generator machine. If it creates more power, it can likely be used somewhere else (lighting, sensors, etc.)

The wells of focus are older (15 + years) and have next to no contaminants or water in the gas stream. The gas stream typically comes out around 500 - 2000 psi and gets fed into a pipeline at 100 psi. There are regulators that control the pressure. Temperature is normally about 100 F.

I thought the simple axial turboexpander would be a good choice, but to your point, the power needed is small and the expander should be sized appropriately.

 

[BrianPetersen]

Solar panel to charge the batteries would be the normal thing to do. Little solar panels up on a pole together with some small electrical gizmocontraption of any sort, are common around here. No moving parts.

 

[Dawsonh4]

Agreed - a lot of wells use solar. Just curious if expander generator is a better more cost effective solution. The added benefit is that there is room to expand into larger wells.

I am still trying to understand the benefits and drawbacks of a scroll vs axial expander. It seems so far that the scroll may be more affected by debris and water, but is more efficient at lower volumes than axial.

 

[EdStainless]

For this size a scroll device would make more sense.
Small turbines run at very high speeds which creates other issues.

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[Dawsonh4]

Thank Ed. Do you recommend scroll device because solely because turbine would run at much higher speeds?

Also for my own clarity. When I think of a turbine I think of a gas turbine like this -

https://www.ge.com/gas-power/services/gas-turbines/upgrades/advanced-gas-path-7f

When I refer to a turbo (axial) expander I am thinking of something more like this -

https://www.openpr.com/news/1034053...ryostar-atlas-copco-samsung-air-products.html

Is there a difference?

 

[BrianPetersen]

Reynolds number effects tend to push smaller turbomachines to be of the centrifugal design. It's easier to get a significant pressure ratio in a single stage with that design. Think of automotive turbochargers. But ... your proposed pressure ratio is off the scale!

The pressures that you are talking about, and yet the tiny power output that you wish to make use of, will not lead to an efficient and practical turbomachine design. It would have to be something the size of a thumbnail spinning at about a million rpm with flow passages the size of a pinhole, and the Reynolds-number effects will be killer, leading to very poor efficiency.

The photo in your second link shows a much bigger machine.

 

[EdStainless]

I was thinking of this

https://airsquared.com/products/scroll-expanders/

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P.E. Metallurgy, consulting work welcomed

 

[Dawsonh4]

Ed,

That was the scroll I was looking at, albeit it is still a little too large.

On the axial (turboexpander) side there is this company offering more of a turbine

https://www.sapphiretechnologies.com/uploads/Sapphire_Technologies_Brochure_617228ecb1.pdf

Assuming both of these were appropriately sized for my application I was curious as to the benefits and drawbacks of each.

 

[BrianPetersen]

https://www.sapphiretechnologies.com/products/

Basic specs are on there.

Maybe you just need to think bigger!

 

[Dawsonh4]

Bigger might be better in this case, but also more expensive.

Still curious to hear anyone's input on the pros and cons of the two types of expanders below

https://www.sapphiretechnologies.com/uploads/Sapphire_Technologies_Brochure_617228ecb1.pdf

https://airsquared.com/products/scroll-expanders/

 

[Dawsonh4]

Following up.

Brian and Ed gave some great input, but didn't quite answer my original question.

Does anyone have any experience with expanders that can explains some of the pro and cons of an axial expander such as this (

https://www.sapphiretechnologies.com/uploads/Sapphire_Technologies_Brochure_617228ecb1.pdf)

compared to a scroll expander such as this (

https://airsquared.com/products/scroll-expanders/)

 

[EdStainless]

a scroll device is better suited to small applications.
It runs at a slower speed, uses conventional bearings, will work reasonably well across a wide range of flows, and can be coupled to any load.
The Sapphire device is very high speed, uses magnetic bearings, has integrated generator and associated electronics so no options.
I would sure like to see the eff vs flow curves first.
There are no tech details on the sapphire page.

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P.E. Metallurgy, consulting work welcomed

 

[Dawsonh4]

Thanks Ed!

From my quick research it looks like magnetic bearings do not have many drawbacks besides being more expensive.

As for the Sapphire device - how can you determine that it runs at a very high speed?

 

[EdStainless]

Why else would they bother with very expensive and delicate magnetic bearings?
The Sapphire device is interesting, on the one hand it is a full package.
But on the other you are stuck with exactly what they decided to build.
There are people out there that will design and build energy recovery systems.
They could package a scroll machine and the appropriate generator/electronics for you.
And in the future this system would be upgradable.
Unless your flow is a fixed temp, pressure, and mass flow make sure to study the eff vs flow curves carefully.
Axial devices are notorious for having very poor turndown.
Even large steam turbines will show this and small devices will be worse.

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[Dawsonh4]

Thanks again Ed. Very helpful!

When you refer to turndown is that the delta in max and min capacity?

Would you have any other companies to suggest for a scroll expander besides AirSquared?

 

[EdStainless]

I have never used one that small so I have no direct experience.
You could reach out to companies that do ORC (organic Rankine Cycle) energy recovery from low temp heat sources (such as geothermal).
I have even seen screw compressors used for this (run backwards).

Yes, that is what I meant by turndown.
With an axial flow device it wouldn't surprise me to see the eff drop faster than the load.
If the unit was 95% eff at 100% rating it might be 35% eff at 50% load.

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P.E. Metallurgy, consulting work welcomed