Scroll Expander vs Axial (turbo) Expander
Scroll Expander vs Axial (turbo) Expander
(OP)
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?
RE: Scroll Expander vs Axial (turbo) Expander
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.
RE: Scroll Expander vs Axial (turbo) Expander
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-co...
Thanks again for the help and patience!
RE: Scroll Expander vs Axial (turbo) Expander
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.
RE: Scroll Expander vs Axial (turbo) Expander
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.
RE: Scroll Expander vs Axial (turbo) Expander
RE: Scroll Expander vs Axial (turbo) Expander
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.
RE: Scroll Expander vs Axial (turbo) Expander
Small turbines run at very high speeds which creates other issues.
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P.E. Metallurgy, consulting work welcomed
RE: Scroll Expander vs Axial (turbo) Expander
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...
When I refer to a turbo (axial) expander I am thinking of something more like this - https://www.openpr.com/news/1034053/global-turbo-e...
Is there a difference?
RE: Scroll Expander vs Axial (turbo) Expander
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.
RE: Scroll Expander vs Axial (turbo) Expander
https://airsquared.com/products/scroll-expanders/
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P.E. Metallurgy, consulting work welcomed
RE: Scroll Expander vs Axial (turbo) Expander
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/Sapph...
Assuming both of these were appropriately sized for my application I was curious as to the benefits and drawbacks of each.
RE: Scroll Expander vs Axial (turbo) Expander
Basic specs are on there.
Maybe you just need to think bigger!
RE: Scroll Expander vs Axial (turbo) Expander
Still curious to hear anyone's input on the pros and cons of the two types of expanders below
https://www.sapphiretechnologies.com/uploads/Sapph...
https://airsquared.com/products/scroll-expanders/
RE: Scroll Expander vs Axial (turbo) Expander
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/Sapph...) compared to a scroll expander such as this (https://airsquared.com/products/scroll-expanders/)
RE: Scroll Expander vs Axial (turbo) Expander
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
RE: Scroll Expander vs Axial (turbo) Expander
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?
RE: Scroll Expander vs Axial (turbo) Expander
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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P.E. Metallurgy, consulting work welcomed
RE: Scroll Expander vs Axial (turbo) Expander
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?
RE: Scroll Expander vs Axial (turbo) Expander
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
RE: Scroll Expander vs Axial (turbo) Expander
OpCos struggle with this problem to generate electric power reliably with next to zero manning for remote satellites; solutions with mixed success include thermo electric generators(TEG), onboard diesel engine generator, and other hybrid solutions (natural gas powered hydraulic power packs for automated valves and battery packs strictly for controls and SCADA). Dedicated power cable from a distant mother platform is expensive but reliable. No easy solutions.
http://ecogenthermoelectric.com/tpl/docs/ecogen_ca...