blending with a stick?

[spaz]

What is general consensus on do continuous blending. I have had two compressor dealers I have talked to tell me that Oxygen can never be introduced upstream, and yet I have seen outfits that bank there nitrox exactly that way. Hmmmm?

[Stone]

spaz once bubbled...
What is general consensus on do continuous blending. I have had two compressor dealers I have talked to tell me that Oxygen can never be introduced upstream, and yet I have seen outfits that bank there nitrox exactly that way. Hmmmm?

[omar]

Well I guess that Dr. Morgan Wells, Director of the National Oceanographic and Atmospheric Association (NOAA) didn't know what he was doing when he developed the process for continuous nitrox blending.

The trick is to ensure a homogenous compressor inlet mixture. All types of compressors can be used with this process.

omar

[spaz]

HMMM...What are some references, for continuous blending? Where can I see Dr. Morgans work?

[rmediver2002]

Some information on Dr. Wells is contained within the NOAA dive manual, a very good source of reference information. Contact your nearest government printing office and you can order a copy for $44.00 (about $20.00 to $40.00 cheaper than most distributers)

http://www.envirodive.com/stikinfo.html A manufacturer site (the Nitrox stik)

http://www.amazon.com/exec/obidos/ASIN/0941332853/inktomi-bkasin-20/002-0178531-2084863 Some additional reference material on the subject

I have been using a continuous blending system for the last 8 months. Compressor is a Eagle Cadet II. I do not use a membrane system, just regulate the amount of stored oxygen going into the compressor inlet with a regulator and needle valve.

Are newest compressor a Bauer Oceanus is labeled do not compress pure oxygen and has the same disclaimer in the manual. I would not doubt companies taking this one step further saying only compress ambient air as a liability measure...

Contact me if you have any questions,

Jeff Lane

[Stone]

rmediver2002 once bubbled...
I do not use a membrane system, just regulate the amount of stored oxygen going into the compressor inlet with a regulator and needle valve.

Are you using a commercial or homemade "Stik" to homogenize the mixture before it reaches the compressor?



Are newest compressor a Bauer Oceanus is labeled do not compress pure oxygen and has the same disclaimer in the manual. I would not doubt companies taking this one step further saying only compress ambient air as a liability measure...


Are you using synthetic oil in the Bauer?

Thanks!

[Genesis]

That "manufacturer site" - the guy's getting $2,000 for a set of baffles in a piece of pipe and some cute labelling!

If that's not the juice of the day I don't know what is.

[omar]

It is not a set of baffles. It is a static inline mixer that provides both clockwise and counter-clockwise motion of the gas streams. This type of mixer is used in industrial applications with very good results.

I am somewhat leery of the baffle setup. The key to not blowing up the compressor is to ensure a homogeneous mix. The static inline mixer will do this.

That being said I can put together a blender setup with the static mixers, regulator, intake filters, flow control (rotameter) for the oxygen input, and other little parts for $325.

This is more than a oxy hacker setup but will work for commercial applications where the question of garage built may make some folks nervous.

omar

[rmediver2002]

We use synthetic oil in all our compressors


We use a commercial system to mix

Jeff Lane

[Genesis]

Ok, so its a set of directional gas jets that produce counter-rotating gas flows so that they mix really well.

Even so..... I think the comment about the price for what amounts to a few pieces of PVC, a precision orifice and a welding regulator is pretty much spot-on :)

[omar]

It is not a set of directional jets..... "static in-line mixer"

You can see the detail on the US patent office site.

Yes, it is expensive. Probably twice as much as it needs to be. But it has not had any real competition to drive the price down.

omar

[Genesis]

does it really do any better of a job, at the outlet, than any of the other means of producing a good "mix" of the gas in the stick?

And can that be quantified, with hard evidence?

If not then it falls into the "gee, that's nice" category - at least for me.

A precision orifice produces a LOT of turbulence in its immediate vicinity. Injected into an area of lower pressure (venturi?) and then having the gas given a good "spin" (perhaps a helix path through the stick down to the bottom, then back up and down the center bore to the exit?) I bet you'd get results indistinguishable from any patented gobbligook when it comes time to take a look at this from analyzing the output for its homogeneous quality (or lack thereof.)

[omar]

The static in-line mixer and the nitrox stik use a helix insert in the tube with multiple elements that reverse the direction of flow in addition to providing the "many turns".

Much simpler than running the gas up and down with injection nozzles to provide the turbulance.

I use mixing tubes with clear PVC, there are 12 elements over 24 inches. The mixing can be observed by running an input of smoke into the unit and just watching.

As I said it is a proven technology in industrial applications. The food industry uses them extensively to blend juice concentrate and water; color additives, blending syrup with carbonated water (pop) etc.

By contrast, just using baffles for mixing if not sized right and with too few of them the mixing will not be as complete.

omar

[Uncle Pug]

I was going to make a blender using PVC pipe and a machinist friend said, "Let me make you one out of stainless steel."

I said, "OK."

So he did and the gas (O2, Helium) is injected into a venturi and there are two delrin baffles in the main bore... with random holes drilled in them. So... I have two baffles over 6" and I have no way of knowing just how thoroughly mixed the gas is by the time it travels the 36" to the first stage of the compressor... but I suspect it is thoroughly mixed by the time it gets to the second stage.... but then I just suspect this to be so... no proof.

Now the post compression analysis seems to be very stable... and I am taking that to indicated that things must be getting mixed up fairly well upstream.

What do you think Omar... should I rework the blender to include more baffles/holes at angles/reverse flows ~ or maybe even a static inline mixer???

[Stone]

If it becomes apparent that I did not correctly baffle my Oxyhacker stik, I will probably spend the extra bucks to incorporate one of the in-line static mixers shown on this Cole-Parmer catelog page (http://www.coleparmer.com/catalog/0304_pdf/A-1076.pdf)

[omar]

well UP

I have always wondered if SK's blender had enough baffles in it for complete mixing. In your case it doesn't matter. This is because of the RIX. For oil lubed compressors I would rather err on the side of caution. An unmixed stream of oxygen could have nasty consequences.

If you look at:
http://www.usplastic.com/catalog/product.asp?catalog_name=USPlastic&category_name=Check+Valves&product_id=Polypropylene+Check+Valve

A nice little check valve to go between the filter and air intake will stop the blow back that we talked about some time ago.

omar

[ericfine50]

Omar,

Do you think SK's dosen't have enough baffels? Silly question, does the length of the "stick" have anything to do with it?

Eric

[FredT]

Absent that lots of turns and flow dividers work. The twisted static mixers are good products where space is critical, and a 24 turn mixer is about a good as it gets for liquids. OTOH space arond a compressor is rarely critical.

For gasses eddies and flow division is a good thing.

A VERY simple mixer can be made from 4" PVC pipe and ping pong balls. 3 or 4 feet of packed pipe will do wonders for mixing gases. Another option is to pack the pipe with loose pack cooling tower packing (VERY high surface to volume ratio with minimum flow restriction). I think the last time I priced a ball mixer for a 20CFM compressor my cost came out under $150, including the 16' of pipe scrapped cause I was just making one.

http://www.usplastic.com/catalog/product.asp?catalog%5Fname=USPlastic&category%5Fname=&product%5Fid=Plastic+Floating+Spheres

The best bet is to get balls that are about 1/3 to 1/4 the inside diameter of the pipe, and another "filler" set at 1/4 the diameter of the first ball. Final screen mesh should not be an even fraction of either size ball.

FT

[spaz]

Thanks for all the great responses.

[oxyhacker]

If you really want to get gas to mix, the trick is to fold the path back and forth over itself, and make it turn at sharp angles, for maximum turbulance and intermixing. This is what a static tube mixer does - each set of vanes is angled opposite from the last one, so the flow constantly is reversing against itself, and this is what the Oxy Hacker mixer does, much more crudely (and cheaply), with the little shelf under the holes. BTW, for those who don't like the baffles system in our mixer (or the material, or the color, or the......) we give info in the book on static mixers too, the builder is free to use whatever method he/she choose.

The problem (if you can call it a problem) with mixers using only a couple widely spaced baffles with holes drilled in them is that if you flow smoke through them, you'll often see (though this depends a lot on the speed) several discrete streams of smoke zigzagging their way from one baffle to the next but never intermixing, looking almost like someone threaded several pieces of rope though the plates.

However our feeling is it doesn't take a whole lot to mix O2 to an acceptable degree so I wouldn't worry about it if the downstream analysis is matching the upstream fairly closely.

We tested ours by making a sensor holder that could be rotated around the tube, and moved in and out, to sample at different spots in the flow path, so see if it made a difference in the reading. It didn't. Not a perfect method, since the sensor acts like an additional baffle, and doubtlessly further homogenizes the mix, but good enough for our purposes.

BTW, even with a perfect mixer and two perfect analzyers, the before and after/upstream and downstream analyzers won't necessarily agree - the moisture removed in the compression process can alter the reading by as much as a percent or two!



Uncle Pug once bubbled...
So he did and the gas (O2, Helium) is injected into a venturi and there are two delrin baffles in the main bore... with random holes drilled in them.

[gaschef]

DEar All

What a load of techobabble.

Firstly does your compressor manufactuerer accept the use of increased oxygen in the input gas. Most DO NOT, the acceptions being those manufacturers like RIX or Aminco who manufacture OIL FREE compressors. If you read the NOAA, USN, RN, or anyone elses manual who knows about the subject OIL FREE compressors are what these people are talking about.

Use high level input O2 at your own risk.

Most international Standards Agencies, Gas Producers, and Professional Diving Agencies(Military & Commercial) accept that above 24% O2 you treat it as 100% O2.

THe limit for oil in air to be mixed with O2 is 0.1mg per cubic metre.
the standards agencies who came up with this figure make no difference between mineral, vegatable or synthetic oils. If your compressor can produce air this clean you will have no trouble.
If you compressor does produce air this clean, get your air testing
done again, as it most be wrong.

If you want to mix O2 online at Low Pressure a simple baffle box is all that is required, The specific gravity of O2 and Air is so near a homogenous mix is gauranteed. in except in very cold conditions

Having spent 20 years mixing gas for the offshore industry and almost as long for the recreational diver, I still cannot believe that the recreational diver cannot accept what has been learnt (painfully in many cases by PROFESSIONAL divers)

In my career I have seen 2 oxygen fires, I pray to god I never see another one. For everyones sake follow the rules, and do not try to cut corners, Oxygen will get up and bite you.

Tim "gaschef" Stevens

Commercial Diving Senior Systems Technician
ANDI Instructor Trainer IT#30

[gaschef]

DEar All

Another thought crossed my mind. If you are going to play with continous blending, make sure that you have some method of isolating the oxygen supply before the compressor is started up.

I would leave it to your imagination as what would happen if the compressor was given 100% oxygen on start up. probably the con rods would get scared of the dark and come out through the crankcase or something similar.

Most commercial online mixing plants have a solenoid valve connected via a timer to the compressor start panel. This will allow the compressor to be up and running before O2 can be entrained into the input air stream. It will also prevent the unintentional escape of O2 into the mixing chamber once the compressor is switched off.

If you are still going to play with this system, please bear in mind some oil free compressors are only rated fo 50% oxygen use, and very few for 100% O2. If you want to play with 100% O2 use a compressor such as a CORBLIN diaphragm pump, which has two steel diaphragms and a crack detection system which will automatically shut the system down if a failure occurs

Any problems email me I will only be to happy to be of assistance

Tim"gaschef" Stevens

[MikeFerrara]

gaschef once bubbled...


THe limit for oil in air to be mixed with O2 is 0.1mg per cubic metre.
the standards agencies who came up with this figure make no difference between mineral, vegatable or synthetic oils. If your compressor can produce air this clean you will have no trouble.
If you compressor does produce air this clean, get your air testing
done again, as it most be wrong.



My air is tested once a quarter and it is always below and that's before th hyper filter. And I try to time the air testing to get it done right before I replace the filters.

[oxyhacker]

This is a common misconception, that because a compressor is oilless it is automatically safe to run O2 through it. However Rix says very emphatically in all its literature that the compressor is for use only with air. It's a great compressor for topping up tanks which are being PP mixed, but it isn't (in the opinion of its manufacturer at least) a nitrox compressor.

And just to confuse things further, I have seen letters from both Coltri and Bauer stating that their compressors can be used for continuous mixing up to 40%!

I agree that it takes great discipline to continuous mix, since letting the O2 spike can cause all sorts of catastrophes.


gaschef once bubbled...
Firstly does your compressor manufactuerer accept the use of increased oxygen in the input gas. Most DO NOT, the acceptions being those manufacturers like RIX or Aminco who manufacture OIL FREE compressors. If you read the NOAA, USN, RN, or anyone elses manual who knows about the subject OIL FREE compressors are what these people are talking about.

[Genesis]

both Bauer and Coltri/MaxAir will tell you that their warranty is void if you run ANY enriched air content through them.

At least this is what they've told me.