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Akimbo
September 11th, 2012, 06:24 PM
What are the leading oxygen concentrator models on the market that are suitable for shipboard mounting? Looking for 15-20 Ft/hour for Nitrox continuous mixing and decompression gas. Power will be 240 VAC single phase, but typically a little dirty off generators.

Wookie
September 11th, 2012, 06:27 PM
Do you have enough power to use a membrane, or do you need a higher concentration of O2 than a membrane can provide?

Akimbo
September 11th, 2012, 07:12 PM
I just got the call this morning and am not sure of the amperage available at this point. Reliability is a big deal since the guy wants to make 3-month cruises in Micronesia. He has an engineering mate that can go to school on the system if necessary.

Definitely looking for high percentage O2 decompression gas, mostly delivered on a hose to a stage. I was considering an LP diaphragm booster for this but still in the early stages.

Wookie
September 11th, 2012, 09:13 PM
If I were doing 3 month cruises in Micronesia and I didn't want a membrane system (40% max) I'd get a dewar of LOX. I'd have it filled on Kwaj. A O2 concentrator (I'm told) makes volumes too small to effectively boost

Akimbo
September 11th, 2012, 10:12 PM
If I were doing 3 month cruises in Micronesia and I didn't want a membrane system (40% max) I'd get a dewar of LOX. I'd have it filled on Kwaj. A O2 concentrator (I'm told) makes volumes too small to effectively boost

I don’t think Dewar insulation will last long enough. One gallon of LOX yields about 115 Standard Ft/Gallon. Evaporation loss was about 3%/day on the 500 gallon LN2 Dewars we used in the North Sea for Helium reclaim systems. Ambient temperature in the tropics “should” make it worse. I understand LOX loss is about the same. Even if you can get LOX on Kwajalein and Guam, it can be an awfully long trip in such a vast area.

I agree that LOX would be an ideal solution if we could make the logistics work. Maybe I can find more suppliers. I thought I read somewhere that The Odyssey in Truk used a concentrator???

I am a little surprised that LOX isn’t used more by dive shops in the US. It is cheaper and safer than HP O2, as long as your consumption exceeds the evaporation rate. Hospitals and decent-size weld shops in the US use LOX almost exclusively.

Thanks for the suggestion.

iain/hsm
September 12th, 2012, 12:32 PM
At 3% LHE loss per day that would have been some time back offshore.
Reclaim went HPG a long time ago with minimal gas loss. Well as long as it was tested with the diver still sitting on the bell stage that is. (Oil Endeavour DSV for Sub Sea International Forties Bravo 1979 Krasburg system)

But for on board oxygen generation you need IMHO to decide first your purity. Most industrial medical units use 90% and argue that in medical terms patents only need 90% not so a diver.

The better blocks run to 93% to 95% and even then the gaseous concentration of CO CO2 need to be agreed with the supplier first as 10% CO and over 400 ppm CO2 are not uncommon, so beware OK for medical use maybe on the surface but for filing rebreather cylinders?

Also be careful as to the percentage tolerance in generators is +/- 3% of purity and they are temperature sensitive. Typical discharge pressure is 9 to 10 psig

For <98% plus purity you will just about double the plant size and increase cost. It adds an additional generator stack and needs more air and produces at higher pressure. Alternative is use a bigger generator than HP compressor and increase the produce discharge pressure to lower the flowrate and increase the purity.

Budget $2000 for a 5 SCFH oxygen generator, and $5000 to rack it up to 2200psi. Working 24/7 at 5 SCFH your power consumption is 0.33 BHP for the generator and 0.50 BHP for the HP compressor.
Both are available single phase off the shelf (stateside) 50 or 60 Hz. Iain Middlebrook

Akimbo
September 12th, 2012, 01:59 PM
At 3% LHE loss per day that would have been some time back offshore.
Reclaim went HPG a long time ago with minimal gas loss. Well as long as it was tested with the diver still sitting on the bell stage that is. (Oil Endeavour DSV for Sub Sea International Forties Bravo 1979 Krasburg system)…

I was talking about the liquid Nitrogen used as a refrigerant to reclaim pure helium from chambers, not closed-circuit surface-based diver recirculating systems like Divex. It's been a while since I installed my first units, but the vacuum on Dewars has not improved significantly. Today, most offshore operations I know of are either using chemical filters for reactive gasses only or membrane helium purifiers for mid-range purity — mostly because they are “good enough” and supplying LN2 offshore is a huge PITA.

Cryomech - Helium Recovery System (http://www.cryomech.com/heliumrecovery.php)

Here is an image of Dewars used offshore:

135223

For others reading this, gas from saturation diving chambers is vented to atmosphere and captured in big flexible bags. It is primarily Helium and mostly “contaminated” with O2, N2, CO2, water vapor, miscellaneous “bio-generated” gasses, and maybe oil vapor. A HP compressor sucks the gas out of the collection bag and runs it through the reclaimer. High-purity reclaimers use chemical filters to capture most of the gasses that solidify at LN2 temperatures — below −196 C (77 K; −321 F) and would clog up the system with “ice”.

What’s left goes into a heat exchanger cooled by LN2 and all the gasses except Helium and trace amounts of Hydrogen condense out as liquids. The helium then heads to HP storage cylinders.

I have seen some Helium Cryogenic units (liquefying the Helium). They produce ultra-high purity, but are higher maintenance and even less efficient (energy and LN2 usage). The LHe then has to be boiled-off (allowed to warm) and then pumped into HP storage.


...But for on board oxygen generation you need IMHO to decide first your purity. Most industrial medical units use 90% and argue that in medical terms patents only need 90% not so a diver.

The better blocks run to 93% to 95% and even then the gaseous concentration of CO CO2 need to be agreed with the supplier first as 10% CO and over 400 ppm CO2 are not uncommon, so beware OK for medical use maybe on the surface but for filing rebreather cylinders?

Also be careful as to the percentage tolerance in generators is +/- 3% of purity and they are temperature sensitive. Typical discharge pressure is 9 to 10 psig

For <98% plus purity you will just about double the plant size and increase cost. It adds an additional generator stack and needs more air and produces at higher pressure. Alternative is use a bigger generator than HP compressor and increase the produce discharge pressure to lower the flowrate and increase the purity.

90+% purity is manageable for open-circuit Nitrox continuous blending and the numbers for open-circuit decompression on 10/90 N2/O2 aren’t bad. Nobody has told me anything about rebreathers or treatment chambers… yet.

The question remains, which units have proven reliable in this application? Reading manufacturer’s literature and listening to salesman isn’t the same as hearing the good, bad, and ugly from people using it at sea out in the middle of nowhere (industrially speaking).

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