making nitrox via membrane without LP compressor?

[scubapanama]

Soooo....

.... to make nitrox with a membrane, all vendors propose a LP scre compressor, drier, membrane and then use the HP compressor to put it into storage cylinders. To have redundancy on your liveaboard, we'd need to of each, including filters ,driers ,etc.

It all adds up.

NOw, do we really need a LP compressor? What about instead of 2 LP , expensive screw compressors with all the asssessories, use a extra HP compressor to fill storage cylinders and via a reducer, produce Nitrox via the membrane. That way, you have practically oil free air meaning longer membrane life, only one type of compressor, great redundancy and you save $$ too.

Screw compresssors use more oil I'm told so they need filters to make the air oil free and , oil that can create problems in the HP compressor.

This setup may not be as efficient but it also means you get a very simply system: 3 HP compressors, great redundancy and very flexible capacity control .

Anything wrong with this picture?

Thorwald

 

[rjack321]

Nice try but you will wear out your HP compressor dang fast.

You need to pump ALOT of gas through the membrane to removed the N2. a dedicated LP compressor is cheaper and will cost you less.

Your HP compressor will be pumping way up at whatever your backpressure reg is set at (1800-2000psi) when it really only needs to make <200psi gas.

 

[fppf]

Yeah, membrane needs a hell of a lot of air, which is why they spec screw compressors.

 

[rcontrera]

As has already been said, yes it can be done. In fact, that is how we run our little portable guy.

However, it will take at least three times the run time for your air compressor to fill a cylinder than normal. For instance, if it takes five minutes of run time to fill an 80 with air, you can count on 15-20 minutes to fill with Nitrox.

 

[scubapanama]

Hi everybody,

Thanks everybody for the feedback. Of course, you're right. I failed to realize that a very big portion of the air that goes into the membrane is vented off. So there's no way it can be very economical to first bring air to a high pressure, then reduce it to put it through a membrane and then put it back up to high pressure (versus the 2-step solution with LP/HP compressors).

So yes Ray, it can be done but in our situation with lots of space and enough power available, we'll go for a LP/HP setup.

Thorwald

--------------------------------------
www . expeditionyacht . org
Building the ultimate expedition yacht
(and liveaboard)

 

[rcontrera]

Man ... I wish more of my customers understood right off the bat like you do. So many people have gone with HP only membrane setup to "save money" only to come back a year or two later and ask about the combo set up.

To decide what system you really need, determine what capacity of HP compressor you need (how many fills per day, how fast you need to fill, bank or no bank, etc.) and then you can match the membrane and LP system to that.

And if you go with a top shelf LP compressor, you will not have an oil usage problem. The rotary compressor that is in our Nitrox Generator uses almost NO oil. I have a dive shop that has run their system for a year and has used less than a cup.

Of course, I work with the Coltri/Nuvair membrane system so I am biased. But there are lots of fine systems and excellent custom system builders out there. And many of them use the Coltri/Nuvair components in their systems.

 

[scubapanama]

Well, if you read the technical documentation of any compressor manufacturer or trade magazines, you can learn that the biggest cost factor in making compressed air is the cost of electricity - 70% - so anything that leads to better efficiency pays itself back pretty fast, even more on a liveaboard where the cost of making electricity is higher than ashore.

We plan to go with a frequency controlled CompAir hydrovane compressors, OMI HTD dryer, activated carbon filter& Aaragon (Parker) membranes and Bauer (IK150 compressor block) compressors with Air-Kool.

Thorwald

 

[iain/hsm]

You are not quite getting the whole story or what you need to know to make an informed choice here. The major factors in compressed gas efficiency are cooling efficiency, gas &#8220;blow-by&#8221; and oil cooling. (Service and spares are next.)

On industrial compressors each stage has &#8220;gas packing that collects this lost gas and re routes it back into the compressor inlet. Granted this is something few scuba divers bother with or on new compressors but as the compressor wears gas loss at best is 10 to 15% of the rated new capacity.

Also if you look at say compressing 30 scfm of air with a four stage oil lubricated compressor you would require a 30 HP motor 22Kw of power due to the &#8220;losses&#8221; in having to cool the hot oil.
For the same output 30 scfm an oil free compressor would require only 25HP 18.5Kw. Saving 4Kw just on the &#8220;hot oil&#8221; cooling loss alone. Granted the saving is less if the compressor is smaller as in most scuba applications. But in industrial applications running and maintenance is.

In addition another major running costs you will uncover with your application is the filtration spares and maintenance. If your 24 diver 4 dive day is calculated out at 7500-8000 cuft of production a day with the type of plant you suggest the IK150 you have a daily running time of 6-7 hours. In a hot humid area like Panama you may need a bigger vessel just to carry the filtration cartridges.

Also not sure about your choice in the carbon filter I have seen numerous parker/permea membranes block up with the fine black powder dust these typical low cost LP compressor filter products produce and as a parker membrane is $2000 replacement cost they are not cheap. Likewise the Hydrovane do you know the oil carryover rate from these models or the model number. I just think that 6-7 hours running every day in a hot humid area you are going to get some expensive surprises when some cheap discharge solenoid blocks and your "oil and water" carry over saturates the membrane and "gums" it up. As most scuba set up's do, time after time after........Iain Middlebrook

 

[Isaac-1]

Ray, with the growing demand for nitrox membrane systems, I can't help but wonder if one of the compressor makers will introduce a small to medium size compressor with an oversized first stage and bleed off system that can be connected to a nitrox membrane for the portable and on board diveboat market.

Ike

 

[veriqster]

Ray, with the growing demand for nitrox membrane systems, I can't help but wonder if one of the compressor makers will introduce a small to medium size compressor with an oversized first stage and bleed off system that can be connected to a nitrox membrane for the portable and on board diveboat market.

Ike

Exactly what i was thinking about, but, i guess there's just not enough demand for now to justify the design and production costs. I think another option would be to "integrate" a 2 cylinder low pressure compressor (to balance it, opposing cylinders) into the HP compressor and find a way to switch it off when needed (split crank? cam drive?).
my guess is that regardless of which way you would go the costs might outweigh the benefits.
anything like this would mean a redesigned unit or it would unbalance when used just for air / run unbalanced when driving the membrane.
honestly, here in north east i do not see dive shops switching any time soon - the demand for nitrox it so small most don't even carry it. maybe that is just a consequence of low availability or price difference???