making nitrox via membrane without LP compressor?

[Isaac-1]

Here along the gulf coast it seems the availability of Nitrox has exploded in the last few years (pardon the pun), it seems almost every dive shop now has it or goes out of business. I suspect a large part of this is due to typical diving depths, etc. Those areas with depths that benefit most from Nitrox are the ones most likely to have it.

Ike

 

[rcontrera]

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

It's far easier to just add in a LP compressor. Our smallest nitrox system with LP and HP compressors isn't cheap. I am sure that the LP compressor is a lot more efficient than the HP would be if tapping pressure off the first stage.

This little gem is fully self contained and is classified as portable but in reality takes two men and a boy to move around as it weighs in at a hefty 385 pounds! Plus, it runs almost eleven grand! Needless to say, not a lot of these are sold to individual divers.

 

[scubapanama]

Iain,

Thanks for sharing your thoughts.

A few observations:

- vane compressors get more efficient after running in (about 5-10%). They are much easier and cheaper to repair (if they need any repair at all).

- we do everything possible (at above typical standards of these kind of installations, even for tropical conditions) to feed dry and clean air in the membrane to extend its life and cool the air before it's goes into the P41 filter so it also lives longer.

Above all I wonder, what is your suggestion? Water lubricated compressors? I haven't really seen them in small capacities. But above all my question is: if this system setup is so problematic, why is it in some form or another on every liveaboard or dive shop? Why do I never hear about water lubricated or other oil-free compressors?

Please enlighten me. I've love to hear your angle.

Thorwald

p.s.: an interesting link comparing vane & screw compressors:
http://www.daveycompressor.com/vanevsscrew.html

 

[Wookie]

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 “blow-by” and oil cooling. (Service and spares are next.)

On industrial compressors each stage has “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 “losses” 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 “hot oil” 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

I'm not too sure I agree with all of your conclusions, Iain.

I have a 24 passenger liveaboard that I operate in Key West. We have a membrane system supplied by a Ingersoll Rand 2545 piston compressor driven by a 10 HP motor, producing 39 CFM at 175 PSI. Note that the pressure is above manufacturers specs, as is the output flowrate. We take excellent cae of it, and use the same chemlube 800 we use in out Mako/Compar 5406 13.1 CFM high ressure machines. The setup is dried by a off-the shelf air dryer who's ratings are short of 39 CFM. I think it is rated at 35 CFM, but I'm unsure of the dewpoint.

The IR piston lp compressor operates from 0530 to 0100, a total of 19.5 hours per day. The high pressure compressors operate about 2 hours/dive, or 10 hours per day. We do not shut down the membrane system between dives, as it takes about 1/2 hour for the system to stabilize when starting. The high pressure compressors are run in stop/start mode.

We replace the filters on the upstream side of the carbon filter once per year. These filters are a .5 micron particulate, a .1 micron particulate, and oil mist. We replace the carbon in the carbon filter once per year. We replace the .1 micron filter between the carbon filter and the heater twice per year, as needed as indicated by the delta P indicator. We have not replaced a membrane in 11 years. The carbon filter holds 20 lbs. of carbon fines. Washed carbon isn't.

Since the Mako is fed by relatively dry gas, we are able to run the dessicant filter and combo filter for 60 hours prior to changing.

It can be done as Thorwald suggests.

 

[scubapanama]

p.s.:

the Hydrovane compressor has a very low oil carry over, even lower than screw compressors.

And of course, every cheap solenoid valve will spoil after a while or is already when its new. Obviously, a serious operator will avoid cheap solenoids and use established brand name product.

Yes, we're in the tropics. And while our compressor area is very well ventilated, just in case our refrigerated dryers are 300% oversized in terms of capacity.

You know a thing or two about the RIX product line from as I can see from your profile Iain. But even if I look at the RIX :

4VX4B-23-NT1 (Nitrox)
4,500 PSI
20 or 26 SCFM
20 or 26 SCFM
20 or 25 HP
230/3/60
4VX4B-23-NT2 (Nitrox)
460/3/60
4VX4B-23-NT3 (Nitrox)
380/3/50


That's 1 CFM/1 HP.

The Bauer PE 500 11 KW (15 HP) makes 18.5 cfm : or 1.23 CFM / 1 Hp.

Looks like the Bauer is more efficient .

Bottom line: only the wrong designed systems “saturates the membrane and gum up”.

 

[iain/hsm]

p.s.:

But even if I look at the RIX :

That's 1 CFM/1 HP.

The Bauer PE 500 11 KW (15 HP) makes 18.5 cfm : or 1.23 CFM / 1 Hp.

Looks like the Bauer is more efficient .

Bottom line: only the wrong designed systems “saturates the membrane and gum up”.

Great post.
But not quite correct with the size of the motor fitted to a compressor being the same as the power required. But this is a very interesting angle and it does show how the motor size specification can be mis-read and how a more detailed explanation should be given to customers.

A 30cfm compressor at 3000psi would require 16-19 BHP or 14Kw
A 30cfm compressor at 3500psi would require 17-20 BHP or 14Kw
A 30cfm compressor at 4500psi would require 18-21 BHP or 15Kw

For all these applications the nearest commercial available size would be the 15Kw or 20HP

Now I didn’t want to drag Rix into this but Rix fits a 25HP or 18.5KW motor but is doing this for engineering practice and not to save $60 in price difference between a 20HP 15Kw and a 25HP 18.5Kw motor.

Now to compare all units properly you also have to consider the piston diameters, piston stroke and compressor RPM.

Bauer PE 680 (24cfm) is 1400 RPM
Rix 4VX4B-23NT3 (26cfm) is 935 RPM

Also if your production gas (nitrox) is being produced at pressure from the membrane
by using the LP membrane and increasing the inlet pressure to the HP compressor and reducing the size of the compressor piston diameters you can reduce the power required dramatically or increase flow or reduce RPM. Again using the 4VX as an example with the same motor but with different piston diameters to lower the RPM you get.

30 scfm with a 7psi inlet RPM 720 or
35 scfm with a 70psi inlet RPM 640 or
40 scfm with a 87psi inlet RPM 640 or
30scfm with a 40psi inlet RPM 350

Now if you really wish to compare we should also look at purity and running but if we compare like for like using your example but with a smaller but fixed output

The Bauer PE 500 11 KW (15 HP) makes 18.5 cfm : or 1.23 CFM / 1 Hp.
The Rix at 18.5 cfm would also require 11Kw 14.1 to 16HP at the 3000 to 5000psi range.

The difference is the Bauer needs 1170RPM to produce 18.5 cfm
The Rix only 500RPM

Clear (as mud) I guess LOL

Iain Middlebrook

 

[Scuba Duck]

While I applaud the idea of saving money by trying to minimize the componets of membrane systems. I would issue a word of caution. I've had the opportunity to fix a couple of membrane systems, and typically this is because the owners took the less expensive route initially, which ended up being the really expensive route.

The membranes are composed of very small hollow fiibers, that allow oxygen to migrate through the walls of the fibers faster than nitrogen. This is what gives the membrane its selectivity and ability to separate gases. Any small particles of oil or dirt will clog up the hollow fibers and ruin the membrane, and you get to purchase a new one. So it is paramount that that all the oil and dirt be removed.

If you have a LP air compressor, you need to have a refrigerated air dry and a very good filtration system. I recently had the pleasure of upgrading one of Nitrox Technologies (Bob Olsens) systems as it did not include the air dryer, the users did not drain the water from the filters and they charged all kinds of nastiness into the membrane. A new membrane, a new refrigerated air dryer, a new auto draining filtration system, and a little duckie magic (control system) and the system will make nitrox all day with spot on accuracy and no user adjustment.

If you use a high pressure feed system (no LP compressor), you are asking for trouble. The air that goes into the membrane needs to be warm for the membrane to work properly. In fact if you put very cold air into the membrane, it will embrittle the little hollow fibers and you run the risk of bursting the fibers, at which point you get to change out the membrane. The adiabatic expansion from high pressure to low pressure absorbs a great amount of heat, meaning the air that goes into the membrane will be very cold (well below freezing), unless heated. All that heat that was generated when compressing the gas, will be absorbed when expanding it. I've already helped replace membranes (UBS systems) for two different dive companies that both blew up their membranes because they forgot to plug in the heater. That brain fart cost them a ton of money. If you use the LP compressor, this risk goes away as the input air will never be that cold.

Nuvair will charge you $8K for the 60 inch grey replacement membrane tube. They will charge you $6K for the shorter (40 inch) grey replacement tube. If you skimp on costs, and end up destroying a membrane, you did not save any money.

LP air compressors are much more cost effective to run from a power consumption standpoint than a HP feed membrane system. Rotary Screw air compressors require less maintenance and run much more trouble free, substantially quieter, and much much much smoother than piston LP air compressors, but cost at least twice as much. You can set your drink on the frame/cabinate of a rotary screw air compresssor and it won't move, try that with a 35 cfm piston style LP air compressor.

The Nuvair systems use Rotair (Italian) Rotary screw air compressors. The Nitrox Technology systems use Sullair (USA) Rotary screw air compressors. Ingersol Rand also makes good Rotary Screw air compressors. You need about 3x as much feed air to the membrane as you get output. This relationship depends on the % of Nitrox you are making as the membrane requires more feed air to make higher percentages.