Advice needed: RIX SA-6A configuration/operation

[spoolin01]

I was hoping for a little expert review and advice on my home compressor rig, a RIX SA-6A, with Robyn gas motor. The compressor is standard configuration from what I can determine from the operating manual, originally used by the Navy. I've used it for almost 5 years, and added a filtration system about two years ago after hearing from LDSs that my tanks were coming in for annual visual with water inside. I don't know that the water came from the compressor, but the coincidence is there. I drain the two coalescers no less often than every 20 minutes. The filter bank was simply inserted between the compressor and fill whip, and contains a moisture scavenger (Lawrence F X44670), CO/moisture indicator, and a triplex CO/carbon/?? element (LF X44420), in that order. I put the indicator between the elements because I thought the most important information would be when the CO element began to get wet air. I had monitored tank fills with an industrial CO meter - putting the meter in a baggie and venting gas into it - for the two years I didn't use the filters, and never got a positive reading. The detection limit of the meter was around 5 ppm, maybe less, so I feel confident CO isn't a problem, I just wanted to drive it as close to 0 as practical. I fill perhaps 20-40 tanks per year, a mix of LP120s, Al80s, and high volume HP tanks.

The feedback I'd hope to get is
1- Is the configuration good? Does anyone think I don't need the moisture filter if I switch over to electric motor drive?
2- Is anyone familiar with the filter housings (they take the Aero Dri/Keco/Gannon filter size)? I'm concerned about their pressure rating. They're not marked for rating or anything else, but I've seen similar looking housings rated 3000 psi in pics of other military surplus. Until now most use has been for LP and AL80 fills, but I do occassionally fill HP tanks and will be doing so more regularly. Should I upgrade to confirmed higher rated housings? I've got a Bauer unit, the small one, Triplex may be the name... but I liked the idea of greater moisture removal capacity. I could buy something else.
3- How should the filters and their plumbing be set up to maximize filter life? I detach the filter bank from the compressor for portability, and cap the bank on both ends, thinking that would prevent ambient moisture from saturating the filters, and from turning the moisture indicator. I notice subtantial air release when uncapping at next use - is that from out-gassing of the filtration materials or beds which don't immediately release all pressure when the system is de-pressurized? However, now I wonder if the way I've closed the system may be actually shortening filter life and turning the indicator. Could moisture release from the first element diffuse across the indicator and into the CO cartridge (this one may have some moisture compound as well, it's hard to tell from the technical info I can find). Should I isolate one or both filters with valves, and/or capping like I've been doing?
4- What sort of life should I expect for the moisture cartridge? I'm near the San Francisco airport where humidity averages 75%, and do fills all year round. Can I regenerate the material with a little heat and vacuum? Does anyone know what chemical it is? It doesn't look like silica gel, calcium chloride, or glass beads but I could be wrong. Both cartridges say they only have a 6 month shelf life. Somewhere I got the impression the CO element is a reactive chemical, but the charcoal and moisture components shouldn't be so time-sensitive should they? I confess I haven't replaced the elements and it's been about 2 years now. The indicator has turned slightly for both moisture and CO exposure, but I'm still trying to confirm that I understand how to read the color strip.
5- Do I need a particle filter downstream from the filters?

I know this is a flurry of questions, but my hope is to get feedback on the utility of this configuration and on managing the filtration aspect.

Mike

 

[spoolin01]

I forgot one important question: the backpressure device is at the end of the fill whip. Should I move it upstream of the filter bank to remove the dead air space, so the 3rd stage operates under load sooner after starting up? For what it's worth, I can't see that fill times are any longer now than when I got the unit, and it wasn't a rebuild that I'm aware of. Also, it's a 6A, not a 6B, I'll try to edit the OP.

 

[pp-1-3-5-7-9]

I forgot one important question: the backpressure device is at the end of the fill whip. Should I move it upstream of the filter bank to remove the dead air space, so the 3rd stage operates under load sooner after starting up? For what it's worth, I can't see that fill times are any longer now than when I got the unit, and it wasn't a rebuild that I'm aware of. Also, it's a 6A, not a 6B, I'll try to edit the OP.

I would recommend you to move the back-pressure device from the end of the fill whip to the end of the two coalescers also follow with a check valve, so that can be avoid the water get into the filter bank but just allow only the moisture vapaor pass through and adsorb by the filter bank to extend the life of the dryer chemical, the check valve can be prevent a back flow get happened when you drain the condensed water from the coalescers.

 

[pescador775]

Place monitors at or near the output. Yes, the desiccant can be regenerated. Place in 475F oven for several hours. The hopcalite(monoxycon) can be reused if not wet. As Peter mentioned, the chemical filters must be sealed off from the environment when idle. They must be functionally separate from the coalescers (moisture trap) when not in use. Placing a backpressure valve at the coalescer output will accomplish this but addition of a check valve is insurance. The outgassing is a mixture of air and carbon dioxide. If you are curious, measure the oxygen content of the vented gas with an analyzer. Go ahead an bleed it off before operating but it is not especially important in the big picture. 5ppm of CO is a good number considering your location and the positioning of the indicator. Use of an electric motor is not relevant to air purity. Running a gasoline engine requires diversion of the air intake to avoid exhaust gas from entering the system. I can't comment on the canister pressure rating. You bought them from LF?

 

[oxyhacker]

Rixes produce wet air. Rix tries to make a virtue of it in their advertising, but it is for most practical purposes a deficiency. Actually, the reason is, water is always produced when you compress air, so its a problem with all compressors. Mechanical separators, like the ones on the Rix, are usually only 75-90% efficient, which means a lot of water makes it past the separator, and usual practice is to follow up with a media cartridge with a dessicant in it to absorb the water that makes it past the separators. Oil-lubed compressors must have such a filter anyhow, to get rid of the oil in the air, and filter media cartridges use the same ingredients to filter out water as they do oil and other bad stuff, so on an oil-lubed compressor one stack does both jobs. Since oil-free Rix compressors produce breathing quality air without any filtration, moisture aside, Rix save a few bucks by not fitting a media stack. But this means the air is still very wet as your shop noticed. So most Rix owners end up having to add a media stack.

The dessicant used in the stack is usually a molecular sieve referred to as 13X. You can regenerate it, and I have done a lot of experimenting doing so, but it really isn't worth the trouble, especially for the amount of air you are pumping. While a Rix generally only needs the dessicant, most Rix users end up using standard cartriges which also have charcoal and maybe hopcalite for CO, and you can probalby get some for your housings from Lawrence Factor.

Your media stacks appear to be surplus, from an air drying rig, and are probably rated for 3000 psi. I would change the plumbing so the air enters at the bottom and leaves from the top so the moisture works its way up as it saturates the media. Actually, I would get rid of one and use just one, you got plenty of capacity there for the amount of gas you are going to be pumping. BTW I usually see these housings hung the other way around, if the the ports are not marked for flow you should ascertain the correct directions and hook them up that way, since cartridges usually have a right and wrong direction for flow.

 

[spoolin01]

Place monitors at or near the output. ... Use of an electric motor is not relevant to air purity.

I'm going to review the existing hardware before asking for some clarification on the check valving and back pressure device. My thought with the monitor was that its best purpose was to confirm the CO scavenger was protected by the upstream moisture cartridge, assuming the moisture unit would be the life-cycle limiter. If the CO material really loses activity on its own, with a 6 month shelf life, perhaps another CO monitor at the output would be useful. I'd love to know if that shelf life is realistic. Do you think the moisture cannister should be isolated from the CO cannister for storage, to prevent diffusion of moisture from the scavenger throughout the rest of the filter bank?

Hah, I didn't present my question about the need for a moisture scavenger very well. As a general rule, is a moisture filter needed on top of the coalescers, to ensure the air is dry enough?

I got the cannisters on eBay from a Florida paintball outfit, with no indication of specific rating. They're massive compared to a (smaller) 5000 psi aluminum cannister I inspected, but I suppose that only says so much.

I appreciate the guidance, thanks.
Mike

 

[oxyhacker]

Shelf life is the result of 3 things:
- the fear that moisture will penetrate the wrappings and eventually reduce the efficiency of the cartridge.
- the fact that many filter media will break down and produce dust etc over time which may also reduce efficiency.
- the desire to sell more cartridges.

So from a profitability point of view, a manufacturer wins both ways by having a short filter shelf life - possibly warranty and liability hassles are reduced, and sales increased. So I wouldn't fixate on shelf life.

The CO catalyst is the least of your worries, since first of all, it lasts almost indefinately, and secondly, won't save your life anyhow if there are lethal amounts of CO - it is very limited in what it can handle. If you are running a gas engine, your first line of defense is having a snorkle to get the intake away from the exhaust (and SA-6s, for some unknown reason, pump faster with a snorkle attached) and being very careful how you position your compressor, not an CO detector or hopcalite in the stack.

You don't want to put your indicator between the separators and the media stack(s), since the gas coming out of the separators is ALWAYS going to be at 100% saturation. That is to say, it always contains the maximum amount of moisture in vapor form that the air can hold given its temperature. Mechanical separators can only remove aerosols - droplets of actual water. They cannot remove gaseous vapor (that's why we use media stacks after the separators). So an indicator after the mechanical separator will always show a high level of moisture.

Oh, while the moisture detector on those eyeball indicators is pretty reliable, the CO one is almost worthless - it wears out much faster the the moisture element, and reponds slowly and erratically.

 

[pescador775]

Hacker, he has placed a moisture/CO indicator between the two filter canisters, not between the separators and the filters. It is difficult to understand his text due to use of strange terminology like "elements" to describe filter cartridges.

As to the desiccant cartridge, the longevity is difficult to estimate because we don't know the capacity or the average temps. LF may provide data on this.

I did a calc to see what Bauer and LF are talking about, and the numbers which they publish for filter capacity do seem to be about right if one is careful to read the fine print. For example, assume an average SCUBA cylinder holds about 2.5 m3 of air/ gas. This is about 87 cu ft. The amount of moisture separated out by the compressor condensator (coalescer) is related to pressure and temperature. At 30C and 150 bar the condensator should remove all but 0.5 gram from 2.5 cubic meters of air. Thus, the desiccant has to deal with that 1/2 gram of water in the course of filling the tank. My experiments showed that 13X can hold 24% by weight water. However, a practical limit is about 20% due to the steep downward curve of adsorption. Thus, I suggest that if one can estimate the weight of desiccant in the filter it is possible to retrace the logic which led to the original recommendations for the filter. If the desiccant in a dryer type cartridge weighs six ozs (170 g) then the adsorption capacity is 170 X .20 = 34 g (water capacity). This is equal to 34/0.5 = 68 tanks = 5900 cf free air. Again, this is at 30C filter inlet temp.

I would guess that your cartridge holds about six ozs or more type 13X molecular sieve but don't know for sure. Desiccant weighs .70 X water weight. Calculate the water volume in ml and multiply by .70 to get weight in grams. Think of it this way, 11 avg tanks per oz of desiccant. That should be conservative because I assumed that the compressor was sucking saturated atmospheric air (New Orleans in summer). I think Bauer sells a cartridge with about 2.5 oz desiccant and claims 3200 cf but don't quote me on a WAG.

In view of statements concerning your SCUBA cylinders, the RIX condensators may not be as efficient as the hypothetical but this method for estimating should be close enough due to some conservatism in the assumptions. Still, LF are the people to ask.

 

[spoolin01]

Thanks everyone for the generous expertise. LOL - sorry about the terminology, I'm in biotech analytical chemistry so filtration and dessication are techniques I'm familiar with in a different context, and even some gas behavior theory from physical chemistry.

If the moisture filter uses a molecular sieve, I assume that some water vapor will evaporate from it when it sits, so I'll probably isolate the water filter from the CO filter for storage, assuming I keep both. There's a lot of good stuff in your advice that I need some time to think through. I'm getting up early to go crabbing tomorrow but I'll get back in a couple of days with some thoughts. Thanks again, Mike

 

[oxyhacker]

I tend to use the "short" 1 lb. cartridge as my standard of comparison for other small cartridges. It holds about 1 lbs/56 cu. in. of media, and is good for about 13,000 cf or air, or 150 80s at standard conditions of 5000 psi/80 F. However, those standard conditions are better than most scuba stacks typically see, so real world performance could easily be half that. Figuring the cartridge fill is about 2/3rds 13x, that's about 8 ounces, or 88 tanks by your rule of thumb, which would seem quite reasonable.

I would guess that your cartridge holds about six ozs or more type 13X molecular sieve but don't know for sure. Desiccant weighs .70 X water weight. Calculate the water volume in ml and multiply by .70 to get weight in grams. Think of it this way, 11 avg tanks per oz of desiccant. That should be conservative because I assumed that the compressor was sucking saturated atmospheric air (New Orleans in summer). I think Bauer sells a cartridge with about 2.5 oz desiccant and claims 3200 cf but don't quote me on a WAG.