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Dew point of scuba air

Discussion in 'Compressors, Boosters and Blending Systems' started by BRT, Jul 2, 2015.

  1. ti325v

    ti325v Dive Shop

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    Ahh..going back a few posts..

    Okay, lets go back to compressors 101...
    Start here.
    Understanding SCUBA Compressors and Filtration

    I quote you here and address these assumptions and questions below.

    That is only changed by the Pressure Maintaining Valve setpoint and the ambient air temperature and the temperature rise on the filter. So I get 1/2 hour of filter life with no PMV and 128 hours if I set my PMV at 240 bar, my fill pressure. It is impossible to fill to 240 bar and maintain pressure over the filter at 1 bar. So if I tell it my PMV is set at 1900 psi/130 bar I get no credit for the time above that. I also think there is a chance the 13x would last longer than 1/2 hour into filling the first tank without a PMV.

    In addition I don't see a way to adjust for ambient humidity. There almost has to be some gain from our dry desert air at 10-20% relative over 80-90% Mexico air unless the calculator is assuming the moisture trap gets either one to the same level???
    That calculator is not screwy in all ,of my uses for it, it has proved extremely accurate.


    Please remember I told you... of the fill stations tested, the VAST majority fail for MOISTURE !

    A significant fact that you do not seem to understand is that ambient humidity plays NO PART IN THIS EQUATION. Once you start compressing air the air, water is squeezed out..hence we have inter stage moisture traps..I bet you if you supplied air to the compressor at 1% humidity and 20*C, you will see water coming out of the traps, not a lot....but there will be some....
    Temperature rise in the FILTER is another misconception that I have researched with Swampdiver and we could find nothing to indicate that M.S. looses its effectiveness at any realistically expected temperatures.

    It is the TEMPERATURE OF THE AIR GOING INTO THE SEPARATOR, which if cooler is denser and drops the moisture much more effectively.
    I use lees than a gram of M.S. per tank fill in 33*C weather (Assuming 22.7% uptake of moisture by the m.s, so .22 g of moisture gets by my separators per fill.)...yes that means with a 33 inch tower holding 1.300 grams of m.s. I get OVER 1,200 fills. I use two final separators, chill the air, and don't think too much about filter changes.

    I agree you get no "Credit" for the time above PMV set point, as it is inconsequential...run the numbers at a pmv pressue of 0, 10, 20, 30....bar and you will begin to see that the curve of the effect is flattening out and it is a case of diminishing returns and increased compressor wear.

    I can say with some certainty that unless you are using some big ass filters, YOU WILL expend your filtration with no pmv VERY quickly.
    I think you are making some serious assumptions here, that are likely outside of your knowledge..give it a try if you think it is wrong...how much water comes out of your final separator in 1/2 hour run ? Is it enough to saturate your M.S. ?
    Remember m.s. is good for 22% by weight of adsoption before it is exhausted. If you have 500 G of M.S. and you get 100 cc of water out of your final separator in 1/2 hour..well there you go..it is finished. No back pressure and a whole lot of that moisture at 100% relative humidity is going into your filters...add the back pressure and you squeeze out a heck of a lot more water..now as the tank fills, it becomes its own PMV...so the effect is not THAT bad....maybe an hour, not a half hour to filter saturation.


    I can put you in touch with a guy I helped here on the internet..all of his steel tanks were ruined...$10,000 because two 5 cent o-rings in his PMV failed and he did not know enough to catch and fix the problem.
    This is why I have pressure gauges before and after my PMV...I can see it open and I can be 100% sure it works.

    This is a whole lot of work for some small flash rust spots in your tanks and with no air test result, not even a report of a 10/20/30% moisture on your compressor...If all is as you say, your filters are too small or you are not changing them often enough..It IS that simple.
     
  2. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    No matter what pressure you are working with the input of the x13 is 100% rh. TYhat is because the mechanical water seperator output of water is from raining in the seperator. May be it comes out 99% but wht the hey. the difference is that at 1 atm 100% is about 2.5% of the 1 atm air volumn. that may be 25,000 ppm of water. at 200 bar that air because of being cmpressed may only be able to hold 125 ppm. when the 200bar air has 125 ppm it to is 100% satuated. Cool it and some other tricks you can et it easily down to say 65 ppm. what once had 25000 now has 65, thats a reduction of water by a factor of almost 400. Then it hits your x13 filter that has a life span of say 10k molocules. that is why with lp air the filter does not last and the higher the psi the longer the x13 lasts. The out put of the x13 is say 40% so if 65 molocules (100%) goes in then you get 25-30 ppm out (40%). remember about 65 ppm is -50f dew point. -85 is a very reasonable dewpoint for most dessicants at least when new. the x13 is just better at it than clay and other dessicants. My rix 6 naver gave me moisture in the tank.

    Back to the moisture thing yout tank has in it perhaps 40% rh air. so if you look up 40% at 70 you eill get a dew point prpobably in the 40's. Yes you will have condensation but from only a few drops. So i guess lower you psi in the tank and you lower the rh cause the air can hold more moisture. Good reason not to store your tank at high pressure in a cold garage.

     
    ti325v likes this.
  3. BRT

    BRT Giant Squid

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    Problem is I store the full tanks in the back of a pickup in the winter as I travel to dive. I understand clearly that the rh is 100% leaving the separator. As of a week or so ago I am cooling the air before the separator. Obviously the cooler the air with 100% rh the less water in the air. I still kind of doubt if my 13x would be full after one tank of air without the back pressure valve. Too much of the pumping time is at pressure. The calculator does not account for that in any way.

    No one has yet shown me any information on what dew point can be reached with 13x at any given pressure nor can I find that info online. If I could see that I would get a -65F dew point at 1900 psi and drop that to -85F by raising my back pressure valve to 2500 psi I'd do that. Don't know if I want to go to 3500 psi, but maybe if that would provide the needed result. Or change my filters at 30 hours instead of 60 hours if that would give me the desired dewpoint. Or go to an extra drying filter if that would do it. I just can't find any hard information that answers the question.

    With all due respect KWS, in SE Texas you don't know if you would have water in the tank at 25F. I test that a couple times a year.

    ti325v, I have pressure gauges on both sides of my back pressure valve and have watched the transition many times. It holds as it reaches 1900 and never drops. The pressure on the other side just comes slowly up. The calculator on ScubaEngineer shows a major difference in filter life with back pressure valve setting. With numbers for my system filled in it goes from 1/2hr with no back pressure valve to 60 hours with a 1900 psi setting to 128 hours with a 3500 psi setting. It is not really a diminishing returns thing according to the calculator. It just doesn't take into account that you can't fill scuba tanks without raising the air pressure over the 13x.

    I understand that with dryer input air I can only reduce the water into the 13x and make it last longer and cannot really output dryer air. So who can tell me how to output -85F dew point air? Not an opinion or a statement that their air doesn't have a problem. I'm more interested in a chart or calculation that shows what pressure I have to have the 13x at to put out air at -85F dewpoint or below?
     
  4. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    BRT

    I don't think you are going to find what you want because it is subject to how much was removed mechanically, chemical dwell time and dessicant ability.
    As far as dew point take any dew point calculator and look at 40% rh at a given temp say 40f water and look at the dew point. Is it possigble that your dew is coming form the valve when you breath off it. The valve would be perhaps significantly cooler than the tank with the expanding of air as it leaves the tank and goes through the first stage.

    Our winters are quite often weeks in the 30's range and some below freezing.

    here is one how accurate it is who knows with pressurized air. Dew Point Calculator
     
  5. BRT

    BRT Giant Squid

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    Dew point calculators are generally available. I find it hard to believe there are no charts or calculators that take temperature, pressure, flow rate, amount of water in the incoming air and so on to tell what dessicants will do. Seems that is true and nobody really knows what is coming out of their compressors unless they have just had an air test. How low of a dew point do the air test show? Do some of you get results back below -65F?
     
  6. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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  7. BRT

    BRT Giant Squid

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    Thanks for the link. I had read it before but did again. He states that 13x can produce -104 dew points but has no backup for that. Probably true though. And if so then good enough. Looks like I need to change my back pressure valve to 2500+ pounds. I have a better thermostat coming for my air cooler so I can hold it closer to freezing without freezing. My current thermostat allows the glycol temperature to run between 33 and 41 degrees and if I try to go lower it may drop down to 27 and then the condensate stoppers the line.

    Thanks for all the help and advise.

    Brian
     
  8. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    I think that air test reports give water content in ppm and not dew point. Im not sure what pressure or temp has to do with dew point. I would guess that if you have x rh then it will require x degrees drop in temp to make it rain. Surely some others may be able to dispute or confirm this thought. I want to say that I think I have read also that 13x does -85 easily. perhaps its the same article as before. I also suspect that no matter what the the other factors are a dessicant can only dry so much. So if clay can only do 60% no pressure or dwell time can make it better. If 13x can do 30% at mosts then a 30 min dwell might produce 45% an hour dwell may produce 35 and 10 days will do no more than 30%. I am assuming that is using new dessicant. so if new can get you to -100 well used 13x would probably still get you the min -50f over a period far exceeding the other media's at the operating temperatures. Cooling appears to to be a major advantage in maximizing the mechanical and chemical moisture removal process. 50 ft of coiled tubing in an ice bucket between the final output and the moisture separator should do it. the cost would be probably recovered by savings on filter cartridges and chemicals.
     
  9. BRT

    BRT Giant Squid

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    PPM converts to dew point at a given pressure. A dew point at atmospheric pressure of -85F converts to a dewpoint of about 11F at 3500 psi. It would be good enough. Cooling the air at the mechanical water filter drops the dew point to that temperature at what ever pressure it at by dropping out all water that condenses. Then the filter has to deal with the approximately 100% RH that is left. An ice bucket would work and I have seen that in use in Mexico but I installed a cooler to do that part. All that is fairly straight forward. What I still don't know is what actual dew point the 13x can bring the air to at a given pressure so that can be converted to the dew point at the tank fill pressure. I guess I'll accept that it is -85 or lower at a 2500 psi back pressure valve setting and see if the problems go away. I have designated one of my cylinders to air fills from my compressor only so if that one gets rust spots I will know it is not enough. If that one stays clean while the others continue to get spotting I'll know it is shop fills doing it.
     
  10. KWS

    KWS ScubaBoard Supporter ScubaBoard Supporter

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    I think that sounds like a a plan. Even here in the winter I see no condensation in my tanks. I even left them in the garage last year. Would one of those inline moisture thingy's work. You know the things after the moisture separation. same contraption as the co checker but for water. Has a disk in it that changes color. Another thing you can check out. I got my lp95 tanks new in 2003 at the first vis I found them rusted. the shop was the only one filling them. when the shop closed I fgot the compressor nad the filtering. he had 2 compressors a main one and another 5k compressor to top off his cascade and fill paint ball tanks. Normally you put the compressor air in the bottom of the water separator and take dry air out the top. He had it plumbed backwards. Any condensate and oil went to the cascade bank. Till then I always wondered why there was no water discharge when I opened the drain's. Now I know. I have a rix 6 and there has never been any rusting from it. Has a 30" filter stack on the out put of the compressor. and The 5k bauer from the shop that I own now has shown no problems either. I think you are probably on to something when you suspect the shop. They may even have one of their tanks 1/2 full of water. I am using a robins filter stack with an I think stc filter cartridge in it. ran it for 5+ years attached to the rix. the bpr is at 2800. Now the rix is down and I have it on the bauer with a new cartridge in it. Neither compressor has shown signs of water issues. and if your dew point at 3500 psi is 11f degrees, that should say a lot in itself.
     

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