quote:Originally posted by Pantera Doug: If I am starting out at 35psi at 70F, and the temperature drops to 15F and I wind up with say 15psi that is measurable, can I calculate the amount of water that was in the air in the tire? Sounds like I am pumping quite a bit of water vapor into these tires at 70F?
Well, the football game is long over so I’m starting to lose interest in the subject and don’t have time to look at the steam/enthalpy tables, but yes it can be calculated. You need to know the water content of the air to begin with. You can assume it is saturated for worst case. If the air was compressed to say 100+ psi and then put into your tire at 35psi, it won’t be saturated. Search “pressure dew point”. I think I can confidently say if you lose that much pressure you also have a leak.
quote:How much of that can be removed by purging and running the compressed air through a good desiccant or better yet do I need to contrive a desiccant filter system to remove a specific amount of compressed water vapor?
With good desicant and low flow, for all intents and purposes (at least in engineering terms, but not chemical terms), virtually all. “Pressure Dew Point” is the measure.
quote:Should that filter be on the intake of the compressor or the exhaust or both?
It would do some good on the intake side but much better on the exhaust side, and after cooling the compressed air. Most folks rely on the tank to cool the compressed air. So in this case you extract it from the tank and put it through the filter.
quote:Do I need to have a proportional desiccant filter size relative to the intake or exhaust of the compressor to achieve this?
It’s usually relative to the rate of flow you are putting through it.
quote:Are there better substances that work as a desiccant than others?
Depends how elaborate you want to get. If it’s only once in a while that you need dry air, silica gel is a common and inexpensive desiccant. It usually looks like fine rock salt and starts blue and becomes clear when it’s spent. It’s cheap but you can bake the water out of it in your kitchen oven and reuse it. Caution: If you have an oil lubricated air compressor, don’t do that, chuck it. There are many other sorbemnts like activated alumina and molecular sieve that are strong desiccators too.
If you want to see best practice in managing compressed air systems for average Joes, look to the guys who paint cars. Water is a real drag that causes paint defects and blushing etc. The norm there would be to have a refrigerative dryer and possibly a regenerative (pressure swing adsorption) or semi-permeable membrane dryer downstream of that. These are common but more expensive but make sense if you need all of your air to be dry.
quote:I can see that in many instances it may be much simpler to drag around tanks of very dry nitrogen to the race track or out onto the runway to top the tires rather than have an extravagant air drying system on a small portable compressor? That could be a player in using nitrogen as well rather than very dry compressed air, i.e., nitrogen in a bottle is actually a whole lot easier to use than drying out the compressed air in a remote location?
Agreed. As a practical matter this may be a more economical source of dry compressed gas depending upon your usage rate, cost to get yourself set up for either N2 tank, or desiccant cartridge, and frequency of use. If you need the gas infrequently or remotely, it may very well be more practical and you do have the benefit of knowing it is dry. The point I was making is it’s more so the dry gas than N2 magic from a pragmatist’s view. If you go this route, I’d suggest you not use the same neoprene compressed air line that’s used to pump water laden air through. Most of these hoses lay around on the garage floor with liquid water in them. Any dry gas will be readily pick up the H20 on the way to your tire. Maybe go buy one of those cheapy polyethylene coiled air hoses to do the job?
quote:Also, not a scuba diver here but is the amount of water vapor that is compressed into a scuba tank a consideration too? Are you changing the safety of the breathen air to the diver by doing that or better yet shortening the life expectancy of very active scuba divers by making them breath an unnatural mixture for extended periods of time? Any MD's care to chime in on that one? Not being paranoid here folks. I blame it all on Ray Herbert, aka, Mr.Wizrd. He ruined me for life?
It’s not really the H20 in the air. We breathe it all the time. In fact, the presence of H20 in air makes breathing it more comfortable. It is an issue and in part the reason for the SCUBA bottle being worn with the valve down. The safety issue is more a matter of cleanliness and preventing corrosion. Water can breed microbes and bacteria. It also can create fouling and freezing in the regulators and breathing devices which is unwelcome at depth. SCUBA compressors are usually multiple stage recip compressors with multiple water traps, inline desiccant filters, and microbial filters at least for the pros and people that sell the service. The standards often go down for live-aboard boats and hobbyists who recharge bottles.
A couple other practical thoughts; to help appreciate how water affects ability to oxidize metals consider how cars age in the desert Southwest compared to coastal Southern Florida or perhaps add salt to the equation in the Midwest rust belt. To further consider how degrading UV light is to polymers and elastomers, have a look at the dashboards in old Arizona cars. Lastly, I just saw a set of mag GT5 wheels sell for North of $10k. Maybe N2 isn’t such a bad idea.
Kelly-Out