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air leaks cold weather

Water in air system and cold weather
#1
It's early, I have too much coffee, and it's too dark to play outside. 

This time of year, the forum gets a fair amount of commentary on air system weirdness and leakage. 

We all know that water in the air system creates rust in the tanks, the rust creates debris that can migrate and clog small orifices, and air valves such as the Height Control Valve, the potties, and all the air operated doors can experience problems. Not to mention the possible contamination of the braking system components. 

But there is another phenomenon that occurs that is a little harder to get your head wrapped around. You'll have to forgive me for getting all geeky with the engineer talk, but here goes. 

We know from experience that when a gas such as air goes from a high pressure to a low pressure that the gas cools. That's why your propane tank gets cool when grilling, it's why an aerosol can gets cooler when spraying the contents, and it's why the freon can gets really cold when adding refrigerant to your system. That cooling is caused by adiabatic expansion. The high pressure side is losing energy by releasing the pressure to the low side. Energy loss results in lower temperature. 

Of course the opposite is true when pumping a gas from a low pressure to a higher pressure. It gets warmer. Just touch the outlet of a running air compressor if you need proof. 

So why bring this up. If you have moisture in the air system, and it is escaping through a small orifice, then at temperatures above freezing, ice can form in the escape path. We don't normally think of ice forming below freezing, but in this circumstance, it can and it will. 

This information is particulary useful in helping to diagnose cold weather related air system weirdness. The first place this can really trip you up is the Height control valve. It is constantly releasing a small amount of air as the suspension moves up and down. In cool temps, you can form ice in that valve. The second place that this can occur is in the automated drain systems associated with the auxiallary air compressors. Those drain systems are prone to a bit of air seepage at the drain. Add the water in the separator bowl, the cooling effect of adiabatic expansion, and you have a recipe for ice forming in the drain valve at temperatures above freezing. 

Before you curse this physical phenomenon, keep in mind that it's the very principle that drives the air conditioning systems. But that's a conversation for another day. 

A common approach used in cold weather by truckers and RV owners is to add methanol to the air system. As Jon pointed out in another post, the methanol lowers the freezing point of the water in the system. However, it does not evaporate the water in the system in any meaningful way. Don't believe the marketing hype, unless the marketer has found a way around the laws of physics. There is no substitute for draining the tanks, and keeping the air dryers in working order.
Richard and Rhonda Entrekin
99 Newell, 512
Subaru Outback Toad
Inverness, FL (when we're home Cool )
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#2
Hi Richard

I have been tracing air leaks the last few months.  Was able to slow the 120v comp to cycle about every 90 minutes.  It was 20 minutes before I found numerous small leaks.  

I have noticed that when the ambient is below 50 degrees the 120v comp cycle time goes down to 15-20 minutes.  As the temp warms to the mid 60's the cycle time goes back to 90 minutes.  We have been in Pensacola since the first of December, and this has been happening since we arrived here.  

The time variation on the cycle time baffles me considering the temp variation is only maybe 25 degrees.

The leak seems to be on the supply side.  The brake air pressure seems to hold for days with no loss.

Any ideas?

Phil
Phil & Sharon
1999 511  41.5
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#3
Yep. Things shrink in cold weather, well at least my stuff does. Seriously, Newell was very fond of the push on air fitting connections. They are more prone to leakage in colder temps. My standard practice, and that of other Gurus, is when is found leaking, to replace it with a compression style fitting. I carry a whole tray of DOT compression fittings of 1/4 and 3/8 sizes just for the occasion.

If you look at the nylon tubing under magnification where it goes into the push on fitting, you will see that over time a small depressed ring forms on the nylon tubing. That is why the push on fittings leak over time. Cold weather makes the connection between the tubing and the collet in the fitting loose. A quick fix for a leaking push on, is to snip about a 1/4 off the tubing and stick it back in.
Richard and Rhonda Entrekin
99 Newell, 512
Subaru Outback Toad
Inverness, FL (when we're home Cool )
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