[Richard]

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.