Tire air Pressure

[Mel Fulks]

I think the gas station compressors get neglected. After paying as much as .75 cents some years back to get as much water as air I bought one.

[John K Jordan]

... When you compress air, it can't hold as much moisture as when it's at a lower pressure. That's why you get water in your tank. And the air compressors in a service station probably compress to at least 150 lbs/sq" so the air coming out of the hose is dryer than the ambient air. It isn't completely dry, but it would be very low humidity. So I'd think that moisture in the air in the tire would be a small factor.

I don't think it works like that. From what I remember from my studies long ago the amount of liquid vs gaseous water in a container is not related to the pressure but dependent on the temperature, i.e., the water doesn't come out of the air because of pressure but from when the air cools to the saturation point (the dewpoint). In that case, water accumulating in the tank would result from the compressor adding more hot, moist air each cycle coupled with the cooling cycles. Since air heats up as it is compressed, if not allowed to cool the air coming through the lines may actually have more moisture in it than the air in the room since it can pick up moisture from any liquid water in the tank. The increased moisture in hot air is why recommended installations have cooling condensers or a long length of line on the outlet of the compressor (along with a gravity trap) to allow the air to cool and water removed before use.

Perhaps a real physicist will comment or I can ask one of my scientist friends.

This page has some interesting information, especially the section at the end discussing why air doesn't actually "hold" water:
http://hyperphysics.phy-astr.gsu.edu...ic/relhum.html

BTW, my shop compressor compresses to about 150 psi. I use a moisture trap, a centrifugal water separator and a desiccant drier before piping the air to the outlets. (The plasma cutter doesn't like wet air.) Even though I drain the tank the desiccant drier picks up a huge amount of water and has to be changed every few months. I suspect some service stations don't even dry the air at all.

But regardless of how the water gets in the tank, my own opinion about filling tires with nitrogen is it is an unnecessary expense with marginal benefit, if any, intended to increase sales. Even if it was beneficial, how will most people add a little air to their tires when needed - hunt for a shop that has nitrogen service? Topping off with "plain" air would add oxygen. When we take our 4wd vehicles in the deep sand (beach) we let air out of the tires to 15 psi or so, then add it back when back on hard pavement - the nitrogen would be vented several times a day!

JKJ

[Mike Henderson]

That opens up an interesting question: "Why do we get water in our air compressor tank?" When the air is compressed it heats up, but once it's in the tank, there's no place for it to absorb more water. And eventually, the air cools to ambient.

So the air going into the compressor is at some level of humidity. Here in CA, it's at a low humidity, maybe 30 to 40%. It goes into the compressor and gets compressed and because of the compression, it heats up. But once it's in the compressor, there's no place where it can absorb more moisture.

Eventually, the air in the tank cools to ambient. And I find water in my tank and have to drain it. I suspect that people who live in high humidity areas have a lot more water than I do.

So if compressed air will hold as much absolute water as uncompressed air, why doesn't the moisture stay in the air? Why do we find it in the bottom of our tank?

I didn't base my comment in my previous post on any science, just on my observation. And because of that, I could be wrong.

Mike

[And because some of the water is left in the tank, I assumed that the air coming out of the tank would have less moisture than the air that went into the tank.]

[Malcolm McLeod]

So if compressed air will hold as much absolute water as uncompressed air, why doesn't the moisture stay in the air? Why do we find it in the bottom of our tank?

As usual, the complete answer is long, but for simplicity's sake, I believe the principal issue is not in the compression of the air, but rather in its's use. As you say, the air in tank cools to ambient. As the pressure in the tank drops due to this usage, the temperature must drop as well (see above reference to the 'ideal gas suggestion'). If the temperature drops below the dew-point of the air in the tank/receiver, you have a mini-monsoon. In the tank.

With extreme pressure drops at point-of-use, you get even more significant cooling ...and you can get even more condensation at the point-of-use.

Industrial processes with a strict no-water requirement, will generally install a refrigerated air dryer head of the receiver. They cool the compressed air below the lowest expected temperature in the system, and so typically establish a 10-15 degF dew point (aka dry air). In a previous life, I worked in a (small) facility with 400hp in compressors. Dry air was a never ending battle.

[John K Jordan]

So if compressed air will hold as much absolute water as uncompressed air, why doesn't the moisture stay in the air? Why do we find it in the bottom of our tank?

Again, the air doesn't "hold" water, the water is introduced and until it condenses exists in the tank as a gas, independent of the air. Water does stay in the air but some of it may and does turn to liquid in normal compressor operation. What water goes where is a dance between the humidity, the pressure, the temperature, and time.

The total pressure in the tank is the sum of the "partial" pressures of each gas in the mixture, water, oxygen, nitrogen, CO2, argon, etc. The maximum partial pressure of water vapor is dependent only on temperature and is called the saturation vapor pressure. If there is more water vapor than this the vapor turns to liquid water.

The "dewpoint" is the temperature at which water as a gas can turn into liquid water. I don't understand all the details but I think it works something like this: The water vapor in the air is compressed in the tank and gets hot. The dewpoint at a certain pressure is a certain temperature. As the tank cools below that temperature, some of the water vapor turns to liquid water. The cooling can be from the tank sitting in the room or from reducing the pressure by letting gas out of the tank. (A compressor that ran continuously would simply pass air at the same humidity as the room.) The dewpoint temperature is actually higher at a higher pressure so at a high pressure liquid water can form sooner than at a low pressure. The way I understand it, gas turns to liquid until it reaches a stable balance for that temperature and pressure. Each cycle of the compressor brings in additional water vapor from the air, more at high humidity levels and less at low, and if even a tiny bit condenses each cycle eventually there will be a lot. If the air coming in is very dry, some of the liquid water can turn into vapor increasing the humidity of that air until it reaches the stable balance. As mentioned, air straight from a compressor can be way too "wet" for a lot of things in industry and science (paint spraying, pharmaceutical processes, etc.) and must be dried. The water can also condense and freeze in pipes in cold environments. This wet air is not considered a problem for things like blowing dust, air tools, and filling tires.

Again, I'm no physicist. I did a quick refresher from a reference book but I don't claim to have a solid handle on the details. Perhaps an expert on this will chime in and straighten out any misconceptions or mistakes. But regardless of the mechanism causing condensation, it does happen and the air from our compressors can be "very" wet even if we drain the tank after every use. I still don't think this will affect tires significantly. I certainly don't worry about it.

BTW, some tires have water added on purpose. Tractor and equipment tires are often filled with liquid for the weight. In climates where it never freezes, people fill them with plain water. Otherwise, people use water mixed with antifreeze, alcohol, calcium chloride, windshield wiper fluid, or even beet juice. Some fluids are more corrosive to the wheels. I have never heard of any fillers which effected the tires. Mine are filled 75% with something but it's been so long I can't remember.

JKJ

[Mike Henderson]

Well, my comment was that the air coming out of our compressors is less "wet" than the ambient air. Let me discuss why I believe that a bit more.

Let's take one cubic foot of air at a standard temperature - let's say 70 degrees. If we compress that to one quarter cubic foot, we know that some of the moisture that was in the cubic foot will come out of the air - because we find water in the tank. (I'm just using round numbers when I say "one quarter cubic foot")

Then, let's expand that one quarter cubic foot back to one cubic foot. The cubic foot still contains some moisture (less than the original one cubic foot because some water stayed in the tank) and the expansion cools the air, causing some of the moisture still left in that one cubic foot to condense out. If we have a water trap on our air line, that moisture will be trapped. When the temperature of that one cubic foot rises to 70 degrees, it will have less moisture than the original one cubic foot of air.

So compared to the original one cubic foot of air, the one cubic foot of air that we get out of the compressor must have less moisture in it since we have two places where it dropped moisture in the form of liquid water.

Mike

[Curt Harms]

Probably Glycerin. It's there for use in applications where there is vibration. Very common in industry.

Like your typical consumer grade air compressor? You're probably right. I recall a "How it's Made" episode about making dial gauges and they used glycerin.

[Kevin Barnett]

Again, the air doesn't "hold" water, the water is introduced and until it condenses exists in the tank as a gas, independent of the air. Water does stay in the air but some of it may and does turn to liquid in normal compressor operation. What water goes where is a dance between the humidity, the pressure, the temperature, and time.

The total pressure in the tank is the sum of the "partial" pressures of each gas in the mixture, water, oxygen, nitrogen, CO2, argon, etc. The maximum partial pressure of water vapor is dependent only on temperature and is called the saturation vapor pressure. If there is more water vapor than this the vapor turns to liquid water.

The "dewpoint" is the temperature at which water as a gas can turn into liquid water. I don't understand all the details but I think it works something like this: The water vapor in the air is compressed in the tank and gets hot. The dewpoint at a certain pressure is a certain temperature. As the tank cools below that temperature, some of the water vapor turns to liquid water. The cooling can be from the tank sitting in the room or from reducing the pressure by letting gas out of the tank. (A compressor that ran continuously would simply pass air at the same humidity as the room.) The dewpoint temperature is actually higher at a higher pressure so at a high pressure liquid water can form sooner than at a low pressure. The way I understand it, gas turns to liquid until it reaches a stable balance for that temperature and pressure. Each cycle of the compressor brings in additional water vapor from the air, more at high humidity levels and less at low, and if even a tiny bit condenses each cycle eventually there will be a lot. If the air coming in is very dry, some of the liquid water can turn into vapor increasing the humidity of that air until it reaches the stable balance. As mentioned, air straight from a compressor can be way too "wet" for a lot of things in industry and science (paint spraying, pharmaceutical processes, etc.) and must be dried. The water can also condense and freeze in pipes in cold environments. This wet air is not considered a problem for things like blowing dust, air tools, and filling tires.

Again, I'm no physicist. I did a quick refresher from a reference book but I don't claim to have a solid handle on the details. Perhaps an expert on this will chime in and straighten out any misconceptions or mistakes. But regardless of the mechanism causing condensation, it does happen and the air from our compressors can be "very" wet even if we drain the tank after every use. I still don't think this will affect tires significantly. I certainly don't worry about it.

BTW, some tires have water added on purpose. Tractor and equipment tires are often filled with liquid for the weight. In climates where it never freezes, people fill them with plain water. Otherwise, people use water mixed with antifreeze, alcohol, calcium chloride, windshield wiper fluid, or even beet juice. Some fluids are more corrosive to the wheels. I have never heard of any fillers which effected the tires. Mine are filled 75% with something but it's been so long I can't remember.

JKJ

Okay. I deal with dew points daily. Compressed air cannot hold as much water vapor as less compressed air. I'm not going into a diatribe about your theory, but if what you said was true, water would go back into vapor after compression - which it typically doesn't.

A cheap compressor will generally lower the humidity to about 9% RH from 40% at room temp.

In this discussion, unless the air line is lower in temp than the compressor tank or the compressor is malfunctioning, the dewpoint of the pressurized tire will be less than typical outside air.

I think some people feel the cold of compressed air and assume it's wet or the humid atmosphere condenses on cold skin giving the impression air that has significant moisture. Fact is, that air might be coming out at 4 degrees Celsius and 30% RH, but that's the same moisture content as room temp air at 10%.

I think a quick way to fool someone about this would be to depressurize a tire and then remove it showing the customer how "wet" it is inside. Fact is, depressurizing it would cool it down allowing moist air to condense once exposed to ambient air.

The chemist in me says 78% nitrogen is the most cost effective way to go here.

[Stewie Simpson]

Jiminy cricket - we are heading towards the 70th post on what should be regarded as simply inflating your tyres with compressed air. Air pressure at 85% of max.tyre rating to allow room for heat expansion. 78% nitrogen !!!!Save the chemistry background for something a little more challenging.

[John Stankus]

Jiminy cricket - we are heading towards the 70th post on what should be regarded as simply inflating your tyres with compressed air. Air pressure at 85% of max.tyre rating to allow room for heat expansion. 78% nitrogen !!!!Save the chemistry background for something a little more challenging.

Stewie - 78% nitrogen is the composition of dry air (21% oxygen 1% other gases). I think Kevin is saying just use air.

The amount of water that can be in the gas phase is the vapor pressure which is a function of temperature. At room temperature that is about 20 torr. (Note 1 atm is 760 torr). The vapor pressure is the maximum amount of water that can be in the vapor state at a particular temperature.