# Track Drying

I’ve gotten a couple questions about why it takes so long for a track to dry and why humid weather should make a difference, so here’s a short explanation.

Air is a mix of gas molecules:  mostly (78%) nitrogen, about 21% oxygen, the rest misc. gases.  The composition is pretty uniform with the exception of how much water there is in the air.   The absolute humidity is the amount of water in some chosen volume of air, for example, how much water vapor is in one cubic meter of air.  Air can only hold so much water vapor and that amount depends on the temperature and pressure.  Dry air would be no ounces of water in a cubic foot of air.  If the vapor is saturated at 30 degrees centigrade (86 degrees Fahrenheit), then the amount of water could be up to three one-hundredths of an ounce of water per cubic foot.

The mechanisms we use to get rid of water on the track are evaporation and possibly boiling.  Evaporation is the same mechanism we use to dry dishes, or even ourselves when we get out a pool and just let the sun dry us.  Evaporation is a liquid changing into a gas.  Boiling is also changing a liquid from a vapor to a gas, but there’s a difference.  Evaporation happens at the surface of a water drop.  Only the outermost few water molecules change from liquid to gas.  Boiling affects the bulk of the water drop.

Regardless of whether we’re talking evaporation or boiling, the water on the track doesn’t exist in a vacuum.  There’s that water vapor in the air.

Nature likes equilibrium.  Equilibrium is when things are equal and concentration is one property that can be equal.  If you pour a glass of red dye into a fish tank full of clear water, the red dye molecules will spread out and uniformly distribute themselves throughout the fish tank.  (Don’t try this if there are fish in the tank, please…)

So we have water molecules in the water drop — a lot of water molecules — and water molecules in the air.  The concentration of water molecules in the air is smaller than the concentration of molecules in the water droplet, but it can vary depending on how humid it is.  The picture below schematically shows three situations in which there are increasing amounts of water vapor in the air surrounding the water drop. The darker the green, the higher the concentration of water molecules.

Nature likes equilibrium, so it would like to have the same concentration of water molecules everywhere.  The rate at which it can move water molecules from the water drop to the air is proportional to the difference in concentrations.

If you have really dry air, there is a big difference in concentrations, and the water from the droplet moves into the air faster.  Have you ever hung your swimsuit out to dry on the balcony of a Florida hotel in July?  It takes forever to dry because the air is so moist.  There isn’t a huge difference between the concentration of water in the air and the concentration of water in the water drop.  If it were relatively dry and we had a rainfall, the track would dry much more quickly than it would with the current conditions:  the humid air is already pretty saturated – relative humidity is how close we are to totally saturated and the numbers have been around 90%.  100% relative humidity means that you absolutely can’t put any more water vapor in the air, so it would take a very, very long time to dry the track.

The jet dryers are literally jet engines trained on the track to speed up evaporation.

The heat from combusting jet fuel is used to dry the track.  The temperature of the combustion fuel is on the order of 1100 degrees F, but it cools pretty quickly as it leaves the dryer (that’s why the jets are so close to the track surface.)  I was trying to get a good number for the temperature of the exhaust, but I haven’t had luck with that.

The numbers I did get (thanks to Jeff Gluck and Nate Ryan) are that they have eight jet dryers.  Each dryer will operate for 50 minutes on 175 gallons of fuel, so if you had all eight jet dryers and it takes 150 minutes to dry the track, we’re talking about

They have over 20,000 gallons of jet fuel available at TMS, which is really good because it looks like they are going to need it.  I noted in a previous post that the Cup race uses somewhere around 5,700 gallons of fuel.  By the time we’re done with the races Monday I suspect we will have used at least double that in jet fuel just trying to dry the track.

I’ve been looking in to why it is so hard to make asphalt that dries faster and it looks like there are some physical laws that make it difficult – more on that after I get in touch with some sources Monday.

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