Evaporation may invoke thoughts of the mundane. It is actually rather fascinating within physics. It is the phase change of a liquid into a gas that is experienced everyday. It allows for our hydrological cycle, temperature regulation in our bodies, cooling our of physical spaces, and the distillation of spirits. Water boils at 100C/212F, but it can evaporate quite successfully at room temperature.
What is really happening to cause this without significant increases in temperature? If you have ever jumped on a trampoline or bounced balls into each other, you have experienced a transfer of kinetic energy. Evaporation is a complex process where matter needs sufficient energy in specific direction to escape the liquid. In a sense, evaporation is very similar to radioactive decay, where particles need sufficient energy to escape the nucleus of an atom.
Evaporation takes several factors to be successful:
- Temperature: increased energy levels (measured as temperature, or enthalpy) increase the rate of evaporation. Add heat to a liquid and it will evaporate more quickly.
- Pressure: lower pressure increases the rate of evaporation. This is why water boils at lower temperatures at higher altitudes. Pressure is the force of other matter pushing downward due to gravity. Less external pressure means less force pushing down on the liquid, allowing it to escape more easily.
- Humidity: the higher the volume of evaporated liquid in the surrounding air slows the pace of evaporation. The air will become saturated and become weighed down with the liquid, preventing more from escaping the liquid. In a hot environment with high humidity, it is difficult to perspire and can lead to heat stroke.
- Surface Area: the greater the surface area of the liquid, the more liquid can evaporate. This is why a wet cloth will dry more quickly when spread out than when balled up.
Surface area is the key aspect of evaporation that I want to focus. Evaporation only happens at the surface of a liquid meeting air. a molecule must be traveling upward with sufficient energy to escape the liquid. If its vector is lateral or downward, it isn’t going to evaporate as it is traveling further into the liquid.
When it comes to aging spirits, many of the practices are steeped in tradition. Barrel aging is one such tradition that dates back to Roman times. Barrels were used to transport and store goods, including water, meat, and wine. Previously, clay amphorae were used, but they were heavy and fragile. Barrels were lighter, more durable and plyable, and could be rolled, making them easier to transport.
Barrels were introduced into spirit aging by accident. Whiskey was transported in barrels that had been used to transport fish, salt, and other goods. The whiskey was found to have taken on the flavors of the previous contents. This was a happy accident as the whiskey was found to be more palatable and complex. In situations with undesirable previous contents, the barrels were charred to remove the previous flavors which introduced another layer of complexity to the whiskey.
When filled, common whiskey barrels weigh in at over 500 pounds (the barrel being over 100 pounds by itself). The weight of the barrel is due to the liquid it contains, the wood, and the metal hoops that hold the barrel together. As they’re round and our motive forces were people, horses, and oxen, they were rolled. Placing the barrel horizontally into its resting place was a natural consequence.
Barrels are porous. As much as the techniques have improved, there are gaps between staves and the barrels heads. Air can pass through the wood and the gaps. This interaction is critical by permitting air transfer between the inside and outside of the barrel. Ethanol vapors escaping the barrel are known as “angel’s share.” This can concentrate compounds that are heavier and can be desireable, but it also represents loss of product.
Initial losses with horizontal resting orientation leave less surface area than a vertical orientation, but further losses will favor vertical orientation.
Doing a quick calculation, ignoring the curvature of the barrel staves, losing 1″ of liquid in a 53 gallon barrel leaves 307.8836 sq. in. of surface area in a horizontal orientation versus 363.05 sq. in. in a vertical orientation. However, moving to a loss of 2″ of liquid yields 424.592 sq. in. in a horizontal orientation versus the same 363.05 sq. in. in a vertical orientation. The surface area increases at a greater rate in a horizontal orientation than in a vertical orientation.
These calculations assume 34″ of inner height of the barrel and 21.5″ of inner diamete at the barrels heads. Both orientations would suffer from calculating the curve of the staves, so this is a simplication.
Calculating the surface area in a vertical orientation is easy, is just the area of a circle with a 10.75″ radius: π×10.75^2 or 363.05 sq. in.
Calculating the surface area in a horizontal orientation is a bit more complex. We have to calculate the horizontal line across the barrel stave using the amount of vertical loss, represented as X. So for 1″ of loss, we have to determine Y, using 10.75″ as the hypothenuse of a right triangle with 10.75 – X and Y as the other two sides. We can use the Pythagorean theorem to solve for Y: (10.75-1)^2 + Y^2 = 10.75^2, or 95.0625 + Y^2 = 115.5626. Then, we solve for Y by subtracting 95.0625 from boths sides and take the square root of each side: Y^2 = 20.5, or Y = 4.5277 sq. in. We then multiple this by 2 and the height of the barrel: 307.884 sq. in.
When we move to 2″ of loss, we have to determine Y again: (10.75-2)^2 + Y^2 = 10.75^2, or 78.0625 + Y^2 = 115.5626. Then, we solve for Y by subtracting 78.0625 from boths sides and take the square root of each side: Y^2 = 37.5, or Y = 6.1237 sq. in. We then multiple this by 2 and the height of the barrel: 424.592 sq. in.
When storing liquor in barrels, the traditional orientation is horizontal. In the American whiskey business, the warehouses resting barrels horizontally are generically called Rickhouses, though this is like calling tissues Kleenex. Warehouses used for resting barrels vertically are called Rackhouses.
A correlated data point is the surface area of the barrel that the spirit contacts. If there is less surface area to contact the air, there more is more contact with the barrel.
In conclusion, evaporative losses are greater for a horizontal orientation as losses increase. A Rackhouse style warehouse which stores barrels vertically will results in more wood interaction and less evaporative losses.
Dustin Dortch 