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The Thermodynamics of a Beer Can

Discovering the remarkable stories of science in every day objects

By Jacob HoodPublished 3 years ago 7 min read
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Last weekend I made a mistake I am sure many of my readers have experienced, which was that I forgot to put enough drinks in the fridge a couple hours before I needed them. For many this would have been a disaster, and unfortunately, they would have been doomed to drinking warm beers until either they concluded the night or waiting for the process of which never seems fast enough, of their fridge naturally bringing their drink down to a frosty and cool 4-5 degrees Celsius (or 39-41 Fahrenheit if you're dead set on using freedom units) for enjoyable consumption.

I however, and a university graduate with a degree in water sciences- and also a year of living in residence under my belt. The latter of which may have been more useful for my predicament then the former. See a simple trick most college students have learned (at least the ones I'm aware of) is to take your warm beers and wrap them in a moist paper towel (metal containers work best, but glass works fine too) before popping them in the freezer for 30 minutes at which point when you full them out and tear off the now icy cocoon that was the old paper towel strip, they are just as cold as if they had been sitting in the fridge all day. Now this is a great party trick, but if you are like me you're probably already formulating different ideas on why this actually works. Go ahead and take your shot at it, but I'm posting the answer below.

A graph showing the temperature change over time between the two cooling methods.

When you were back in elementary school you probably heard that heat moves in three ways: Radiation, convection, and conduction. The beer cooling trick uses the latter two as the more powerful heat transfer methods since heat transfer by radiation is proportional to the fourth power of the material's absolute temperature, and in most situations, we come across in daily life, pretty negligible. If we compare the beer cooling trick to the normal method used of just plopping the beer cans in the fridge for a couple hours, we see there are two big differences. The first is that the can is placed in the freezer, not the fridge. This is important, but probably not for the reason you think it is. Secondly, the drink is wrapped in a wet paper towel, again its important that the paper towel is wet- its the liquid property of water that seems important here since just placing it against ice cubes might increase the rate of cooling we see in the can, but seemingly not at the same rate. These two variables actually work together in a fascinating way to propel the rapid cooling of your drinks to new entirely new levels.

The three methods of heat transfer visualized.

Before we dive deep into this topic however, its important to know some basics about what is going on. When we cool our beer, we aren't "adding cold" into the beer can achieve a colder beer. Adding cold is in all reality an impossible task since coldness is simply the absence of heat. What we are trying to do when we place a beer in the fridge, is let the fridge suck the heat out of the beer so that the temperature of the beer reaches equilibrium with that of the air inside the refrigerator. When we want to measure just how much heat energy (because remember, heat is an expression of energy) needs to be removed from the beer inside the can, we employ the metric of something referred to as specific heat capacity. Specific heat capacity is a fancy term used by chemists and physicists to specify how much energy it takes to warm a unit of an object by a certain temperature. For example, the specific heat capacity of water is 4.184 J/gK which might sound complicated but is actually super simple if you break it down into small parts. J/gk means joules per gram per degree kelvin. when we're referring to water, a gram is 1cm^3, or 1ml. Kelvin is the same as Celsius, and a joule is the amount of energy is takes to move lift a 1-kilogram object 1 meter. So, if you want to take 25ml of water and take it from 0C (freezing temperature) to 100C (boiling temperature) then it would take 10,460J to achieve this (4.184J/gK *25g *100K). Using this knowledge and knowing that beer is mostly water (especially if you're drinking beers brewed by my American friends) we can calculate that for a 355ml can of beer (12 oz) we will need to remove about 26,710 joules of energy from the can before it goes from room temperature to drinking temperature. So 26,710 J is the magic number, now the question is, how can we drain that energy the fastest?

The first law of physics that this method exploits would be that of Dalton's law of evaporation, which states that evaporation continues until the saturation vapor pressure in the air is reached, which basically means that a dry environment will evaporate more water than a wet environment (duh). The way our fast beer cooling trick utilizes this principle is simple, due to the way your freezer works, cycling air from the freezer and condensing it with the use of a refrigerant over and over again, a deep freezer can actually be one of the driest places on earth, basically the air has all the water "squeezed" out of it, and is so cold, that any exposed surfaces (say for example a wet paper towel saturated with water and a high surface area) will quickly evaporate any water out until the air inside the freezer is saturated- which if you remember back a few sentences, since the air is constantly being "squeezed" by the refrigerator, will take much longer than it usually would in such a cold environment.

The rapid evaporation of the water off the paper towel also compounds with another factor effecting the cooling of the drink- water absorbs heat very well (remember its specific heat capacity is 4.18J/gK) but aluminum is one of the better common heat conductors, in fact its about 300x better than water. Aluminum does not take nearly as much energy to heat up (its specific heat is 0.897J/ gK) and its thermal conductivity is about 205 W/m K. Just to get a sense of how insane that is, water's thermal conductivity is 0.6 W/m K, and the best natural thermal conductor in the world that we know of is diamond, and its conductivity is 1000 W/m, so about 5 times better than aluminum, and about 1700 times better than water.

If you are visualizing this as we go, the energy in a beverage in the fridge follows the following route. The beer's thermal energy starts at a relatively high density inside the can, and as the cool air of the fridge comes into contact with the excellent thermal conductor of the aluminum can, it allows some of the heat to slowly leak through from the warm beer to the now warm can. The cool air molecules in the fridge come into contact with the warm metal and conduction moves the heat energy into them. The warm air around the can now raises up through convection and cool air around it swoops into replace it. Most of the energy escapes slowly through conduction with the low-density air particles coming into contact with the beer can, and very little heat escapes through radiation.

Now the college method of cooling your beer works with some of the same principles in play, the metal can is conducting the heat from your warm beer in the freezer very quickly, however instead of slowly leaking into the low-density air directly around it, it comes across liquid water- water which happens to have a specific heat of about 4 times higher than that of air, and also about 830 times more dense. As the water inside the paper towel readily takes in the heat energy from the beer inside the can, it starts to evaporate into the dry cold air of the freezer. As this happens, the paper towel actually dries slightly and looses enthalpy (a measurement of the internal energy of the object) at a rapid rate. The evaporation cools the aluminum can, allowing more heat to quickly drain into the aluminum and then the water in the paper towel, causing more evaporation and moving the action along, with more and more heat being dumped at a faster rate due to the dry air (more room for moisture to evaporate) in the freezer being at a colder temperature (more room for heat) and the properties of the can, the towel, the water, and the air in the freezer all cooperating together to suck as much heat out of the beer as quickly as possible so you can enjoy a cold one on your Friday night with friends... or writing a science Vocal post.

Either way if you found this interesting and informative, go ahead and like the post, if you want part 2 on what happens inside the can when you leave your beer in the freezer for too long (another sin I am guilty of) go ahead and click the follow button. I wouldn't be able to write so much without the community’s support.

Cheers!

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About the Creator

Jacob Hood

I love science so I went to school for it.

I have an interest in business, so I'm starting my own company.

I enjoy writing so I split my time between here and my Quora account.

You like my articles, so you clicked my profile :)

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