A can cooler is a handheld insulated container that holds a beverage bottle or can. The can cooler allows you to drink from the can or bottle while keeping it cold for much longer than it would if you held it in your hand. Can coolers are often called coozies or koozies, although Koozie is a brand name. They’re also know in some areas as huggers, sleeves, jackets, coolies, cozies and in Australia, stubby holders.
Can coolers are made of an insulating material. An insulator reduces the flow of energy from one place to another. In this case, the energy is heat, and the can cooler blocks the heat from the air, the sun and from your hand from reaching your beverage.
Vacuum-insulated can coolers are usually made of 304 (also called 18/8) stainless steel. Stainless steel is strong, relatively inexpensive, low maintenance and food safe. It would normally be a terrible insulator, but stainless steel can coolers are built with a double wall (a vessel within a vessel), and the air is removed from between the wall to create a vacuum. A vacuum is the best insulator we can create, barring some exotic materials and structures.
How are vacuum-insulated can coolers made?
An outer vessel and a slightly smaller inner vessel are formed from stainless steel tubing. A small hole is punched in the bottom of the outer vessel. The two vessels are welded together at the rim. The joined vessels are placed upside down in a vacuum furnace with glass bead covering the hole in the bottom of the outer vessel. The air is pumped out of the furnace, which sucks all the air out of the space between the two vessels and and pass the glass bead. The furnace then heats up and melts the glass bead, which seals the hole and preserves the vacuum.
Why is vacuum such a good insulator?
When you hold a drink can in your hand, your body heat warms the molecules of the can which in turn warm the molecules of contents. This is conduction.
Vacuum is an excellent insulator because it prevents conduction of heat through physical contact and the loss of heat to the air via convection. Heat is transferred in materials when molecules on the warm side of a substance are energized by the heat. They start moving more and bump into their neighboring molecules, transferring some of their energy. This cascade continues until the temperature of the object is the same as the temperature of the surrounding environment.
The opposite of an insulator is a conductor. Silver and copper are excellent conductors, coming in at a value of around 400 for thermal conductivity. Aluminum rates a 237. Stainless steel, while still considered a “good” conductor of heat, only scores a 14. Air, on the other hand, is a terrible conductor/excellent insulator of heat with a conductivity rating of about 0.25 (as long as the air is trapped and still). A vacuum is the champ, however, with a conductivity rating of zero. (These number are watts per meter in Kelvin, so yeah, pretty esoteric.)
Vacuum almost completely blocks conduction because there are very few molecules of air left in a vacuum to transfer heat (a perfect vacuum is very difficult to achieve). You hand stays warm, and your beer stays cold with a vacuum-insulated can cooler.
Conduction, convection and radiation
We’ve have talked a lot about conduction, but there are two more ways heat gets transferred into your drink: convection and radiation.
We briefly mentioned convection before. Convention happens when the flow of air moves heat around. To stop convection, you have to stop air from moving. In a can cooler, this accomplished by having some sort of seal around the can. Typically, the can cooler will fit tightly around the sides of the container while exposing the top, or the can cooler will have some sort of collar that holds the can or bottle in while creating a seal. The more of the can or bottle that’s inside the cooler, the better. Any air trapped inside the can cooler will act as an insulator once it is down to the temperature of your beverage. You want a can cooler that forms a good seal around your beverage, and the more of your beverage that’s inside the container, the better.
Finally, there’s radiation. Not x-ray or gamma but infrared. If you leave your drink out in the sun or near a fire, it will be warmed by infrared radiation. Vacuum will not stop radiated energy. The heat from radiation can only be reflected away or absorbed. In fact, the better a conductor a material is, the better a reflector it makes. Silver, copper and aluminum are all excellent reflectors, but their high cost, tendency to oxidize and weakness (compared to steel) make them poor can cooler material.
No matter how shiny it is, reflective metal will still get hot if left out in the sun. The sun pumps a massive amount of energy into the earth, and the metal cannot reflect all the energy that is coming at it. A good bit will get absorbed, heating the metal surface.
An uncoated stainless steel can cooler will reflect some radiated heat away, but remember, stainless is not as good a conductor as many other metals so it’s also not the best reflector. One solution is to plate the outside of the inner vessel of the can cooler with a reflective metal such as copper. The copper will stay shiny because it is inside the vacuum where there is no air (specifically no sulphur) to cause it to tarnish.
Why are can coolers made of stainless steel?
There are a number of reasons stainless steel is a good choice for a can cooler. It doesn’t rust so it’s good around wet drinks. It’s attractive, food safe and durable. Stainless steel is much stronger than alternatives like aluminum, which is important because the metal used in a vacuum-insulated can cooler must be able to stand up against the negative pressure of the vacuum. Stainless steel can easily withstand the heat of the vacuum furnaces used to make the can coolers, while aluminum cannot. Stainless steel just 0.5mm thick is typically used for can coolers. Aluminum would have to be significantly thicker to serve in the same role, making it much more difficult to work and form.
Stainless steel isn’t the best metal for conducting heat. This is significant because eventually your beer will get warm even in the best can cooler. That’s because the stainless steel exterior is connected to the stainless steel interior. Over time, heat travels from the exterior skin and over the rim where the inner and outer vessels meet to the inner skin. The inner skin then transfers warmth to the air trapped inside the cooler or directly to the beverage if it is in direct contact with the inner wall. For this reason, it’s better to have a can cooler that uses some kind of space to keep the can or bottle from contacting the cooler wall.