Chilly’s bottles have become extremely popular and this is no doubt because they are very good at what they do, keeping drinks hot and cold for long periods of time. From my own experience they can actually be too good, leading to a burnt mouth when out and about. What interests me about Chilly’s is why are they so good and also is it possible for them to still be as effective in the more extreme climates of the world as they are in the relatively mild North West of England.
Heat transfer in any application is driven by a temperature difference. In the case of Chilly’s this is the temperature difference between the drink inside and the air that surrounds it. What limits heat transfer in this application is the mechanisms of heat transfer taking place. Chilly’s are made of 18/8 stainless steel, this in itself does not have brilliant thermal properties. However it is not the stainless steel that makes the product so good. It is that they are constructed with two layers that sandwich a thin vacuum. As the vacuum contains nothing or virtually nothing, the methods of heat transfer that are associated with everyday heating and cooling; conduction (through solids) or convection (through moving liquids) do not occur in this space. The only heat transfer occurring in the vacuum is through radiation. This tends to be associated with heat sources of very high temperatures such as the sun, or fires and will be limited in the temperatures at play within Chilly’s.
So we know why the heat transfer in Chilly’s is so low, but we need something that quantifies how quickly heat passes through Chilly’s specifically. This is a heat transfer coefficient and has the units W°C-1, this is typically applied to houses and details the heat lost dependent on the temperature difference between in and out. Through a crudely devised experiment involving a fridge, a food thermometer and a temperature sensor I calculated the heat transfer coefficient to be 0.014W°C-1 when keeping a drink cool and 0.018W°C-1 when keeping a drink hot.
Using the calculated heat transfer coefficient and open source weather data I was able to model a Chilly’s containing a hot drink with and without milk in the extreme winter of Alaska in January against a comparatively mild Mancunian winter climate.
As there is an element of personal preference in hot drink temperature, I’ve assumed that when the temperature drops below 50°C it is no longer a pleasant drinking experience. In this case the bottle performs well with the drink still “hot” after 12 hours in both climates providing it has not had milk added that chills the initial brew temperature. Next up, a cold drink in the blistering heat of Dubai in August against a warm Mancunian summers day.
When the drink stops being “cold” I think is a lot more subjective as it is dependent on the environmental that it is being drunk. Whilst a drink of water at 23°C might not sound that pleasant in the UK, in the sweltering Dubai desert I’m sure it would be quite refreshing. Notably even on a summers day in Manchester where peak temperatures of 28°C can occur in the middle of the day, in the early hours of the morning the ambient temperature can drop below the temperature of the drink causing it to cool.
Chilly’s boast that they can keep drinks hot for 12 hours and cold for 24 hours. Based on this exercise I think this claim should hold for even the most extreme climates on earth. However, it’s bad luck if you’re a particularly picky snow plough worker in Alaska who likes milk in their coffee and works +12 hour shifts.