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Thermal insulation/cover for heating water bottle Answered

I am building a water bottle that can heat water inside up to 80 degree Celsius for my engineering project. The design of the bottle is as follows:

1. an aluminium or steel flask

2. heating coil at the bottom of the flask(for heating)

3. battery and circuit for the heating coil is located below the heating coil.

I want to design a cover such that a person can hold the bottle when the water is hot. Also, i want to protect the circuit and battery at the bottom from the heating coil. How can I do this? (As of now, i was thinking to spray polyurethane foam around the bottle and use a plastic cover. Also, using a polystyrene board below the heating coil to separate it from circuit board/battery).

Are there any other insulating materials that can be used? Considerations for cover and insulation: 1. Should be able to hold the bottle 2. Wash the bottle (hence, waterproof)


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Jack A Lopez
Jack A Lopez

2 years ago

This seems like kind of an oddball application: using batteries to heat your beverage.

The reason why it seems odd to me is there are competing methods, that are seemingly cheaper and easier. Some that come to mind are vacuum bottles, and chemical heaters. Uh... lemme supplement that wild assertion with some links into the Wikipedia,



just in case you, or someone else reading this, have never heard of either of those things.

Regarding insulating materials, that are cheap, I think these polymer foams you mentioned (canned spray polyurethane foam and expanded polystyrene board) are both good.

There are tables out there, that list common materials and their thermal conductivity (or resistivity)



By the way, some numbers in these tables that are maybe misleading, are those materials that are non-solids. Like for example, argon gas superficially looks like a good insulator, with k = 0.016 W/m/K, or even dry, 1 atm, air with k=0.0262 W/m/K. However any non-solid material is going to be capable of moving heat by convection, which means we cannot just compare, for example air and EPS (expanded polystyrene) board, just by comparing the bulk thermal conductivity. We kind of know that EPS will be the better insulator, because the air can move heat by convection, in addition to conduction.

I don't know if that spoils the surprise, but when when you look through tables like this, you quickly realize that common, fluffy insulating materials, like EPS, fiberglass, feathers, etc, all seem to have thermal conductivity getting asymptotically close to the value for dry air, which is around k=0.03 W/m/K. As a consequence of this, there is a wide variety of fluffy, air pocket containing, materials to choose from, like polymer foams, or even organic materials like feathers, or rice hulls, that all have roughly the same value for thermal conductivity, all getting close to around k=0.03 W/m/K, near the value for air itself.

So how can you beat that, in an engineered material? Answer: make a material that is light and fluffy, but fill the voids with vacuum instead of air. I am not sure if such a thing actually exists. However in that first table I linked to, I noticed there are two entries for perlite, described as, "perlite, atmospheric pressure" and "perlite, vacuum", and the thermal conductivity of the vacuum perlite is significantly lower, "0.00137" versus "0.031"

I dunno. I guess I am just saying that there are a lot of materials that will work, for the application you are contemplating.