When making my thermos, I need to understand how to minimize the 3 types of heat transfer
- radiation – transfer of heat through electromagnetic waves
- conduction – transfer of molecule movement through a solid
- convection – movement of heat through a liquid or a gas
Other criteria I had to meet for this grade 7 assignment is that it needs the practical (easy to hold) and it needs to be usable carrying at least 250mL or 1 cup of water.
Problem Definition and Considerations
To understand the bottom line stated above, someone told me to google how heat flows and to search things that are insulated.
The 3 forms of heat loss or transfer listed above came from this website (click) which explains it well with pictures and words.
We recently had our home upgraded and we insulated our attic. Here is a website that helped me understand that better (click). The measure of how good insulation is is called its R value. Here is a website (click) that define he R value and it has a table of materials with good to bad R values. Polyurethane foam, I was told was the first item on the list I could get a local hardware store that had the highest R value (1.2-1.4). This is the rigid foam panels (R=1.23) that new houses are lined with for insulation. It also comes in spray-foam varieties but R=0.93-1.1 . Due to cost, we went with polystyrene rigid foam (R=0.88-0.95) for the lid and spray foam (practicality and it fills all gaps).
Aluminum foil wraps nicely and stops electro-magnetic radiation before it can get to the foam. The foam and polystyrene boards do the rest limiting air movement (convection) and slowing conduction (it is hard for heat to travel through the foam due to the bubbles). The outer cover protects the foam and interesting duct tape was used to do this and integrate a handle into the design.
Using a glass jar would make sure it can be easily cleaned and it already comes with a lid. Most expensive thermoses are made of glass on the inside.
One thing that is important is that polystyrene and polyurethane will “melt” if the wrong kind of glue is used. We used glue that said it was compatible with polystyrene and polyurethane.
Construction & Design:
A good container to use was the glass Taster’s Choice instant coffee jar. The lid is square and therefore cannot “slip” when a lid is glued to it. A glass container can hold all sorts of liquid with minimal damage and easy cleaning for years to come.
To stop the radiation heat transfer, tinfoil was wrapped around the jar.
Heat from the contents will conduct through the glass and easily through the tin foil, but is immediately slowed down by the final insulative layers. Looking at insulative properties of materials (the R value) the higher the R value, the better. The most practical material at the top of the list is polyurethane. This is the same material used in insulating houses. A squarish hole can be cut to match the lid of the coffee container quite easily. Using 2 layers of this glued together keeps the heat loss out of the top to a minimum. Heat rises, so there could be a lot of heat loss.
Next, the sides were done a little oddly. Using parchment paper wrapped around a large tomato juice can gave a form in which to place the foil-wrapped jar and then we would fill the void with squeeze-foam used to seal holes for drafts. Steps were
- wrap the tin can with parchment
- clear-tape wrap the parchment around the tin can
- remove the tin can
- fill the bottom of this form with foam (about 3 cm is all that is needed)
- once it it dried, a bread knife was used to make a 3cm puck
- re-make the form with the tin can, parchment and tape but with the puck already in the bottom
- place the can on the puck and in the parchment form
- fill the void between the outer foil-wrapped jar and the parchment with more spray foam
- remember the foam expands to 2X its size so it does not need to be filled to the top
- wait for it to dry
- take form off
- cut top of the foam even with the shoulder of the jar and test the lid
- cut the lid so it is the same circumference as the new thermos
- wrap everything in duct tape integrating an old shoulder bag handle into the duct tape so it is easy to carry
The thermos stops radiation and conductive heat and the foam bubbles (spray polyurethane and polystyrene foam) slow the convection of heat between the duct-tape outer layer and the foil. That is how the thermos was made to combat all 3 forms of heat loss.