Keeping food hot in food delivery bag

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Hello physics forums
I'm looking to make a food delivery bag and to keep the food as hot as possible as long as possible. I want to use the cheapest materials I can find. My knowledge is limited and so far my plan is to sandwich some cardboard between aluminum foil. I was then going to line the inside of the bag with some fabric to give it a more finished look and to protect the foil. Finally, wrap the whole thing in a backpack type shell. Would this work well? Will the fabric inside prevent the thermal radiation from reflecting off the foil? Would a thin piece of packing foam add any major benefits? If the aluminum foil gets wrinkled, would this drastically change its ability to retain heat? Is there a better way to do this using cheap materials? Thanks for any info
Food-Delivery-Bag.jpg
 

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  • #2
CWatters
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Various types of insulation exist for houses. They have different thermal and mechanical properties. Some are rigid foam boards, some are semi rigid wool like and others are more like laminated foils (so called multifoils). If you are trying to make a bag rather than a rigid box I would look at a combination of Rockwool and multifoil. Perhaps a sandwich of multifoil - rockwool - multifoil.

If you are lucky you might be able to find offcuts at a local building site and get it free.

If the Rockwool insulation is too thick you might be able to cut it using an electric carving knife or perhaps even a bread knife.
 
  • #3
CWatters
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PS If you were making a rigid box I would look at rigid foam based insulation. For example PIR/PUR or EPS foam boards. PIR would have the best thermal properties (Aerogel would be better but way too expensive and hard to find).
 
  • #4
kuruman
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Try multiple layers of aluminized mylar. If you have ##n## layers, the fraction of the heat per unit time (power) transferred from the inside to the outside ideally goes as ##\frac{1}{n+1}## so you don't buy much after 5-6 layers. Furthermore, the layers are thin. If aluminized mylar is too expensive (price it), try alternate layers of aluminum foil and polyurethane film. BTW aluminized mylar is used to super-insulate liquid nitrogen dewars.
 
  • #5
anorlunda
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I agree. A rigid foam board is your best choice. Styrofoam is the cheapest, but other foams give much better insulation.

You can buy a single small closed cell foam board at the hardware store. Loose insulation like rock wool are sold in big bags.

Do you want to make one bag or many?
 
  • #6
berkeman
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Welcome to the PF. :smile:
I'm looking to make a food delivery bag and to keep the food as hot as possible as long as possible
Whatever you use, be sure it is food safe. If it keeps food hot but uses materials that are not food safe, that's probably a bad combination.

You could re-task a small styrafoam cooler, for example. Presumably the material they use is good at insulating and also food safe. :smile:
 
  • #7
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Ok thanks for the replies. I just want to make one bag and have very little money (like $40). Does foam board crack easily?. Basically I'm making a cube and putting it in a custom backpack so whatever forces are acting on it; The weight of the food inside etc. I might build a thin plywood/veneer support cube and line it with foam board. Would layering aluminum foil and plastic bags work as well? Would it "cold bridge"?
 
  • #8
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Welcome to the PF. :smile:

Whatever you use, be sure it is food safe. If it keeps food hot but uses materials that are not food safe, that's probably a bad combination.

You could re-task a small styrafoam cooler, for example. Presumably the material they use is good at insulating and also food safe. :smile:
Thanks! Yeah I'm also concerned about offgasing of these materials.. I need to find a large foam cooler that can accommodate large pizzas and the ones I've seen are just too small :(
 
  • #9
berkeman
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You could slice a larger one in half, and seal it from the outside away from the food. That should keep it food safe, I would think. :smile:
 
  • #10
Tom.G
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For the foil I suggest metalized Mylar as used in Space Blankets or Emergency Blankets. Plain metal foil tends to get holes in it unless it is well protected. You can use the metalized Mylar as the inside surface, making it relatively easy to clean.

A minor variation is if you can find a plastic storage box of the right size, you could use that as the inner shell and wrap insulation around. The storage boxes I'm thinking of are made of either polyethylene or polypropylene and are found as storage boxes for under a bed or as food storage containers.

For the main insulation ordinary corrugated cardboard works surprisingly well but weighs more than plastic foam. Home refrigerators used to use several layers of cardboard before plastics took over. (they also used less electricity than the 'New Light Weight' ones with plastic foam)

If you decide to use corrugated cardboard, layer it so the corrugation in adjacent layers runs across the corrugation of the previous layer. This gives the assembly extreme strength against bending, especially if you glue or tape the edges together. Depending on usage you may still need a hard outer shell for mechanical protection against impacts though.

Let us know how it all turns out!

Cheers,
Tom
 
  • #11
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Kind of cheating, but might be helpful: the food will lose lot of heat to warm up the interior of the bag before the insulation kicks in. If you preheat the bag that will help a lot. So, just keep a bottle of hot water inside before loading the food...
 
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  • #12
sophiecentaur
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Will the fabric inside prevent the thermal radiation from reflecting off the foil
The reflective foil will probably be more effective if it's the 'first line of defence', right next to the food package. The food package will be the hottest thing in the box and, if you are only storing the food for a short while, it's unlikely that the thermal gradient through all the layers will have established itself. (It will not behave like a school experiment measuring thermal conductivity of materials, where things are steady state) We are not dealing with a steady state situation here so it's not straightforward and we are looking at a transient effect.
Of course, the food will be cooling down from the get go but the insulation (which has finite thermal capacity) will initially be warming up and the Infra red radiation will be reflected back by the foil instantly, whereas the surface of foam etc will need to warm up before it sends back any IR to the food.
 
  • #13
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The reflective foil will probably be more effective if it's the 'first line of defence', right next to the food package. The food package will be the hottest thing in the box and, if you are only storing the food for a short while, it's unlikely that the thermal gradient through all the layers will have established itself. (It will not behave like a school experiment measuring thermal conductivity of materials, where things are steady state) We are not dealing with a steady state situation here so it's not straightforward and we are looking at a transient effect.
Of course, the food will be cooling down from the get go but the insulation (which has finite thermal capacity) will initially be warming up and the Infra red radiation will be reflected back by the foil instantly, whereas the surface of foam etc will need to warm up before it sends back any IR to the food.
Great info. Thank you
 
  • #14
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A couple things occur to me. First, the temperature will be low enough that thermal conduction, not radiation will be the main mechanism of heat loss. No problem with reflective foil like mylar but the impact will be small. If you really like shiny stuff, I would avoid aluminum foil due to weight compared to mylar. If the air is allowed to move, this will increase heat loss. If the walls are not rigid, flexing will cause some "breathing" of the walls as well as the food storage volume and increase heat loss. An airtight closure is important. Layers of mylar only invite flexing which will move air around and increase heat loss. Something that holds air still like foam would be a good choice. I would advise you to go to some place that sells picnic coolers, etc. Those people have spent a lot of time building things to do this job with the contents cooler, not hotter. Same problem. If you need flexible walls how do others do that? What flexible material holds air still inside without compressing and allowing breathing? (think of squeezing a sponge and how water is lost.)

On cardboard, notice that when someone ships you an item needing to be kept cold, cardboard is the shipping container. Inside will usually be a foam box with a lid or they will have wrapped the contents with something holding air in cells to be still as possible. The spaces are filled with this type of material. I suspect the main reason is impact protection but it also matters that the air stays still. Besides, cardboard is relatively heavy.

You need to consider whether or not all foods are at the same temperature. If not, how to you manage that? Hotter in the center? Extra foam layers to protect the hot material? Individual foam containers? How long will you need to hold the food at temperature? How hot can you start? What temperature will customers accept. Does the delivery person have to wear it on his back and remove it for delivery? Does he carry it in a vehicle and put it on his back to deliver? Can the box be rigid? I suspect there are a lot of questions that we would need to answer to really help you.
 
  • #15
256bits
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air is allowed to move, this will increase heat loss.
A good point.
The curtailment of convection from open air to a closed space is probably more than half the battle.
Add a layer of insulation and the contents should keep heat for the duration of delivery.
 
  • #17
Tom.G
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I'm a bit suspicious, quoting from that link:
  • No shaking or kneading required
  • To activate - remove warmer from outer package, shake to activate.
 
  • #18
anorlunda
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You don't say if the container holds one item, or multiple items added or removed one-at-a-time. In the latter case, much of the heat can be lost during adding/removal.

With cold storage, like ice boxes, a major improvement is seen when it opens from the top. With a side opening door (like on a house refrigerator), most of the cold air spills out every time the door is opened. Then the warm replacement air must be cooled.

For warm storage, the opposite should be true. Opening it from the bottom to remove some food would preserve heat better than opening from the side. Obviously, opening from the bottom without spilling the food is trickier than opening from the top.

Even opening from the side, a plastic strip screen analogous to the one in the picture, could reduce air spillage. They use screens like that for walk-in freezers to reduce heat losses while people and goods go in and out.
images?q=tbn:ANd9GcRKnxz16WoIPVmj1TUXZLnqymIMgz3b6KP0Feg8WqFxEKp2Cx_cOg.jpg
 

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