What is the best material to retain heat? (for public usage)

[Jan]

Hello,

is there any material or substance or combination of both that will keep warm for at least 30 minutes after absorbing it from external source? Must be able to use for public purpose, in reasonable costs, no toxic materials.

 

Most materials I know from real life that are used can be heated, but the heat goes off pretty quickly.

 

Thanks,

Jan

[DrP]

I think you need to look for materials with a high specific heat capacity.  

I think people used to heat rocks in a fire then put them in their beds to pre warm them.

What is the application?

 

PS - water is quite high iirc - thus the humble hot water bottle.

[Jan]

Thanks DrP for your answer. I'd like to create special dishes for my father's restaurant. Their plates become cold very fast so they have to heat them up a lot. Waiters can't even carry them with bare hands and customer's food is cold within 5 minutes. If the plate can be warm for up to 30 minutes it would improve customers satisfaction a lot.

[StringJunky]

You could use plastic covers  that sit on the edge of the plate over the food until it is served to keep the heat in.

[Sensei]

In local Vietnamese restaurants there are used so called "Hot Plates", which look like this:

 

It keeps hot meal for much longer than porcelain plates.

They are made of cast iron and wood.

Google for "hot plate cast iron" or "lava cast iron kitchenware". 60 ukp per piece.

ps. There is needed layer of onions at the bottom, otherwise chicken would be dry and too burned.

 

Edited January 11, 2018 by Sensei

[EdEarl]

Seems to me many restaurants have similar requirements for something to keep food hot. If it were easy, you could get it. You really didn't specify enough.

What temperatures, low and high, are acceptable, how much mass must be kept hot, what kind of mass, and how quickly does it cool now degrees/min. Can the mass be completely insulated, or will some of it be exposed to the environment. All that may not be enough, it is necessary to know how many calories/minute are lost to the environment, which you may not be able to measure.

I have a stainless steel thermos-cup that keeps my tea warm for about 12 hours, starting too hot to drink. It is closed on top, but opened enough to sip occasionally. If you can fully insulate, it will probably be your least expensive option. If insulating is not an option, a solution may be quite expensive.

[pavelcherepan]

10 hours ago, DrP said:

I think you need to look for materials with a high specific heat capacity.  

I'd like to amend this by adding that you need some material with high specific heat capacity and low heat conductivity, therefore it will be able to store a lot of heat energy, but will be reluctant to give it away.

Nice, I learned a new term here- thermal effusivity and looking at some numbers, DrP was right at recommending water or if you want something more solid things like PVC, wood, silicone or natural rubber can do the job. Neoprene is also good.

https://thermtest.com/materials-database

[EdEarl]

Water is heavy. To reduce mass it is possible to get more stored heat with less mass by melting the mass used to store heat. The heat released while melting is "heat of fusion." The heat of fusion of water is almost 80 times its specific heat; thus, you can use 1/80th as much water if the temperature you need is about 0C. Other substances melt at various temperatures, so you might find a paraffin or salt that freezes and melts at your desired temperature.

You may need to use an insulator and another substance to store heat.

Heat of vaporization is higher than heat of fusion in some substances.

Because there are many engineering choices, designing your solution requires information about you project. If you are reluctant to provide the necessary information, you can study university level thermodynamics which requires a bit of calculus.

 

[DrP]

7 hours ago, pavelcherepan said:

DrP was right at recommending water...

Although it doesn't make a very good plate. lol 

[EdEarl]

I made a mistake. Heat is absorbed during melting and released during freezing.

[swansont]

21 hours ago, Jan said:

 is there any material or substance or combination of both that will keep warm for at least 30 minutes after absorbing it from external source? 

Anything that fits this bill is going to be difficult to warm up

[Jan]

Thanks everyone for replies. I didn't thought that it will be easy. I am quite sure that it would involve some innovation and smart brains - that's why I put the question here

What about some substance that would be inside the plate (without access to air) that will keep it hot from inside when it absorbs it from environment? Something like those things: https://www.amazon.com/HotHands-Hand-Warmer-Value-Pack/dp/B0749NTTZN

 

EdEarl:

What temperatures: average to high, so the food has pleasurable temperature

How much mass must be kept hot: plate with 300-400g of food/soup

What kind of mass: food - meat, vegetables, soup, etc.

How quickly does it cool: keep warm at least 20-30 minutes

It would be exposed to the environment

[EdEarl]

I'd recommend starting no hotter than 50C for safety. Large surfaces like plates loos more heat per second than a smaller surfaces like bowls. Some dishes like porcelain conduct heat poorly; whereas, metal dishes conduct heat quickly. You want to conduct some heat to the food, but limit the heat going to the environment. For example, put your heater under a porcelain plate because the entire plate will heat up to keep the food warm, but it will slow heat loss better than a steel plate. Selecting the right dishes is probably important to success of this project, and a variety of dishes might be tested and data taken.

I think this is a bigger project than can be done on this forum. I have no estimate of the cost per meal. Style is important for food service. I suspect both cost and style requirements will be difficult to achieve. I am not the right person to attempt this project, perhaps someone else will speak up.

Edited January 12, 2018 by EdEarl

[mistermack]

You can buy hand warmers that work by oxidising lighter fluid. They don't use a flame, you light them initially, and the flame goes out, and then a catalyst keeps an oxidising reaction going without the flame, till the fuel is exhausted. (about five or six hours later !)

I own three and they work well. They get too hot to hold, they come in a cloth sleeve to make them cosy to use. I paid about three pounds each for them brand new, made in China, bought on ebay. 

You would need to work out how to place them under the plate, and keep the heat in. But it's one suggestion that might work.