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I bought a new flask today (life in the fast lane), it'll keep hot things hot for 8 hours and cold things cold for 24 hours - why such a big difference?

asked in Flasks, hot, cold



macavity answers:

Not sure on the technical answer but things that are hot begin to cool from the start where as things that are cold do not get colder as such but rise much slower from fridge or frezzer coolness and more likely to suffer with stagnation if left for long periods of time


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seacommander answers:

I think the difference can be explained by the temperature difference between the contents and ambient temperature.
For example if we consider a cool drink to be at say 5 degrees C and the ambient at 20 then there is only a 15 degree difference and heat transfer will be relatively slow.
If we make a hot drink it may be at 80 degrees C for example at and at an ambient temperature of 20 degrees the difference will be 60 degrees and heat transfer will be much quicker.
Another factor that may be involved is the direction of heat transfer. For the cold substance the heat transfer is from outside of the flask to the inside and the opposite for the hot drink. Heat to warm the cold substance can only be transferred from air to the outer surface of the flask which may not be as an efficient means of transfer as a hot substance directly in contact with the inner surface.


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rainchild answers:

Because you live in cold England. If you lived in Cairns it would be the reverse!


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robinsamuels answers:

Exactly as said above. The flask is just a layer of insulation, there is no flask out there which can provide a complete vacuum.

So, the contents slowly absorb heat/radiate warmth from/to to outside air. If the air temperature is around 10 degrees and the contents are 90 degrees, the difference is much greater, therefore allowing for faster absorption of lost heat by the surrounding air, than if the contents were at 5 degrees.

Just picture putting a bag of frozen prawns in a pan of cold water. If it's a small pan, the prawns can freeze the water before they fully defrost because they lower the temperature faster than they themselves defrost. If you were to put them in a pan of hot water, the water would cool, but the prawn would reach the water temperature faster.


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