Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: paul.fr on 24/04/2008 16:29:10
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Im sure most of use prefill the flask with hot water, then 10 minutes or so later fill it with fresh hot water, tea, etc. But do you need to preheat a thermos flask?
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Paul no you do not need to reheat your Thermos flask as the hot
soup or hot drink will heat it up as you pour it in.
But the same goes for cold drinks as well but for the reverse reasons, of course e.g. cold juice., cool beer, water etc;
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The thermal capacity of the flask is very small compared with that of the liquid with which you are filling it so pre heating or cooling will make very little difference
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If the thermos liner is glass then no, if it stainless steel then I would recommend it.
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Thermos flasks, aka vacuum flasks, are designed to have very thin walls specifically to reduce the thermal capacity of the wall structure, so in practice pre-heating/cooling them won't make much difference. Ideally, the flask should be made out of an insulator to reduce conductive losses through the neck join to a minimum and these days many are made out of plastics. Glass was the preferred material to make vacuum flasks because it was rigid enough to withstand the vacuum between the closely separated walls of the flask without collapsing and in addition could be silvered to reduce radiative losses. Modern plastics though, are much easier to work with, are just as strong and just as effective as glass as an insulator. However, some of the plastics used are susceptible to long-term heat degradation and can become brittle over time whereas a glass vacuum flask will not.
Stainless steel flasks are generally only used where extreme durability is required because, being a conductor, it will readily transmit the heat to the 'weakest' link in the chain where the loss rate is highest, generally around the opening or neck of the flask. For this reason, a stainless steel vacuum flask will not be as effective as a glass or plastic vacuum flask.