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Vacuum flask knowledge

A vacuum flask (also known as a Dewar flask, Dewar bottle or thermos) is an insulating storage vessel that greatly lengthens the time over which its contents remain hotter or cooler than the flask's surroundings. Invented by Sir James Dewar in 1892, the vacuum flask consists of two flasks, placed one within the other and joined at the neck. The gap between the two flasks is partially evacuated of air, creating a near-vacuum which significantly reduces heat transfer by conduction or convection.

Vacuum flasks are used domestically, to keep beverages hot or cold for extended periods of time, and for many purposes in industry.
 

Inside a vacuum flask

Diagram showing how the parts of a vacuum flask keep it either hot or cold

Let's take a quick look at what's going on inside a flask:

  1. Screw-on stopper.
  2. Outer plastic or stainless steel case.
  3. Outer layer of glass coated with reflective material (or stainless steel in unbreakable flasks).
  4. Vacuum.
  5. Inner layer of glass (or stainless steel in unbreakable flasks).
  6. One or more supports keep inner vacuum container in place.
  7. Additional insulation reduces heat losses and cushions flask against impacts.

Stainless steel vacuum flask

Keeps all these features prevent virtually all heat transfer by either conduction, convection, or radiation. But with new recyle materials 18/8 stainless steel. The vacuum prevents conduction. The tight stopper prevents air from entering or leaving the flask, so convection isn't possible either. What about radiation? When infrared radiation tries to leave the hot liquid, the reflective lining of the inner chamber reflects it straight back in again. There's virtually no way heat can escape from a vacuum flask and a hot drink stored inside will stay steaming hot for several hours.

Flasks also work for cold drinks. If heat can't escape from a vacuum flask, it follows that heat can't penetrate into a flask from outside either. The sealed stopper stops heat getting in by convection; the vacuum stops conduction, and the metal lining between the outer case and the inner chamber stops heat radiating in either.

Whether you like your coffee piping hot or icy cold, vacuum flasks are an absolutely brilliant way to keep your drinks just the way you want. Some heat still escapes (or gets in) eventually, mostly through the stopper, but flasks like this are still a vast improvement on virtually every other kind of insulated drinks container.
APPLICATIONS OF HEAT TRANSFER - ppt video online download

Try order some good vacuum flask from begu.

 



 

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