2. Thermos flask, also known as a vacuum or
Dewar-flask, is a double-walled glass vessel;
that minimizes the heat transfer between the
fluid inside and the surrounding outside. It
keeps a hot liquid hot and a cold liquid cold for
an extended period.
3.
4. You already know that to reduce the heat
transfer, you must increase the resistance to
heat transfer. As a result, insulating materials
that minimize heat transfer
through conduction, convection, and
radiations come into the application.
5. Outer casing: Outer casing made of plastic or
stainless steel protects the glass walls.
The outer layer of glass or stainless steel: The
inner portion of this glass layer coated with
reflective material reflects any incoming
radiation.
Vacuum space: This space maintained at
almost a vacuum prevents heat convection.
6. The inner layer of glass or stainless steel: The
outer portion of this glass layer coated with
reflective material prevents heat transfer
through radiation.
Plastic or cork stopper: The stopper made of
plastic or cork acts as a seal to prevent fluid
from going out and as insulation for heat
conduction and convection.
Support structure: The structure made of
insulation material acts as a rigid connector
that keeps the inner and outer flask rigidly in
its place.
7. A thermos flask is nothing but one bottle inside
another bottle, separated by a vacuum layer in
between. The vacuum layer prevents heat transfer
through conduction and convection; since
conduction and convection require the presence of
any medium to transfer heat. And as a result,
radiation is the only possible way of heat transfer
remaining. The outer part of the inner glass layer
and the inner part of the outer glass layer are
coated with reflecting material. The reflective
coating minimizes the emissivity or absorptivity of
the material. And it acts as a poor radiator and a
good reflector of heat radiation.
8. The Stefan-Boltzmann equation shows that the
heat transfer Q through radiation is directly
proportional to the material’s emissivity ε.
Q=εσAT4s
Thus, coating with the reflective layer reduces
the emissivity value and decreases the
radiation heat transfer.
The additional insulating material between the
outer glass layer and the casing further adds
resistance to conduction heat transfer.
9. As a result, to minimize the heat transfer from
the liquid to the atmosphere outside the
thermos flask and vice versa, the thermos flask
restricts all modes of heat transfer. The
conduction is prevented by inserting a
thermally insulating material, convection is
prevented by inserting the vacuum layer, and
radiation is prevented by coating reflecting
material on the glass surfaces.
Different layers in the thermos flask act as
resistance to all the modes of heat transfer, as
shown in the figure below.
11. The application of thermo-flask is not limited to
household purposes but extends to scientific labs
and experiments. Also known as the Dewar flask,
Scottish scientist Sir James Dewar invented the
thermos flask in 1892. Dewar worked in cryogenics
(science that deals with phenomena at low
temperatures) and needed vessels to store
chemicals at extremely low temperatures and in
stable conditions. Therefore it leads to the
invention of the flask; by placing one glass bottle
in another larger one and evacuating the air
between the bottles.
12. A thermos flask, a double-walled glass vessel, keeps
hot fluid hot and cold fluid cold for an extended
period. It contains a combination of layers that prevent
heat transfer due to conduction, convection, and
radiation.
The key learnings from this post:
Vacuum layer: The vacuum layer prevents the heat
transfer through conduction and convection. The lack
of medium ensures that there is no conduction and
convection.
Reflective coating: The reflective coating on the glass
ensures that there is no radiation heat transfer. The
reflective coating reduces the value of emissivity of the
material.
