Thermal Conductivity &
Thermal conduction is responsible for transporting heat through a solid.
The rate at which energy is transported (in Joules per second) depends
on three factors
|Air (@ 20 oC)
The term thermal conductivity is associated with the first of these. Heat
can pass through some materials, particularly metals, very easily. These
materials have a high thermal conductivity. Other materials, such as fiberglass
have a lower thermal conductivity, they will pass heat only slowly. Calling
the thermal conductivity K, the area A, the thickness d, and the inside
and outside temperatures T1 and T2 then the rate
at which heat is conducted is given by
What the solid is made from.
The temperature gradient, which itself depends on
the difference in temperature between the inside and outside surfaces
the thickness of the material
The area of the solid
Q. What is the rate at which heat is transported through glass
which measures 30 cm by 50 cm with a thickness of 3 mm, if the inside temperature
is 280 K and the outside temperature is 325 K?
A. There are three parts to this calculation
The temperature difference is 325 - 280 = 45 K, and so the temperature
gradient is 45 K / 0.003 m = 1.5 x 104 K/m.
The area is 0.3 m * 0.5 m = 0.15 m2.
The rate at which heat passes Q/t = K A (T2-T1) /
d = 0.8 * 0.15 * 1.5 x 104 = 1800 J/s = 1800 W.
So how does this apply to refrigerators?
Once a refrigerator has cooled down to its normal
operating temperature the only task for the cooling system is to remove
any heat which enters the refrigerator. the more heat enters the refrigerator,
the harder the cooling system has to work. Part of the heat input is inevitable,
from opening and closing the door. However, a major portion arises from
heat being conducted through the refrigerator walls. If we can reduce this
amount of heat then the cooling system can be cycled 'off' for as long
as possible, thereby cutting energy usage and cost. The means for doing
this is to surround the refrigerator by a layer of material which has a
low thermal conductivity, a practice known as lagging.
Generally speaking the best materials to use
are those which trap air, since the air is a very poor conductor of heat.
Fiberglass wool is a good material for precisely this reason, its fibbers
will trap considerable amounts of air, providing it is not squashed, since
compressing it will squeeze the air out like squeezing water out of a sponge.
Another common material is urethane foam, which contains millions of tiny
air bubbles, and which also has the advantage of being solid when cured,
and difficult to compress.
How Things Work | Refrigerators