How Things Work - Thermal Conductivity & Insulation

Thermal Conductivity & Insulation

Thermal Conduction

Thermal conduction is responsible for transporting heat through a solid.

Thermal Conductivity

Material	Thermal Conductivity (J/s/m/K)
Aluminum	238
Copper	397
Iron	79.5
Air (@ 20 ^oC)	0.0234
Glass	0.8
Water	0.6
Ice	2
Wood	0.08

The rate at which energy is transported (in Joules per second) depends on three factors

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

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

Q/t = K A (T₂-T₁) / d

Calculation

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