Principles of heat transfer

When a hot surface is surrounded by an area that is colder, heat will transfer until both are at the same temperature. This transfer of heat takes place by one or more of 3 methods :

• Conduction
• Convection
• Radiation

Conduction. A process by which heat flows by molecular transportation along or through a material or from one material another. Conduction requires contact between 2 materials for heat transfer to occur. It can occur in solids, liquids or gases.

Convection. A process that can only occur in liquids and gases which involves a change in density of the liquid or gas resulting from a change in temperature. If in contact with a solid, heat loss would initially be through direct contact(conduction) but resulting density change of the liquid or gas would cause it to be displaced and a continuous flow(or convection) would result.

Radiation. A process by which heat is emitted and transmitted across space as energy. It requires no contact or medium such as air to transfer and readily occurs across a vacuum. All bodies emit radiant energy and the rate of еmission is governed by temperature difference, the distance and the emissivity of the surfaces.

How insulation works

In order to perform effectively, insulation must restrict heat flow by any and preferably all three methods of heat transfer. Most insulants (but not all) adequately reduce convection and conduction by having a cellular structure. The radiation component is reduced by absorption into the body of the insulant and can be further reduced by the application a bright foil on the outer facing of the product.

To reduce heat transfer by conduction, an insulant should have a small ratio of solid volume to void volume. Additionally, a thin wall matrix, a discontinuous matrix or a matrix with minimum point contact are all beneficial. Conduction across the voids can be further reduced by the use of inert gases rather than still air.

To reduce heat transfer by convection, an insulant should have a cellular structure or a high vold content. Small cells or voids inhibit convection within them and are less prone to transfer energy to adjacent cells.

To reduce heat transfer bу radiation, insulation is placed in close contact with the hot surface. Radiation may still penetrate an open cell material but rapidly absorbed within the immediate matrix and energy is converted to conductive or convective heat flow.