How is Heat Transferred?

Any temperature difference within a body or between different bodies or systems leads to the transfer of heat, until a temperature equilibrium is reached. This heat transfer can take place in three different ways:

• through conduction
• through convection
• through radiation

In real situations, heat transfer takes place simultaneously but not equally in all three ways.

Conduction is the transfer of heat by direct contact of particles. It takes place between solid bodies or between thin layers of a liquid or gas. Vibrating atoms give off a part of their kinetic energy to the adjacent atoms that vibrate less.

Convection is the transfer of heat between a hot solid surface and the adjacent stationary or moving fluid (gas or liquid), enhanced by the mixing of one portion of the fluid with the other. It can occur as free convection, by natural movement in a medium as a result of differences in density due to temperature differences. It can also occur as forced convection with fluid movement caused by mechanical agents, for example a fan or a pump. Forced convection produces significantly higher heat transfer as a result of higher mixing velocities.

Radiation is the transfer of heat through empty space. All bodies with a temperature above 0°K emit heat by electro-magnetic radiation in all directions. When heat rays hit a body, some of the energy is absorbed and transformed to heat up that body. The rays that are not absorbed pass through the body or are reflected by it. In real situations, heat transmission is the sum of the simultaneous heat transfer through conduction, convection and radiation.

The logarithmic mean temperature difference is defined as the relationship between the temperature differences at the heat exchanger's two connection sides according to the expression: