How is the heat transfer calculated

Heat Transfer: Types, Practical Examples & Calculation


Heat transfers through liquids, gases and bodies. Physically, however, in order to move and reproduce, heat needs a host on which it can “saddle up”.

This condition applies to all three possible types of heat transfer, which differ in terms of heat conduction, convection and heat radiation. For more information, see the following:

Overview of types of heat transfer

Heat conduction:

Heat transfers through solid bodies. This thermal energy depends on the condition and conductivity of the transmitter used. In physics, this type of heat transfer is based on the temperature difference that results from the participation of different substances.

Example:

A saucepan is on the stove and warms up over the stove top. After a while, the saucepan and even the handle will be hot or warmed up. Metals, in particular, are good heat conductors.

Convection (heat flow):

The second way heat can take is by flowing. This occurs in liquids and gases. Movable and moving transport media take the heat “with them”. The best known and most typical implementation is heat transfer by convection. Here, flowing air serves as a vehicle.

Example:

Warm air from the radiator rises and warms the living space.

Thermal radiation

No immediate transport aid is required in the case of thermal radiation. Electromagnetic waves move through space similar to light radiation. The heat transfer only begins when the waves hit solid bodies. This conversion is similar to the principle of a heat exchanger. Physical energy is converted from a primarily unusable state into usable heat by solid bodies.

Example:

Infrared heaters use infrared rays to heat ceilings, walls and furniture. These give off the heat absorbed in this way to the room.

What is the scope of the transport based on?

With every heat transfer, the point in time of transfer between different media or hosts decides on the degree of energy utilization and the amount of heat. In all types of heat transfer, it is composed of several factors that form a heat transfer coefficient. It is also known as the heat transfer coefficient. This value summarizes the material properties of output energy, hosts and heat exchangers:

Thermal conductivity of the substances involved

  • Density of the substances involved
  • Specific heat capacities
  • Flow behavior with fluid substances
  • Size and area of ​​heat-exchanging surfaces
  • Type and condition of the surfaces and elements involved

While individual media, substances and hosts have fixed values ​​such as conductivity and losses, the heat transfer coefficient is a variable. I have it calculated using fixed values. The result shows how much of the transported or arrived energy is converted into heat. The proportion describes the extent of heat transfer at material transitions. A clear picture forms the surface of a radiator. The coefficient describes the heat that has arrived in the passing air at the point of "transfer".

Calculation types and formulas

There are basic calculation formulas for all three types of heat transfer. However, every “bare” formula must be supplemented in everyday use in order to do justice to the specific ways of heat transfer.

For the basic calculation, the size of the contact areas between the media or hosts is included. Then there are the temperatures of the substances involved and the time factor. For flowing and thermal transmission paths, the conductivity of the host, the surface temperature and size of the emitting energy source and the density of the flowing material must be taken into account. Similar factors apply to convection heat transfer. For the calculation, the applicable collection of formulas contains individual calculation factors and values ​​that lead to approximate values. Mobility always leads to fluctuations, so that an absolute value would only be possible under unrealistic laboratory conditions. Strictly speaking, the coefficient can only represent an average value.

Practical applications in heating technology

In practical application, efficient and successful heat transfer requires a good transfer ratio of the energy between the substances involved.

Common and typical types of heat transfer are examples of how this ratio is optimized in heating technology:

  • Ventilation accelerates the air in order to dissipate the “injected” heat more quickly
  • Large-area radiators "touch" larger amounts of air passing by
  • Flat radiators have several panels mounted one behind the other and multiply the impact surfaces of the ambient air
  • Surfaces and bodies that are irradiated with heat are provided with ideally reflective and / or heat-storing surfaces
  • In high rooms, the transfer of radiant heat to the ceiling prevents heat loss or waste due to rising heat

A decisive aspect in all constructions is the speed and duration of the need.