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3.1 Heat transport

Heat flows arise in bodies whose temperature level is not balanced. The heat basically flows from places with a higher temperature to places with a lower temperature, i.e. in the direction of the temperature gradient. If someone with a body temperature of 37 ° C leans against a cold window pane with a surface temperature of 5 ° C, he gives off heat and freezes. The term “cold” is a subjective sensation. From a physical point of view, every substance contains a certain amount of heat; colloquially it is “cold” below 0 ° C.

If a substance had 0 K (Kelvin), it would contain 0 J (Joule). However, this is only conceivable in theory.

Thermal energy can be transferred in three different ways:

• Thermal conduction

• heat transfer (convection)

• Thermal radiation.

3.1.1 Thermal radiation

Warmed bodies emit electromagnetic waves with a high proportion in the infrared range. No matter is required for their transmission. The sun's rays are made up of 46 percent long-wave heat rays (infrared, wavelength 780 to 3000 nm), 47 percent visible light rays (wavelength 380 to 780 nm) and 7 percent ultraviolet rays (wavelength below 380 nm) and are both air-filled and in vacuum space such as B. transmitted in space (Fig. 3-1). When the rays hit matter such as wood, glass, skin, stone, etc., the radiation energy is converted into heat movement of the molecules (energy conversion). Humans perceive radiant heat as the most pleasant form of heat transfer. It is absorbed by the heat receptors in our skin and converted into body heat in the tissue. A comfortable radiant heat climate, such as from the tiled stove (approx. 90% radiation), corresponds to the climate in nature in summer. The absorption capacity of a body exposed to thermal radiation depends on the nature of the surface.

Bodies with dark and rough surfaces absorb (absorb) more heat (black collector surface), light and smooth ones reflect (reflect back) more. For this reason, light-colored clothing is worn in summer, which reflects the sun's rays.

The most important heat producer for us humans is the sun. By means of radiation it delivers approx. 1.37 kW / m² min (solar constant) in summer when it hits vertically.

The emission ratios of the material surfaces are given in comparison to the total black body, which has the highest radiation constant with CS = 5.67 W / m² · K4 (Tab. 3-1). It is noticeable that bare metals have an extremely low emission ratio, non-metals a high emission ratio. Only with the metal coating of the inner pane did the energy loss of the glazing decrease immensely (Fig. 3-2).

3.1.2 Heat transfer (convection)

This type of heat transfer occurs through mass transport (flow). It is therefore only possible in gases and liquids.

Currents can be caused by self-buoyancy z. B. warm air (thermal), warm rising water (gravity heating) or forced circulation z. B. caused by a fan or pump.

Examples of convection:

Hair dryer, hot air blower, hot water pipe with gravity or circulation pump, fan in the drying chamber for technical wood drying, Gulf Stream in the Atlantic.

The gas or liquid flow transports the heat away from the heat source to less warm surfaces such as walls, ceilings, floors (Fig. 3-4).

The ratio of radiation to flow is determined by the surface of the radiator on the one hand and by its temperature on the other. The larger the surface, the higher the proportion of radiation. The higher the temperature, the higher the proportion of convection.

In buildings, the heat dissipation has a particularly negative effect when joints z. B. in windows, doors or when connecting roof-wall are not made tight and it then "pulls", d. H. leads to large ventilation heat losses. Convection is useful when the moving air in a ventilated wall cladding or a ventilated roof skin absorbs any moisture and transports it away.

3.1.3 Thermal conduction

This form of heat transport is linked to the presence of matter; the heat is transferred from particle to particle in solid, liquid and gaseous bodies without any mass being transported. The kinetic energy of the molecules and atoms at a higher temperature is transferred to those at a lower temperature through collision processes. The material itself remains unmoved, only the energy is passed on. (Fig. 3-5).

The speed of heat transport depends on the material. High density fabrics e.g. B. Metals such as silver, copper, iron etc. conduct heat quickly, they are good heat conductors.

Wood, polyurethane foam, polystyrene foam (e.g. Styrofoam), mineral wool etc., gases such as (still) air and, above all, light gases such as argon or xenon conduct heat slowly - they are known as poor heat conductors. Vacuum insulates completely.

Examples where conduction occurs:

Stovetop, iron, veneer press, soldering iron, concrete wall. Warm room air in buildings is largely given off to the cold outside air via thermal conduction in the building envelope.

Kuno Schlatter

BM online 04 | 2003

Tags convection temperature

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