Are there really convection currents?
Lexicon> letter K> convection
Definition: the circulation of a gas or a liquid, often driven by temperature differences
Categories: Basic Terms, Physical Basics, Heat and Cold
Author: Dr. Rüdiger Paschotta
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Original creation: 08/03/2011; last change: 03/14/2020
convection initially simply means a circulation (flow) of a gas (e.g. air) or a liquid. This can be driven by a fan, for example (forced convection). Often, however, convection is also driven solely by temperature differences (free convection). For example, the air in the vicinity of a radiator is heated, expands and flows upwards because its density is reduced so that the buoyancy is slightly greater than its weight. Cooler air flows in from below and is then also heated. This creates a flow pattern throughout the room (see Figure 1): The air flows up the radiator, along the ceiling to the opposite wall, down there and back along the floor. In this way, the air can heat e.g. B. on the ceiling, walls and floor. Convection is a mechanism by which heat is transported from the radiator to the whole room. This heat transport is much stronger than that through heat conduction, since the heat conductivity of the air is very low.
Heat transport by convection is particularly dominant in heavily ribbed radiators; this is why such radiators are also called Convectors. As the temperature difference between the radiator and the room air increases, the convection becomes considerably stronger: more air is circulated and the rising air also becomes warmer. This results in a relatively high value for the so-called radiator exponent.Cold bodies can also drive convection, just in the opposite direction!
Cold bodies can also drive convection. For example, air can be cooled at a window, thereby sinking downwards and thus causing convection in the opposite direction as with a radiator. If there is a seat in the front of the window, it can get uncomfortably cool there. Often, a radiator is installed below the window, which forces convection upwards and thus prevents chills at the seat, although of course it also increases the heat loss at the window. It is better to use a window with good thermal insulation so that a good level of comfort is achieved even without a radiator underneath.Insulation materials often contain a lot of air and additional material that prevents convection.
Materials for thermal insulation often contain a lot of air, as this has a low thermal conductivity. At the same time, however, the convection of this air must be prevented. This is e.g. B. achieved in foams that the air is held in many small bubbles that are not connected to each other.
Convection also occurs in liquids, such as water. There are still isolated central heating systems in which the heating water is circulated solely by convection due to the temperature differences, i.e. without a heating circulation pump. One speaks of one Gravity systemSince gravity is essential for the convection mechanism. However, this principle requires generous pipe cross-sections and a high flow temperature and is therefore unfavorable in terms of energy, especially if a heat pump or a condensing boiler is used as the heat source. That is why such gravity systems are rarely built today. The energy requirement of a modern heating circulation pump is far less than the efficiency losses of a gravity system.
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See also: heat, radiator, central heating, conduction, gravity system, radiator exponent
as well as other articles in the categories of basic concepts, physical principles, heat and cold
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