Convection cooling occurs when fluid motion transfers heat and can be natural or forced. Natural cooling occurs from changes in fluid density, while forced convection occurs when an external force moves the fluid. Conduction is heat transfer through a material without movement. Natural convection occurs in the Earth’s atmosphere and oceans, while forced cooling is common in homes and businesses. Fans and evaporative cooling systems can improve cooling performance.
Convection cooling is any heat transfer that occurs from a fluid motion. Both liquids and gases can exhibit convection cooling and the effect can be natural or forced. Natural cooling occurs from heat transfer due to changes in fluid density, such as warm air rising and cooler air sinking. Forced convection occurs when an external force is added to move the fluid, such as a fan moving the air or a spoon stirring a liquid.
Heat transfer occurs mainly by conduction and convection. Conduction is a transfer of heat through any material, without movement of the material. An example of conduction is a metal pan heated by a flame on a gas stove. The gas flame heats the bottom of the pan and conduction will transfer the heat to the rest of the pan. When the heat is out, the convection will cool the pan as the heat transfers and rises to the air around the pan.
Natural convection cooling occurs in the Earth’s atmosphere and oceans. The air is heated by ground heating and rises. As air rises, it cools and returns to the surface, creating global air circulation and weather patterns. Ocean currents carry warm water to cooler oceans and cooler water sinks and move to warmer regions. Sunlight adds energy by heating air or water, and the rotation of the Earth imparts energy, but the motion is considered natural rather than forced.
Convection cooling also occurs within the Earth. The molten core, heated by the decay of radioactive elements, rises to the outer crust of our planet. Convection cools the molten core material and slowly returns to the core. This motion causes our continents to drift slowly above the molten core, a phenomenon called tectonic plate motion.
Forced cooling is common in homes and businesses. Air conditioning and heating systems used forced convection of air to move heat into or out of a building. Fans inside electronic equipment used forced convection cooling to move air over the electronic components. Refrigeration systems use fans both to remove heat from the condensing coils, but also to cool the compressor and move the air inside the refrigerated compartment.
Cooling fans have demonstrated convection effects for centuries. With the invention of electricity, electric motors could be used to drive table and ceiling fans. Since the end of the 20th century, sophisticated ceiling fans equipped with thermostats for automatic control of their functions have been available. All are based on the same principle of cooling the human body by moving air through the skin.
Convection can be combined with evaporation to improve cooling performance. Evaporative cooling systems, which use forced air passed through a moist evaporative pad to cool interior spaces, are popular in areas with very low humidity. These systems cool the interior space by removing heat from the incoming air through evaporation. The resulting airflow is colder, but contains more moisture. Evaporative systems don’t work well in areas with higher relative humidity, because cooling won’t occur and the interior could get quite humid.
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