Heat can move up, down, or sideways, depending on the situation. The idea that hot air has an inherent desire to flow up is a misconception that we in the data center airflow management business would like to see dissipate.
Temperature difference is the major factor with regards to the direction and rate of heat transfer. Because air tends to move towards thermal equilibrium, it is important to maintain physical separation of hot and cold air in data centers; the need for hot and cold air separation was the reason that the data center containment industry came into existence. The laws of thermodynamics state that air moves from areas of higher temperature towards areas of lower temperature. Air is a fluid that accounts for both density and buoyancy. When air is heated the molecules move around faster, which causes it to expand, and as it expands its density becomes lower. The warmer, lower density air will rise above the denser, cooler air.
Pressure is another determining factor when looking at air movement. The flow of air from areas of high pressure to areas of low pressure is an embodiment of Newton’s third law. Equilibrium is what also drives movement between areas of differing pressure, so uninhibited air will continuously move from high to low pressure until equilibrium is reached. This movement towards equilibrium is also known as expansion.
Principles of air movement:
1) Heat Transfer:
a. Conduction: Air flows from a higher temperature region to a lower temperature between mediums
that make physical contact.
b. Convection: Heat transfer due to the movement of a fluid; can be free/natural, or forced.
2) Air flows from a higher pressure to a lower pressure
What does this have to do with data center airflow management?
The data center containment industry has been inundated with graphs depicting airflow, most of which show large, sweeping lines indicating the flow of air. In most cases, the airflow depicted is a result of a mechanical device, usually a fan. The data presented by these graphs tends to lead one to believe that mechanically induced airflow will sufficiently separate hot exhaust air from cold intake air. In real-world scenarios, air curtains are inefficient and ineffective.
Modern mechanical air conditioning systems rely on four sided duct systems to deliver supply air to the source of the heat load, and the return is moved by the same means. This is the only way to ensure the separation of supply and return airflow. Systems administrators and building managers should be dubious of airflow management systems that require an increase in energy to accomplish air separation. Instead, it is best to apply the simplest principles of airflow when designing a system aimed at full separation of supply and return airflow.
If you would like to learn more about the flow of air, please see the following link:
Learn How Air Moves Through This Incredible Optical Device
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