THERMAL MANAGEMENT
Liquid cooling for precise temperature control Electronics that generate a lot of heat in a small volume often benefit from swapping a cooling fan for a system based on a liquid coolant. Greg Ducharme • Laird Thermal Systems
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specific heat of water is higher than the specific heat of air—about 4,200 J/kg°C vs. 1,005 J/kg°C. This is another way of saying that air performs better as a thermal insulator than as a thermal conductor. That, in a nutshell, is why liquid cooling can be more effective than air cooling. Liquid cooling systems can dissipate a large amount of heat in densely packed electronic enclosures to facilitate more complex system designs. Liquid cooling systems combine a high capacity for transferring waste heat with a high coefficient of performance (COP, the ratio of useful cooling provided to work required). There are two types of liquid cooling setups. First are liquid heat exchanger systems. They use either a liquid-to-liquid heat exchanger or a liquid-to-air heat exchanger to cool the coolant in a liquid circuit. In the first case, the coolant is often cooled below ambient temperature using a facility (often called primary) coolant loop. Alternatively, the aircooled system will cool the coolant to near ambient temperature. The remainder of both systems consist of a pump to circulate the coolant, often a tank to give the pump a constant supply of coolant, and a liquid circuit to transfer coolant from the heat source to the liquid cooling system. Second are liquid chillers (recirculating chillers). They use a compressor system instead of a liquid heat exchanger assembly. A chiller uses a vapor compression mechanical refrigeration system that connects to the coolant system through a device called an evaporator. Refrigerant circulates through an evaporator, compressor, condenser and expansion device. The evaporator functions as a heat exchanger such that heat captured by the process coolant flow transfers eeworldonline.com
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to the refrigerant. As the heat-transfer takes place, the coolant evaporates, changing from a low-pressure liquid into vapor. The coolant then flows to a compressor, which ensures the pressure in the evaporator remains low enough to absorb heat at the correct rate and raises the pressure in outgoing coolant vapor to ensure its temperature remains high enough to release heat when it reaches the condenser. The coolant returns to a liquid state at the condenser. The latent heat given up as the refrigerant changes from vapor to liquid is carried away from the environment by a cooling medium (air or water). Liquid cooling systems have several advantages over conventional air-cooled systems: High heat-pumping capacity: Liquid heat exchangers can reduce the thermal resistance of conventional heat sink fan dissipation
Liquid cooling systems can dissipate a large amount of heat. For example, in a medical X-ray system, a liquid heat exchanger system cools the coolant in a liquid circuit by the use of a liquid-to-air heat exchanger. The system contains a pump that circulates coolant and a liquid circuit to transfer coolant from the heat source to the liquid cooling system. An expansion device enables the cooling circuit to be completely sealed from the outside environment and compensates for the thermal expansion of the fluid over its wide operating temperature range.
Liquid-to-liquid cooling
10 • 2020
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