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Air Cooling of Electronics Enclosures A Technical Briefing

Learn about the challenges and advancements in air cooling for electronics enclosures, heat transfer mechanisms, fan characteristics, and how servers are cooled. Discover the importance of thermal management and key strategies. Get insights into forced convection, fan theory, and best practices for efficient enclosure cooling.

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Air Cooling of Electronics Enclosures A Technical Briefing

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  1. Air Cooling of Electronics Enclosures A Technical Briefing

  2. Topics • Background and problem description • Heat transfer • Fan characteristics • How servers are cooled • Wrap-up

  3. Air Cooling of EnclosuresBackground and Problem Description • Advances in electronics are pushing cooling issues to the forefront • More applications • Smaller components More heat in less volume • Faster speed • Previous methods of thermal management are becoming insufficient

  4. Air Cooling of EnclosuresBackground and Problem Description Major causes of electronic failures Source: BCC, Inc. Report GB-185R

  5. Air Cooling of EnclosuresBackground and Problem Description Thermal management requirements forData/Communications Centers are increasing From Industry Cooling Consortium: IBM, Lucent, HP, Dell, Nortel, Cisco, and Others

  6. Air Cooling of EnclosuresBackground and Problem Description Thermal management protects against premature failure MIL-HNBK-217 and Bellcore Standard

  7. Air Cooling of EnclosuresBackground and Problem Description 2% Clockspeed Increase for every 100C CPU Reduction Data for a 1,500 MgHz CPU Kyrotech and IBM Report

  8. Air Cooling of EnclosuresHeat Transfer • Three types of heat transfer – conduction, convection and radiation • Conduction is heat transfer by molecular interaction (think of direct contact) • Convection is heat transfer by a combination of fluid mixing and conduction (think of heat transfer from a surface to a fluid like air) • Free convection – fluid movement caused by density changes • Forced convection – fluid movement caused by a pump or other device • Radiation is emission of electromagnetic waves (for example: the sun) • Conduction and convection are the primary heat transfer mechanisms involved in enclosure cooling – radiation is a small player • Our focus today is on forced convection (air cooling) • Convection is most easily controlled by users • Conduction is a function of the enclosure construction and location in the facility

  9. Air Cooling of EnclosuresHeat Transfer • Convection heat transfer from a surface to an airflow is governed by the equation q = hA(Tw – Tf) Where q = the heat transfer rate h = the convection heat transfer coefficient A = the surface area Tw = the temperature of the surface Tf = the temperature of the fluid • In enclosure cooling, the factors h, A and Tw are determined by the server • We can affect q (the rate at which heat is transferred to the air) by helping determine Tf (the temperature of the air entering the server)

  10. Air Cooling of EnclosuresFan Characteristics • Fan theory • Fans work against a resistance called static pressure • Static pressure can be likened to blowing through a drinking straw – how hard the fan has to push to get the air to move through the path • Each air path has its own unique static pressure (think of different sized drinking straws) • Fan performance is dependent on the static pressure of the air path

  11. Air Cooling of EnclosuresFan Characteristics • Axial fans • Most commonly used fans in electronics cooling • Wide variety of sizes and capacities are used • Good application for air movement in low static pressure environments • Centrifugal fans • Work well against higher static pressure • More readily adaptable to ducting to provide for better delivery of air • More readily adaptable to filtration • Typically more efficient than axial fans – as power requirements rise in server facilities, these fans will become more popular • Typically allow operation at lower speeds which helps manage noise

  12. Enclosure with few servers Additional cooling air that a centrifugal blower provides Air Cooling of EnclosuresFan Curve Centrifugal Blower Static Pressure Axial Fan Airflow (CFM)

  13. Enclosure with more servers The more resistance, the better a blower works compared to a fan Air Cooling of EnclosuresFan Curve Centrifugal Blower Static Pressure Axial Fan Enclosure with few servers Airflow (CFM)

  14. Air Cooling of EnclosuresHow Servers Are Cooled • Servers (and other electronic equipment designed for rack-mounted use) are typically fan cooled • Fans are integral to the servers • Air inlet is at the front of the server • Air inlet temperature is affected by room temperature and enclosure front door configuration • Air inlet temperature typically varies from the top of the enclosure to the bottom • Air discharge is at the rear of the server • Discharge path often congested by cable management • Warm discharge air not forcibly exhausted very well from the enclosure

  15. Air Cooling of EnclosuresHow Servers Are Cooled • Airflow is low within the server • Small fans • Congested air path • Server room air conditioning provides the cool air the servers use • Cooling the entire room requires tons of refrigeration • As servers require more cooling, adding room air conditioning capacity without improving airflow to the server is: • Costly • Inefficient • Can make the room uncomfortable for workforce • Adds to room noise • Improving server cooling needs a more direct solution

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